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Improving the efficiency and sustainability of direct arylation polymerization for conjugated polymer synthesis
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Improving the efficiency and sustainability of direct arylation polymerization for conjugated polymer synthesis
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Content
IMPROVING THE EFFICIENCY AND SUSTAINABILITY OF DIRECT
ARYLATION POLYMERIZATION FOR CONJUGATED POLYMER SYNTHESIS
by
Liwei Ye
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
CHEMISTRY
May 2022
Copyright 2022 Liwei Ye
ii
Dedication
To my family and friends.
iii
Acknowledgments
I would first like to thank my family for their support throughout my life.
Specifically, to my parents for showing me how to keep my head up during difficult times,
and to always keep moving forward. I would like to also thank my grandparents who raised
me and encouraged me throughout the years.
I would next like to thank my doctoral advisor, Prof. Barry C. Thompson. You have
taught me so much and have given me many opportunities to pursue different avenues of
chemistry during my time here at USC. I especially thank you for giving me the chance
and freedom to develop my own research creativity. It was in your lab and through your
motiviation I was able to develop scientifically, professionally, and personally.
Additionally, I would like to thank the committee members Prof. Megan Fieser and Prof.
Andrea Armani for agreeing to serve on my qualifying examination, and defense
committees.
To Dr. Nemal Gobalsingham, Dr. Robert Pankow, and Dr. Sanket Samal, I thank
you all for guiding me throughout the years, training me in lab and on instrumentation, and
helping me through the challenging times of graduate school. Without your help my
success would not have been possible. Additionally, Robert and Sanket, you have become
great friends, and I look forward to the continued growth of our friendship. I would like to
especially thank Dr. Robert Pankow for your mentorship in lab and intelligent discussion
on Direct Arylation Polymerization. Together we have really pushed the boundaries of this
project and have achieved so much with it. I look forward to becoming your colleague
again at Northwestern University. Pratyusha, Negar, Alex, Qingpei, and Tanin, you have
all been great colleagues and friends. I thoroughly enjoyed your company, and you were
iv
always there for me at the time of need. The work in this dissertation would not have been
possible without the assistance of the following individuals: Dr. Robert Pankow (project
creation and design, monomer synthesis, polymer synthesis and characterizations in
Chapters 2, 3, 4), Alexander Schmitt (monomer synthesis, polymer synthesis in Chapter
3, 4), Mami Horikawa (monomer synthesis in Chapter 2), Tanin Hooshmand (monomer
synthesis and polymer synthesis in Chapter 6, 7).
v
TABLE OF CONTENTS
Dedication ii
Acknowledgments iii
List of Tables xi
List of Figures xiii
List of Schemes xxiv
Abstract xxix
Chapter 1: Improving the Efficiency and Sustainability of Catalysts for Direct
Arylation Polymerization 1
1.1 Introduction 1
1.2 Highly Efficient Pd-Catalysts for Direct Arylation Polymerization 4
1.2.1 Pd-Catalysts in coordinating solvents 4
1.2.2 Pd-Catalysts in non-coordinating solvents (Ozawa-derived conditions) 28
1.3 Efficient Catalysts for Oxidative Direct Arylation Polymerization (Oxi-DArP) 50
1.3.1 Pd-Catalysts for Oxi-DArP 50
1.3.2 Thiazole (Tz)-C-2 Directed Cu-Catalyzed Oxi-DArP 54
1.4 Sustainable Cu-Catalysts for Direct Arylation Polymerization 56
1.4.1 Small-molecule Cu-Catalyzed Direct Arylation 56
1.4.2. Development of Cu-Catalyzed Direct Arylation Polymerization 58
1.5 Conclusion and Outlook 65
1.6 References 69
vi
Chapter 2: Green Solvent Processed Amide-Functionalized Conjugated Polymers
Prepared via Direct Arylation Polymerization (DArP) 106
2.1 Introduction 106
2.2 Experimental 109
2.3 Results and Discussion 111
2.4 Conclusions 116
2.5 References 117
Chapter 3: Synthesis of Conjugated Polymers using Aryl-Bromides via Cu-
Catalyzed Direct Arylation Polymerization (Cu-DArP) 124
3.1 Introduction 124
3.2 Experimental 128
3.3 Results and Discussion 132
3.4 Conclusion 133
3.5 References 134
Chapter 4: An Efficient Precatalyst Approach for the Synthesis of Thiazole-
Containing Conjugated Polymers via Cu-Catalyzed Direct Arylation Polymerization
140
4.1 Introduction 140
4.2 Experimental 146
4.3 Results and Discussion 149
4.4 Conclusion 151
vii
4.5 References 152
Chapter 5: p-Cymene: A Sustainable Solvent that is Highly Compatible with Direct
Arylation Polymerization (DArP) 158
5.1 Introduction 158
5.2 Experimental 161
5.3 Results and Discussion 166
5.4 Conclusions 168
5.5 References 169
Chapter 6: “In-Water” Direct Arylation Polymerization (DArP) under Aerobic
Emulsion Conditions 175
6.1 Introduction 175
6.2 Experimental 178
6.3 Results and Discussion 184
6.4 Conclusion 185
6.5 References 186
Chapter 7: Recycling Heterogenous Catalysts for Multi-Batch Conjugated Polymer
Synthesis via Direct Arylation Polymerization 193
7.1 Introduction 193
7.2 Experimental 196
7.3 Results and Discussion 201
7.4 Conclusions 202
viii
7.5 References 203
Biographical Sketch 209
Appendix A 210
A.1 General 210
A.2 Synthesis 212
A.3
1
H-NMR and
13
C-NMR for monomers 2, 3a, 3b. 217
A.4 Polymer NMR 221
A.5 FTIR spectra 228
A.6 Polymer Solubility Tests 229
A.7 Detailed Procedure for UV-Vis Film Preparations 230
A.8 UV-VIS Data and Spectra in Solutions 232
A.9 Space-charge Limited Current (SCLC) Mobility Measurements. 235
A.10 Cyclic Voltammetry (CV) measurements. 237
A.11 GPC Traces 239
A.12 DSC Traces 243
A.13 GIXRD Spectra 244
A.14 References 245
Appendix B 246
B.1. General 246
B.2 Additional optimization of Cu-DArP conditions and polymerization results 247
B.3 Monomer Synthesis 250
ix
B.4 Representative polymerization procedures 252
B.5 NMR of Monomers 258
B.6 Polymer NMR 260
B.7 References 264
Appendix C 266
C.1 General 266
C.2 Monomer Synthesis 267
C.3 Cu-precatalyst Synthesis 269
C.4 Representative polymerization procedures 271
C.5 Monomer NMR 275
C.6 NMR of Cu-precatalysts 277
C.7 Polymer NMR 279
C.8 Proposed end-group assignments 282
C.9 References 282
Appendix D 285
D.1 General 285
D.2 Monomer Synthesis and NMR 286
D.3 General Polymerization Procedure 288
D.4 Polymer NMR 293
D.5 Detailed NMR analysis for P2, P3, and PCDTBT 298
D.6 UV-Vis Spectra 301
x
D.7 References 303
Appendix E 305
E.1 General 305
E.2 Visualization of emulsions and DArP in emulsion conditions 307
E.3 General polymerization procedures 308
E.4 Polymer NMR 311
E.5 UV-Vis Spectra 315
E.6 References 316
Appendix F 317
F.1 General 317
F.2 Preliminary results for catalyst-recycling experiments 318
F.3 General polymerization procedures 319
F.4 Additional Recycling Experiments using SiliaCat Pd-DPP 326
F.5 The testing of other commercially-available heterogenous Pd catalysts 327
F.6 Polymer NMR 328
F.7 Detailed NMR analysis for representative PEDOTF 337
F.8 UV-vis spectra 338
F.9 References 342
xi
List of Tables
Table 2.1 DArP conditions and polymerization results.
......................................................................................................................................... 109
Table 3.1 Cu-DArP conditions for the synthesis of PDOF-OD and polymerization results
. ........................................................................................................................................ 127
Table 4.1 Cu-DArP conditions for the synthesis of PF-5BTz and polymerization results.
. ........................................................................................................................................ 145
Table 5.1 DArP conditions for the synthesis of P1-P3 depicted in Scheme 5.1. ............ 162
Table 6.1 Optimization for the synthesis of P1 using emulsion conditions depicted in
Scheme 6.2.. ..................................................................................................................... 179
Table 7.1 Initial testing of the recyclability of SiliaCat® Pd-DPP using DArP conditions
depicted in Scheme 7.2.. .................................................................................................. 195
Table 7.2 Further optimization of the recycling experiments using SiliaCat® Pd-DPP
using DArP conditions depicted in Scheme 7.2.. ............................................................ 198
Table A.1 P1 (Table 2.1, Entry 5, Mn = 15.4 kDa) maximum solubilities in different green
solvents.. .......................................................................................................................... 229
Table A.2 Summary of optical bandgaps, λmax values in various solvents. .................... 235
Table A.3 SCLC-Hole Mobility Data. ............................................................................ 237
Table B.1 Additional polymerization results for the synthesis of P1 (PDOF-OD) using Cu-
DArP. ............................................................................................................................... 240
Table B.2 Optimization of Cu-DArP conditions for the synthesis of TPD-copolymers.248
Table F.1 Testing of recyclability of SiliaCat Pd-DPP using DArP conditions with the use
of neodecanoic acid (NDA) in place of PivOH. .............................................................. 326
xii
Table F.2 Testing of recyclability of FibreCat and Pd/C using DArP conditions shown in
Scheme F.2. ..................................................................................................................... 327
xiii
List of Figures
Figure 1.1 Proposed catalytic cycle for direct arylation via the SEAr mechanism and the
role of the amide-coordinating solvent (DMF) in this system proposed by Antoine et al….
............................................................................................................................................. 7
Figure 1.2 Proposed Pd-catalytic cycle for direct arylation via the concerted-metalation-
deprotonation (CMD) step.. ............................................................................................... 14
Figure 1.3 Enthalpic contributions to the CMD transition state energy, which is divided
into bond distortion energy (Edist) and interaction energy (Eint) proposed by Fagnou et al..
........................................................................................................................................... 15
Figure 1.4 Plausible mechanism of suppressing β-defects using a bulky carboxylate ligand.
........................................................................................................................................... 28
Figure 1.5 Summary of commonly used non-polar solvents in DArP with their boiling
points and sustainability.. .................................................................................................. 36
Figure 1.6 Mechanism for direct arylation via the Ozawa-derived condition and origin of
homocoupling defects from a trans-configuration of the transition state. ......................... 41
Figure 1.7 Plausible mechanism of suppressing β-defect-formation by using a bulky
phosphine ligand involved in the direct arylation of a DPP monomer.. ............................ 44
Figure 1.8 Proposed Mechanistic insights on the β-defect formation assisted by the ester
directing group. .................................................................................................................. 46
Figure 1.9 Depiction of the C-H abstraction transition states. ......................................... 49
Figure 1.10 Plausible mechanism for the synthesis of P26 via Oxidative Direct Arylation
Polymerization. .................................................................................................................. 51
xiv
Figure 1.11 Summary of regioselective C-H activation by different metal catalysts on the
thiazole (Tz) unit. .............................................................................................................. 52
Figure 1.12 Plausible mechanism for Cu-catalyzed Direct Arylation Polymerization using
a polyfluorinated arene as the C-H substrate. .................................................................... 61
Figure 1.13 Plausible mechanism for Cu-catalyzed Direct Arylation Polymerization using
a Tz-C-2 directing group strategy. ..................................................................................... 64
Figure 2.1 Summary of different approaches towards green solvent processing of
conjugated polymers... ..................................................................................................... 107
Figure 2.2
1
H-NMR (500 MHz, CDCl3, 25 °C) of DArP polymers P1 (Table 2.1, Entry 3,
top) and P1 (Table 2.1, Entry 5, bottom)... ...................................................................... 113
Figure 2.3 UV-vis absorbance for films of polymers P1 (DCB, ethanol, and butanol) and
P2 (DCB)... ...................................................................................................................... 114
Figure 3.1
1
HNMR analyses of representative P1 (PDOF-OD) (Table 3.1, Entry 6) and P4
(Table 3.1, Entry 8) polymers. ......................................................................................... 132
Figure 4.1 Summary of Pd-DArP and Cu-DArP development.. .................................... 141
Figure 4.2 (a) Summary of different polymerization methods for BTz-containing
conjugated polymers; (b) Summary of small-molecule selective direct arylation on thiazole
unit... ................................................................................................................................ 142
Figure 4.3
1
HNMR analyses of representative polymers: (a) PF-5BTz (Table 4.1, entry 4)
synthesized using Cu(phen)(PPh3)Br and (b) PF-5BTz (Table 4.1, entry 3) synthesized
using CuI, phen.. .............................................................................................................. 150
Figure 5.1 Summary of commonly used solvents in DArP with their boiling points and
sustainability... ................................................................................................................. 159
xv
Figure 5.2
1
H NMR analyses of P1 synthesized using p-cymene (Table 5.1, entry 3),
CPME (Table 5.1, entry 4), and toluene (Table 5.1, entry 5) as the solvent... ................ 166
Figure 6.1
1
H NMR analyses of P1 synthesized using H2O/p-cymene 9:1 (v:v) emulsion
in air and under N2 atmosphere (Table 6.1, entry 7 and 8, respectively).. ...................... 184
Figure 7.1
1
H NMR analyses of 5 batches of PEDOTF synthesized from the recycling of
SiliaCat® Pd-DPP (Table 7.2). ........................................................................................ 201
Figure A.1
1
H NMR of compound 2 in CDCl3 at 25 °C and 500 MHz... ...................... 217
Figure A.2
1
H NMR of compound 3a in CDCl3 at 25 °C and 500 MHz... .................... 218
Figure A.3
13
C-NMR of Compound 3a in CDCl3 at 25 °C and 500 MHz... .................. 218
Figure A.4
1
H-NMR of Compound 3b in CDCl3 at 25 °C and 500 MHz... ................... 219
Figure A.5
13
C-NMR of Compound 3b in CDCl3 at 25 °C and 500 MHz.. ................... 220
Figure A.6
1
H NMR of P1 (Table 2.1, Entry 2) in CDCl3 at 25 °C and 500 MHz.. ...... 221
Figure A.7
1
H-NMR of P1 (Table 2.1, Entry 3) in CDCl3 at 25 °C and 500 MHz.. ...... 221
Figure A.8
1
H-NMR of P1 (Table 2.1, Entry 4) in CDCl3 at 25 °C and 500 MHz.. ...... 222
Figure A.9
1
H-NMR of P1 (Table 2.1, Entry 5) in CDCl3 at 25 °C and 500 MHz. ....... 222
Figure A.10
1
H-NMR of P1 (Table 2.1, Entry 6) in CDCl3 at 25 °C and 500 MHz. ..... 223
Figure A.11
1
H-NMR of P1 (Table 2.1, Entry 7) in CDCl3 at 25 °C and 500 MHz. ..... 223
Figure A.12
1
H-NMR of P2 (Table 2.1, Entry 8) in CDCl3 at 25 °C and 500 MHz.. .... 224
Figure A.13
1
H-NMR of P2 (Table 2.1, Entry 9) in CDCl3 at 25 °C and 500 MHz. ..... 224
Figure A.14
1
H-NMR of P2 (Table 2.1, Entry 9) in C2D2Cl4 at 25 °C and 600 MHz. .. 225
Figure A.15
1
H-NMR of P2 (Table 2.1, Entry 9) in C2D2Cl4 at 75 °C (348 K) and 600
MHz.. ............................................................................................................................... 225
xvi
Figure A.16
1
H-NMR of P2 (Table 2.1, Entry 9) in C2D2Cl4 at 100 °C (373 K) and 600
MHz.. ............................................................................................................................... 226
Figure A.17
1
H-NMR of P2 (Table 2.1, Entry 9) in C2D2Cl4 at 125 °C (398 K) and 600
MHz.. ............................................................................................................................... 226
Figure A.18 Expanded region of
1
H-NMR of P2 (Table 2.1, Entry 9) in C2D2Cl4 at 75 °C
(348 K), 100 °C (373 K) and 125 °C (398 K) at 600 MHz. ............................................ 227
Figure A.19 FTIR spectra of P2 (Table 2.1, Entry 9).. ................................................... 228
Figure A.20 Solution of P1 (Table 2.1, Entry 2, Mn = 10.4 kDa) in green solvents (ethanol,
1-butanol and anisole) and DCB used for spin-coating.. ................................................. 231
Figure A.21 Films of P1 (Table 2.1, Entry 2 or 3) processed by green solvents (EtOH,
anisole, 1-butanol, and EtOH:H2O (88:12)) on glass slides. ........................................... 231
Figure A.22 UV-VIS spectra of solutions of P1 (in DCB, EtOH, 1-Butanol), and P2 (in
DCB).. .............................................................................................................................. 232
Figure A.23 Comparison of UV-VIS spectra of P1 both in solutions (in DCB) (black) and
as cast on film (with DCB) (red).. ................................................................................... 233
Figure A.24 Comparison of UV-VIS spectra of P1 both in solutions (in 1-Butanol) (black)
and as cast on film (with 1-Butanol) (red).. ..................................................................... 233
Figure A.25 Comparison of UV-VIS spectra of P1 both in solutions (in EtOH) (black) and
as cast on film (with EtOH) (red).. .................................................................................. 234
Figure A.26 Comparison of UV-VIS spectra of P2 both in solutions (in DCB) (black) and
as cast on film (with DCB) (red). .................................................................................... 234
xvii
Figure A.27 Cyclic Voltammetry (CV) measurement of P1 (Table 2.1, Entry 5).
Determined by cyclic voltammetry (vs. Fc/Fc
+
) in 0.1 M TBAPF5 in acetonitrile solution..
......................................................................................................................................... 237
Figure A.28 Cyclic Voltammetry (CV) measurement of P2 (Table 2.1, Entry 9).
Determined by cyclic voltammetry (vs. Fc/Fc
+
) in 0.1 M TBAPF5 in acetonitrile solution.
......................................................................................................................................... 238
Figure A.29 Gel Permeation Chromatography (GPC) trace of P1 (Table 2.1, Entry 2)..
......................................................................................................................................... 239
Figure A.30 Gel Permeation Chromatography (GPC) trace of P1 (Table 2.1, Entry 3)..
......................................................................................................................................... 239
Figure A.31 Gel Permeation Chromatography (GPC) trace of P1 (Table 2.1, Entry 4).
Determined as Mn = 8.3 kDa, PDI = 1.5. ........................................................................ 240
Figure A.32 Gel Permeation Chromatography (GPC) trace of P1 (Table 2.1, Entry 5).
Determined as Mn = 15.4 kDa, PDI = 1.5. ...................................................................... 240
Figure A.33 Gel Permeation Chromatography (GPC) trace of P1 (Table 2.1, Entry 6).
Determined as Mn = 10.8 kDa, PDI = 2.0. ...................................................................... 241
Figure A.34 Gel Permeation Chromatography (GPC) trace of P1 (Table 2.1, Entry 7).
Determined as Mn = 13.5 kDa, PDI = 1.5. ...................................................................... 241
Figure A.35 Gel Permeation Chromatography (GPC) trace of P2 (Table 2.1, Entry 8).
Determined as Mn = 7.9 kDa, PDI = 1.5. ........................................................................ 242
Figure A.36 Gel Permeation Chromatography (GPC) trace of P2 (Table 2.1, Entry 9).
Determined as Mn = 11.6 kDa, PDI = 1.9.. ..................................................................... 242
Figure A.37 DSC trace of P1 (Table 2.1, Entry 5). No Tm, Tc were found.. .................. 243
xviii
Figure A.38 DSC trace of P2 (Table 2.1, Entry 9). No Tm, Tc were found. ................... 243
Figure A.39 GIXRD diffraction pattern of P1 (Table 2.1, Entry 5)... ............................ 244
Figure A.40 GIXRD diffraction pattern of P2 (Table 2.1, Entry 9). .............................. 244
Figure B.1
1
H NMR of 9,9-bis(octyl)-2,7-dibromofluorene (1).. ................................... 258
Figure B.2
1
H NMR of 9,9-bis(hexyl)-2,7-dibromofluorene (3). ................................... 259
Figure B.3
1
H NMR of 2,5-dibromothiophene (4).. ....................................................... 259
Figure B.4
1
H NMR of P1 (PDOF-OD) synthesized using the conditions in Table 3.1
(Entry 6).. ......................................................................................................................... 260
Figure B.5
1
H NMR of P1 (PDOF-OD) synthesized using the conditions in Table B.1
(Entry 1) ........................................................................................................................... 260
Figure B.6
1
HNMR (500 MHz) of P2 (Table B.2, Entry 2) in CDCl3 at 25 °C. ............ 261
Figure B.7
1
HNMR (500 MHz) of P2 (Table B.2, Entry 3) in CDCl3 at 25 °C. ............ 261
Figure B.8
1
HNMR (500 MHz) of P3 (Table B.2, Entry 5) in CDCl3 at 25 °C. ............ 262
Figure B.9
1
HNMR (500 MHz) of P4 (Table B.2, Entry 6) in CDCl3 at 25 °C. ............ 262
Figure B.10
1
HNMR (500 MHz) of P4 (Table 3.1, Entry 8) in CDCl3 at 25 °C. ........... 263
Figure B.11
1
HNMR (500 MHz) of P5 (Table 3.1, Entry 9) in CDCl3 at 25 °C. ........... 263
Figure B.12
1
HNMR (500 MHz) of P6 (Table 3.1, Entry 10) in CDCl3 at 25 °C. End groups
denoted with *. ................................................................................................................ 264
Figure C.1
1
H NMR of 9,9-bis(2-ethylhexyl)-2,7-dibromofluorene. ............................. 275
Figure C.2
1
H NMR of 5-Tributylstannylthiazole. ......................................................... 276
Figure C.3
1
H NMR of 5,5’-Bithiazole (5-BTz). ........................................................... 276
Figure C.4
1
H NMR of Cu(phen)(PPh3)Br. .................................................................... 277
Figure C.5
1
H NMR of Cu(neocup)(PPh3)Br. ................................................................ 277
xix
Figure C.6
1
H NMR of Cu(bipy)(PPh3)Br. .................................................................... 278
Figure C.7
1
H NMR of Cu(phen)(PPh3)I.. ..................................................................... 278
Figure C.8
1
H NMR of PF-5BTz synthesized using the conditions in Table 4.1 (entry 3).
......................................................................................................................................... 279
Figure C.9
1
H NMR of PF-5BTz synthesized using the conditions in Table 4.1 (entry 4).
......................................................................................................................................... 279
Figure C.10
1
H NMR of PF-5BTz synthesized using the conditions in Table 4.1 (entry 5).
......................................................................................................................................... 280
Figure C.11
1
H NMR of PF-5BTz synthesized using the conditions in Table 4.1 (entry 9).
......................................................................................................................................... 280
Figure C.12
1
H NMR of PC-5BTz synthesized using the conditions in Scheme 4.2. .... 281
Figure C.13 Proposed end-group assignments based on the model compounds S1, S2, S3,
the
1
H NMR spectrum is that of Table 4.1 (entry 4) ....................................................... 282
Figure D.1
1
H NMR of dimethyl 3,4-thiophenedicarboxylate.. ..................................... 287
Figure D.2 P1 synthesized using p-cymene (Table 5.1, entry 3), CPME (Table 5.1, entry
4) and toluene (Table 5.1, entry 5).. ................................................................................ 289
Figure D.3 P2 synthesized using p-cymene (Table 5.1, entry 6) and toluene (Table 5.1,
entry 7). ............................................................................................................................ 290
Figure D.4 P3 synthesized using p-cymene (Table 5.1, entry 8).. .................................. 291
Figure D.5
1
H NMR of P1 synthesized using the conditions in Table 5.1 (entry 1).. .... 293
Figure D.6
1
H NMR of P1 synthesized using the conditions in Table 5.1 (entry 2). ..... 294
Figure D.7
1
H NMR of P1 synthesized using the conditions in Table 5.1 (entry 3). ..... 294
Figure D.8
1
H NMR of P1 synthesized using the conditions in Table 5.1 (entry 4). ..... 295
xx
Figure D.9
1
H NMR of P1 synthesized using the conditions in Table 5.1 (entry 5). ..... 295
Figure D.10
1
H NMR of P2 synthesized using the conditions in Table 5.1 (entry 6). ... 296
Figure D.11
1
H NMR of P2 synthesized using the conditions in Table 5.1 (entry 7). ... 296
Figure D.12
1
H NMR of P3 synthesized using the conditions in Table 5.1 (entry 8).. .. 297
Figure D.13
1
H NMR of PCDTBT synthesized using the conditions in Scheme 5.2. ... 297
Figure D.14
1
H NMR analyses of P2 synthesized using p-cymene (Table 5.1, entry 6), and
toluene (Table 5.1, entry 7) as the solvent. ...................................................................... 298
Figure D.15
1
H NMR analyses of P3 synthesized using p-cymene (Table 5.1, entry 8) as
the solvent. ....................................................................................................................... 298
Figure D.16
1
H NMR analyses of PCDTBT synthesized using p-cymene (Scheme 5.2) as
the solvent. ....................................................................................................................... 300
Figure D.17 UV-vis spectra of P1 (Table 5.1, entry 3-5) synthesized by using p-cymene
(Table 5.1, entry 3, black), CPME (Table 5.1, entry 4, red), and toluene (Table 5.1, entry
5, blue). ............................................................................................................................ 301
Figure D.18 UV-vis spectra of P2 (Table 5.1, entry 6-7) synthesized by using p-cymene
(Table 5.1, entry 6, black), and toluene (Table 5.1, entry 7, red). ................................... 302
Figure D.19 UV-vis spectra of P3 (Table 5.1, entry 8) synthesized by using p-cymene.
......................................................................................................................................... 302
Figure D.20 UV-vis spectra of PCDTBT (Scheme 5.2) synthesized by using p-cymene..
......................................................................................................................................... 303
Figure E.1 Comparison between K-EL 2 wt% H2O:toluene (9:1 v/v) emulsion (left) and
K-EL 2 wt% H2O:p-cymene (9:1 v/v) emulsion (right)... ............................................... 307
Figure E.2 DArP conducted using K-EL 2 wt% H2O:p-cymene (9:1 v/v) emulsion... .. 307
xxi
Figure E.3 The breakage of emulsion (separated into two layers) as a result of increasing
the loading of K2CO3 to 40 equiv., which increases the density of the aqueous solution
(Table 6.1, entry 9)... ....................................................................................................... 308
Figure E.4
1
H NMR of P1 synthesized using the conditions in Table 6.1 (entry 3).. .... 311
Figure E.5
1
H NMR of P1 synthesized using the conditions in Table 6.1 (entry 5)... ... 312
Figure E.6
1
H NMR of P1 synthesized using the conditions in Table 6.1 (entry 7).. .... 312
Figure E.7
1
H NMR of P1 synthesized using the conditions in Table 6.1 (entry 8)... ... 313
Figure E.8
1
H NMR of P1 synthesized using the conditions in Scheme 6.3. ................. 313
Figure E.9
1
H NMR analyses of P1 synthesized using H2O/p-cymene 9:1 (v:v) emulsion
in air and under N2 atmosphere (Table 6.1, entry 7 and 8, respectively). ....................... 314
Figure E.10 UV-vis spectra of P1 (Table 6.1, entry 3, 5, 7, 8) synthesized by using
emulsion conditions. ........................................................................................................ 315
Figure E.11 UV-vis spectra of PPDTBT (Scheme 6.3) synthesized by emulsion conditions.
......................................................................................................................................... 315
Figure F.1
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 1).
......................................................................................................................................... 328
Figure F.2
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 2)...
......................................................................................................................................... 329
Figure F.3
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 4)..
......................................................................................................................................... 329
Figure F.4
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 5)...
......................................................................................................................................... 330
xxii
Figure F.5
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 6)..
......................................................................................................................................... 330
Figure F.6
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 7).
......................................................................................................................................... 331
Figure F.7
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 8).
......................................................................................................................................... 331
Figure F.8
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 9)..
......................................................................................................................................... 332
Figure F.9
1
H NMR of PEDOTF synthesized using the conditions in Table F.1 (entry 1).
......................................................................................................................................... 332
Figure F.10
1
H NMR of PEDOTF synthesized using the conditions in Table F.1 (Cycle
1)... ................................................................................................................................... 333
Figure F.11
1
H NMR of PEDOTF synthesized using the conditions in Table F.1 (Cycle
2)... ................................................................................................................................... 333
Figure F.12
1
H NMR of PEDOTF synthesized using the conditions in Table F.1 (Cycle
3). ..................................................................................................................................... 334
Figure F.13
1
H NMR of PEDOTF synthesized using the conditions in Table F.1 (Cycle
4).. .................................................................................................................................... 334
Figure F.14
1
H NMR of PEDOTF synthesized using the conditions in Table F.1 (Cycle
5)... ................................................................................................................................... 335
Figure F.15
1
H NMR of PEDOTF synthesized using the conditions in Scheme F.3 (using
FibreCat as the catalyst for 24 hours of reaction time)... ................................................. 335
xxiii
Figure F.16
1
H NMR of PEDOTF synthesized using the conditions in Table F.2 using
Pd/C as the catalyst (Cycle 1)... ....................................................................................... 336
Figure F.17
1
H NMR analyses of PEDOTF synthesized using the reaction conditions listed
in Table 7.1, entry 7 (third cycle of a round of catalyst-recycling experiments).. .......... 337
Figure F.18 UV-vis spectra of PEDOTF (Table 7.1, entry 1, 2, 4). . ............................ 338
Figure F.19 UV-vis spectra of PEDOTF (Table 7.1, entry 5-9). .................................... 339
Figure F.20 UV-vis spectra of PEDOTF (Table F.1, entry 1, 4)... ................................. 340
Figure F.21 UV-vis spectra of UV-vis spectra of PEDOTF (Table 7.2, Cycle 1-5)... ... 340
Figure F.22 UV-vis spectra of UV-vis spectra of PEDOTF (Table F.2, using Pd/C)... . 341
Figure F.23 UV-vis spectra of UV-vis spectra of PEDOTF (Scheme F.3, using pristine
FibreCat). ......................................................................................................................... 341
xxiv
List of Schemes
Scheme 1.1 Catalysts used in traditional cross-coupling reactions (Stille and Suzuki),
small-molecule direct arylation, and in DArP... .................................................................. 2
Scheme 1.2 Representative early reports on Pd-catalyzed direct arylation via the proposed
SEAr mechanism.. ................................................................................................................ 5
Scheme 1.3 Representative early reports on Pd-catalyzed direct arylation. ....................... 6
Scheme 1.4 Some representative syntheses of electron-rich EDOT or ProDOT-based
conjugated polymers via Lemaire-derived DArP conditions... ......................................... 10
Scheme 1.5 (a) Development of the early Fagnou-derived conditions
and their application
in DArP using (b) 1,2,4,5-tetrafluorobenzene and (c) 2,2’,3,3’,5,5’,6,6’-octafluorobiphenyl
as the C-H monomer by Kanbara et al... ........................................................................... 11
Scheme 1.6 (a) Small-molecule direct arylation studies featuring pentafluorobenzene and
2-hexylthiophene via two different monomer functionalization strategies and (b)
application of the optimal strategy in DArP reported by Thompson et al... ...................... 13
Scheme 1.7 High reactivity of benzene using carboxylate-assisted direct arylation
conditions developed by Fagnou et al... ............................................................................ 13
Scheme 1.8 DMA as the optimal solvent for electron-rich thiophenes via DArP, but unable
to provide polymer products with electron-deficient thiophenes, as reported by Kanbara et
al.. ...................................................................................................................................... 16
Scheme 1.9 DMA as the optimal solvent for electron-rich thiophenes via DArP, but only
provided oligomeric products with electron-deficient thiophenes, as reported by Ling et
al... ..................................................................................................................................... 17
xxv
Scheme 1.10 Ligation of DMA to the Pd catalyst by displacing a phosphine ligand, which
lowers the energy barrier for the C-H activation, reported by Hartwig et al... .................. 18
Scheme 1.11 Selected amide solvents investigated by Thompson et al. to evaluate their
influence on the synthesis of P3HT via Fagnou-derived DArP.. ...................................... 19
Scheme 1.12 Generation of Pd(0) through reduction of Pd(II) via C-H/C-H homocoupling
of 2,2’-bithiophene under the Fagnou-derived conditions, reported by Kanbara et al... ... 21
Scheme 1.13 Generation of Ni(0) through reduction of Ni(II) via C-H/C-H homocoupling
of benzothiazole similar to Pd catalysts, as reported by Itami et al... ................................ 22
Scheme 1.14 Ultra-lowing of the Pd-catalyst to minimize homocoupling defect of P3HT,
reported by Thompson et al.. ............................................................................................. 24
Scheme 1.15 Strategies of suppressing homocoupling defects for donor-acceptor (DA)
copolymer (P15) synthesized using the Fagnou-derived DArP conditions.. ..................... 25
Scheme 1.16 (a) Unsuccessful synthesis of P16 using the Fagnou-derived DArP conditions
reported by Kanbara et al. (b) Small-molecule model study on 2,2’-bithiophene with 1-
bromo-4-methylbenzene. (c) Synthesis of P17 using the Fagnou-derived DArP conditions
by Kanbara et al... .............................................................................................................. 26
Scheme 1.17 Synthesis of β-defects-free P3HT using a bulky carboxylic acid additive,
neodecanoic acid (NDA) reported by Thompson et al.. .................................................... 27
Scheme 1.18 Synthesis of rr-P3HT using non-coordinating solvent, THF, reported by
Ozawa et al. Structures of Pd(Herrmann), L1, and L2 are denoted.. ................................ 30
Scheme 1.19 Synthesis of DA copolymer P7 via the Ozawa-derived conditions. Structure
of the Pd2(dba)3 catalyst is denoted.. ................................................................................. 31
xxvi
Scheme 1.20 Application of the L1-based Ozawa-derived conditions to the synthesis of
TPD-based copolymer P18 reported by Ozawa et al. and Leclerc et al... ......................... 32
Scheme 1.21 Comparison between two classes of DArP conditions (Fagnou-derived and
Ozawa-derived) to prepare P19 by Thompson et al.. ........................................................ 33
Scheme 1.22 Catalytic active species in the Ozawa-derived condition in small-molecule
direct arylation (top), and the role of hemilabile ligand L1 in explaining the high reactivity
of the L1-based Pd-catalytic system, proposed by Ozawa et al... ..................................... 34
Scheme 1.23 Examples of highly efficient DArP protocols in sustainable solvents
(Thompson et al.) or in biphasic conditions (Leclerc et al.).. ............................................ 37
Scheme 1.24 Synthesis of P21 using regular Ozawa-derived conditions, which led to
insoluble cross-linked material reported by Leclerc et al. and Ozawa et al. Utilizing a
“mixed ligand” approach using TMEDA as a co-ligand suppressed defect-formation as
reported by Ozawa et al... .................................................................................................. 40
Scheme 1.25 Investigating the effect of phosphine ligand bulkiness on the minimization of
defect-formation of DArP by Leclerc et al... ..................................................................... 43
Scheme 1.26 (a) Synthesis of P22 via DArP resulted in a significant amount of β-defects
in the polymer structure due to the incorporation of an ester directing group and (b) defect-
free synthesis of P23 with an electron-rich donor with less-reactive C-Hβ bonds, reported
by Thompson et al.. ........................................................................................................... 45
Scheme 1.27 Synthesis of amide-functionalized polythiophenes P24 and P25 reported by
Thompson et al... ............................................................................................................... 47
xxvii
Scheme 1.28 Synthesis of P26 using two different monomers: 5-bromo-3-hexylester
thiophene (with a directing group adjacent to the C-H activation site) and 2-bromo-3-
hexylester thiophene (without a directing group adjacent to the C-H activation site.. ...... 48
Scheme 1.29 Synthesis of P26 via Oxidative Direct Arylation Polymerization assisted by
an ester directing group, reported by Thomson et al... ...................................................... 50
Scheme 1.30 Examples of thiazole (Tz)-C-5 assisted Pd-catalyzed Oxidative Direct
Arylation Polymerization.. ................................................................................................ 53
Scheme 1.31 Examples of thiazole (Tz)-C-2 assisted Cu-catalyzed Oxidative Direct
Arylation Polymerization.. ................................................................................................ 55
Scheme 1.32 (a-c) Examples of small-molecule direct arylation catalyzed by Cu(I) catalyst,
reported by Daugulis and You et al.
29,30,34
(d) Proposed active Cu-catalytic species
(complex 7) for Cu-catalyzed direct arylation (left) and comparison to active Pd-catalytic
species (complex 1) for Pd-catalyzed direct arylation.. ..................................................... 57
Scheme 1.33 First examples of Cu-catalyzed Direct Arylation Polymerization developed
by Thompson et al.. ........................................................................................................... 58
Scheme 1.34 Isolation of a fluoroarylcopper intermediate (complex 8) by Daugulis et al.,
which can be subsequently arylated... ............................................................................... 60
Scheme 1.35 (a) Synthesis of P8 via Cu-catalyzed Direct Arylation Polymerization using
an aryl-bromide as the monomer with the use of a co-solvent system, as reported by
Thompson et al. (b) Synthesis of P33 via Tz-C-2 directed Cu-catalyzed Direct Arylation
Polymerization using an efficient precatalyst approach reported by Thompson et al... .... 62
Scheme 2.1 Synthesis of P1 and P2 via DArP ................................................................ 108
Scheme 3.1 Summary of Cu-DArP development.. ......................................................... 125
xxviii
Scheme 3.2 Synthesis of PDOF-OD (P1) using 2,7-dibromo-9,9-dioctylfluorene .. ...... 126
Scheme 3.3 Synthesis of TPD-copolymers using aryl-bromides as donor units .. ......... 130
Scheme 4.1 Synthesis of PF-5BTz using 2,7-dibromo-9,9-bis(2-ethylhexyl)fluorene and
5-BTz ............................................................................................................................... 144
Scheme 4.2 Synthesis of PC-5BTz. ................................................................................ 149
Scheme 5.1 Synthesis of P1-P3 using DArP conditions listed in Table 5.1. .................. 161
Scheme 5.2 Synthesis of PCDTBT using p-cymene as the solvent.. .............................. 165
Scheme 6.1 Development of DArP in water-compatible/sustainable conditions.. ......... 176
Scheme 6.2 Synthesis of P1 via DArP using emulsion conditions listed in Table 6.1.. . 178
Scheme 6.3 Synthesis of PPDTBT via DArP using the optimal emulsion conditions listed in Table
6.1, entry 7... ...................................................................................................................... 182
Scheme 7.1 Brief summary of Pd-catalyzed CP synthesis.... ......................................... 193
Scheme 7.2 Synthesis of PEDOTF using SiliaCat® Pd-DPP as the heterogenous
catalyst..... ........................................................................................................................ 196
Scheme A.1 Monomer Synthesis.. .................................................................................. 212
Scheme F.1 Preliminary results for the catalyst-recycling experiments using Ozawa-
derived conditions.. .......................................................................................................... 319
Scheme F.2 Synthesis of PEDOTF using FibreCat or Pd/C as the heterogenous catalyst..
......................................................................................................................................... 327
xxix
Abstract
Improving the Efficiency and Sustainability of Direct Arylation Polymerization for
Conjugated Polymer Synthesis
By
Liwei Ye
Doctor of Philosophy in Chemistry
Conjugated polymers are attractive semiconducting materials for applications such
as organic photovoltaics (OPVs), organic light emitting diodes (OLEDs), organic
transistors, batteries, and bioelectronics. Traditional synthetic methodologies such as
Stille-Migita and Suzuki-Miyaura polymerization are often utilized to prepare these
important materials, which require the use of toxic, pyrophoric reagents and challenging
(if not impossible) monomer purifications. In recent years, Direct Arylation Polymerization
(DArP) has emerged as a facile, eco-friendly pathway for the synthesis of conjugated
polymers by reducing the number of synthetic steps and highly toxic organotin or
organoboron by-products. By circumventing monomer functionalization with toxic
xxx
transmetallating reagents such as organostannane and organoboron required for Stille-
Migita and Suzuki-Miyaura polymerization methods, DArP proceeds through a metal-
catalyzed C-H activation pathway for the preparation of high performance conjugated
polymer materials. The rapid development of DArP protocols has allowed the preparation
of well-defined conjugated polymers via a sustainable, atom-economical pathway.
However, despite its inherent sustainability, there are many problematic aspects of
DArP that seek to be addressed to further improve both its efficiency and sustainability.
For instance, as the major component in these reactions, the solvents most prevalently
employed for DArP are hazardous and produced from unsustainable sources. However,
little development of employing sustainable solvents or “greener” synthetic methods for
DArP has been reported. Furthermore, sustainable processing of conjugated polymers
synthesized using DArP requires attention. Moreover, DArP relies on the expensive, rare
metal, Pd, as the catalyst. With the growing demand for industrial-scale conjugated
polymer synthesis and the development of continuous flow methods, the need for less-
expensive metal catalysts or recyclable heterogenous catalysts for DArP has become
urgent. In this dissertation, strategies for improving the efficiency and sustainablity of
DArP by addressing all the problems enlisted above are presented to synthesize functional,
well-defined conjugated polymers with the development of several novel DArP protocols.
Chapter 1 evaluates the development of several classes of efficient
catalysts/catalytic systems from small-molecule studies to polymerizations, including the
mechanisms involved in these transformations and how they inspire catalyst and monomer
design for defect-free conjugated polymer synthesis. Recent advances in developing more
sustainable first-row transition metal catalysts for DArP are also highlighted, and the
xxxi
fundamental understanding of these efficient and sustainable catalysts motivates our
pursuit for the next generation of novel reaction protocol design to enable more effective
and environmentally-friendly conjugated polymer synthesis detailed in Chapters 2-7.
In Chapter 2, the first class of poly (3-alkylamidethiophenes) (P3AAT) is prepared
via the sustainable method of DArP that can be processed using green, sustainable solvents.
The unprecedented synthesis of P3AAT reveals the superiority of DArP, as P3AAT can be
readily prepared in only three simple steps with molecular weights (Mn) up to 15.4 kg/mol
and yields up to 90% exclusively with this methodology. The tertiary amide, poly(N-hexyl-
N-methylthiophene-3-carboxamide-2,5-diyl) (P1) has excellent solubility in the green
solvents ethanol, 1-butanol, and anisole. Processing of P1 in 1-butanol is shown to provide
comparable SCLC hole mobility versus dichlorobenzene and commensurate photophysical
properties. Also, the secondary amide, poly(N-(2-ethylhexyl)thiophene-3-carboxamide-
2,5-diyl) (P2) was successfully synthesized, demonstrating excellent functional-group-
tolerance for DArP, while showing hydrogen bonding features and similar SCLC hole
mobility as P1. This study provides a facile synthetic strategy for a novel structural motif
that can be processed in sustainable solvents without a compromise in performance.
In Chapter 3, the first synthesis of conjugated polymers via Cu-catalyzed DArP
using aryl-bromides is reported. This work has significantly advanced the development of
Cu-DArP, enabling the synthesis of a broad scope of alternating donor-acceptor
copolymers using aryl-bromides with catalytic copper, completely replacing expensive,
unstable aryl-iodides previously employed. Through optimization of Cu-DArP conditions,
less-reactive aryl bromides can be successfully polymerized with different coupling
xxxii
partners using catalytic quantities of Cu (15 mol%), allowing the preparation of perfectly-
alternating conjugated copolymers with Mn up to 17.3 kg/mol using a co-solvent approach.
In Chapter 4, we explore the utility of a well-defined, easy-to-prepare, highly-
soluble and stable precatalyst, Cu(phen)(PPh3)Br, as an alternative to the CuI, 1,10-
phenanthroline catalytic system previously used for Cu-DArP. We report a drastic
improvement of Cu-DArP methodology for the synthesis of 5,5’-bithiazole (5-BTz)-based
conjugated polymers enabled by an efficient precatalyst approach, affording polymers with
good Mn (up to 16.5 kg/mol) and excellent yields (up to 79%).
1
HNMR studies reveal the
exclusion of homo-coupling defects, which further verifies the excellent stability of
Cu(phen)(PPh3)Br compared to CuI. Furthermore, the Cu catalyst loading is decreased
from 15 mol% to only 5 mol% (Mn of 11.8 kg/mol, 64% yield), which is unprecedented
when aryl-bromides are employed for Cu-DArP. Significantly, 5-BTz is shown to be
inactive under various of Pd-DArP conditions, which demonstrates the high compatibility
of Cu-DArP as the only pathway for the C-H activation of the 5-BTz unit and a clear case
demonstrating an advantage of Cu-DArP relative to Pd-DArP.
In Chapter 5, we report the application of a sustainable, naturally-sourced, high-
boiling aromatic solvent, p-cymene, to DArP for the first time. p-Cymene was found to
display excellent solubilizing ability in the synthesis of a broad scope of alternating
copolymers with Mn up to 51.3 kg/mol and yields up to 96.2%, outperforming those
prepared using cyclopentyl methyl ether (CPME) and toluene. Structural analysis revealed
the exclusion of defects in these polymers prepared by using p-cymene as the solvent,
which in the case of a 2,2’-bithiophene monomer, challenging to access through the use of
conventional solvents for DArP, such as DMA and toluene. This report demonstrates the
xxxiii
significant potential of p-cymene as a green solvent to replace traditional hazardous,
unsustainable solvents for DArP.
In Chapter 6, we report the first synthesis of conjugated polymers via DArP using
emulsion reaction media. Importantly, these polymerizations were found suitable under
aerobic conditions. By using an “oil-in-water” strategy with a H2O/p-cymene 9:1 (v:v)
emulsion medium stabilized by a surfactant, we were able to significantly decrease the
amount of organic waste generated per kg of conjugated polymer synthesized (E-factor) by
10 times compared to conventional DArP methods, preparing well-defined conjugated
polymers with Mn up to 14.5 kg/mol. This work demonstrates a significant enhancement
in the sustainability aspect of DArP, and the novel emulsion-DArP method presented is
promising towards environmental-compatible, low-cost industrial-scale conjugated
polymer synthesis.
In Chapter 7, the first report on the recycling of heterogenous catalysts for CP
synthesis using DArP is presented. We found SiliaCat® Pd-DPP to be a highly efficient
and recyclable catalyst for multi-batch CP synthesis providing CPs with Mn up to 82
kg/mol even after being recycled 3 times. Batch-to-batch variations were further optimized
to afford up to 5 batches of polymers with Mn of 25±2.5 kg/mol without structural disparity.
Significantly, this work discloses among the most sustainable CP synthesis protocols to
date and presents the critical concept of catalyst-recycling to the important field of organic
semiconducting polymers, which potentially enables access to truly low-cost flow
chemistry for industrial-scale CP synthesis.
1
Chapter 1: Improving the Efficiency and Sustainability of Catalysts for Direct
Arylation Polymerization
1.1 Introduction
Conjugated polymers are electroactive materials based on an extended π-
conjugated aromatic backbone. Compared to inorganic semiconductors, these organic
macromolecules are advantageous for being low-cost, soluble, and readily processed to
form lightweight, flexible thin-films for industrial-scale devices.
1,2
Owing to their unique
electronic, photophysical, and mechanical properties, conjugated polymers are found to be
widely applicable in fields such as organic photovoltaics (OPVs),
3,4
organic light emitting
diodes (OLEDs),
5
organic field effect transistors (OFETs),
6
chemical sensors,
7
batteries,
8
electrochromic devices,
9
energy storage,
10
bioimaging,
11
drug delivery,
12
and
bioelectronics.
13
To satisfy the growing demand for finely-tuned, high-performance conjugated
polymers, a broad range of synthetic methodologies have rapidly developed during the past
several decades. Transition metal-catalyzed polycondensation reactions have proven to be
reliable and robust tools to access the C-C bonds formation between aryl groups, affording
high-quality conjugated polymers used in the aforementioned applications. Typically, aryl
halides are coupled with aryl units functionalized with organometallics, such as
organomagnesium (Kumada),
14
organozinc (Negishi),
15
organoboron (Suzuki-Miyaura),
16
and organotin (Stille-Migita).
17
Over the years, the development of these methodologies to
synthesize versatile conjugated polymer structures, such as homopolymers or alternating
copolymers, has been enabled by research in efficient transition metal catalysts
(predominantly, Pd and Ni catalysts).
18,19
However, the necessity in these synthetic
2
protocols to pre-activate one of the coupling sites using stoichiometric, toxic
transmetallating reagents raises concerns over their low sustainability.
17
In addition,
conventional cross-coupling methods such as Stille-Migita polymerization involve
monomers equipped with organostannanes, which are highly susceptible to decomposition,
extremely challenging to purify (and in many cases, impossible to purify), and difficult to
handle (often exist as viscous oil). Thus, the demand for a more convenient,
environmentally-friendly, and lower-cost method to prepare these polymers has rapidly
emerged.
Scheme 1.1 Catalysts used in traditional cross-coupling reactions (Stille and Suzuki), small-
molecule direct arylation, and in DArP.
For a few decades, advanced research in transition metal-catalyzed C-H activation
has offered an ideal alternative approach for aryl-aryl cross-coupling reactions.
20–24
The
term “direct arylation” is often used to define the direct coupling of a halogenated arene or
heteroarene and a non-functionalized aryl or heteroaryl C-H bond to form biaryl products,
requiring pre-activation of only one coupling partner.
22
In the past few decades, direct
3
arylation has developed into a promising synthetic method compared to traditional cross-
coupling methods because it occurs with less synthetic steps, higher atom economy, and
forms only environmentally-benign byproducts. While a variety of transition metals have
been discovered to promote these transformations (Scheme 1.1),
21,24–26
including Pd,
22
Cu,
27–34
Ru,
35–39
Rh,
40–43
Ir,
44,45
Ni,
46–49
Co,
50–52
Fe,
53–55
Au,
56,57
literature reports in this
field has been dominated by Pd-catalyzed direct arylation due to its high efficiency and
broad scope. As a consequence, the recent ten years have witnessed the intense
development of Pd-catalyzed direct arylation as a powerful tool to prepare well-defined π-
conjugated polymers.
58–81
Since the emergence of a robust direct arylation polymerization
(DArP) in 2010,
82
several classes of highly efficient catalytic systems have been explored
to synthesize high performance conjugated polymers.
20,21,83,63,79
Moreover, the versatility
in the choices of metal catalysts for direct arylation compared to traditional cross coupling
methods (Pd for Stille and Pd, Ni for Suzuki) has allowed the development of new DArP
protocols using more sustainable metal catalysts alternative to Pd (Scheme 1.1).
77
Though
several comprehensive reviews have been published to summarize the development of
reaction conditions and polymer structure-function relationships with the DArP method,
58–
60,63
an extensive review to evaluate different catalysts/catalytic systems used for DArP has
yet to be written. In this chapter, we detail the development of efficient catalysts employed
for the synthesis of conjugated polymers via DArP, including the development of
polymerization conditions inspired by small-molecule synthesis, mechanistic
understanding of different classes of DArP conditions, the suppression of defect-formation
and the effect of directing groups on the performance of the catalysts. Moreover, recent
4
advances on exploring sustainable alternative metal catalysts to replace Pd shed light on
the potential of using first-row transition metals for DArP.
1.2 Highly Efficient Pd-Catalysts for Direct Arylation Polymerization
1.2.1 Pd-Catalysts in coordinating solvents
1.2.1.1 Early Development of Small-Molecule Direct Arylation (via SEAr/Heck-like
Mechanism)
In a typical organometallic-functionalized cross-coupling reaction (such as a Stille-
Migita coupling), the organometallic aryl bond reacts as a nucleophile during a
transmetalation step to attack a Pd(II)-intermediate generated after an oxidative addition to
an aryl-halide compound.
17
This led to an assumption in early research in direct arylation
that the coupling partner featuring a reactive C-H bond should exhibit nucleophilic
reactivity, and that the mechanism of direct arylation might undergo a proposed
electrophilic aromatic substitution (SEAr).
22,84
Hence, direct arylation of π-electron-rich
heteroaromatics such as azole compounds, benzothiophenes, benzothiazoles, etc., were
extensively investigated, with one of the earliest reports showed by Ohta et al. in 1989 that
indole derivatives could be selectively arylated at the 2-position by a chlorinated pyrazine
compound (Scheme 1.2a).
85
Noticeably, a majority of these early reports on the
transformation involve conditions initially developed for Heck coupling reactions:
“ligandless” Pd catalysts (such as Pd(OAc)2), a polar amide solvent (dimethylformamide
(DMF) or dimethylacetamide (DMA)), a carbonate or acetate base (such as K2CO3,
Cs2CO3, KOAc), and extremely harsh reaction condition (typically 150-180 °C).
22,84,86,87
Another example (Scheme 1.2b) studied by Ames and co-workers in 1983 shows that
5
intramolecular direct arylation is feasible to activate an unreactive C-H bond on benzene
rings to form a family of five-membered-ring benzofurans (Scheme 1.2b).
88
The potential
SEAr mechanism of these transformations was showcased by Rawal et al. using phenol-
substituted benzene derivatives which were more reactive towards intramolecular direct
arylation, presumably due to a promoted nucleophilicity from a deprotonated phenol
functionality, while a diminished reactivity from a methoxy-substituted substrate was
obtained (Scheme 1.2c).
89
These studies set the stage for the following two decades of
research on small-molecule direct arylation, predominantly utilizing variations of an
amide-solvent-coordinated Pd-catalytic system.
Scheme 1.2 Representative early reports on Pd-catalyzed direct arylation via the proposed S EAr
mechanism, including (a) direct arylation of an indole compound reported by Ohta et al.,
85
(b)
intramolecular direct arylation of unreactive benzenes to form benzofurans by Ames et al.,
88
(c)
difference in reactivities of phenol and methoxy-substituted substrate towards intramolecular direct
arylation reported by Rawal et al.
89
6
In 2006, Antoine and co-workers reported a regioselective small-molecule direct
C-H arylation of the electron-rich thiophenes 3-methoxy-thiophene and 3,4-
ethylenedioxythiophene (EDOT) with halogenated benzenes via a presumed SEAr
mechanism (Scheme 1.3a).
90
Importantly, the amide-solvent-coordinating pattern of Pd
catalysts was revealed by the authors as to promote the electrophilicity (DMF-assisted
ionization) of the Pd species generated after the oxidative addition step (Figure 1.1).
90
Scheme 1.3 Representative early reports on Pd-catalyzed direct arylation, including (a) C-H
arylation of electron-rich thiophenes reported by Antoine et al.,
90
(b) first report of direct arylation
of thiophene with bromobenzene by Ohta et al.,
91
(c) possible Heck-like mechanism for direct
arylation,
92,93
(d) first synthesis of conjugated polymer via DArP using a Lemaire-derived
condition.
98
The direct arylation of thiophene, one of the most important building blocks for conjugated
polymers, was first reported in 1990 by Ohta et al. using 5 mol% of Pd(PPh3)4 with DMA
at 150 °C (Scheme 1.3b).
91
Interestingly, instead of the SEAr mechanism, it was proposed
to undergo a Heck-type mechanism, in which an insertion of thiophene on the Pd-catalyst
7
takes place, followed by an α-proton-elimination to generate the α-arylated product
(Scheme 1.3c).
92,93
This was later supported by density functional theory calculation (DFT)
studies performed by Davis et al. in 2005 showing that the C-H activation on aromatic rings
does not involve a positively-charged Wheland intermediate, dismissing the possibility of
the SEAr mechanism.
94
Moreover, the involvement of a Heck-type pathway in direct
arylation was strongly evidenced from DFT studies and kinetic isotope effect (KIE)
measurements by Itami et al. in 2010
95
and Larrosa et al. in 2016
96
showing that
benzothiophene could be selectively arylated at the β-position aided by a Ag additive.
Figure 1.1 Proposed catalytic cycle for direct arylation via the S EAr mechanism and the role of the
amide-coordinating solvent (DMF) in this system proposed by Antoine et al.
90
1.2.1.2 Development of the Lemaire-derived DArP Conditions
Developed under the assumption of a Heck-type mechanism, in 1999, Lemaire et
al. adopted and further utilized conditions for Heck-coupling reactions for the
polymerization of 2-iodo-3-alkylthiophenes monomers, which is the first ever DArP
reported in the literature.
97,98
This novel synthesis of a benchmark conjugated polymer,
poly(alkylthiophenes) (P3AT) (P1) via DArP was performed using the combination of a
8
Pd(OAc)2 catalyst and DMF as the coordinating amide solvent, K2CO3 as the base, and
tetrabutylammonium bromide (TBAB) as a phase transfer agent at 80 °C (Scheme 1.3d).
98
Although only oligomeric P3AT materials (number average molecular weight (Mn) = 3.0
kg/mol, dispersity (Ð) = 2.1) with a moderate regioregularity (rr) (~90%) was obtained,
Lemaire-derived DArP conditions were further developed since 2009, which are typically
performed using a Pd(II) catalyst (almost exclusively Pd(OAc)2) in a coordinating amide
solvent (DMF, DMA, or N-Methyl-2-Pyrrolidone (NMP)), a carbonate or acetate base
(K2CO3, KOAc, NaOAc), and an optional phase transfer agent such as TBAB.
Presumably due to the SEAr or Heck-like character of these Lamaire-derived DArP
conditions, they were found to be applicable for DArP reactions using electron-rich
thiophene substrates, such as alkyl-substituted thiophenes or dioxythiophene derivatives.
90
Following the studies by Antoine et al. and Lemaire et al., several research groups
attempted to evolve these small-molecule transformations into polymerization methods for
the syntheses of EDOT- or 3,4-propylenedioxythiophene(ProDOT)-based conjugated
polymers, which have shown to be excellent materials for electrochromic devices, light-
emitting devices, and batteries.
8,99
Previously, these EDOT- or ProDOT-based polymers
were most commonly synthesized via oxidative polymerization method using oxidizing
agents such as iron salts or via Grignard metathesis (GRIM), which prompted several
problems such as lower atom economy, tedious synthetic procedures, and highly stringent
polymerization conditions.
100,101
In 2009, Kumar et al. first reported the synthesis of a poly(ProDOT-alt-EDOT)
copolymer (P2) using a Lemaire-derived catalytic system: Pd(OAc)2 (10
mol%)/NaOAc/TBAB/DMF at 70 °C (Scheme 1.4a).
102
Although only low molecular
9
weight polymers were obtained (Mn = 4.6 kg/mol), this represents the first thiophene-based
alternating copolymer synthesized via DArP.
102
In the following year, Kim et al. continued
the study of the Lemaire-derived catalytic system on electron-rich thiophene substrates
such as 3,4-dialkyl-thiophene and EDOT, paired with a bislactam-based halogenated-
monomer (Scheme 1.4b).
103
Despite the use of an elevated temperature (130 °C) in a
similar catalytic system (Pd(OAc)2 (10 mol%)/NaOAc/DMA), the molecular weights of
these polymers are still quite low (Mn = 7.6 kg/mol for P3 and Mn = 8.2 kg/mol for P4). By
using a higher Pd-catalyst loading (20 mol%), Lee et al. successfully synthesized another
bislactam-EDOT-based copolymer (P5) with a high molecular weight (Mn = 43.0 kg/mol)
and good yield (80%) using a Lemaire-derived DArP condition (Scheme 1.4c).
104
It is
worth-noting that in these earlier DArP reports, the loading of Pd-catalysts are typically
higher (10-20 mol%) than traditional cross-coupling polycondensation methods (for
example, 1-4 mol% typically for Stille polymerization
17
), which demonstrates the
limitations of these early DArP conditions. Although Kanbara et al. reported the synthesis
of a fluorene-EDOT-based copolymer (P6) with an excellent molecular weight (Mn = 147
kg/mol) (Scheme 1.4d) using a microwave-assisted Lemaire-derived DArP condition
105
,
more efficient, better mechanistically-understood DArP catalytic systems that utilize lower
Pd-catalyst loading are highly desirable for the preparation of high quality conjugated
polymer materials.
10
Scheme 1.4 Some representative syntheses of electron-rich EDOT or ProDOT-based conjugated
polymers via Lemaire-derived DArP conditions.
1.2.1.3 Early Fagnou-derived DArP Conditions of Polyfluorinated Arenes
The development of new catalytic systems for DArP was fundamentally limited by
a clearer mechanistic understanding of direct arylation. With early methods, the most
commonly understood mechanisms of this C-H activation process were the proposed SEAr
and Heck-like pathways, which only involve electrophilic Pd catalysts with electron-rich,
nucleophilic aromatic substrates. Consequently, problems associated with these early
DArP methodologies include limited substrate scope (only capable to activate electron-rich
11
C-H bonds in building blocks such as EDOT or alkylthiophenes), and low reactivity and
selectivity of these Pd species (high catalyst loading or harsh reaction condition is
required). In 2006, Fagnou and co-workers first reported the direct arylation of electron-
deficient polyfluorinated arenes with high yields (up to 98%) using a low Pd catalyst
loading (1-5 mol%) at 120 °C (Scheme 1.5a).
106
Significantly, detailed computational
studies (DFT) suggested for the first time a concerted-metalation-deprotonation (CMD)
mechanism, in which the deprotonation step is dependent on the acidity of the C-H bond,
indicating a complete inversion of reactivity compared to the SEAr and Heck-like
pathways.
106
Moreover, in this proposed CMD pathway, the insertion of the Pd-complex
to the fluorobenzene substrates is suggested to be assisted by an anionic carbonate ligand
for the first time. Competition experiments reveal that more electron-deficient arenes react
preferentially, in contrast to what was proposed for SEAr or Heck-like mechanisms, where
more electron-rich substrates are preferred for the C-H activation process. Therefore, this
study revealed a new Pd catalytic system that favors electron-deficient arenes for direct
Scheme 1.5 (a) Development of the early Fagnou-derived conditions
106
and their application in
DArP using (b) 1,2,4,5-tetrafluorobenzene
107
and (c) 2,2’,3,3’,5,5’,6,6’-octafluorobiphenyl as the
C-H monomer by Kanbara et al.
108
12
arylation, and the CMD pathway remains the most prevalently accepted concept to this
day.
106
This Pd-catalytic system is herein referred to as the “early Fagnou-derived
conditions”, which consists of a Pd catalyst (Pd(OAc)2 is most commonly employed), a
phosphine ancillary ligand (alkyl-phosphine ligand is usually used in the form of a BHF4
salt, such as P
t
Bu2Me-HBF4), a carbonate base (K2CO3 is most commonly used in this
catalytic system), and an amide-coordinating solvent (DMA is often used).
These conditions for small-molecule direct arylation of fluorobenzenes developed
by Fagnou et al. were successfully transcribed to DArP by Kanbara et al. in 2011 (Scheme
1.5b).
107
Despite using a slightly lower temperature (100 °C), 1,2,4,5-tetrafluorobenzene
paired with 2,7-dibromo-9,9-dioctylfluorene was copolymerized to afford P7 with a high
Mn of 31.5 kg/mol and a 81% yield by using the early Fagnou-derived catalytic system
(Pd(OAc)2 (5 mol%), P
t
Bu2Me-HBF4 (10 mol%), K2CO3 (2 quiv.), and DMA as the amide
solvent).
107
The importance of the amide-coordinating solvent (DMA) was further
demonstrated by Kanbara et al. since the utilization of a non-coordinating solvent such as
toluene leads to suppression of the polymerization, although the mechanistic role of DMA
was not studied in these reports.
106,107
Subsequently, in 2012, Kanbara et al. further utilized
a more reactive fluorobenzene, 2,2’,3,3’,5,5’,6,6’-octafluorobiphenyl as the monomer.
108
By using the exact same DArP condition derived from Fagnou et al., P8 was afforded with
a higher Mn of 43.2 kg/mol, and a higher yield of 95% (Scheme 1.5c).
108
In 2015, Thompson et al. studied the synthesis of a fluorinated copolymer, P9, via
DArP by investigating the influence of functionality on a model small-molecule system
(Scheme 1.6).
109
In this model system, one of the coupling partners between a 2-substituted
thiophene and a pentafluorobenzene was brominated, and a significantly higher reactivity
13
is observed for direct arylation between a brominated thiophene and a unfunctionalized
fluorinated benzene (92% in yield compared to the reversed functionalization of coupling
partners, which results in a 23% yield).
109
The result from this model system is in consistent
with Fagnou’s work, in which more electron-deficient fluorinated arenes were found to
react preferentially, and the acidity of the C-H bonds is critical for the CMD step of direct
arylation.
106,109
By applying the optimal condition from the model system, which consists
of Pd(OAc)2 (5 mol%), P
t
Bu2Me-HBF4 (10 mol%), Ag2CO3 (1.1 quiv.) in DMA to the
DArP reaction, P9 was afforded with Mn of 12.0 kg/mol and a yield of 42% (Scheme 1.6).
In agreement with the small-molecule model study, the attempt of polymerization for the
synthesis of P9 using a reversed monomer functionalization failed.
Scheme 1.6 (a) Small-molecule direct arylation studies featuring pentafluorobenzene and 2-
hexylthiophene via two different monomer functionalization strategies and (b) application of the
optimal strategy in DArP reported by Thompson et al.
109
Scheme 1.7 High reactivity of benzene using carboxylate-assisted direct arylation conditions
developed by Fagnou et al.
110
14
1.2.1.4 Carboxylate-Assisted Fagnou-Derived Conditions via the CMD Mechanism
Fagnou’s work on the direct arylation of electron-deficient fluorobenzenes reveals
that the reactivity of direct arylation is dependent on the acidity of the C-H bond instead of
the arene nucleophilicity.
106
More importantly, this seminal study proposed for the first
time of the role of anionic ligand (carbonate base) assisted C-H bond cleavage mechanism
in the critical CMD step. Inspired by this proposal, Fagnou et al. incorporated a pivalic acid
additive for the first time to the direct arylation of an inactivated simple arene, benzene.
110
The addition of 30 mol% of pivalic acid to the catalytic system involving Pd(OAc)2 (3
mol%), DavePhos (3 mol%), K2CO3 (2.5 eqv.) in DMA at 120 °C exhibited unprecedented
high reactivity for the direct arylation of benzene with 100% conversion, albeit the need
for the large excess of the benzene substrate (Scheme 1.7). By comparison, the reaction
Figure 1.2 Proposed Pd-catalytic cycle for direct arylation via the concerted-metalation-
deprotonation (CMD) step.
110
15
conducted without the carboxylic acid additive under the same reaction condition provided
no isolated arylation product. Mechanistic studies using DFT analysis support a proposed
pathway involving the binding of Pd species to the pivalate anion in the CMD step by
demonstrating a decrease of transition state energy of 1.3 kcal/mol with the use of pivalate
anion (Figure 1.2).
In following studies, Fagnou et al. developed a comprehensive analysis towards the
understanding of the carboxylate-assisted CMD mechanism over a broad range of aromatic
substrates.
111,112
The CMD transition state was being analyzed and the enthalpic
contribution to the CMD transition state energy was divided into the factors of bond
distortion energy (Edist, the bending and elongation of the Pd-ligand bond and the C-H
bond) and interaction energy (Eint, the interaction between the aromatic substate, Pd
catalyst, and the carboxylate additive) (Figure 1.3). Evaluation by DFT calculations
indicated the reactivity of electron rich substrates is dictated by Eint, and direct arylation of
electron-rich substrates occurs at the most nucleophilic position with the assistance of a
Figure 1.3 Enthalpic contributions to the CMD transition state energy, which is divided into
bond distortion energy (E dist) and interaction energy (E int) proposed by Fagnou et al.
111,112
16
carboxylate additive.
111,112
Electron-deficient aryl substrates, however, are highly reactive
due to a lower Edist barrier of the C-H bond, and therefore, the selectivity of these substrates
for direct arylation is preferred at the most acidic C-H bond. Significantly, this study shows
that the involvement of the carboxylate-assisted CMD mechanism is much more broadly
implicated to include both electron-rich and electron-deficient substrates for direct
arylation, in contrast to the aforementioned SEAr and Heck-like mechanisms.
1.2.1.5 Role of the Amide Solvent in Fagnou-Derived Conditions
Scheme 1.8 DMA as the optimal solvent for electron-rich thiophenes via DArP, but unable to
provide polymer products with electron-deficient thiophenes, as reported by Kanbara et al.
113
In DArP, when a coordinating amide solvent such as DMA is used, Kanbara and
co-workers reported that electron-rich thiophene substrates exhibit superior reactivity
compared to electron-deficient thiophene derivatives with more acidic C-H bonds, even
when a carboxylate additive is present to facilitate the process.
113
In a model system using
2,7-dibromo-9,9’-dioctylfluorene as the dibrominated monomer, DMA was found to be the
suitable solvent for electron-rich thiophenes such as EDOT, 3,4-dialkoxy-thiophene, and
17
3,4-dimethylthiophene (P10-P12), but failed to provide any polymer products when
electron deficient thiophenes such as 5-(2-ethylhexyl)-thieno-[3,4-c]-pyrrole-4,6-dione
(TPD) and 3,4-dicanothiophene were used (Scheme 1.8).
113
Similar effects of amide
solvent on substituted thiophene substrates can be observed in the study by Ling et al.,
where the electron-rich substituted thiophene derivative, 3,4-dimethoxy-thiophene
provides polymer (P13) with optimal Mn (24.0 kg/mol) for DArP using DMA as the solvent
while electron deficient substrate 3,4-dicanothiophene only gives oligomeric product (P14)
(Mn = 5.9 kg/mol) (Scheme 1.9).
114,115
These results are in contrast to the efficient
polymerizations of P7 and P8 involving C-H activation of electron-deficient fluorinated
benzenes using DMA as the solvent (Fagnou-derived condition).
107,108
It is possible that
the reactivities of the electron-deficient substituted thiophene substrates are different than
those of fluorinated benzene substrates under the Fagnou-derived condition. Although
mechanistic insights regarding this reactivity of DArP using polar amide solvent were not
provided in these reports, investigation on the effect of the amide solvent on the Pd catalysts
is valuable for the development of DArP to prepare high quality conjugated polymers.
Scheme 1.9 DMA as the optimal solvent for electron-rich thiophenes via DArP, but only provided
oligomeric products with electron-deficient thiophenes, as reported by Ling et al.
114
18
Scheme 1.10 Ligation of DMA to the Pd catalyst by displacing a phosphine ligand, which lowers
the energy barrier for the C-H activation, reported by Hartwig et al.
116
In a report by Hartwig et al. in 2011, a phosphine-ligated aryl-Pd carboxylate
complex [P(t-Bu)3Pd(Ar)-(O2CR)] was isolated, but was found unable to form any biaryl
products with benzene by direct arylation (Scheme 1.10).
116
In contrast, when the
phosphine ligand in the organometallic Pd complex was displaced or consumed to form a
“ligandless” aryl-Pd carboxylate species, the reactivity of the direct arylation reactions
significantly enhanced. In this case, the DMA solvent is found to ligate with the Pd catalyst
to form an amide-solvent‐coordinated Pd complex [(DMA)Pd(Ar)(O2CR)], which is
shown to be the catalytically highly-active species for the C-H cleavage step.
116
In addition,
amide-solvent-coordinating Pd-catalyst [(DMA)Pd(Ar)(O2CR)] possesses a much lower
activation barrier of C-H abstraction (31 kcal/mol) compared to that of P(t-
Bu)3Pd(Ar)(O2CR)] (42 kcal/mol), showcasing the highly efficient phosphine-free
Fagnou-derived DArP conditions consist of Pd(OAc)2 as Pd source, K2CO3 as the base,
and an amide-coordinating solvent (DMF or DMA). The phosphine ligated Pd-complex
possesses a much higher CMD energy barrier possibly due to the increased steric hindrance
from the P(t-Bu)3 ligand, compared to the highly-reactive [(DMA)Pd(Ar)(O2CR)]
complex.
116,117
19
With the role of a ligand being recognized for amide-coordinating solvents, their properties
such as the steric size of the substituents on the amide moiety would be expected to
influence the reactivity of the [(amide-solvent)Pd(Ar)(O2CR)] complex. In an effort to
study this hypothesis, Thompson et al. investigated the influence of the amide solvent
structure on the reactivity of DArP catalytic system by synthesizing the benchmark
conjugated polymer poly(3-hexylthiophene) (P3HT).
118
By using a Fagnou-derived
catalytic system for DArP using Pd(OAc)2 as the catalyst, K2CO3 as the base, neodecanoic
acid (NDA) as the carboxylate additive, various of amide solvents possessing different
substituents on the amide were investigated for their impact on the synthesis of P3HT via
DArP (Scheme 1.11). The authors observed that DArP is sensitive to the steric bulk on the
amide, where enough steric bulk of the alkyl substituent on the carbonyl carbon and
nitrogen atom of the amide solvent is required to promote the high reactivity of the Pd
catalyst, yet excessive steric bulk of the amide solvent is postulated to inhibit efficient
ligation to the Pd center.
118
Amide solvents possessing an acyclic aliphatic structure such
as DMA, N,N-dimethylpropionamide (DMP) and N,N-diethylacetamide (DEA) were
Scheme 1.11. Selected amide solvents investigated by Thompson et al. to evaluate their influence
on the synthesis of P3HT via Fagnou-derived DArP.
118
20
found to be optimal solvents for the synthesis of P3HT (Mn = 23.6-32.5 kg/mol) using
DArP, and the amide solvents were found to be sensitive to atmospheric water content,
likely due to the hydrolysis of the amide.
118
Additionally, cyclization of the amide solvent
such as NMP was found detrimental to DArP for the synthesis of P3HT, although NMP
was found to be an efficient solvent in a Fagnou-derived DArP condition for the synthesis
of a donor-acceptor (DA) alternating conjugated polymer.
119
Nevertheless, the study on the
choice of amide solvents conducted by Thompson et al. provides valuable insights to the
ligation of the amide solvents and the effective Pd species in the Fagnou-derived DArP
conditions.
1.2.1.6 Minimization of Defect-Formation in Fagnou-Derived DArP Methods
Suppressing Homocoupling Defects Using Fagnou-Derived DArP Conditions:
It is most commonly accepted that the active Pd catalyst entering the catalytic
cycles for cross-coupling polymerizations is a Pd(0) species.
20–22,111
For the case of Stille
polymerizations, Pd(0) catalysts such as Pd(PPh3)4 or Pd2(dba)3 are often used, which
alleviate the need for a reduction of Pd(II) catalysts to generate the active Pd(0) species.
17
For DArP conditions, especially Fagnou-derived DArP conditions, air-stable Pd(II)
precatalysts such as Pd(OAc)2 are most commonly employed.
22,23,110–112,118
These Pd(II)
species often undergo a reduction by electron-rich phosphine ligands through the
generation of a PdL2(OAc)2 intermediate, and subsequently generate an active Pd(0)
species for the oxidative addition step in the catalytic cycle.
120–123
In the case of the Fagnou-
derived DArP conditions, however, phosphine ligands are often not utilized and are absent
in the reactive [(DMA)Pd(Ar)(O2CR)] catalytic species, as shown by Hartwig et al.
116
21
Therefore, the pathway Pd(II) catalysts undergo to generate Pd(0) species in these systems
with the absence of any potential reductants or phosphine ligands was unclear. To
investigate the pathway to generate the active Pd(0) species in the phosphine-free Fagnou-
derived conditions, Kanbara and co-workers reported a small-molecule study on the
homocoupling of bithiophene under a typical Fagnou-derived condition (Pd(OAc)2 as the
Pd source, K2CO3 as the base, 100 °C in DMA). As a result, the reaction between 1
equivalent Pd(OAc)2 and 5 equivalents of 2,2’-bithiophene in the absence of phosphine-
ligands affords quarterthiophene as a product, accompanied by the formation of Pd black
(Scheme 1.12, top).
124
The authors explained this phenomenon with a proposed mechanism
involving a CMD pathway followed by an oxidative C-H/C-H homocoupling to give the
corresponding dimerized product, accompanied by the generation of a Pd(0) species
(Scheme 1.12, bottom). Due to the lack of stabilizing ligand for the Pd(0) species, Pd black
is formed and precipitated from the reaction. The authors found that the use of a minimal
amount of the Pd catalyst (1 mol%) successfully suppressed the undesired C-H/C-H
Scheme 1.12 Generation of Pd(0) through reduction of Pd(II) via C-H/C-H homocoupling
of 2,2’-bithiophene under the Fagnou-derived conditions, reported by Kanbara et al.
124
22
homocoupling and thus improved the yield and selectivity of direct arylation between
thiophene substrates and bromobenzenes.
124
Parallel catalytic behavior can be observed with another transition metal, Ni,
towards the direct C-H arylation of azoles. Although Ni-catalyzed DArP has not been
demonstrated, Ni has been shown to be an effective metal for catalyzing small-molecule
direct arylation, especially using azole substrates with acidic C-H bonds.
46,49
Similar to Pd,
Ni is proposed to involve a Ni(0)/Ni(II) redox catalytic mechanism, with the Ni(0) species
being the active catalytic species for the direct arylation.
49
Unlike Pd(0) sources such as
Pd(PPh3)4 or Pd2(dba)3 precatalysts, which can be handled in air, commercially-available
Ni(0) catalysts such as Ni(cod)2 are highly unstable in air and suspectable to oxidation
under air and moisture.
20,21
Therefore, the Ni(0) active species is proposed to be generated
in situ from stable Ni(II) catalysts such as Ni(OAc)2 in Ni-catalyzed direct arylation of
azole compounds. In a study by Itami et al., experiments showed that an oxidative C-H/C-
H homocoupling of benzothiazole occurred with the presence of Ni(OAc)2 and afforded a
Ni(0) species as a result (Scheme 1.13), which is highly similar to the behavior of the Pd
catalysts reported by Kanbara et al.
49,124
However, likely because of a lower catalytic
reactivity of Ni compared to Pd towards direct arylation, 10 mol% of Ni catalyst loading
Scheme 1.13 Generation of Ni(0) through reduction of Ni(II) via C-H/C-H
homocoupling of benzothiazole similar to Pd catalysts, as reported by Itami et al.
49
23
is required for the desired arylation to occur, which prohibits the potential of lowering the
catalyst loading of Ni to improve selectivity. Instead, azole substrates are being employed
in a higher equivalence with a portion of the azole being sacrificed to generate azole dimers
and the active Ni(0) species, which is problematic compared to Pd catalysts studied by
Kanbara et al.
49,124
The study by Kanbara and colleagues on the selectivity of small-molecule direct
arylation is valuable to examine the potential defect-formation of DArP using Fagnou-
derived conditions. In particular, homocoupling of thiophenes to generate the active Pd(0)
species may lead to a decrease in the regioregularity (rr) of the polymer.
58,63
To improve
the rr of P3HT using Fagnou-derived DArP conditions, Thompson et al. reported the
minimization of auxiliary reagent loadings for the synthesis of P3HT, including the loading
of the catalyst (Pd(OAc)2), the carboxylate additive (NDA), and the solvent (DMA).
125
Specifically, with an ultra-low loading of Pd(OAc)2 of 0.0313 mol% along with an elevated
temperature at 160 °C, the authors were able to synthesize P3HT with Mn of 24.2 kg/mol
and yield of 91% (Scheme 1.14).
125
Importantly, the P3HT synthesized in this report
achieved a high rr of up to 96.5%, which is the highest rr of P3HT using Fagnou-derived
DArP conditions and is superior compared to Stille P3HT polymers (up to rr = 94%).
125
This report suggests that, in the phosphine-free Fagnou-derived conditions, minimization
Scheme 1.14 Ultra-lowing of the Pd-catalyst to minimize homocoupling defect of P3HT,
reported by Thompson et al.
125
24
of Pd catalyst loading is important for the suppression of homocoupling defects embedded
in the polymer structure, consistent with the small-molecule study performed by Kanbara
et al.
117,124
Scheme 1.15 Strategies of suppressing homocoupling defects for donor-acceptor (DA) copolymer
(P15) synthesized using the Fagnou-derived DArP conditions.
126
Identifying homocoupling side reactions are not only critical for the regioregularity
of homo-polymers such as P3HT, but also important for DA alternating copolymers
synthesized using DArP protocols. Donor-donor and acceptor-acceptor homocouplings are
important potential defects that are detrimental to the structural, optical, and electronic
properties of the resulting polymers.
63
In 2014, Sommer et al. reported the synthesis of P15
(Scheme 1.15) using a Fagnou-derived condition: Pd(OAc)2 as catalyst, K2CO3 as base,
PivOH/PivOK as the carboxylate additive, and DMA as the solvent.
126
Although in some
experiments, the authors utilized a DMA/toluene, or DMA/tetrahydrofuran (THF) co-
solvent system, DMA is still present as the only amide coordinating solvent that is
potentially ligated to the Pd catalyst, and therefore, the DArP conditions in this report are
herein categorized as Fagnou-derived conditions. The authors demonstrated that the
addition of a phosphine ligand (PCy3) suppressed the formation of 4,7- bis(4-hexyl-2-
25
thienyl)-2,1,3-benzothiadiazole (TBT-TBT) homocoupling defects, while the reduction of
the reaction temperature helped prevent 2,7-dibromo-9-(1-octylnonyl)-9H-carbazole (Cbz-
Cbz) homocouplings (Scheme 1.15). These results reveal that phosphine ligands can play
a significant role in the prevention of defect-formation in Fagnou-derived DArP conditions.
Although it has been demonstrated by Hartwig et al. that the coordination of the Pd metal
center is governed by the amide-solvent regardless of the presence of phosphine ligands,
116
and phosphine-free syntheses of homopolymers such as P3HT via the Fagnou-derived
DArP conditions have been reported in several reports by Thompson et al.,
74,118,125,127,128
Sommer et al. reason that the dependence on the phosphine ligands in suppressing
homocoupling defects for DA polymers might be specific to the monomer combination
investigated. In the absence of phosphine ligands, the sterically-less demanding acetate
ligands might be able to bridge two palladium centers, which results in homocoupling of
two Cbz units with terminal Pd complexes. Unlike the synthesis of P3HT reported by
Thompson et al., which showed the high-temperature-tolerance of the phosphine-free
Fagnou-derived DArP conditions,
118,125
donor-donor homocoupling defects are potentially
temperature-dependent for the Fagnou-derived conditions involving phosphine ligands.
126
Suppressing β-defects using Fagnou-Derived DArP Conditions:
In 2012, Kanbara et al. attempted to use the Fagnou-derived DArP protocol (2
mol% Pd(OAc)2 as the Pd source, K2CO3 as the base, pivalic acid (PivOH) as the
carboxylate additive, in DMA at 100 °C) for the polymerization of 2,7-dibromo-9,9’-
dioctylfluorene with 2,2’-bithiophene monomers (Scheme 1.16a).
129
This led to the
formation of insoluble materials, presumably due to significant amounts of β-defects
(branching defects) and likely crosslinking of the unsubstituted 2,2’-bithiophene motif. To
26
examine the formation of β-defects, a small-molecule model study was conducted by
Kanbara et al., which utilized a mono-brominated compound with the 2,2’-bithiophene
monomer under the same phosphine-free Fagnou-derived Pd-catalyzed direct arylation
conditions (Scheme 1.16b). As a result, trace amounts of branching defect structures were
identified by using the same polymerization conditions, which likely explains the potential
β-defects embedded in the polymer structure of P16, since only trace amounts of β-defect
in small-molecule coupling can result in a significant degree of branching formation when
transforming the reaction conditions to polymerizations.
129
This is further supported by a
successful polymerization of P17 with a 3,3’,4,4’-tetramethyl-2,2’-bithiophene monomer
(Mn = 31.8 kg/mol and yield of 91%), circumventing the β-branching at the β-positions
Scheme 1.16 (a) Unsuccessful synthesis of P16 using the Fagnou-derived DArP conditions
reported by Kanbara et al.
129
(b) Small-molecule model study on 2,2’-bithiophene with 1-bromo-
4-methylbenzene. (c) Synthesis of P17 using the Fagnou-derived DArP conditions by Kanbara et
al.
130
27
(Scheme 1.16c).
130
These results illustrate that besides the problematic C-H/C-H
homocoupling defects arising from the higher loading of Pd-catalysts while reacting with
the 2,2’-bithiophene monomer (discussed above), the Fagnou-derived DArP protocol is
also suspectable to C-H activation of unwanted positions on the thiophene substrates.
Indeed, Pd-catalyzed direct arylation can potentially lead to unselective C-H
activation on the β-positions of thiophenes, as supported by the free energy calculations
for C-H activation of thiophene via the CMD pathway. In a report by Fagnou et al., under
the Pd catalyzed condition, the free energy of the CMD step for the α-position of thiophene
is 25.6 kcal mol
-1
while the β-position of thiophene is 29.9 kcal mol
-1
.
111
The low transition
state energy of the CMD pathway due to the highly-reactive Fagnou-derived condition
assisted by the coordinating solvent and the carboxylate additive can lead to undesired C-
H activation and branching defects.
110–112,116
While in the case of small-molecule reactions,
Scheme 1.17 Synthesis of β-defects-free P3HT using a bulky carboxylic
acid additive, neodecanoic acid (NDA) reported by Thompson et al.
74,127,131
28
such defects can be removed during purification, they are embedded in the resulting
polymer structures. In 2013, Thompson et al. reported the influence of the structure of
carboxylic acid additive on the properties of P3HT synthesized via Fagnou-derived DArP
conditions.
74,127,131
In particular, the authors discovered that with the use of a bulky
carboxylic acid, NDA, in place of the traditionally employed PivOH, β-defects of the
prepared P3HT can be completely suppressed (Scheme 1.17), as evidenced by the
1
H NMR
analysis.
131
On the contrary, polymerizations employing PivOH as the carboxylate additive
resulted in 0.16% β-defect concentration, which was demonstrated to be detrimental to the
crystallinity and solar-cell performance of the resulting P3HT polymers.
128,131
As proposed
by Thompson et al., the bulkiness of NDA (a mixture of neopentyl C10 carboxylic acids
with bulky substituents) likely sterically inhibits the Pd-catalysts from the β-position of the
thiophene substrates and therefore prevents the C-H activation of the undesired branching
formation (Figure 1.4).
131
The strategy of incorporating a bulky carboxylic acid not only
eliminates the formation of β-defects for Fagnou-derived DArP protocols involving amide-
coordinating solvents, but has been extended to DArP conditions that utilize non-
coordinating solvents,
61
which will be discussed in more detail in Section 1.2.2.4.
1.2.2 Pd-Catalysts in non-coordinating solvents (Ozawa-derived conditions)
1.2.2.1 Development of Ozawa-Derived DArP Conditions
Figure 1.4 Plausible mechanism of suppressing β-defects
using a bulky carboxylate ligand.
29
As mentioned in the above section, the amide-solvent‐coordinated Pd complex
[(DMA)Pd(Ar)(O2CR)] is a highly-active catalytic species for direct arylation, as identified
by Hartwig et al.
116
The development of small-molecule direct arylation using a
coordinating amide solvent was proposed by Ohta et al.
85,91
and Lemaire et al.
97,98
via SEAr
or Heck-like mechanism, which were further improved and optimized by Fagnou et al. via
the proposed CMD mechanism.
106,110–112
The Fagnou-derived condition was subsequently
applied to the synthesis of rr-P3HT by Thompson et al.,
74,118,125,128,131
as discussed above.
These phosphine-free Fagnou-derived DArP conditions are advantageous with their ultra-
low loadings of auxiliary reagents, and are highly cost-effective and result in low residual
metal content. However, direct arylation conducted via the Fagnou-derived conditions is
highly solvent-dependent, where the increase of steric bulk on the amide-moiety
118
or the
absence of the coordinating amide solvent
109
can result in suppressed polymerization
outcomes. This is due to the crucial ligation of the amide solvents to the Pd species in this
class of conditions, regardless of the addition of a phosphine ligand.
116
Additionally, polar
amide coordinating solvents such as DMF and DMA exhibit low solubilizing ability for
conjugated polymers (especially for DA alternating copolymers with more complex π-
conjugated backbones) compared to non-coordinating solvents such as THF and
toluene.
63,64,79
Therefore, DArP conditions that utilize non-coordinating solvents are highly
attractive.
79
Since non-polar solvents such as THF and toluene are not ligated to the Pd-
catalysts, the employment of different ligands can play significant roles in the catalytic
efficiency of the Pd species towards direct arylation and can be fine-tuned to accommodate
a broad scope of reaction substrates (Ln = PR3 in Figure 1.2). Conversely, the catalytically-
active Pd species in the Fagnou-condition are more likely to coordinate to the amide
30
solvents rather than phosphine ligands (Ln = amide solvent in Figure 1.2), according to the
report by Hartwig et al.
116
In 2010, Ozawa et al. first attempted the synthesis of rr-P3HT by employing the
non-coordinating solvent, THF.
82
By directly replacing the base the K2CO3 in the Fagnou-
derived condition with the stronger base Cs2CO3 and switching the solvent from DMA to
THF, the polymerization did not proceed successfully.
82
This revealed that the catalytic
system (Pd(OAc)2, carboxylate additive, phosphine-free) only applies to the Fagnou-
derived DArP condition, and does not apply to DArP conditions without coordinating
solvents.
79
Through extensive optimization of the Pd-catalysts and the phosphine ligands,
the authors developed a highly-efficient novel catalytic system for DArP, which consists
Scheme 1.18 Synthesis of rr-P3HT using non-coordinating
solvent, THF, reported by Ozawa et al. Structures of
Pd(Herrmann), L1, and L2 are denoted.
82
31
of a Pd-Herrmann catalyst, P(2-MeOC6H4)3 (L1), Cs2CO3 as the base, and superheated
THF as the solvent, which afforded P3HT with Mn of 30.3 kg/mol and rr of 93% (Scheme
1.18).
82
The Pd-Herrmann catalyst is a palladacycle (structure shown in Scheme 1.18)
derived from Pd(OAc)2 and P(o-tolyl)3, which exhibits high thermal stability and is widely
used in Pd-catalyzed cross-coupling reactions.
132
The synthesis of P3HT via this Ozawa-
derived DArP condition was further improved by using P(2-Me2NC6H4)3 (L2) instead of
L1, which provided highly regioregular P3HT (rr = 98%) with an improved Mn (30.6
kg/mol).
82
The authors also attempted various of phosphine ligands other than L1 and L2,
however, these led to polymers with unsatisfactory Mn and rr, demonstrating the significant
role of L1 and L2 in the highly-efficient Pd catalytic system utilized in the Ozawa-derived
conditions.
82
Scheme 1.19 Synthesis of DA copolymer P7 via the Ozawa-derived conditions. Structure of the
Pd 2(dba) 3 catalyst is denoted.
133
The Ozawa-derived DArP condition was successfully applied to the synthesis of an
alternating DA copolymer P7,
133
which was previously synthesized via the Fagnou-derived
condition by Kanbara et al. with Mn of 31.5 kg/mol.
107
A highly efficient catalytic system
(Pd2(dba)3·CHCl3/L1/PivOH/Cs2CO3) was revealed, showing a high catalytic performance
by affording P7 with high Mn of 347.7 kg/mol and yield of 96% (Scheme 1.19).
Importantly, the employment of a carboxylate additive (PivOH or AcOH) was proven to
be benifitial for the Ozawa-derived DArP catalytic system. This shows that, the
32
carboxylate-assisted CMD pathway for C-H activation proposed by Fagnou et al. likely
also applies to the Ozawa-derived condition, when the Pd species is ligated to the
phosphine ligand instead of the amide solvent (Figure 1.2). The authors also discovered
that the choice of the phosphine ligand L1 is crucial for this catalytic system, as even its
stereoisomer P(4-MeOC6H4)3 resulted in the formation of oligomers (Mn = 3.1 kg/mol).
133
Other phosphine ligands such as P(o-tolyl)3, P(t-Bu)3, PCy3, SPhos were also ineffective
towards the desired polymerization.
133
This L1-based Pd-catalyst was proven to be effective for the synthesis of TPD-
based copolymer P18 by Ozawa et al. (Mn = 36.8 kg/mol, yield > 99%)
134
and Leclerc et
al. (Mn = 56 kg/mol, yield = 96%).
135
using the Ozawa-derived DArP conditions (Scheme
1.20). In both reports, the use of L1 was indispensable regardless of the Pd-catalysts
employed such as Pd2(dba)3·CHCl3, PdCl2(MeCN)2, and Pd(OAc)2.
134,135
For comparison,
Ozawa et al. also prepared P18 via Fagnou-derived DArP condition using DMA as the
solvent, which resulted in a lower Mn (15.1 kg/mol) and a bimodal peak in the gel
Scheme 1.20 Application of the L1-based Ozawa-derived conditions to the synthesis of TPD-
based copolymer P18 reported by Ozawa et al.
134
and Leclerc et al.
135
33
permeation chromatography (GPC) profile.
134
This demonstrates the advantage of using
non-polar solvents in the Ozawa-derived condition owing to the superior solubilizing
ability of these solvents for conjugated polymers, compared to polar, coordinating solvents
in the Fagnou-derived condition. Similarly, Thompson et al. studied the synthesis of
PPDTBT (P19) using both classes of DArP conditions (Scheme 1.21).
136
With the Fagnou-
derived condition (Pd(OAc)2/K2CO3/NDA/DMA), P19 was provided with Mn of 14
kg/mol, and a low yield of 28%, while the Ozawa-derived condition
(Pd2(dba)3/L1/NDA/Cs2CO3/THF) afforded P19 with similar Mn of 15 kg/mol, but a much
improved yield (78%).
136
These results show that while the Fagnou-derived catalytic
system is effective for the synthesis of conjugated polymers, the Ozawa-derived condition
is suitable for a broader scope of DA copolymers due to the use of better solvents for
conjugated polymers.
134–136
Moreover, a variety of the Pd-catalysts were found applicable
in the Ozawa-derived condition, including Pd2(dba)3, PdCl2(MeCN)2, Pd(Herrmann),
Pd(OAc)2, PdCl2(PPh3)2, etc., which allow fine-tuning of Pd-catalysts to optimize
polymerization outcomes for each specific substrates.
62,79,82,133–136
Conversely, the Fagnou-
Scheme 1.21 Comparison between two classes of DArP conditions (Fagnou-derived and Ozawa-
derived) to prepare P19 by Thompson et al.
136
34
derived DArP condition almost exclusively utilizes Pd(OAc)2 as the Pd source ligated to
the amide-solvent, which limits the optimization parameters to achieve higher quality
conjugated polymers.
74,110,128–131
1.2.2.2 Mechanistic Understanding of the Ozawa-Derived Conditions
Early results with Ozawa-derived conditions demonstrated that the choice of
phosphine ligand is significantly more critical to DArP, when compared to the Fagnou-
derived conditions.
133,134
To probe the role of phosphine ligands in the Ozawa-derived
catalytic system and how they affect the DArP reactivity, Ozawa et al. conducted a study
on a small-molecule direct arylation system (Scheme 1.22, top).
137,138
They observed that
in non-coordinating solvents, the Pd catalyst forms an equilibrium between the
catalytically-inactive tetrameric Pd species (complex 2, n = 4), the dimeric Pd species
(complex 2, n = 2), and the monomeric active Pd catalytic species (complex 3). The
catalytic activity of monomeric complex 3 was verified by reacting with 2-methylthiophene
to afford the arylated thiophene product. It is worth noting that the monomeric Pd active
Scheme 1.22 Catalytic active species in the Ozawa-derived condition in small-molecule
direct arylation (top), and the role of hemilabile ligand L1 in explaining the high reactivity
of the L1-based Pd-catalytic system, proposed by Ozawa et al.
137,138
35
species (complex 3 in Scheme 1.22) is structurally in agreement with complex 1 in Figure
1.2 proposed by Fagnou et al., where Ln in complex 1 is replaced by a phosphine ligand
(PAr’3) in the Ozawa-derived condition.
111,112,137,138
By screening different phosphine
ligands, Ozawa et al. observed that the increase of steric bulk of Ar facilitates the formation
of the catalytic active complex 3, which improves the reactivity of direct arylation.
Furthermore, in another account, Ozawa et al. reported that the increase of electron-
deficiency of PAr’3 facilitates the direct arylation of electron-rich thiophenes, while
electron-rich PAr’3 ligands promote the direct arylation of electron-poor benzothiazole.
139
DFT calculations for complex 3 confirmed that the PAr’3 ligands remained coordinated
with the Pd species throughout the catalytic cycle of the reaction in THF (Ln = PAr’3 in
Figure 1.2).
Although Pd complexes employing bulky Ar’ group on the phosphine ligand tend
to form the monomeric Pd species (complex 3), Ozawa et al. found that the reactivity of
the Pd catalyst bearing a compact Ph group was remarkably lower due to its tendency
toward aggregation to form complex 2.
137,138,140
Conversely, this aggregation was
completely suppressed with the use of L1,
140
which explains the significantly high
reactivity of the Pd/L1 catalytic system towards DArP.
133,134
To investigate this
phenomenon, L1-coordinated aryl-Pd-acetate complexes were isolated (Scheme 1.22,
bottom). In the solid state, complex 4 was isolated in its dimeric form, however, this
complex was converted into two different types of monomeric complexes in solution
(complex 5 and complex 6).
140
X-ray studies revealed that L1 is a hemilabile ligand,
exhibiting a weak coordination of the Pd center to the O atom of L1 (complex 5), which
allows a rapid interchange of the coordination mode of Pd and L1 (between complex 5 and
36
complex 6).
140
This particular solution behavior of the Pd-L1 catalytic system effectively
suppresses the aggregation towards inactive complex 2, thus giving rise to the high
reactivity of the L1-based Ozawa-derived DArP conditions.
79,133,134,136,140
1.2.2.3 Pd Catalysts in Sustainable Solvents and Water-Compatible Conditions
The development of Ozawa-derived DArP conditions circumvents the necessity of
employing amide-coordinating solvents, allowing conjugated polymers to be prepared
using non-polar solvents, such as THF or toluene. This further allows researchers to explore
the potential utilization of sustainable solvents for DArP, since alternative green solvents
almost exclusively lie in the category of non-coordinating solvents (a sustainable amide-
coordinating solvent has yet to be discovered to the best of our knowledge).
141–146
Although
DArP provides an inherently sustainable pathway for conjugated polymer synthesis by
avoiding stoichiometric toxic acute hazards such as organostannanes, the solvents being
employed for DArP are present in the highest quantity in these reactions and are still
Figure 1.5 Summary of commonly used non-polar solvents in DArP with
their boiling points and sustainability.
37
hazardous for health and environment.
77
Moreover, solvents such as THF or toluene are
produced from unsustainable sources and require extensive energy for their production
from fossil sources.
142,144
On the other hand, sustainable solvents such as 2-MeTHF and
cyclopentylmethyl ether (CPME) have drawn significant attention since high catalytic
efficiencies for small-molecule direct arylation in these solvents were realized (Figure
1.5).
147,148
Although Sommer et al.
149
and Marks et al.
61
reported the synthesis of high Mn
conjugated polymers using 2-MeTHF as the alternative to THF, 2-MeTHF requires a large
number of synthetic steps and is more prone to organic peroxide formation compared to
THF, despite being capable of derivation from biomass.
141,147
In an effort to investigate the
use of a variety of sustainable solvents for DArP, in 2018, Thompson et al. reported that
CPME is the optimal green alternative solvent for the synthesis of P19, providing polymers
with high Mn (41 kg/mol) and high yields (up to 98%) (Scheme 1.23a).
150
For comparison,
Scheme 1.23 Examples of highly efficient DArP protocols in sustainable
solvents (Thompson et al.)
150,154
or in biphasic conditions (Leclerc et al.)
157
.
38
P19 synthesized using CPME as a sustainable solvent almost triples the Mn compared to
that synthesized using the conventional solvent THF (15 kg/mol).
136,150
This result
demonstrates that the L1-based Pd catalytic system displays more outstanding performance
in the sustainable solvent CPME.
141,143,150,151
Besides the use of 2-MeTHF and CPME, the
use of bio-derived solvent anisole has also emerged for DArP.
152,153
Recently, Thompson
et al. also identified a bio-renewable, naturally-sourced aromatic solvent, p-cymene
(Figure 1.5), in the application of DArP for the first time.
154
Although aromatic solvents
are an important class of non-coordinating solvents for DArP displaying excellent
solubilizing ability for conjugated polymers and exhibiting intriguing behavior such as
chain termination,
149,155,156
sustainable replacement for this class of solvent remained
unexplored for DArP. As a sustainable alternative to aromatic solvents such as toluene or
xylenes, p-cymene has a high boiling point (177.1 °C), which allows DArP to be conducted
without a pressurized reaction setting, while other sustainable solvents such as 2-MeTHF
and CPME have lower boiling points and are usually run in a pressurized DArP condition
(Figure 1.5). The L1-based Pd-catalyst displayed excellent performance in the sustainable
solvent while p-cymene provides high solubilizing ability for conjugated polymers, which
allowed P20 to be synthesized in high Mn of 51.3 kg/mol (Scheme 1.23b), which is higher
than that prepared by the conventional aromatic solvent, toluene (47.4 kg/mol).
154
In 2017, the water-compatibility of the Ozawa-derived DArP catalytic system was
realized by Leclerc et al.
157
The authors demonstrated that, strict anhydrous, air-free
Schlenk techniques are not necessary for DArP using the Ozawa-derived conditions.
Similarly, Kanbara et al. showed the syntheses of TPD-based polymers under aerobic
conditions using non-anhydrous toluene as the solvent (Ozawa-derived DArP
39
conditions).
158
By comparison, Thompson et al. reported that when Fagnou-derived DArP
conditions were used for the synthesis of P3HT, an amide solvent containing traces of water
could drastically decrease the reactivity of DArP since amide solvents may be hydrolyzed
to dialkylammonium carboxylate compounds.
118
Although the report by Leclerc et al. did
not demonstrate an improvement of the sustainability aspect of DArP, since the amount of
conventional toxic solvent (toluene) utilized remained the same compared to traditional
DArP methods, the authors showcased that the Ozawa-derived conditions are water and
air-insensitive as well as being more user-friendly compared to the Fagnou-derived
conditions. As an example, Leclerc et al. synthesized P18 using a L1-based Pd catalytic
system with an excellent Mn of 72 kg/mol and a high yield of 97% in a toluene/water
biphasic solution (Scheme 1.23c). The biphasic DArP system allowed the use of
inexpensive, more accessible “wet” reagents and solvents, though a saturated K2CO3
aqueous solution (40 equiv. of K2CO3) is required to provide the optimal results.
157
1.2.2.4 Suppression of Defect-Formation in Ozawa-Derived DArP Methods
“Mixed Ligand” Approach in Suppressing Homocoupling Defects
The improvement of sustainability using the Ozawa-derived conditions has
undoubtedly marked the major development of DArP. However, similar to the Fagnou-
derived conditions, undesired branching defects, homocoupling defects and debromination
events are major concerns and obstacles that limit the general compatibility and broad
capacity of the Ozawa-derived DArP conditions.
63,79
Specifically, although the L1-based
catalytic system has demonstrated its high efficiency for DArP in noncoordinating
solvents, direct arylation at undesired C-H positions and C-Br/C-Br homocoupling defects
have been observed in the examples of several diketopyrrolopyrrole (DPP)-based
40
copolymers, as documented by Sommer et al.,
159,160
Wang et al.,
161
and Leclerc et al.
162
Since the emergence of the Ozawa-derived conditions in 2010, several approaches have
been utilized to minimize the defect-formation of this synthetic methodology.
Ozawa et al. first developed a “mixed ligand catalyst” approach and found that the
combined use of L1 and an affordable TMEDA (tetramethylethylenediamine) ligand
effectively suppressed the formation of branching defects and significantly reduces the
homocoupling defects resulting from debromination events.
163,164
The synthesis of TPD-
based copolymer P21 via DArP was first examined by Leclerc et al.,
165
who reported the
formation of insoluble material, an indication of potential branching and cross-linking
defects embedded in the polymer structure. Ozawa et al. attempted the reaction at a lower
temperature (decreased from 120 °C to 100 °C) with a different Pd catalyst (changed from
Pd2(dba)3·CHCl3 to Pd(Herrmann)) while the ligand of choice remained the same (L1),
however, 74% of the polymer product was isolated as insoluble materials while the soluble
Scheme 1.24 Synthesis of P21 using regular Ozawa-derived conditions, which led to insoluble
cross-linked material reported by Leclerc et al.
165
and Ozawa et al. Utilizing a “mixed ligand”
approach using TMEDA as a co-ligand suppressed defect-formation as reported by Ozawa et
al.
163,164
41
portion of the product contained a large amount of defects, as determined by
1
H NMR
analysis.
163,164
Remarkably, the use of a “mixed ligand” approach by using TMEDA as a
co-ligand in addition to L1 decreased the amount of homocoupling defects (from 4.9% to
1.0%) accompanied by the disappearance of the insoluble materials (Mn = 20.0 kg/mol,
yield = 88%) (Scheme 1.24). Importantly, it was observed that a bidentate nitrogen-
containing ligand was required for such an effect in the “mixed ligand” approach, as
monodentate ligands such as Et3N proved ineffective as a co-ligand. Ozawa et al. further
extended this “mixed-ligand” approach to the prevention of branching linkages and
homocoupling defects by using TMEDA as a co-ligand in the synthesis of various DA
copolymers.
166–168
Ozawa et al. proposed some mechanistic insights for the minimization of homocoupling
and branching defects by incorporating the “mixed ligand” approach with a TMEDA co-
Figure 1.6 Mechanism for direct arylation via the Ozawa-derived condition and origin of
homocoupling defects from a trans-configuration of the transition state. Utilizing TMEDA is
proposed to inhibit the trans route and reduce the defect-formation.
164,167
42
ligand.
164,167
Similar to complex 1 proposed in Figure 1.2, the catalytic cycle for direct
arylation proceeds via a [(Ln)Pd(Ar)(O2CR)] (Ln = PAr’3 for Ozawa-derived conditions)
intermediate after an oxidative addition followed by carboxylate anionic exchange (Figure
1.6). Subsequently, the C-H activation of the Ar’-H unit following the CMD transition state
can happen in both the cis configuration and the trans configuration. According to this
mechanistic proposal, while the cis configuration transition state leads to the desired cross-
coupling product, the trans CMD configuration undergoes an intramolecular protonation
to form the Ar-H product, which exemplifies the debromination of the Ar-Br monomer.
This undesirable debromination event can be followed by a C-H activation with another
monomer (Ar’-H) to form a homocoupling defect (Ar’-Ar’), which would be embedded in
the polymer structure. On the other hand, the introduction of the co-ligand TMEDA, which
exhibits stronger coordination ability with Pd (stronger basicity) compared to the
carboxylate ligand, ultimately inhibits the trans route and minimizes the homocoupling
defects. Through
1
H NMR analysis, Ozawa et al. further observe that the formation of
homocoupling defects is followed by the formation of branching defects. Therefore, the
authors conclude that the prevention of homocoupling defects can effectively lead to the
suppression of branching defects.
Minimization of β-Defects Using Bulky Ligands
As mentioned earlier, the utilization of bulky carboxylate ligands (additives) such
as NDA is an highly effective approach to inhibit the formation of β-defects for Fagnou-
derived DArP protocols.
118,131
It would be expected that such approach can be adopted by
Ozawa-derived DArP conditions, since the catalytic direct arylation cycles for both classes
43
of conditions proceed via a critical CMD transition state (Figure 1.2),
63
only differentiated
by a different ligation of the Pd catalyst. Leclerc et al.,
169
Marks et al.,
61
and Thompson et
al.
136,170
successfully demonstrated that the use of bulky carboxylic acid additives can be
expanded to Ozawa-derived conditions thereby suppressing the C-H activation of
undesired positions and preventing the formation of β-defects. Besides the carboxylate
ligands, another important ligand coordinated to the Pd center in the Ozawa-derived
conditions is the phosphine ligand. Therefore, it can be expected that the increased
bulkiness of the phosphine ligands might positively influence the selectivity of the C-H
activation. Leclerc et al. expanded the L1-based Ozawa-derived DArP condition by
increasing the alkoxy-substituent on the phosphine ligands (Scheme 1.25).
162
From the
methyl substituent of L1, the authors explored isopropyl, 2-ethylhexyl, cyclopentyl,
cycloheptyl, and methylcyclohexyl substituents. Polymerization results showed that the
increased bulkiness of the phosphine ligands suppress the formation of homocoupling
Scheme 1.25 Investigating the effect of phosphine ligand bulkiness on the minimization of defect-
formation of DArP by Leclerc et al.
162
44
defects and β-branching defects. Figure 1.7 shows an example of a plausible transition
state involved in the direct arylation of a DPP monomer with the C-H bond for desired
activation and an undesirable C-Hβ bond for potential β-linkages. The increased steric bulk
of the phosphine ligand might invoke significant steric hinderance from the alkyl side
chains of the DPP unit, thus inhibiting the unwanted C-Hβ activation.
1.2.2.5 Directing-Group Effect on the Performance of Pd-Catalysts: A Double-Edged
Sword
Recently, directing group effects on Pd-catalysts in regards to the selectivity and
defect-formation of the C-H activation has been realized by Thompson et al.
171
In an effort
to prepare P22, an ester-functionalized polythiophene derivative that demonstrates good
performance in organic solar cells with fullerene or non-fullerene acceptors,
172
the authors
found an unexpected high reactivity of the L1-based Ozawa-derived DArP catalytic system
in sustainable solvent, CPME. In particular, Thompson et al. reported that the ester moieties
adjacent to the C-H functionalization sites can act as directing groups, which significantly
enhanced the reactivity of the Pd catalyst towards C-H activation. However, the ester
Figure 1.7 Plausible mechanism of suppressing β-defect-formation
by using a bulky phosphine ligand involved in the direct arylation of
a DPP monomer.
45
directing groups not only enhance the reactivity towards the desired C-H activation, but
also affect the C-Hβ bonds on the halogenated monomer, which causes a significant
reduction of the direct arylation selectivity. By using the a
PdCl2(PPh3)2/L1/NDA/Cs2CO3/CPME catalytic system, the synthesis of P22 resulted in a
complete gelation of the reaction mixture within only 2 hours of reaction time (Scheme
1.26a). Only insoluble materials were recovered for this polymerization, which indicated
that even though an enhanced Pd reactivity was achieved from the ester directing group,
significant degree of cross-linking or β-branching defects were formed.
These observations from Thompson et al. show that, despite the presence of a bulky
carboxylic acid (NDA) in the Pd catalytic system, directing groups such as an ester moiety
are capable of facilitating C-H activation of distal protons (C-Hβ) on adjacent aryl groups
forming undesired β-defects. Esters have been demonstrated to offer an effective
Scheme 1.26 (a) Synthesis of P22 via DArP resulted in a significant amount of β-defects in
the polymer structure due to the incorporation of an ester directing group and (b) defect-free
synthesis of P23 with an electron-rich donor with less-reactive C-H β bonds, reported by
Thompson et al.
171
46
coordination for the Pd center, displacing the carboxylate ligand and allowing the C-H
activation on the distal protons via a seven-membered cyclopalladation.
173
Inspired by this
proposal, Thompson et al. suggested a similar seven-membered palladacycle CMD
intermediate coordinating the ester moiety and the adjacent thiophene aryl group to the Pd
center (Figure 1.8). Therefore, being displaced by the ester directing group, the presence
of a bulky carboxylic acid does not prevent the formation of β-defects. To verify this
conjecture, the authors replaced the use of the 2,2’-bithiophene monomer to a more
electron-rich thieno[3,2-b]thiophene (TT) aryl unit with a β-proton of lower reactivity,
which provided defect-free synthesis of P23 with Mn of 26.4 kg/mol and a yield of 90%
without the formation of insoluble materials (Scheme 1.26b). This shows that in the
presence of directing groups, undesired C-H activation can be suppressed by careful
selections of the halogenated monomers with less-reactive C-Hβ bonds, which inhibits the
formation of the palladacycle CMD intermediate coordinated to the directing group. To
further confirm the assumption, Thompson et al. utilized a more electron-deficient
Figure 1.8 Proposed Mechanistic insights on the β-defect formation assisted by the ester
directing group.
47
halogenated monomer, 2,2’-bithiazole (2-BTz), which resulted in an acceleration of the
branching defects (Figure 1.8).
In 2019, Thompson et al. reported the first synthesis of an amide-functionalized
polythiophene derivative using sustainable solvent, CPME. P24, a tertiary amide
functionalized polymer, was prepared via DArP with good Mn of 15.4 kg/mol and yield of
77% (Scheme 1.27, top).
174
Importantly, this polymer was found to exhibit good thin-film
processability in green alcohol solvents such as ethanol and 1-butanol for UV-vis
spectroscopy and space-charge-limited current (SCLC) hole mobility measurements. The
authors also successfully synthesized the secondary amide, P25, which demonstrates the
excellent functional group tolerance of the DArP catalytic system (Scheme 1.27, bottom).
Specifically, the replacement of Cs2CO3 with a weaker base K2CO3 was found to be critical
to inhibit the potential Pd-catalyzed aryl-amination reactions of the N-H bond.
Interestingly, Thompson et al. observed a highly regioregular structure (>99%) of P24
Scheme 1.27 Synthesis of amide-functionalized polythiophenes
P24 and P25 reported by Thompson et al.
174
48
revealed by
1
H NMR analysis, which suggested an improved site selectivity for C-H
activation presumably due to the adjacent carbonyl functioning as a directing group.
This phenomenon implies that, carbonyl directing groups such as an amide or an
ester moiety can potentially perform as a valuable handle for enhancing the site selectivity
for DArP. Although the use of ester directing groups has been demonstrated previously by
Thompson et al. to trigger undesired activation of C-Hβ bonds,
171
the introduction of these
directing groups in monomer design for DArP is a double-edged sword. If exploited
judiciously, these directing groups adjacent to the C-H bond designated for activation can
significantly enhance the Pd reactivity while improving the C-H selectivity by
circumventing the homocoupling defect-formation, as evidenced in small-molecule C-H
activation studies
175–178
and observed by Thompson et al. with the amide-functionalized
polythiophenes (P24)
174
. To elucidate how this functionality can affect the inhibition of
defect-formation via DArP, Thompson et al. studied the synthesis of an ester-
Scheme 1.28 Synthesis of P26 using two different monomers: 5-bromo-3-
hexylester thiophene (with a directing group adjacent to the C-H activation site)
and 2-bromo-3-hexylester thiophene (without a directing group adjacent to the
C-H activation site, reported by Thompson et al.
179
49
functionalized thiophene-homopolymer P26, using two different monomers: 2-bromo-3-
hexylester thiophene (without a directing group near the site of C-H activation), and 5-
bromo-3-hexylester thiophene (with a directing group adjacent to the C-H activation
site).
179
By comparison, the authors reported that the inclusion of a directing group adjacent
to the site of C-H activation improved the Mn (12.9 kg/mol vs. 11.2 kg/mol) and yield (94%
vs. 85%), demonstrating an improved reactivity of the Pd catalyst likely facilitated by the
ester moiety (Scheme 1.28). More importantly, Thompson et al. found a significant
improvement in rr of P26 (from 94% to >99%), which matches the observation with the
synthesis of the amide-functionalized P24, providing a near absolute exclusion of homo-
coupling defects. The authors concluded that rational monomer designs such as the
inclusion of directing groups can limit defect-formation for DArP and such a strategy can
be extended towards the synthesis of other conjugated polymers with directing groups
(Figure 1.9).
175,177,178,180,181
Figure 1.9 Depiction of the C-H abstraction transition states of (a) 5-bromo-3-hexylester
thiophene monomer with higher C-H selectivity through effective coordination of directing
group to Pd and (b) 2-bromo-3-hexylester thiophene monomer with lower C-H selectivity due
to limited interaction of directing group to Pd, as proposed by Thompson et al.
179
50
1.3 Efficient Catalysts for Oxidative Direct Arylation Polymerization (Oxi-DArP)
1.3.1 Pd-Catalysts for Oxi-DArP
1.3.1.1 Ester-Directing-Group Assisted Pd-Catalyzed Oxi-DArP
The approach of implementing directing groups has been effective to improve the
reactivity and selectivity of Pd catalysts via the C-H/C-X cross-coupling pathway.
175–178
In
recent years, the directing group effect has been utilized for the synthesis of conjugated
polymers via transition-metal-catalyzed Oxi-DArP methodologies (sometimes referred to
as dehydrogenative polycondensation), which proceeds through an oxidative C-H/C-H
cross-coupling pathway. These protocols are considered more simplified and
straightforward compared to conventional DArP methods with an improved atom-
economy.
182–184
However, the reduction of synthetic steps without the necessity of the
halogenation of monomers comes with a cost: the use of a stoichiometric amount of
terminal oxidant is required for these reactions. Different from the catalytic cycle of a
conventional Pd-catalyzed direct arylation, due to the absence of an oxidative addition step
of a Pd(0) species to Ar-X to form a Pd(II) intermediate, a terminal oxidant is needed to
Scheme 1.27 Synthesis of P26 via Oxidative Direct Arylation
Polymerization assisted by an ester directing group, reported by
Thomson et al.
185
51
convert the Pd(0) species (generated from the reductive elimination step) to Pd(II), as
depicted in Figure 1.10.
In 2016, Thompson et al. report the first synthesis of a regioregular ester-
functionalized polythiophene (P26) via Oxi-DArP, eliminating any functionalization of the
monomer required for traditional direct arylation methods.
185
The authors were able to
optimize the reaction conditions using the unsymmetrical monomer 3-hexylester thiophene
to afford P26 with Mn of 14.6 kg/mol and yield of 68%, and a rr of 85% (Scheme 1.29).
Although being used primarily as an oxidant, Ag2CO3 was demonstrated to be vital for the
Oxi-DArP catalytic system, since the use of other oxidants such as Cu(OAc)2 suppressed
the polymerization results (6.5 kg/mol, 33% yield) and rr (75%). Studies by Sanford et
al.
186
and Luscombe et al.
187
reveal a more profound role of Ag rather than simply serving
as an oxidant for the Pd-catalyst. A dual Ag-Pd catalytic system might be participating in
the proton abstraction step and facilitating the C-H activation through a bimetallic pathway,
Figure 1.10 Plausible mechanism for the synthesis of P26 via Oxidative Direct Arylation
Polymerization, including a Ag-mediated transition state and a directing-group-assisted proton
abstraction step.
52
as depicted in the first transition state of the plausible mechanism in Figure 1.10. On the
other hand, the ester moiety functions as a directing group to enhance the selectivity of the
C-H activation through the coordination with the Pd catalyst, thus improving the rr of the
synthesized P26. The directing group effect could be evidenced by the unsuccessful
polymerization of P3HT via Oxi-DArP using the same optimized condition reported by
Thompson et al. Subsequently, Chen et al. reported the synthesis of a polythiophene
derivative via Oxi-DArP with a similar reaction condition using a sulfone-directed
thiophene monomer (Mn = 9.6 kg/mol and yield = 98% with a reported rr of 99%).
188
1.3.1.2 Thiazole (Tz)-C-5 Directed Pd-Catalyzed Oxi-DArP
In a following report, Thompson et al. prepared two families of random
copolymers, which featured symmetrical comonomers with directing groups in the
polymer backbone of P26.
189
For P27, 5% of the TPD monomer with carbonyl directing
group was incorporated in the polymer structure, and it was synthesized via Oxi-DArP with
Mn = 13.9 kg/mol and yield = 54%. It is worth-noting that TPD homopolymer has
previously been prepared by Oxi-DArP using Pd(OAc)2 as the catalyst and Cu(OAc)2 as
Figure 1.11 Summary of regioselective C-H activation by different metal catalysts
on the thiazole (Tz) unit. While Pd is more selective for the C-H activation of the C-
5 position of Tz, Cu catalysts are C-2 directed by the coordination of the N-3 site.
53
the oxidant.
190
Another aryl unit the authors incorporated to prepare P28 was the 4,4’-
dimethyl-2,2’-bithiazole monomer. Thiazole (Tz) and bithiazole (BTz) are important
electron-deficient building blocks for high-performing polymers in organic electronics that
display attractive charge-transport properties and enhanced stability.
191–194
Synthetically,
Tz can function as directing groups for regioselective C-H activation using different
catalysts. As shown in Figure 1.11, small-molecule C-H activation of the Tz unit reveals
an interesting reactivity pattern: Pd-catalysts are highly regioselective towards the C-H
activation of the C-5 position due to a lower activation energy barrier compared to the C-
2 position (23.7 kcal/mol vs. 26.3 kcal/mol).
195–197
On the other hand, the C-2 position of
the Tz is more selective for Cu catalysts through the strong Cu-coordination at the N-3 site,
which effectively increases the acidity of the C-H bond at the C-2 position.
28,30,198
Therefore, in order to conform with the Pd-catalytic system used in the Oxi-DArP of P25,
the 4,4’-dimethyl-2,2’-bithiazole monomer was used with C-5 sites available for the C-H
Scheme 1.30 Examples of thiazole (Tz)-C-5 assisted Pd-catalyzed Oxidative
Direct Arylation Polymerization, including the (a) synthesis of a random
copolymer P28 incorporated with 5% of bithiazole unit reported by Thompson et
al.,
189
and (b) synthesis of Tz-functionalized conjugated polymer reported by You
et al.
199
54
activation. P28 with 5% of the BTz unit was synthesized with Mn of 11.7 kg/mol and a
yield of 68% (Scheme 1.30a).
189
Using a similar approach, You et al. reported the synthesis
of a series of conjugated polymers via Tz-C-5 directed Pd-catalyzed Oxi-DArP method.
199
By using O2 instead of Ag salt as the terminal oxidant, this methodology is considered more
sustainable while its high Pd-catalytic efficiency was allowed by flanking each monomer
with two Tz units. These Tz-functionalized monomers have two C-H bonds available at
the C-5 position of Tz for site-selective activation, as shown in Scheme 1.30b with the
synthesis of P29 as an example.
1.3.2 Thiazole (Tz)-C-2 Directed Cu-Catalyzed Oxi-DArP
The aforementioned Tz-C-2 directing pattern for Cu-catalysts have also been
exploited for Oxi-DArP methodologies. Pioneering work of Miura et al.
200
and You et al.
201
reported small-molecule oxidative dehydrogenative cross-coupling reactions of azole
compounds (C-2 directed) using Cu(OAc)2 as the catalyst, and in some cases, Cu(OAc)2
serves as an oxidant as well. These small-molecule studies demonstrated that the use of C-
2 directed azole compounds is critical for Cu-catalyzed Oxi-DArP protocols, confirming
the strong and unique coordinative ability of the nitrogen atom (N-3 site) to the Cu metal
center.
198
Following these reports, You et al. developed a Pd/Cu(I) co-catalytic system with
stoichiometric amount of Cu(OAc)2 as the oxidant for the synthesis of BTz-based
copolymers via the Tz-C-2 directed Oxi-DArP method.
202
As an example, P30 was
55
prepared with Mn of 33.9 kg/mol and yield of 96% using a Tz-functionalized A-B-A type
monomer (Scheme 31a). Subsequently, Kanbara et al. utilized a similar monomer design
for the development of an aerobic Cu-catalyzed Oxi-DArP methodology (for example, the
synthesis of P31, Scheme 31b).
203
In this case, oxygen in air is used as an oxidant and
therefore the amount of metal catalyst/oxidant is significantly reduced (only 10-20 mol%
of Cu(OAc)2 is required), aligning with the principles of sustainable chemistry.
Although recent advances of this protocol have demonstrated examples of Oxi-DArP
without the assistance of any directing group, which significantly broaden the scope of this
methodology,
204–206
the use of stoichiometric metal oxidants in most cases remains a major
concern. Conversely, the inclusion of directing groups in the design of monomers has
Scheme 1.31 Examples of thiazole (Tz)-C-2 assisted Cu-catalyzed Oxidative
Direct Arylation Polymerization, including the (a) synthesis of a Tz-
functionalized polymer P30 using a Pd/Cu co-catalytic system by You et al.,
202
and (b) synthesis of Tz-functionalized polymer P31 using Cu(OAc) 2 as a sole
catalyst with oxygen in air as oxidant, reported by Kanbara et al.
203
56
significantly promoted the efficiency of the catalysts towards highly-regioselective
polymerizations and enhanced the sustainability of the method by allowing the use of lower
metal loadings and milder conditions.
1.4 Sustainable Cu-Catalysts for Direct Arylation Polymerization
1.4.1 Small-molecule Cu-Catalyzed Direct Arylation
The development of efficient Pd catalysts has allowed DArP to evolve into a
powerful synthetic technique to prepare high quality conjugated polymers on industrial
scales.
207
Pd, however, is one of the most rare metal species and occurs in the earth’s crust
at only 0.015 part per million (ppm) in concentration.
26,208,209
By comparison, the
concentration of first-row transition metal such as Cu (60 ppm), Co (30 ppm), and Ni (16
ppm) are orders of magnitude greater than that of Pd.
26,208
Therefore, exploring the
potential of utilizing more sustainable first-row transition metals as catalysts for DArP is
highly attractive and plausible, as encouraged by small-molecule studies of direct C-H
activation catalyzed by a variety of alternative metals to Pd.
22-59
In 2013, Kanbara et al.
reported the first employment of Ru as an alternative metal catalyst to Pd for DArP.
210
By
using a 2-pyrimidinyl substituted pyrrole as the monomer, the directing-group assisted Ru-
catalyzed DArP protocol offers a first example of Pd-free synthesis of conjugated polymer
via DArP. However, Ru is present in the earth’s crust at an even lower concentration
(0.0004 ppm) compared to Pd and therefore not considered a sustainable metal.
208,209
As a first step to replace Pd in conventional DArP protocols, the potential of Cu
was recognized as a sustainable metal catalyst for DArP due to its high abundance, low
cost, and extensive small-molecule direct arylation studies pioneered by Miura et al.
27
and
57
Daugulis et al.
28–30
Although Cu(II) catalysts such as Cu(OAc)2 has been used as the sole
catalyst for Oxi-DArP (as mentioned in the above section),
203
these methodologies are
limited to homo-polymerizations while stoichiometric oxidants are required. Therefore, it
became of great interest to develop Cu-catalyzed DArP with the conventional C-X/C-H
coupling methods to allow the preparation of a broad scope of DA copolymers. As shown
in Scheme 1.32, representative Cu-catalyzed direct arylation reported by Daugulis et al.
29,30
and You et al.
34
employ aryl iodides or aryl bromides with either fluorinated benzenes
(without directing groups) or azole compounds (C-2 directed) as the coupling partners
(Scheme 1.32a-c). In these conditions developed by Daugulis et al. and later on by You et
al., Cu(I) catalysts such as CuI are usually employed combined with the use of a bidentate
amine ligand such as 1,10-phenanthroline (phen), in vast contrast to the conditions utilized
Scheme 1.32 (a-c) Examples of small-molecule direct arylation catalyzed by Cu(I)
catalyst, reported by Daugulis and You et al.
29,30,34
(d) Proposed active Cu-catalytic
species (complex 7) for Cu-catalyzed direct arylation (left) and comparison to active
Pd-catalytic species (complex 1) for Pd-catalyzed direct arylation from Figure 2.
58
for Cu-catalyzed Oxi-DArP where Cu(II) species is usually used (without the use of a
ligand). Similar to Fagnou-derived Pd-catalyzed direct arylation, amide-coordinating
solvents such as DMF are crucial for Cu-catalyzed direct arylation to proceed, indicating a
more profound role of the amide-solvent in the Cu-based catalytic system. A
[(DMF)Cu(Ar)(phen)] complex (complex 7 in Scheme 1.32d) is likely the active species
in the catalytic cycle for the C-H activation, drawing a parallel to the aforementioned
[(DMA)Pd(Ar)(O2CR)] catalytic species (complex 1, Ln = DMA) proposed by Hartwig et
al. for the Fagnou-derived Pd-catalyzed counterpart (Scheme 1.32d).
111,116
1.4.2. Development of Cu-Catalyzed Direct Arylation Polymerization
1.4.2.1. Cu-Catalyzed DArP Using Thieno-[3,4-c]-pyrrole-4,6-dione (TPD) Substrate
Through extensive optimization of reaction conditions and careful selection of substrates,
in 2018, Thompson et al. developed the initial report of Cu-catalyzed DArP for the
preparation of various TPD-based copolymers with Mn up to 10.1 kg/mol and yields up to
55% (P32) (Scheme 1.33a) with an absence of structural defects.
211
However, the
Scheme 1.33 First examples of Cu-catalyzed Direct Arylation Polymerization developed by
Thompson et al., including (a) using a TPD monomer using 50 mol% catalyst loading
211
and
(b) using a polyfluorinated monomer with only 5 mol% Cu-catalyst loading.
212
59
conditions developed in this initial study required the use of a sub-stoichiometric amount
of the Cu-catalyst (50 mol%), which could be attributed to the low reactivity of the C-H
bond in TPD towards Cu-catalyzed DArP. These results demonstrated that, despite
employing a monomer which contains a directing group to facilitate the Cu-catalyzed C-H
activation, carbonyl as a directing group does not provide strong enough coordination to
the Cu-catalyst, in contrast to the highly effective ester and amide directing groups to the
Pd-catalyst (mentioned above).
171,174,179
In order to establish more efficient Cu-catalyzed
protocols using lower catalyst loadings, monomers with higher C-H acidity or stronger
directing group effect should be considered, according to small-molecule studies by
Daugulis et al.
28–30
1.4.2.2 Cu-Catalyzed DArP Using Polyfluorinated Arene and Mechanistic
Considerations
In the following report by Thompson et al., octofluorobiphenyl monomer was chosen to be
the coupling partner of an iodinated fluorene monomer, affording P8 with Mn of 16.4
kg/mol in 54% yield (Scheme 1.33b) with a minimization or exclusion of defects.
212
Remarkably, the use of a polyfluorinated arene as the C-H substrate allowed a significant
decrease of the Cu-catalyst loading from 50 mol% to only 5 mol%, which is the same
catalyst-loading reported by Kanbara et al. for the preparation of P8 when a Pd-catalyst
was used (5 mol% of Pd(OAc)2, see Scheme 1.5c).
108
Consistent with the findings from
small-molecule Cu-catalyzed direct arylation, the amide-coordinating solvent DMA is vital
for Cu-DArP to occur.
211,212
This further verifies the assumption that DMF or DMA serves
a more intimate role in the Cu-catalytic system rather than simply a reaction solvent, similar
to the Fagnou-derived Pd-catalytic system.
111,116
60
In Pd-catalyzed cross-coupling reactions to form biaryl products, it is generally accepted
that these reactions proceed first with an oxidative addition of aryl halides, followed by the
critical CMD step and reductive elimination step to afford the product.
110–112
For Cu-
catalyzed direct arylation, however, DFT studies by Lin et al. revealed a potentially
different mechanism compared to Pd.
33
The overall energy barrier calculated for the
mechanism involving oxidative addition as the first step (37.3 kcal/mol) is substantially
higher than that for the mechanism in which CMD is the first step (27.9 kcal/mol).
Experimentally, Daugulis et al. successfully isolated complex 8 (Scheme 1.34), which is a
presumed Cu-catalyzed direct arylation intermediate featuring a pentafluorophenylcopper-
phenanthroline structure.
29
Additionally, complex 8 was able to react with aryl iodides to
afford biaryl coupling products in high yield.
213,214
Although complex 8 has been isolated
by Daugulis et al., it is reasonable to propose a DMA-coordinated Cu complex (complex 7
in Scheme 1.32) as the active species for Cu-catalyzed DArP, since both small-molecule
Scheme 1.34 Isolation of a fluoroarylcopper intermediate
(complex 8) by Daugulis et al.,
29
which can be subsequently
arylated. This provides important evidence for proposing
plausible catalytic cycle for Cu-catalyzed Direct Arylation
Polymerization.
61
and polymerization studies find that the use of amide-solvents is critical for these reactions
to occur.
34,211
Combining these observations, a plausible mechanism for the synthesis of
P8 via Cu-catalyzed DArP might proceed through first a base-promoted proton abstraction
step and metalation step to form a fluoroarylcopper species followed by its reaction with
the aryl halide to afford the coupling product (Figure 1.12). The Cu-catalyst is proposed
to undergo a Cu
I
/Cu
III
oxidation states, as suggested by Daugulis et al. and mechanistic
studies by Lin et al.
29,33
1.4.2.3 Cu-Catalyzed DArP Using Aryl-bromides
Figure 1.12 Plausible mechanism for Cu-catalyzed Direct Arylation Polymerization using a
polyfluorinated arene as the C-H substrate, which undergoes a base-promoted proton
abstraction and metalation step suggested by Daugulis et al.
29
and Lin et al.
33
62
A major limitation of these initial Cu-catalyzed DArP reports by Thompson et al.
is the necessity to employ aryl-iodides, which lack practicality for large-scale polymer
synthesis and exhibit low stability.
211,212,215,216
You et al. studied the influence of solvents
on the small-molecule Cu-catalyzed direct arylation of caffeine using aryl-bromides as the
coupling partners, and discovered that the use of a DMF/m-xylene (1:1) co-solvent system
provided the optimal results.
34
Indeed, a similar pattern of solvent selection can be found
in the studies by Daugulis et al. where most reactions involving aryl-iodides employed
DMF as the solvent, while most reactions involving aryl-bromides utilized a DMF/m-
xylene co-solvent system.
29,30
When this co-solvent approach was introduced to Cu-
catalyzed DArP by Thompson et al., P8 was successfully synthesized with Mn = 17.3
kg/mol and yield = 54% by using brominated fluorene as one of the monomer (Scheme
1.35a) with no detectable structural defects.
217
DFT studies by Lin et al. might provide
some mechanistic insight into explaining the dependence of solvent in these reactions.
33
Scheme 1.35 (a) Synthesis of P8 via Cu-catalyzed Direct Arylation Polymerization
using an aryl-bromide as the monomer with the use of a co-solvent system, as
reported by Thompson et al.
217
(b) Synthesis of P33 via Tz-C-2 directed Cu-catalyzed
Direct Arylation Polymerization using an efficient precatalyst approach reported by
Thompson et al.
219
63
Two distinct pathways for Cu-catalyzed direct arylation were proposed, one of which
undergoes a neutral pathway and prefers a non-polar solvent. Therefore, it is likely that Cu-
catalyzed direct arylation with aryl-bromides proceeds via a neutral pathway, which is
promoted by the introduction of a non-polar solvent, m-xylene, as a co-solvent.
1.4.2.4 Tz-C-2 Directed Cu-Catalyzed DArP Using An Efficient Precatalyst Approach
While the co-solvent approach allowed conjugated polymers to be prepared via Cu-
catalyzed DArP using aryl-bromides, the Cu-catalyst loading was raised up from 5 mol%
to 15 mol% to accommodate the lower reactivity of the monomers. To improve the
efficiency of the Cu-catalytic system when aryl-bromides are used, Thompson et al. turned
their attention to C-H monomers with a strong directing group effect. Specifically, due to
the strong C-2 directing effect of Tz units (as discussed previously in Cu-catalyzed Oxi-
DArP), developing new Cu-catalyzed DArP protocols using a 5,5’-bithiazole (5-BTz)
monomer would be particularly intriguing, since the co-polymerizations of this monomer
in DArP using Pd-catalysts were unsuccessful, as reported by Sommer et al.
218
However,
the authors found that directly applying the previous Cu-catalyzed DArP condition using a
CuI/phen catalytic system to the 5-BTz monomer only provided oligomeric product for
P33 (3.8 kg/mol).
219
Experimentally, Thompson et al. observed a formation of green
inorganic solids after the polymerization, indicating the oxidation of Cu(I) catalyst to
Cu(II). This is supported by
1
H NMR analysis of the polymer, revealing a significant
amount of acceptor-acceptor homocoupling defect, which is catalyzed by Cu(II) salts via
the Tz-C-2 directed oxidative C-H/C-H coupling (discussed in section above). This result
demonstrated the low stability of Cu(I) salts under the harsh reaction conditions required
for Cu-catalyzed DArP (140 °C), which is a major drawback of the Cu-catalyzed
64
methodology to potentially replace Pd.
220
By replacing the CuI salt with a bench-stable,
chemically well-define Cu(I) precatalyst, Cu(phen)(PPh3)Br,
221,222
the efficiency of the Cu-
catalyzed synthesis of P33 via DArP was drastically improved (16.5 kg/mol with 79%
yield) (Scheme 1.35b). Importantly, the
1
H NMR spectrum of the synthesized polymer
showed a defect-free structure for P33, indicating the high thermal stability of the
Cu(phen)(PPh3)Br precatalyst. This highly-efficient Cu-precatalyst approach further
allowed the authors to decrease the loading of the Cu-catalyst to only 5 mol% (Mn = 11.8
kg/mol, yield = 64% for P33), which is the lowest Cu-catalyst loading for DArP when aryl-
bromides are used.
Although a more soluble Cu-precatalyst was employed, Thompson et al. reported
that the use of the amide-coordinating solvent DMA is still critical for the polymerization.
Figure 1.13 Plausible mechanism for Cu-catalyzed Direct Arylation Polymerization using a Tz-
C-2 directing group strategy, which undergoes a coordination-promoted proton abstraction and
metalation step suggested by and Lin et al.
33
65
Therefore, the [(DMA)Cu(Ar)(phen)] complex is likely still the active species for the
catalytic cycle (Figure 1.13), despite the introduction of a phosphine ligand in the structure
of the precatalyst.
33
The use of a phosphine ligand allowed the formation of the
Cu(phen)(PPh3)Br precatalyst to provide a stable Cu(I) source, yet might be displaced by
DMA with a stronger coordinating ability upon catalytic direct arylation, similar to the
study by Hartwig et al. on the Pd counterpart.
116
Another mechanistic consideration is
suggested by computational studies of Lin et al. that, the proton-abstraction and metalation
step is likely assisted by the coordination of the metal center to the Tz (N-3 site) (Figure
1.11) through a CMD transition state, as opposed to the base-promoted deprotonation-
metalation step in the case of a polyfluorinated arene (Figure 1.12).
33
Additionally, similar
to the Cu-catalyzed DArP using TPD as the C-H monomer with a carbonyl directing group,
the use of a carbonate base (K2CO3) is vital for the Tz-C-2 directed Cu-catalyzed
methodology. In contrast to the case of a polyfluorinated arene monomer where the use of
the K3PO4 base afforded the optimal result,
212,217
changing the base from K2CO3 to K3PO4
completely suppressed the directing group assisted Cu-catalyzed DArP (in both cases with
TPD and 5-BTz monomers).
211,219
This indicates that the carbonate base in its anionic form
might serve as a ligand for the C-H abstraction of the CMD step, since the CMD step of a
Pd-catalyzed direct arylation assisted by an anionic carbonate ligand was realized by
Fagnou et al.
106
Conversely, the deprotonation-metalation step of the highly acidic C-H
bond on the polyfluorinated arene is base-promoted instead of directing-group-assisted as
proposed by Daugulis et al.,
29
thus, K3PO4 with a stronger basicity is preferred.
1.5 Conclusion and Outlook
66
The development of efficient catalysts/catalytic systems used for DArP has been
presented in this chapter, which allows the preparation of a broad range of conjugated
polymers with unique structure-function relationships. Starting from early reports of small-
molecule direct arylation, the past few decades have led to drastic improvements in the
design of catalytic systems and understanding of reaction mechanisms toward effective
synthesis of conjugated polymers. Specifically, for DArP conditions that rely on amide-
coordinating solvents, such as the Fagnou-derived conditions, the catalytically-active Pd
species was identified as having a strong interaction with the solvent. From the aspect of
improving the sustainability of DArP, the high efficiency of these conditions are
advantageous for allowing ultra-low loading of the Pd-catalyst and auxiliary reagents. For
DArP conditions that involve non-coordinating solvents, on the other hand, such as the
Ozawa-derived conditions, Pd catalytic systems are more subject to be fine-tuning for the
optimized preparation of a broad scope of conjugated polymers. These conditions also
allow the improvement of sustainability of DArP by employing solvents sourced from
green, renewable resources, making DArP significantly more sustainable and user-friendly
especially when compared to other polymerization methods, such as Stille or Suzuki.
Exploring new catalytic systems such as designing novel ligands, however, will be critical
for these DArP conditions in non-polar solvents, since the substrate scope of these
protocols remains limited and highly-dependent on specific phosphine ligands. Moreover,
designing catalysts/ligands that exhibit catalyst-transfer (“ring-walking”) behavior for the
synthesis of low-dispersity conjugated polymers with controlled molecular weights via
DArP will be highly attractive.
223
67
A major challenge for developing efficient catalysts for DArP is the fine-tuning of
the catalytic systems for suppressing unselective C-H activation or homocoupling defects
to generate high quality polymers to rival well-explored methods such as Stille and Suzuki.
To this end, both classes of DArP conditions can be optimized through careful selection of
the catalytic systems such as the choice of ligands to provide superior polymer structural
regularity with minimization of defects. Future studies should continue to design catalytic
systems from an organometallic perspective to guide these efficient catalysts to even higher
regioselective C-H activation and prepare polymers for a wider range of applications.
Another promising pathway to promote the regioselectivity of DArP that emerged recently
is the judicious utilization of directing groups in the monomer design. Although a variety
of directing groups have been widely incorporated in small-molecule C-H activation to
improve the reactivity and selectivity,
175–178,224,225
the employment of such a strategy in
DArP remains limited. Moreover, broadening and refining efficient catalytic systems for
DArP to be compatible and tolerable with different functional groups and structural motifs
will continue to be an attractive subject of interest,
226
since such functionalized monomers
are generally incompatible with Stille or Suzuki.
The emergence of Oxi-DArP in recent years has given this methodology much
attention from the fields of conjugated polymers and organic electronics due to its
unprecedented high atom- and step-economy. Importantly, there are aspects of Oxi-DArP
that lack fundamental studies and optimization. For example, unlike conventional DArP
protocols, the role of the solvent remains unrealized for Oxi-DArP. Additionally, exploring
the use of different ligands (for example, phosphine ligands for Pd catalysts and nitrogen-
based ligands for Cu catalysts) might exhibit outstanding potential of Oxi-DArP. In future
68
studies, fine-tuning of monomer design, further optimization of reaction conditions and
exploring more sustainable sources of oxidants are critical for enhancing the efficiency and
sustainability of the catalytic systems employed in this method.
The sustainability aspect of the source of transition metal catalyst should also be
considered. Recently, the use of heterogenous Pd-catalysts (Pearlman’s catalyst,
Pd(OH)2/C) in DArP has emerged, which offers recyclability to the synthetic method as
demonstrated by the preparation of regioregular P3HT.
227
Leclerc et al. also utilized silica-
supported heterogeneous Pd catalysts, SiliaCat DPP-Pd and SiliaCat Pd0 in DArP.
157
These
heterogeneous Pd-catalysts allow easy-separation from the reaction mixture, which offers
great appeal especially in industrial settings with improved sustainability and cost-
effectiveness of the catalysts.
83,228–230
Additionally, replacing Pd with first-row transition
metals, such as Cu, for DArP is enormously attractive for the field by allowing conjugated
polymers to be prepared with catalytic quantities (5 mol% of loading) of Cu. Optimizing
the Cu-catalyzed DArP conditions by further designing more efficient Cu-precatalysts and
utilizing different directing groups might expand the scope of this methodology to
potentially rival or even replace Pd for DArP. Moreover, as shown in Figure 1.1, compared
to other polymerization methods such as Stille or Suzuki, transition metals that are able to
catalyze direct arylation are vast, yet, a very limited number of metals alternatives to Pd
have been utilized for the polymerizations. The pursuit of sustainable alternative metals for
DArP is imperative and still much room exists to explore.
69
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Chapter 2: Green Solvent Processed Amide-Functionalized Conjugated Polymers
Prepared via Direct Arylation Polymerization (DArP)
2.1 Introduction
Conjugated polymers have emerged as promising low-cost, non-toxic materials
compared to their inorganic counterparts for applications such as organic photovoltaics
(OPVs), organic light emitting diodes (OLEDs), organic field effect transistors (OFETs),
chemical sensors, bioelectronics, and biological cell imaging.
1–6
However, solution
processing of conjugated polymers often requires the use of highly toxic, carcinogenic
halogenated solvents such as chloroform, and 1,2-dichlorobenzene (DCB). Aside from
water, as shown in Figure 2.1, the best choice for green solvents for processing conjugated
polymers are simple alcohols, such as ethanol and 1-butanol, or anisole, which can be
sourced from biomass.
7
Typically, it has been thought that ionic or highly polar terminal
groups such as ammonium, sulfonate, phosphonate (class 1), or functional side chains such
as oligo(ethylene glycol) (class 2) need to be incorporated to enhance solubilities of
conjugated polymers in green solvents.
8–11
The introduction of these ionic/ polar terminal
groups on the side chains were found to be detrimental to the performances of devices and
also significantly complicated monomer synthesis.
9,10
107
Figure 2.1 Summary of different approaches towards green solvent processing of conjugated
polymers.
Poly(alkylthiophenes) (P3AT) and related analogs are a class of conjugated
polymers that attract great interest, due to their relative ease of synthesis, desirable
electronic properties, and highly modular functionalization.
12–14
For example,
polythiophenes with a tethered amide functional group have been electrochemically
polymerized for applications as biological DNA binders and sensors.
15,16
In an effort to
simplify the structures of conjugated polymers capable of green solvent processing, we
were emboldened to devise a simple, scalable monomer, and developed the amide
functionalized polythiophenes, poly(N-hexyl-N-methylthiophene-3-carboxamide-2,5-
diyl) (P1) and poly(N-(2-ethylhexyl)thiophene-3-carboxamide-2,5-diyl) (P2), depicted in
Figure 2.1 and Scheme 2.1. It is worth-noting that synthesizing P3AAT via Stille-
polymerization requires extremely challenging (if not impossible) synthesis and
108
purification of the stannyl-monomers. More importantly, for secondary-amide
functionalized P2 with an N-H bond, stannylating the monomer using a strong base such
as LDA or application of polymerization methods such as Grignard metathesis (GRIM)
will react with the secondary amide functional group. Therefore, we concluded that these
polymers can only be prepared via direct arylation polymerization (DArP) to target highly
regio-regular amide-functionalized polythiophenes, which would provide a streamlined
synthetic pathway that does not incorporate any functionalization of monomer and the use
of highly toxic alkylstannanes.
17–25
Herein, we report the first conjugated polymer (P1)
prepared by DArP that demonstrates green-solvents-solubility and processability, and the
first secondary-amide functionalized conjugated polymer synthesized via DArP (P2) that
shows hydrogen bonding, which can be potentially useful for controlling polymer self-
assembly and morphorlogy.
26,27
Significantly, this study can easily be extended via DArP
to the synthesis of more complex copolymer structures such as donor-acceptor (D-A)
alternating copolymers or random copolymers, providing conjugated polymers with
exciting new properties such as green-solvent-processability or hydrogen-bonding.
28,29
This study presents a simple methodology for developing simple, low-cost, easy-to-prepare
conjugated polymers, which can be readily synthesized in only three simple steps (Scheme
2.1) with good Mn, yields, and structural fidelity.
Scheme 2.1 Synthesis of P1 and P2 via DArP.
109
2.2 Experimental
Complete synthetic procedures and characterization for the monomers and
polymers can be found in Appendix A. As shown in Table 2.1, using conditions originally
developed by Ozawa and Leclerc, our first attempt for the synthesis of P1 was performed
by using Pd2dba3 as Pd catalyst source using high-pressure THF (Table 2.1, Entry 1).
30,31
This did not lead to polymer formation, which suggested that Pd2dba3 is not an efficient
Pd-catalyst source for this particular monomer. By changing to PdCl2(PPh3)2, which has
been shown as an efficient catalyst for polythiophene synthesis via DArP, the desired
polymer was obtained in good Mn (10.4 kDa) and satisfactory yield (70%) (Entry 2).
32,33
Our previous study investigated the use of sustainable solvent cyclopentyl methyl ether
(CPME), which shows great compatibility with DArP.
24
However, the replacement of THF
Entry Polymer Solvent
a
Pd source
b
(Mol%)
b
Base
c
Time
(h)
Mn (kDa)
d
, Ð
d
Yield
e
(%)
1 P1 THF Pd2dba3 (4) Cs2CO3 16 NP NP
2 P1 THF PdCl2(PPh3)2 (4) Cs2CO3 16 10.4, 1.7 70
3 P1 CPME PdCl2(PPh3)2 (4) K2CO3 48 8.1, 1.6 76
4 P1 CPME PdCl2(PPh3)2 (4) Cs2CO3 48 8.3, 1.5 85
5 P1 CPME PdCl2(PPh3)2 (6) Cs2CO3 72 15.4, 1.5 77
6 P1 CPME Pd(OAc)2 (3) K2CO3 24 10.8, 2.0 80
7 P1 CPME Pd(OAc)2 (5) K2CO3 24 13.5, 1.5 73
8
P2 CPME PdCl2(PPh3)2 (4) Cs2CO3 48 7.9, 1.5 90
9
P2 CPME Pd(OAc)2 (3) K2CO3 24 11.6, 1.9 53
f
a
All polymerizations were conducted with 0.2 M concentration and reaction temperature of 110
°C.
b
For Pd 2dba 3 and PdCl 2(PPh 3) 2, tris(o-methoxyphenylphosphine) was used with Pd: ligand
ratio of 1:4; For Pd(OAc) 2, P(t-Bu) 2Me-HBF 4 was used with Pd: ligand ratio of 1:2. 0.5
equivalents of neodecanoic acid was used for all polymerizations.
c
1.5 equivalence of base was
employed.
d
NP indicates no polymer formation.
e
Determined after purification via Soxhlet
extraction.
f
insoluble material was obtained from this polymerization after Soxhlet extraction,
which is not included in the calculation for yield.
Table 2.1 DArP conditions and polymerization results.
110
with CPME reduced the molecular weight to 8.1 kDa, albeit with a slight increase in yield
(76%) (Entry 3). Cs2CO3 improved Mn (8.3 kDa) and yield (85%) slightly (Entry 4).
Increasing PdCl2(PPh3)2 catalyst loading to 6 mol%, while prolonging reaction time
provided the best molecular weight as it almost doubled the Mn from Entry 4 to 15.4 kDa
(Entry 5). As an alternative route, Pd(OAc)2 with a phosphine ligand was demonstrated by
Sommer et al. and Wang et al. as an efficient DArP condition, thus, we explored the use of
Pd(OAc)2 with P(tBu)2Me-HBF4 as a ligand (Entry 6).
21,34
We found this reaction condition
in CPME to be highly efficient as the polymerization was completed in only 24 hours (due
to precipitation of polymer and decomposition of palladium catalyst “palladium black”
observed), providing polymer with high Mn (10.8 kDa) and good yield (80%). Increasing
the loading of Pd(OAc)2 to 5 mol%, which is the catalyst loading reported by Wang et al.,
increased the Mn to 13.5 kDa with good yield (73%) (Entry 7).
For the synthesis of P2, application of a condition used for P1 (Entry 3) provided
lower Mn polymer (7.9 kDa) (Entry 8), albeit in excellent yield (90%). We were, however,
concerned about the use of Cs2CO3 as a base, since this has been shown by Buchwald et
al. to facilitate aryl-amination reactions, which may lead to defects embedded within the
polymer for P2.
35
Although it has been shown in a small molecule study that secondary
amides can be preserved in a direct arylation reaction, transcribing to polymerizations still
exhibits great synthetic challenges, such as the use of unbalance stoichiometric aryl bromides
(2:1 to substrate), and unfavorable solvents for conjugated polymer synthesis (2 wt % SPGS-
550-M in water).
36
Unlike small molecule syntheses, even a small degree of side-reaction
can result in major defects embedded in the polymer structure, which cannot be removed
in a purification (such as Soxhlet extraction) and can be easily evidenced from
1
HNMR. In
111
fact,
1
H-NMR studies (see Appendix A and discussion below) of this polymer (Entry 8)
show impurities in the aliphatic region (Figure A.12), which we believe to be resultant of
N-arylation of the secondary amide. Therefore, instead of applying the optimal condition
for P1 (Entry 5) to the synthesis of P2, we concluded that the reaction condition needed to
be altered to improve polymer purity. Employing Pd(OAc)2 with P(t-Bu)2Me-HBF4 as a
ligand and K2CO3 as a base, provided P2 with a satisfactory Mn (11.6 kDa) and yield (53%)
(Entry 9). The decrease in yield (53%) was due to a fraction of insoluble polymer, which
is likely due to lower solubility from intramolecular hydrogen bonding as described by
previous reports.
26,37,38
Also, the
1
H-NMR shows no apparent impurity in the aliphatic
region, and N-H resonance (δ 5.82 ppm) remains after polymerization (Figure A.13). It is
important to note that CHCl3 soluble fraction of P2 (Entry 9) exhibits good solubility in
halogenated organic solvents. The synthesis of P2 marks an unprecedented conjugated
polymer synthesis of a homo-polymer containing secondary amide with N-H bonds
preserved, exhibiting superior functional group tolerance for these DArP conditions.
2.3 Results and Discussion
To confirm the desired polymer structures for P1 and P2,
1
H-NMR analysis was
performed (complete polymer NMR spectra provided in Appendix A). Regarding the
assignments of P1, the spectra shown in Figure 2.2 depicts that both α-protons adjacent
to amide functional group, N-CH2 (A1 and A2) and N-CH3 (B1 and B2) form rotamers,
which are observed for alkyl amides, resulting in two distinct singlets for each of the
aforementioned protons.
39,40
Specifically, the rotamer resonances attributed to the N-CH2
in monomer 3a (δ 3.46 and 3.32 ppm, Figure A.2), are assigned to the resonances at δ 3.53
and 3.16 ppm in the polymer. A similar pattern was observed for the N-CH3 (methyl) with
112
monomer 3a at δ 3.03 ppm (Figure A.2), the resonances in the corresponding polymer P1
are assigned to B1 and B2 at δ = 3.10 ppm and δ = 2.86 ppm (Figure 2.2). Structural fidelity
is critical for polymers composed of unsymmetrical repeat units, since defects can
negatively influence the electronic and physical properties of the polymer.
41,42
Previous
reports on poly(3-hexylthiophene) (P3HT) and poly(3-hexylesterthiophene) (P3HET)
showed that the ratios of head-to-head (HH) and branching (β) defects content can be
readily determined from the aliphatic region in
1
H-NMR.
30,43
It is well-studied that for
P3HT, while the desired head-to-tail (HT) couplings are found δ = 2.82 ppm, HH and β-
branching signals are found in the range of δ = 2.56-2.58 ppm and δ = 2.18-2.38 ppm
respectively.
44
Similarly, HT coupling of P3HET is detected at δ = 4.30 ppm, with HH
coupling shows at δ = 4.13 ppm.
45
Therefore, a similar shift upfield is expected if any HH
coupling (~ 0.2 ppm) or β defects (~ 0.6 – 0.8 ppm) are to occur. As shown in Figure 2.2,
P1 exhibits no quantifiable signal of homocoupling or β defects in the expected regions,
indicating a highly regio-regular polymer structure. With regards to end-groups, we
observe a correlation between the GPC-Mn and the relative integrations for resonances
assigned to a, b with the major resonances (Figure 2.2). Specifically, we observe a decrease
in integration ratio by roughly half the original value when the Mn of P1 nearly doubles
from 8.1 kDa (Table 2.1, Entry 3) to 15.4 kDa (Table 2.1, Entry 5). A similar trend was
113
observed for other entries as well (see Appendix A for annotated spectra), providing
further evidence for a and b as likely end-groups.
46
Figure 2.2
1
H-NMR (500 MHz, CDCl 3, 25 °C) of DArP polymers P1 (Table 2.1, Entry 3, top)
and P1 (Table 2.1, Entry 5, bottom).
To confirm the presence and retention of the N-H bond for P2, FT-IR spectroscopy
was obtained (Figure A.19), which shows a broad N-H stretch between 3300 cm
-1
to 3700
cm
-1
. To confirm the presence of hydrogen bonding, VT-NMR experiments were
performed (Figure A.14-A.17). The
1
H-NMR signal of the N-H peak was observed to shift
upfield with increasing temperature: δ = 5.82 ppm at T = 348 K, δ = 5.80 ppm at T = 373
K, δ = 5.78 ppm at T = 398 K (Figure A.18). The signal intensities of this peak increased
significantly upon the elevation of temperature. This observation, previously observed in
polymers containing amides or other hydrogen-bonding contents, confirms the formation
of hydrogen bonding in P2.
37,38
Interestingly, the major aromatic peak shifts downfield (δ
= 7.50 ppm at T = 348 K; δ = 7.54 ppm at T = 373 K; δ = 7.58 ppm at T = 398 K) as the
114
temperature increases, which is in agreement to literature report.
38
This can be explained
by the reducing of hydrogen-bonding interactions between polymer side chains upon the
elevation of temperature,
37,38
thus removing electron density from the aromatic C-H bond
and results in a deshielding effect. It should be noted that DSC and GIXRD experiments
indicate these polymers (P1 and P2) to be fully amorphous, as no Tm, Tc, or diffraction
pattern was observed (See Figure A.37-A.40).
Figure 2.3 UV-vis absorbance for films of polymers P1 (DCB, ethanol, and butanol) and
P2 (DCB). See Appendix A for complete details.
For both P1 and P2 the electrochemical HOMO, UV-vis absorption, and space-
charge limited current (SCLC) mobilities were measured (see SI for tabulated data and
experimental conditions). Similar to P3HET (-5.98 eV), both P1 (-5.96 eV) and P2 (-5.90
eV) exhibit much deeper HOMO levels compared to P3HT (5.27 eV).
44,45
In addition to
DCB, P1 was found to have excellent solubility in the green solvents ethanol, 1-butanol,
and anisole (Figure S20), allowing for the formation of thin-films via spin-coating (Figure
A.21). To better quantify the solubility of P1 in green solvents, the maximum solubilities
of P1 with the highest Mn (Table 2.1, Entry 5, Mn = 15.4 kDa) in green solvents were
115
measured and summarized in Table A.1. Outstanding maximum solubility of P1 was found
in 1-butanol (18 mg/mL), while ethanol and anisole also gave good solubility results (9
mg/mL for ethanol and 14 mg/mL for anisole). For P1 with lower molecular weight (Table
2.1, Entry 3, Mn = 8.1 kDa), good solubility in ethanol/water mixture (5 mg/mL) was found.
However, we found 1-butanol and ethanol provided uniform films, while anisole and
ethanol/water mixtures did not (Figure A.21). P2 was found to only be soluble in
chlorinated solvents, such as DCB.
UV-Vis absorbance spectroscopy experiments were conducted by spin-casting
solutions of P1 and P2 (Figure 2.3). With regards to optical bandgap, λmax, and absorption
coefficient, films of P1 prepared from solutions of ethanol (2.16 eV, 468 nm, and 37.4 ×
10
-3
cm
-1
) and 1-butanol (2.18 eV, 471 nm, and 32.1 × 10
-3
cm
-1
) gave similar results to
those prepared from DCB (2.18 eV, 468 nm, and 35.7 × 10
-3
cm
-1
). P2 (1.89 eV, 475 nm,
and 35.7 × 10
-3
cm
-1
) provided a narrower bandgap, comparable λmax, but lower absorption
coefficient than P1, respectively. As previously observed in conjugated polymers
containing secondary amides, the decrease in absorption intensity of P2 compared to P1
can be attributed to the disruption of p-stacking and aggregation due to the introduction of
hydrogen-bonding side-chain interactions.
37
UV-Vis absorbance spectroscopy data was
also collected for P1 and P2 in solution (For P1, DCB, ethanol, and 1-butanol were used;
For P2, DCB was used). By comparison, both thin-film and solution measurements gave
similar UV-Vis spectra and comparable results for λmax (See Figure A.22-26, Table A.2
for details).
SCLC hole mobility measurements were conducted for P1 and P2, and are provided
in Table A.3. The SCLC device architecture was chosen for hole mobility measurements
116
because the same technique is reported for measuring hole mobilities of P3HET and
alcohol-processed conjugated polymers have been evaluated via the SCLC device
architecture.
11,45
P1 and P2 were found to have similar hole mobilites for films processed
from DCB (4.74 × 10
-6
and 5.42 × 10
-6
cm
2
V
-1
s
-1
, repsectively). For P1 in 1-butanol, hole
mobilities were found to be comparable (8.48 × 10
-6
cm V
-1
s
-1
), but working devices could
not be obtained using ethanol. Additionally, the μhmax for P1 processed in 1-butanol (2.74
× 10
-5
cm
2
V
-1
s
-1
) is comparable to that of P3HET prepared via Stille and processed using
DCB (2.09 × 10
-5
cm
2
V
-1
s
-1
).
45
2.4 Conclusions
In conclusion, we have developed a sustainable, simple route to novel amide-
functionalized conjugated polymers (P1 and P2) with Mn values up to 15.4 kDa and yields
up to 90%. This is the first report of amide functionalized polythiophenes (P3AAT) being
successfully synthesized via DArP, including a secondary amide, displaying an
unprecedented functional group tolerance for DArP.
1
H-NMR analysis reveals a highly
regio-regular polymer structure of P1, and FT-IR and NMR for P2 indicate the presence of
hydrogen bonding. P1 is shown to be the first conjugated polymer prepared via DArP that
has good solubility and processability in green-solvents, such as ethanol and 1-butanol,
providing similar absorption properties and SCLC mobility results when compared to
DCB. P2 is the first conjugated homo-polymer containing a secondary amide ever
synthesized, and was found to retain good hole mobility with the introduction of N-H
bonds. This study provides a simple pathway for the preparation of conjugated polymers
capable of green-solvent-processing and hydrogen bonding, which will provide significant
117
impact in organic electronic and bioelectronic applications. Future work will focus on
extending P3AAT homo-polymers to D-A alternating copolymers or random copolymers
incorporating a certain percentage of monomers 3a or 3b to obtain desired new properties
while improving charge mobilities and crystallinity of conjugated polymers.
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Chapter 3: Synthesis of Conjugated Polymers using Aryl-Bromides via Cu-
Catalyzed Direct Arylation Polymerization (Cu-DArP)
3.1 Introduction
Conjugated polymers are promising materials that attract interest for wide-range of
applications such as organic photovoltaics (OPVs), organic light emitting diodes (OLEDs),
organic field effect transistors (OFETs), chemical sensors, and bioelectronics.
1–6
Conventional routes towards the synthesis of these materials often include methodologies
such as Stille, or Suzuki polymerizations, which rely on challenging monomer syntheses
and generate toxic, stoichiometric by-products.
7,8
Direct Arylation Polymerization (DArP)
has emerged as a simplified and effective synthetic methodology in recent years by directly
activating unfunctionalized C-H bonds to achieve efficient aryl-aryl couplings, allowing
polymers to be synthesized at low cost with minimized homocoupling or branching
defects.
9–14
Although effort has been made to further improve its sustainability, the vast
majority of DArP methodologies rely on low abundance and expensive metal catalysts,
such as Pd.
15–17
As a more sustainable, low cost metal catalyst, copper has drawn significant interest
and has been studied as an alternative catalyst to Pd for conjugated polymer synthesis by
oxidative homo-coupling methodology.
18,19
However, the scope of these polymerizations
are limited to homo-polymers, and the use of stoichiometric oxidants has prohibited such
conditions from overtaking Pd-catalyzed conjugated polymer synthesis. As an effort to
demonstrate copper as an efficient catalyst for the most prevalent perfectly alternating
125
donor-acceptor type conjugated copolymers, recently, we reported the first copper-
catalyzed DArP (Cu-DArP) for the synthesis of various thieno[3,4-c]pyrole-4,6-dione
(TPD) donor-acceptor copolymers.
20
In a follow-up study, we reported a Cu-DArP
methodology for the synthesis of a fluorinated arylene conjugated copolymer, poly[(9,9-
dioctylfluorene-2,7-diyl)-(2,2′,3,3′,5,5′,6,6′-octafluoro-4,4′-diphenylene)] (PDOF-OD)
with high molecular weights (Mn up to 24.5 kDa) and low loadings of copper catalyst (only
5 mol%) (Scheme 3.1).
21
Scheme 3.1. Summary of Cu-DArP development.
However, the necessity of employing aryl iodides in these studies limited such Cu-
DArP strategies to achieve lower cost and greater sustainability, since aryl iodides were
found to be much more challenging and expensive to synthesize and purify.
22
Specifically,
iodination of aromatic C-H bonds often requires unstable, hazardous, and expensive
iodination reagents such as N-iodosuccinimide or periodic acid under harsh reaction
conditions, and in addition, aryl-iodides generally lack commercial availability compared
with their aryl-bromide counterparts.
20,22
More importantly, aryl-iodides exhibits much
126
higher reactivity and lower stability compared to aryl-bromides. Additionally, C-I bonds
can undergo radical-based photolytic-cleavage leading to decomposition of the monomer
or potential undesired side reactions.
23,24
This poses significant challenges to the
practicality of Cu-DArP methodology such as chemical storage and handling, especially
when larger scale or industrial settings are envisioned. With the aim of extending Cu-DArP
to a significantly more facile, low-cost, and practical synthetic approach, herein, we report
the first application of Cu-DArP to the synthesis of conjugated polymers using aryl-
bromides.
Pioneering studies in Cu-catalyzed small molecule bi-aryl cross-coupling led by
Daugulis et al. and Miura et al. reveal that aryl-bromides often lead to no arylation product
or much lower yields compared to aryl-iodides, especially when less reactive coupling
partners are utilized.
25–28
You et al. and Gao & Zhou et al. reported Cu-catalyzed direct
arylations with low copper catalyst loadings and high yields using aryl-bromides, however,
substrates investigated in these studies are limited to azole compounds such as caffeine and
benzoxazoles, and reactions are often conducted in highly concentrated solutions (such as
1.2 M) and with a stoichiometric excess of aryl-bromide, which are not consistent with
conjugated polymer synthesis.
29,30
However, studies on ligands and solvents in these
reports provided insight and inspiration for us on how to achieve polymeric products using
aryl bromides with copper catalysts. The synthesis of PDOF-OD (P1) was first selected as
Scheme 3.2 Synthesis of PDOF-OD (P1) using 2,7-dibromo-9,9-dioctylfluorene 1 (Conditions
listed in Table 3.1).
127
our model system (Scheme 3.2), since our previous report demonstrated that it could be
readily synthesized with high Mn and yields using catalytic copper without directing-group
assistance an aryl iodide.
21
Monomer 2 (2,2′,3,3′,5,5′,6,6′-octafluorobiphenyl) was selected
over 1,2,4,5-tetrafluorobenzene because of its ease of handling (much higher boiling point
and low volatility), and better comparison can be made to our previous report synthesizing
PDOF-OD using the same monomer.
21
Additionally, Daugulis et al. demonstrated that
fluoroarenes can be arylated with high yields using aryl-bromides and catalytic amount (10
mol%) of CuI, albeit with a stoichiometric excess of fluoroarene.
26
Table 3.1 Cu-DArP conditions for the synthesis of PDOF-OD and polymerization results
Entry Polymer
a
Ligand
b
Base
c
Solvent (Conc.)
d
Time (hr.)
M n (kDa)
e
, Ð
e
Yield
e
(%)
1 P1 1, 10-
Phenanthroline
K 3PO 4 DMA (0.5 M) 72 - 0
2 P1 Dmbpy K 3PO 4 DMA (0.5 M) 72 - 0
3 P1 Neocuproine K 3PO 4 DMA (0.5 M) 72 - 0
4 P1 PPh 3 K 2CO 3 DMA (0.5 M) 72 - 0
5 P1 PPh 3 K 3PO 4 DMA (0.5 M) 72 - 0
6 P1 1, 10-
Phenanthroline
K3PO4 DMA/m-xylene
(1:1) (0.5 M)
16 17.3, 2.4 55
7 P1 1, 10-
Phenanthroline
K 3PO 4 DMA/o-xylene
(1:1) (0.5 M)
72 - 0
8 P4 1, 10-
Phenanthroline
K2CO3 DMA/m-xylene
(1:1) (0.4 M)
72 9.0, 1.7 65
9 P5 1, 10-
Phenanthroline
K 2CO 3 DMA/m-xylene
(1:1) (0.4 M)
72 5.1, 1.4 26
10 P6 1, 10-
Phenanthroline
K 2CO 3 DMA/m-xylene
(1:1) (0.4 M)
72 6.4, 1.8 35
a
All polymerizations were conducted using 15 mol% copper catalyst loading. All polymerizations were
conducted at 140 °C. 99.999%-Puratrem Cu(I) iodide was used as the copper source.
b
Ligand loadings
were 1:1 ratio to Cu(I) iodide except for PPh3 (3:1 ratio to CuI was used). Dmbpy = 4,4’-Dimethyl-2,2’-
bipyridine.
c
All polymerizations were conducted using 4 equivalents of base.
d
N,N-dimethylacetamide =
DMA.
e
Determined for polymer products after purification.
128
3.2 Experimental
As shown in Scheme 3.2 and Table 3.1, our initial efforts focused on extending
our previous work by directly replacing 2,7-diiodo-9,9-dioctylfluorene (which gave Mn =
20.4 kDa, yield = 71% using 15% Cu-catalyst loading) with 2,7-dibromo-9,9-
dioctylfluorene as aryl-donor (Table 3.1, Entry 1). This led to no polymeric precipitation
in MeOH, although formation of oligomeric product might have occurred as suggested by
the color change that occurred during the reaction. This result supports a much lower
reactivity of aryl-bromides compared to aryl-iodides in catalytic direct arylation, which is
consistent to reports by Fagnou et al.
31
Buchwald et al. demonstrated that more electron-
rich phenanthroline derived ligands could stabilize Cu(III) intermediates in the N-arylation
catalytic cycle, thus accelerating the rate limiting aryl halide activation step.
32
However,
dimethyl-substituted phenanthroline derived ligands such as Dmbpy and Neocuproine did
not yield polymer product (Table 3.1, Entry 2, 3). Gao & Zhou et al. reported a highly
efficient Cu-catalyst system, CuI/PPh3 (1:3), forming metal complex Cu(PPh3)3I in situ for
direct arylation of benzoxazoles and benzoimidazoles with high yields (up to 95%) using
a catalyst loading of 5 mol%.
30
However, this catalytic system did not yield polymeric
product for PDOF-OD using either K2CO3 or K3PO4 (Table 3.1, Entry 4, 5), and we
postulate that the Cu(PPh3)3I complex might only be efficient for C-2 arylation of azole
type substrates.
While our previous work utilized DMA for Cu-DArP reactions, Daugulis et al.
reported the best arylation results for fluoroarenes with aryl-bromides when a DMF/m-
xylene (1:1) co-solvent system was used, and lower conversions were obtained in DMF.
26
Similar trends can be observed throughout these studies that reactions with aryl iodides
129
achieve high yields when DMF is used as the only solvent, however, for aryl bromides,
DMF/m-xylene (1:1) appears to be a required reaction parameter.
25,27
You et al. further
confirmed this observation by studying the ratio of DMF/m-xylene co-solvent systems and
finds that a 1:1 ratio provides the best yield for arylations of aryl-bromides with caffeine.
29
Since our first Cu-DArP report found DMA gives higher Mn and yields compared to DMF,
our attention turned to a DMA/m-xylene (1:1) co-solvent system, while keeping CuI/1,10-
Phenanthroline (15 mol%) and K3PO4 (4 equiv.) as the base constant (Table 3.1, Entry 6),
which gave PDOF-OD of good Mn (17.3 kDa) and good yield (55%) in only 16 hours (due
to precipitation of polymerization observed). Interestingly, replacement of m-xylene with
o-xylene completely inhibited the polymerization (Table 3.1, Entry 7), indicating a specific
requirement for m-xylene as a co-solvent partner with polar amide solvents (DMF, DMA)
for Cu-DArP. These observations are consistent with reports by Daugulis and You et
al.
26,27,29
Density functional theory (DFT) studies on small molecule Cu-catalysed direct
arylation reveals a possible neutral catalytic cycle that aryl-bromides undergo, which
favours non-polar solvents and may explains the role of m-xylene.
33
This result (Table 3.1,
Entry 6) marks the first catalytic Cu-DArP synthesized conjugated polymer with good Mn
and yield using aryl-bromides. To gain a further comparison with our previous report, we
increased the Cu-catalyst loading to 50 mol%, which improved Mn to 22.0 kDa with 67%
yield (Table B.1, Entry 1), however, decreasing the loading to 5 mol% did not give
polymer product (Table B.1, Entry 2), in contrast to our previous work with aryl iodides
in DMA.
With an optimized condition in hand, we were interested to explore the scope of
Cu-DArP using other aryl-bromides. We selected thieno[3,4-c]pyrrole-4,6-dione (TPD)
130
co-polymers based on their extensive use in conjugated polymers and the lower acidity of
TPD monomer (compared to 2).
34
In addition, our first report of Cu-DArP focused on TPD
monomers co-polymerized with a variety of aryl-iodides (see Scheme 3.1), and we
proposed that the carbonyl groups on TPD units can act as directing groups to facilitate C-
H activation.
20
Therefore, it is of great interest to extend this work toward the more
sustainable and facile synthetic route via aryl-bromides (Scheme 3.3).
As shown in Table B.2 and Scheme 3.3, we initially focused on extending the
optimized polymerization condition using 50 mol% Cu-catalyst from our first report to this
study with aryl-bromides. Through careful optimization of reaction conditions (See
Appendix B), TPD-copolymer P4 was successfully synthesized with good Mn of 10.4 kDa
and yield 72% (Table B.2, Entry 6), which is the highest Mn reported for TPD co-polymers
synthesized via Cu-DArP. The DMA/m-xylene (1:1) co-solvent system was further
Scheme 3.3 Synthesis of TPD-copolymers using aryl-bromides as donor units (Conditions listed
in Table 3.1 and Table B.1).
131
confirmed to be critical for Cu-DArP of aryl-bromides. Moreover, the introduction of a
non-polar solvent, m-xylene, was found to not only increase the reactivity of aryl-bromides
toward Cu-DArP, but improve the solubility of the resulting copolymers, allowing highly
concentrated (0.4 M) Cu-DArP reactions to occur without premature precipitation.
Surprisingly, decreasing the Cu-catalyst loading from sub-stoichiometric 50 mol% to
catalytic 15 mol% gave comparable Mn (9.0 kDa) and yield (65%) of P4 (Table 3.1, Entry
8). This result indicates the first TPD-copolymer synthesized via catalytic CuI, which is a
remarkable step forward for Cu-DArP especially when compared with our first report, in
which the more reactive aryl iodides required 50 mol% Cu-catalyst loading for
polymerizations to proceed. It is worth-noting that TPD-acceptor monomers are found to
be less reactive towards C-H activation, since even Pd-catalyzed DArP requires di-iodide
functionalized aryl-donors.
35
With optimal conditions in hand, we were emboldened to further broaden the scope
of aryl-bromides in Cu-DArP. Thiophene based aryl bromides 4 and 5 (Scheme 3.3) were
chosen because of the prevalence of such TPD-copolymers (P5, P6) in organic
electronics.
34,36
With only 15 mol% Cu-catalyst loading, P5 was synthesized with Mn = 5.1
kDa and yield = 26% (Table 3.1, Entry 9), which is similar to that obtained from the di-
iodide counterpart with 50 mol% Cu-catalyst loading.
20
5,5'-dibromo-2,2'-bithiophene (5)
was co-polymerized with the TPD monomer to provide P6 with improved Mn (6.4 kDa)
and yield (35%) (Table 3.1, Entry 10). We attributed the lower Mn and yields of P5 and P6
to lower solubility of these polymers in highly concentrated solutions as reported in
previous studies, and it is worth-noting that even with the highly reactive Stille-
132
polycondensation, P6 was synthesized with Mn of 7.08 kDa, which is only slightly higher
than our Cu-DArP protocol.
34,36
3.3 Results and Discussion
Figure 3.5.
1
HNMR analyses of representative P1 (PDOF-OD) (Table 3.1, Entry 6) and P4 (Table
3.1, Entry 8) polymers. Potential resonances for end groups and defects are denoted. d = donor-
donor homocoupling (d7.89), a = acceptor-acceptor homocoupling (d7.88), b = branching defects.
Collected in CDCl 3 at 25 °C and 500 MHz.
133
To validate the proposed structures synthesized via Cu-DArP using aryl-bromides,
1
HNMR spectra were collected. Complete
1
HNMR spectra can be found in Appendix B,
and all analyses performed were referenced to our previous Cu-DArP studies.
20,21
Figure
3.1 shows detailed peak-assignments and end group analyses of two representative
polymers from Table 3.1 (P1, Entry 6 and P4, Entry 8). For P1 (PDOF-OD) synthesized
in this study, the major resonances located at δ7.94 and δ7.59 match identically to our
previous report as well as a literature report.
21,37
End group assignments were performed
based on our previous report, where detailed end group analyses were conducted based on
model compounds.
21
As shown in Figure 3.1, integral ratios of the end group c, c′ and the
polymeric protons C, C′ can be determined as 1:23, which is very similar to repeat units
given by GPC Mn value (1:25 for 17.3 kDa). Based on previous reports, this indicates a
likely absence of β branching defects.
21,37
For TPD copolymers, as shown in Figure 3.1,
the representative polymer P4 provides identical major resonance signals (δ8.22-8.27 and
δ7.86 for HA-HB’’) compared to our first Cu-DArP report.
20
Potential structural defects
such as donor-donor (d) (d7.89), acceptor-acceptor (a)(d7.88) coupling, or branching
defects (b) (d7.89) were not observed.
20
3.4 Conclusion
In conclusion, we report the first Cu-DArP methodology for the synthesis of
conjugated polymers using aryl-bromides. Replacing aryl-iodides is of significant
importance for Cu-DArP because expensive, synthetically-challenging, and unstable aryl
iodides prohibit Cu-DArP from becoming a more general and practical method. To
overcome the lower reactivity of aryl-bromides in Cu-DArP, we discovered from the
synthesis of PDOF-OD (P1) that the use of a DMA/m-xylene (1:1) co-solvent system is
134
critical for polymerizations to occur, providing P1 with Mn = 17.3 kDa and good yield 55%
with a catalytic 15 mol% of CuI. The DMA/m-xylene (1:1) co-solvent system was further
confirmed as an efficient solvent of choice for Cu-DArP by providing P4 with satisfactory
Mn of 9.0 kDa, and 65% yield using catalytic 15 mol% CuI, which is the first TPD-
copolymer synthesized via catalytic copper.
1
HNMR of both P1 and P4 confirm the
proposed polymer structures, showing minimization or absence of homo-coupling or
branching defects. This study has significantly advanced Cu-DArP, demonstrating the
capacity of this synthetic methodology for a broad scope of conjugated polymers with a
variety of aryl-bromides using catalytic copper.
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140
Chapter 4: An Efficient Precatalyst Approach for the Synthesis of Thiazole-
Containing Conjugated Polymers via Cu-Catalyzed Direct Arylation Polymerization
(Cu-DArP)
4.1 Introduction
For decades, conjugated polymers have been explored as an effective class of
materials for a wide-range of applications such as organic photovoltaics (OPVs), organic
light-emitting diodes (OLEDs), organic field effect transistors (OFETs), chemical sensors,
and bioelectronics.
1–5
In recent years, Direct arylation Polymerization (DArP) has been
found to provide a facile and environmentally-benign pathway for conjugated polymer
synthesis by eliminating the need for toxic tin- or boron-functionalized monomers.
6–11
However, current DArP methodologies are reliant on catalysis by Pd, which is 4000 times
less earth-abundant and 16 times more expensive in comparison to Cu.
12
To address this
important issue of sustainability in DArP, we developed the first Cu-catalyzed DArP (Cu-
DArP) conditions that allowed conjugated polymers to be prepared with high molecular
weights (Mn up to 24.5 kDa) and minimized homo-couplings or branching defects.
13–15
Through careful optimization of reaction conditions, we were able to lower the Cu-catalyst
loading and successfully employ aryl-bromides in place of the less stable and more
expensive aryl-iodides, significantly improving and practicality of Cu-DArP towards the
replacement of Pd-catalysts.
15
141
Figure 4.1 Summary of Pd-DArP and Cu-DArP development.
However, unlike Pd-catalyzed protocols, which have been extensively studied to
employ stable, efficient Pd-complexes as precatalysts such as Pd2(dba)3, PdCl2(PPh3)2, and
Pd-(Herann-Beller) (Figure 4.1a),
16
the use of Cu-precatalysts in Cu-catalyzed small
molecule direct arylation has not been reported to the best of our knowledge.
17
Pioneering
studies in the field led by Daugulis et al., Miura et al., and You et al. utilized CuI as catalyst
and phenanthroline or PPh3 as ligand without any use of Cu-precatalysts.
18–21
When
transcribing such conditions to polymerization methodology, the nature of CuI as an
inorganic salt exhibits certain drawbacks, which hinder Cu-DArP from potentially
overtaking conventional Pd-catalyzed methodologies (Figure 4.1b). First, Cu(I) salts
display low solubility in organic solvents,
17
which likely leads to the need for polar amide
solvents (such as N, N-dimethylacetamide, i.e. DMA) in our previous reports.
13-15
Another
drawback is the low stability of Cu(I) salts under the harsh conditions required for Cu-
142
DArP (140 °C),
17
which agrees with our experimental observations that green inorganic
salts were formed after polymerizations in our previous studies (likely Cu(II) salts).
13-15
These major drawbacks of Cu(I) salts previously adapted for Cu-DArP may have resulted
in undesired side reactions and the need for higher Cu-catalyst loadings in our reports,
especially when aryl-bromides were employed (15-50 mol%).
15
Considering that the
employment of Pd-precatalysts in Pd-catalyzed DArP has contributed considerably the
development of the field, we envisioned the use of a well-defined, soluble, and stable Cu-
precatalyst could enable us to access a more effective and versatile Cu-DArP methodology.
Cu(phen)(PPh3)Br, is a chemically well-defined Cu(I) complex (Figure 4.1c), that
was first synthesized and employed by Venkataraman et al. as a highly efficient Cu-
precatalyst for the formation of aryl-nitrogen, aryl-oxygen, and aryl-carbon bonds.
22
However, the use of Cu(phen)(PPh3)Br in biaryl coupling reactions has not been reported
Figure 4.6 (a) Summary of different polymerization methods for BTz-containing conjugated
polymers; (b) Summary of small-molecule selective direct arylation on thiazole unit.
143
to the best of our knowledge. It can be readily prepared simply by the addition of 1,10-
phenanthroline (phen) ligand to a solution of tris(triphenylphosphine) copper (I) bromide.
This Cu-precatalyst has been shown to display excellent stability in air and moisture, and
is highly soluble in common organic solvents.
22
Additionally, Cu(phen)(PPh3)Br exhibits
high structural resemblance to our previous Cu-DArP catalytic system (CuI, phen).
Therefore, we identified Cu(phen)(PPh3)Br as an ideal candidate for the further pursuit of
our Cu-DArP studies.
While our previous Cu-DArP studies focused on fluorinated monomers such as
2,2′,3,3′,5,5′,6,6′-octafluoro-4,4′-diphenylene,
14,15
we turned our attention to thiazole-
containing conjugated polymers due to their prevalence in organic electronics.
23
Specifically, electron-deficient bithiazole (BTz)-based conjugated polymers display
improved charge-transport properties and stability in a variety of applications owing to
their low highest occupied molecular orbital (HOMO) levels.
24
Furthermore, due to the
reduced steric hinderance between repeating units and strong intermolecular S-N
interactions, BTz units afford Donor-Acceptor (D-A) copolymers with more planar
backbones and higher degrees of crystallinity.
25
Therefore, BTz units such as 2,2’-
bithiazole (2-BTz) and 5,5’-bithiazole (5-BTz) have been proven to be attractive building
blocks for materials applied in OPVs and OFETs.
23,25,26
However, while numerous reports have demonstrated highly effective Pd-DArP
conditions for the synthesis of 2-BTz copolymers,
27–29
the regio-isomeric 5-BTz unit is
reported to be surprisingly inactive under a variety of Pd-DArP protocols by Sommer et al.
(Figure 4.2a).
29
Although Kanbara et al. successfully activated the C-H bonds at the C-2
position of the thiazole unit via Cu-catalyzed oxidative direct arylation polymerization
144
(oxi-DArP), the resulting polymeric outcomes are identical to 2-BTz copolymers
synthesized via Pd-DArP (Figure 4.2a).
30
To the best of our knowledge, the 5-BTz
monomer has not been polymerized via DArP, presumably due to the lack of catalytic
activity of Pd towards the C-H bond at the C-2 position.
Our research in the literature of small-molecule direct arylation studies reveals a
similar reactivity pattern of the thiazole unit (Figure 4.2b). While Pd-catalysts are highly
regio-selective towards C-5 arylation due to a lower activation energy barrier (23.7 kcal
mol
-1
vs. 26.3 kcal mol
-1
(C-2), respectively),
31,32
C-2 arylation is selectively preferred
when Cu(I) catalysts or Pd/Cu(I) co-catalytic systems are applied.
18,33
This behavior can
be explained by the strong Cu(I)-coordination from the N-3 site, which increases the acidity
of the C-H bond at the C-2 position.
31
Operating under this hypothesis, we were
emboldened to pursue Cu-DArP as potentially the only methodology to access the C-H
activation of the 5-BTz monomer (Scheme 4.1). Herein, we present the first 5-BTz
copolymer, poly[(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl)-alt-(5,5’-bithiazole)] (PF-
5BTz), synthesized with high Mn and yields via Cu-DArP using a robust Cu-precatalyst
approach.
Scheme 4.1 Synthesis of PF-5BTz using 2,7-dibromo-9,9-bis(2-ethylhexyl)fluorene and 5-
BTz (Conditions listed in Table 4.1).
145
entry catalyst/ligan
d
a
cat.
mol %
Base
b
solvent
c
(conc.)
temp.
(°C)
time
(h)
Mn (kDa)
d
,
Ð
d
yield
d
(%)
1
e
CuI/phen 15 K3PO4 DMA/m-
xylene (0.4 M)
140 72 - 0
2
e
CuI/phen 15 tBuOLi DMA/m-
xylene (0.4 M)
140 72 - 0
3
e
CuI/phen 15 K2CO3 DMA/m-
xylene (0.4 M)
140 72 3.8, 1.5 36
4 Cu(phen)(PP
h3)Br
15 K2CO3 DMA/m-
xylene (0.4 M)
140 16 16.5, 1.6 79
5 Cu(phen)(PP
h3)Br
15 K2CO3 DMA/m-
xylene (0.2 M)
140 72 13.1, 1.7 65
6 Cu(neocup)(P
Ph3)Br
15 K2CO3 DMA/m-
xylene (0.4 M)
140 16 - 0
7 Cu(bipy)(PPh
3)Br
15 K2CO3 DMA/m-
xylene (0.4 M)
140 16 - 0
8 Cu(phen)(PP
h3)I
15 K2CO3 DMA/m-
xylene (0.4 M)
140 16 3.2, 1.7 29
9 Cu(phen)(PP
h3)Br
5 K2CO3 DMA/m-
xylene (0.4 M)
140 16 11.8, 1.8 64
10 Cu(phen)(PP
h3)Br
5 K2CO3 m-xylene (0.4
M)
140 16 - 0
11
f
Cu(phen)(PP
h3)Br
5 K2CO3 DMA/m-
xylene (0.4 M)
140 16 - 0
12
g
Pd(OAc)2 2 K2CO3 DMA (0.04 M) 70 16 - 0
13
h
Pd2(dba)3/P(o
-anisyl)3
1 Cs2CO3 THF (0.2 M) 120 16 - 0
14
h
PdCl2(PPh3)2/
P(o-anisyl)3
2 Cs2CO3 CPME (0.2 M) 110 16 - 0
a
For CuI, 99.999%-Puratrem Cu(I) iodide was used. phen = 1,10-Phenanthroline. neocup =
Neocuproine. bipy = 2,2’-Bipyridine.
b
All Cu-DArP (entries 1-10) were conducted using 4 equivalence
of base. All Pd-DArP (entries 11-13) were conducted using 3 equivalence of base.
c
DMA
= N,N-
dimethylacetamide. THF = Tetrahydrofuran. CPME = Cyclopentyl methyl ether.
d
Determined for
polymer products after purification.
e
Ligand loadings were 1:1 ratio to Cu(I) iodide.
f
2-BTz was used
instead of 5-BTz.
g
0.3 equiv. of neodeconoic acid was used as an additive.
h
Ligand loadings were 2:1
ratio to Pd-catalysts. 0.5 equiv. of neodeconoic acid was used as an additive.
Table 4.1 Cu-DArP conditions for the synthesis of PF-5BTz and polymerization
results.
146
4.2 Experimental
As shown in Scheme 4.1, 2,7-dibromo-9,9-bis(2-ethylhexyl)-9H-fluorene was
selected as a coupling partner to 5-BTz for our model study. Detailed description for the
synthesis of Cu-precatalysts, monomers, and polymers can be found in Appendix C. A
DMA/m-xylene (1:1) co-solvent system was used based on our previous report, which
demonstrated that it is critical for Cu-DArP to proceed using aryl-bromides.
15
Daugulis et
al. reported that the C-2 position of thiazole can be arylated with high yields using CuI,
phen as the catalytic system and K3PO4 or tBuOLi as base.
18,19
However, desired
polymerizations did not proceed under either conditions (Table 4.1, entry 1, 2), contrasting
our previous reports in which K3PO4 provides the optimal conditions for Cu-DArP.
14,15
After changing the base to a milder base, K2CO3, while keeping the same catalytic system
(15 mol% CuI, phen), PF-5BTz was afforded with Mn of 3.8 kDa and a low yield (36%)
(entry 3).
1
HNMR reveals a significant amount of acceptor-acceptor homo-coupling (a)
(detailed discussion is provided below), potentially due to the oxidative thiazole (C-
H)/thiazole (C-H) coupling catalyzed by Cu(II), as reported by Kanbara et al.
30
This
illustrates the instability of the CuI, phen catalytic system, which is prone to oxidation to
form Cu(II).
17
Remarkably, switching the Cu-catalyst to the aforementioned bench-stable,
soluble Cu-precatalyst, Cu(phen)(PPh3)Br, drastically improved the efficiency of the
polymerization while keeping the same catalyst loading (15 mol%), affording PF-5BTz
with an excellent Mn (16.5 kDa) and a good yield (79%) in only 16 hours (entry 4).
1
HNMR
confirms no sign of a homo-coupling, which is likely a result of an improved stability of
Cu(phen)(PPh3)Br, providing a stable Cu(I) source. By comparison with entry 3, this result
further verifies reports by Venkataraman et al. that Cu(phen)(PPh3)Br is a highly efficient,
stable and soluble Cu-precatalyst for Pd-free cross-coupling chemistry.
22
Decreasing the
147
concentration of the polymerization while prolonging the reaction time to 72 hours did not
improve the Mn or yield (13.1 kDa, 65%, respectively) (entry 5).
Venkataraman et al. also reported two analogous Cu-precatalysts,
Cu(neocup)(PPh3)Br and Cu(bipy)(PPh3)Br, which can be prepared in the same fashion as
Cu(phen)(PPh3)Br (see Appendix C for detailed synthesis of Cu-precatalysts).
22
These
species were also found to be robust in catalyzing cross-coupling reactions such as the
formation of aryl-nitrogen and aryl-oxygen bonds.
22
Despite the suppression of the desired
polymerization found utilizing these two Cu-precatalysts (entry 6, 7), the ease of structural
modification of Cu(phen)(PPh3)Br can potentially impact the future development of Cu-
DArP. This is in agreement with report by You et al. that phen is the most effective bidentate
pyridinic ligand for Cu-catalyzed direct arylation.
21
Additionally, we synthesized the iodo-
counterpart of the Cu-precatalyst, Cu(phen)(PPh3)I, which afforded PF-5BTz with a much
lower Mn (3.2 kDa) and yield (29%), accompanied by the visible decomposition of the
precatalyst (entry 8). By comparison, this demonstrates the thermal stability of
Cu(phen)(PPh3)Br, even under relatively harsh condition (140 °C).
Subsequently, with the optimal condition (entry 4) in hand for 15 mol% catalyst
loading, we decided to probe a lower catalyst loading of 5 mol%, which afforded PF-5BTz
with a good Mn of 11.8 kDa and a yield of 64% (entry 9). This result marks the first
conjugated polymer synthesized by DArP using only 5 mol% of Cu-catalyst when an aryl-
bromide is used as the coupling partner, which has significantly improved upon our
previously disclosed Cu-DArP report.
15
Next, we were interested to investigate the
potential of using non-polar solvents such as m-xylene for the newly-developed Cu-DArP
protocol without the use of an amide solvent (DMA), since Cu(phen)(PPh3)Br was found
148
to be soluble in common organic solvents, such as toluene.
22
As a result, no polymer
product was obtained, although oligomers likely formed based on an observed color change
(entry 10). This result indicates that the role of DMA in Cu-DArP may not be limited to
the improvement of the solubility of Cu-catalysts. DMA has been demonstrated to form
Pd-complex and serve as a critical component of the Pd catalytic system.
34
Similarly, DMA
also likely facilitates Cu-DArP by playing a similar role as a ligand to form DMA-Cu(I)-
complexes during the catalytic cycle. A control experiment was then performed by
replacing the 5-BTz unit with a 2-BTz monomer, which completely inhibited the
polymerization (entry 11). By comparison, a conclusion can be drawn that Cu-DArP
conditions preferably activate the C-2 positions of the 5-BTz unit, as opposed to their
inability to activate the C-5 positions of the 2-BTz unit, which is in agreement with small-
molecule studies (Figure 4.2b).
18,31
Having achieved an unprecedented 5-BTz copolymer synthesis via DArP using a
Cu-precatalyst strategy, we were intrigued to further confirm the inactivity of the 5-BTz
monomer under Pd-DArP protocols, as reported by Sommer et al.
29
Three different Pd-
DArP conditions were attempted for the synthesis of PF-5BTz (entry 12-14). The selected
conditions have been proven to be versatile to afford a variety of conjugated polymer
structures, including homo-polymers and D-A copolymers with minimal homo-couplings
or defects using different Pd-catalysts.
10,12
Interestingly, no polymer product was obtained
when these Pd-DArP conditions were employed (entry 12-14). These results verify our
assumption that the C-H activation of the 5-BTz monomer unit can only be achieved by
Cu-catalysts, which is a significant step forward for Cu-DArP to potentially rival Pd-
catalyzed methodology.
149
Finally, to probe the scope of the developed Cu-DArP method, we turned our focus
to the synthesis of a carbazole-containing conjugated polymer, poly[(N-9’-heptadecanyl-
2,7-carbazole)-alt-(5,5’-bithiazole)] (PC-5BTz) (Scheme 4.2). By using only 5 mol% of
Cu(phen)(PPh3)Br, PC-5BTz was afforded with a moderate Mn of 9.7 kDa and yield of
59%. Taking into account that a 9-heptadecyl side-chain was used instead of the more
soluble bis(2-ethylhexyl) side-chain in PF-5BTz, we presumed a lower solubility of PC-
5BTz was the main reason for a slightly lower Mn and yield.
4.3 Results and Discussion
1
HNMR spectroscopy studies were performed to characterize and confirm the
proposed polymer structures. All recorded
1
HNMR data for Table 4.1 and Scheme 4.2 is
available in Appendix C. Figure 4.3 shows detailed peak-assignments, including end-
group and defect analyses of two representative polymers (PF-5BTz) synthesized using
Cu(phen)(PPh3)Br (Table 4.1, entry 4) and CuI, phen employed in our previous reports
(Table 4.1, entry 3).
13–15
The major resonances of PF-5BTz match with that previously
synthesized via Pd-catalyzed oxi-DArP with resonances centered at δ8.02 and δ7.84
(ppm).
35
The integration ratio of these major resonances can be accurately assigned to
protons of PF-5BTz repeat unit (see Figure C.8-11 in Appendix C). This indicates the
absence of branching and cross-linking defects embedding in the polymer structure,
according to report by Kanbara et al.
30
The assignments of end groups were performed by
comparing the collected PF-5BTz spectra to those of model compounds with similar
Scheme 4.2 Synthesis of PC-5BTz.
150
structures.
35
a homo-coupling was assigned by comparing the
1
HNMR spectra of PF-5BTz
to a BTz-based homo-polymer.
36,37
A detailed discussion regarding these assignments can
also be found in the SI (see section 8 of Appendix C and Figure C.13).
Figure 4.3
1
HNMR analyses of representative polymers: (a) PF-5BTz (Table 4.1, entry 4)
synthesized using Cu(phen)(PPh 3)Br and (b) PF-5BTz (Table 4.1, entry 3) synthesized using CuI,
phen. Potential resonances for end groups and defects are denoted. a = acceptor-acceptor
homocoupling (d7.96). Collected in CDCl 3 at 25 °C and 500 MHz.
151
Importantly, as shown in Figure 4.3a, PF-5BTz synthesized via the newly-
developed Cu-precatalyst approach (entry 4) display the absence of an a homo-coupling
signal, which would be expected if a Cu(II) species were generated during the reaction
based on the report by Kanbara et al.
30
This result suggest Cu(phen)(PPh3)Br as a highly
robust precatalyst to provide a stable Cu(I) source, preventing oxidation to Cu(II) from
occurring. The addition of a PPh3 ligand to the Cu-catalyst structure likely improve the
oxidative stability of Cu(I), since similar behavior of phosphine ligands has been reported
to stabilize Pd(0)/Pd(II) species in cross coupling reactions.
38
By comparison, PF-5BTz
prepared using the previous Cu-DArP methodology (entry 3) exhibits a significant amount
of a homo-coupling defect (δ7.96) (Figure 4.3b). Furthermore, the absence of the terminal
5-BTz end-groups (d, d’, e), a low Mn(3.8 kDa), and a significant amount of the fluorene
end-groups (a-c’) indicate the presence of the thiazole (C-H)-thiazole (C-H) oxidative
coupling, likely catalyzed by Cu(II) generated from an unstable Cu(I) source (CuI).
30
4.4 Conclusion
In summary, a novel precatalyst approach for Cu-DArP methodology has been
presented, which allows for a drastic improvement for the synthesis of PF-5BTz. By
replacing our previously reported CuI, phen catalytic system with an easy-to-prepare,
soluble, and stable Cu-precatalyst, Cu(phen)(PPh3)Br, the Mn of PF-5BTz was
substantially improved from 3.8 kDa to 16.5 kDa with a yield increased from 36% to 79%.
Structural analysis using
1
HNMR spectroscopy reveals the absence of a homo-coupling
when Cu(phen)(PPh3)Br was used, suggesting it as a highly stable Cu(I) source compared
to CuI. This newly-developed Cu-DArP condition allows a decrease of Cu-catalyst loading
from 15 mol% to 5 mol% to afford PF-5BTz with a good Mn of 11.8 kDa and a yield of
152
64%, which is the lowest when aryl-bromides were employed. Impressively, Cu-DArP was
demonstrated as the only methodology to achieve the C-H activation of the 5-BTz unit,
since the synthesis of PF-5BTz under various of Pd-DArP conditions were proven
unsuccessful, which is in agreement with literature reports including polymer syntheses as
well as small molecule studies. These results also demonstrate a clear example of a case
where Cu-DArP proves more effective than Pd-DArP. Future work will focus on designing
new Cu-precatalysts and further optimizing polymerization conditions to seek a broader
substrate scope.
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Oxygen as the Sole Oxidant: Facile, Green Access to Bithiazole-Based Polymers.
ChemSusChem. 2016, 9, 2765–2768.
(36) Kuwabara, J.; Kuramochi, M.; Liu, S.; Yasuda, T.; Kanbara, T. Direct Arylation
Polycondensation for the Synthesis of Bithiazole-Based Conjugated Polymers and Their
Physical Properties. Polymer Journal. 2017, 49 (1), 123–131.
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as Critical Side Reactions in Direct Arylation Polycondensation. ACS Macro Lett. 2014, 3
(8), 819–823.
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158
Chapter 5: p-Cymene: A Sustainable Solvent that is Highly Compatible with Direct
Arylation Polymerization (DArP)
5.1 Introduction
π-conjugated polymers are promising electroactive materials widely implemented
in various of applications, such as organic photovoltaics (OPVs), organic light-emitting
diodes (OLEDs), organic field effect transistors (OFETs), electrochromic devices, and
chemical sensors.
1–5
To prepare these polymers, conventional cross-coupling methods such
as Stille-Migita (Stille) and Suzuki-Miyaura (Suzuki) polymerizations are employed,
which require the functionalization of monomers with organometallics such as organotin
and organoboron.
6,7
Besides the challenging synthetic accessibility, the generation of
stoichiometric, acutely toxic, and hazardous by-products from these methods poses
significant environmental drawbacks. In vast contrast, the emergence of Direct Arylation
Polymerization (DArP) in the past decade offers a streamlined, sustainable, low-cost
pathway for the preparation of defect-free, high-performance conjugated polymers,
circumventing the necessity for hazardous, pyrophoric reagents and highly toxic
byproducts.
8–12
Despite being established as a “greener” synthetic approach, the sustainability
aspect of this methodology still remains problematic.
12
Specifically, DArP protocols are
reliant on less earth-abundant Pd catalysts and toxic, hazardous solvents produced from
unsustainable, non-renewable sources.
13,14
Recently, progress has been made to potentially
replace Pd with earth-abundant, low-cost transition metal catalysts for DArP, such as Cu.
15–
18
However, given the fact that the solvent is used in the highest quantity, exploring non-
159
toxic, sustainable solvents for DArP is of the utmost urgency, especially considering the
potential of DArP for industrial-scale conjugated polymer synthesis.
19,20
Recently, we
identified cyclopentyl methyl ether (CPME) as a sustainable alternative to toxic ethereal
solvents such as tetrahydrofuran (THF), providing poly[(2,5-bis(2-
hexyldecyloxy)phenylene)-alt-(4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole]
(PPDTBT) with excellent molecular weights (Mn) (up to 41 kg/mol) and yields (up to
98%).
21
Subsequently, CPME was demonstrated to be the optimal solvent for the syntheses
of amide- and ester-functionalized polythiophenes with exceptionally high regioregularity
(>99%).
22,23
Figure 5.1 Summary of commonly used solvents in DArP with their boiling points and
sustainability.
However, ethereal solvents such as THF, 2-MeTHF, and CPME are classified as
organic peroxide-formers over their long-term storage.
24
Furthermore, they possess low
boiling points (66.0 °C, 80.2 °C and 106.0 °C for THF, 2-MeTHF and CPME, respectively,
see Figure 5.1), which are typically lower than the temperature required for DArP
(typically 100-140 °C).
12,21
Thus, highly-pressurized reaction vessels are generally
160
required when these ethereal solvents are utilized, and polymerization results carried by
these low-boiling solvents might be irreproducible due to the inconsistency in the solvent
volume from different reaction setups.
25
Conversely, aromatic solvents such as toluene,
xylenes, mesitylene and chlorobenzene are non-coordinating, high-boiling solvents that are
also exempt from any peroxide-formation.
14,24,26
More importantly, aromatic solvents are
demonstrated to exhibit superior solubility for conjugated polymers synthesized via
DArP.
26
What remains unresolved for aromatic solvents, however, is their high toxicity
(carcinogenicity in some cases) to human health and the environment, and the intensive
energy required for their industrial production from fossil sources.
13,14,24
Therefore, it
would be a significant advance for DArP to explore new sustainable aromatic solvents that
are sourced from renewable feedstock and do not present toxic hazards to health, while
preserving important properties such as high boiling point and good solubility for
conjugated polymers.
p-Cymene, a non-toxic aromatic compound that structurally resembles xylenes and
mesitylene (Figure 5.1), can be produced in large quantities as a side product from citrus
fruit processing.
14,27,28
As a naturally-occurring compound, p-cymene is present in
significant quantity in the essential oils of more than 100 plant species and 200 foods, such
as fruit peels, cilantro, and green pepper.
27
Moreover, p-cymene is used in herbal drugs
from medicinal plants to prevent coughs, as well as in the cosmetics industry for the
production of perfumes and fragrances.
27
Being recognized as a bio-renewable solvent, p-
cymene has recently been studied for organic transformations such as esterification and
olefin-metathesis, however, the application of p-cymene in the field of conjugated polymer
has yet to be exploited.
28,29
Considering the significant potential of p-cymene as a
sustainable alternative to conventional aromatic solvents (such as toluene, xylenes) with a
161
high boiling point (177.1 °C), we were emboldened to employ p-cymene for the synthesis
of conjugated polymers via DArP. Herein, we disclose for the first time that the high
compatibility of the new sustainable solvent, p-cymene, in DArP can be realized by
providing conjugated polymers with excellent Mn (up to 51.3 kg/mol) and yields (up to
96.2%) with minimal detectable defects.
Scheme 5.1 Synthesis of P1-P3 using DArP conditions listed in Table 5.1.
5.2 Experimental
Kanbara et al. reported toluene to be the optimal solvent for DArP using 2,7-
dibromo-9,9-dioctylfluorene as the monomer paired with various of electron-deficient
thiophenes.
30
Hence, as the sustainable alternative to aromatic solvents such as toluene, we
first applied p-cymene for DArP using a similar model system (Scheme 5.1). An electron-
deficient thiophene unit, dimethyl 3,4-thiophenedicarboxylate, was selected as the first
coupling partner since the ester directing group was found to facilitate the C-H activation
of thiophenes.
22,23,31
By using 4 mol% of Pd(OAc)2 as the catalyst, which is the best-
performing catalyst in Kanbara’s study, in combination with 16 mol% of P(o-anisyl)3 as
the ligand, 3.2 equiv. of Cs2CO3 as the base, and 1.0 equiv. of neodecanoic acid (NDA) as
an additive (Scheme 5.1), P1 was afforded with a Mn of 22.4 kg/mol and a moderate yield
of 69.1% (entry 1). The use of 4 equiv. of P(o-anisyl)3 ligand relative to Pd was inspired
162
from the optimization of DArP protocols by Leclerc et al.
9
Replacing the Pd-catalyst with
PdCl2(PPh3)2 increased the Mn to 31.9 kg/mol and yield to 75.8% (entry 2). By increasing
the concentration of the polymerization to 0.2 M, the Mn of P1 was further improved to
33.8 kg/mol accompanied by a significant improvement in yield (96.2%) (entry 3). It is
worth-noting that the optimal Mn of P1 (33.8 kg/mol) synthesized using p-cymene as the
solvent is higher than that synthesized using a polar, unsustainable solvent DMA (23.4
kg/mol) via DArP reported by Ling and co-workers.
32
To perform a more complete comparison, we attempted the synthesis of P1 using
the sustainable solvent CPME previously disclosed for ester-directed thiophene
substrates.
23,31
As a result, P1 was obtained with a lower Mn (20.4 kg/mol) and yield
entry polymer solvent
b
concentration (M) Pd-catalyst Mn (kg/mol)
c
, Ð
c
yield
d
(%)
1 P1 p-cymene 0.1 Pd(OAc)2 22.4, 1.53 69.1
2 P1 p-cymene 0.1 PdCl2(PPh3
)2
31.9, 1.49 75.8
3 P1 p-cymene 0.2 PdCl2(PPh3
)2
33.8, 1.60 96.2
4 P1 CPME 0.2 PdCl2(PPh3
)2
20.4, 1.53 79.3
5 P1 toluene 0.2 PdCl2(PPh3
)2
23.8, 1.83 87.1
6 P2 p-cymene 0.2 PdCl2(PPh3
)2
51.3, 1.73 72.6
7 P2 toluene 0.2 PdCl2(PPh3
)2
47.4, 1.86 59.3
8 P3 p-cymene 0.2 PdCl2(PPh3
)2
42.0, 5.21 89.2
a
Reactions are conducted using the general conditions described in Scheme 5.1.
b
CPME = Cyclopentyl
methyl ether. All solvents are used in their anhydrous forms followed by 15 minutes of degassing prior to
use.
c
Estimated by GPC (80 °C, 1,2,4-trichlorobenzene) calibrated with polystyrene standards.
d
Polymer
products were purified via Soxhlet extraction using MeOH, hexanes, and collected by CHCl3.
See
Appendix D for detailed experimental procedure.
Table 5.1 DArP conditions for the synthesis of P1-P3 depicted in Scheme 5.1.
a
163
(79.3%) (entry 4), demonstrating the higher compatibility of p-cymene for DArP compared
to other sustainable solvents. Another comparison was drawn between p-cymene and the
conventional, toxic aromatic solvent, toluene, which provided P1 with a lower Mn of 23.8
kg/mol and a lower yield of 87.1% (entry 5). Significantly, it was observed that during the
polymerizations using both CPME and toluene as the solvents, premature precipitation
occurred after 24 h, while the reaction using p-cymene as the solvent was conducted
smoothly without any polymer precipitation, and the reaction solution displayed a much
darker green color. These observations led us to the assumption that p-cymene is a
sustainable aromatic solvent highly suitable to solubilize conjugated polymers with high
Mn.
This assumption was further supported by the synthesis of P2, a 5-(2-ethylhexyl)-
thieno-[3,4-c]-pyrrole-4,6-dione (TPD)-based conjugated polymer, using p-cymene as the
solvent with the optimized DArP condition (entry 3), which afforded P2 with an
exceptional Mn of 51.3 kg/mol, and a good yield of 72.6% (entry 6). Consistent with the
synthesis of P1, toluene as the traditional aromatic solvent provided P2 with a slightly
decreased Mn (47.4 kg/mol) and a significant decrease in yield (59.3 %) (entry 7). The
decrease in yield is due to a portion of P2 with lower Mn being removed from the hexanes
fraction of the Soxhlet extraction, likely a consequence of premature precipitation of the
polymer in toluene during the reaction. On the contrary, similar to our observations with
P1, p-cymene also displayed an excellent solubilizing ability for P2 by allowing the
polymerization to be conducted without any obvious precipitation of polymer.
Interestingly, several sustainable constituents of essential oils including p-cymene have
been demonstrated to be the most suitable solvents to dissolve high Mn polystyrene for
waste recycling process.
33
This might explain the high dissolution ability of p-cymene for
164
conjugated polymers exhibited in this report. It is also imperative to note that the boiling
point of p-cymene (177.1 °C) is much higher than the reaction temperature (110 °C), which
allows these reactions to run without being pressurized, as opposed to those employing
conventional DArP solvents such as THF, CPME, or toluene (Figure 5.1). For this reason,
p-cymene is highly advantageous among all the sustainable solvents available for DArP,
especially in a large, industrial scale setting.
Encouraged by these results, we were interested to examine the capacity of p-
cymene as a sustainable solvent for DArP using a 2,2’-bithiophene monomer without any
electron-withdrawing substituent or carbonyl directing groups on the thiophene substrate
(Scheme 5.1, P3). The synthesis of P3 via DArP was found to be extremely challenging
due to the absence of orienting groups or steric protection on the β-positions of 2,2’-
bithiophene, which leads to insoluble materials being obtained presumably due to
significant amounts of embedded β-defects.
34,35
As reported by Kanbara et al. and Leclerc
et al, both the polar solvent DMA (after 3 hours of reaction time) and the non-polar solvent
toluene (after 210 minutes of reaction time) afford insoluble cross-linked materials with a
Mn of P3 not exceeding 28 kg/mol.
34,35
Using the optimized DArP condition developed in
this report for P1 and P2 (entry 3 and entry 6), P3 was afforded with a high Mn of 42.0
kg/mol and a yield of 89.2% (entry 8). Importantly, even after 24 hours of reaction time,
no insoluble material was obtained after Soxhlet extraction. Polymer P3 was found to be
completely CHCl3-soluble, which along with evidences from
1
H NMR spectroscopy
(discussed below), suggests a defect-free synthesis of P3 using p-cymene. Notably, the Mn
of P3 provided by using p-cymene as the solvent (42.0 kg/mol) surpassed those prepared
using toluene as the solvent via DArP (28 kg/mol) or Suzuki (19 kg/mol) polymerization
methods.
35
However, premature precipitation of P3 was observed in p-cymene during the
165
reaction, which is likely due to the limited solubility of P3, as evidenced by a large
polydispersity (Ð = 5.21).
35
Scheme 5.2 Synthesis of PCDTBT using p-cymene as the solvent.
To further expand the scope of p-cymene as a new green solvent for DArP, we
investigated the compatibility of this newly developed DArP condition in the synthesis of
poly[(9-(heptadecane-9-yl)-9H-carbazole)-alt-(4,7-di(thiophen-2-
yl)benzo[c][1,2,5]thiadiazole)] (PCDTBT), a perfectly alternating copolymer widely
explored in OPVs due to its high-efficiency in organic solar cells.
36
Conventionally,
PCDTBT is prepared using the Suzuki polymerization method, however, only moderate
Mn (ranging from 5.1 kg/mol to 11.1 kg/mol) of this material is obtained by utilizing
different DArP conditions, potentially due to its limited solubility in traditional hazardous
solvents such as DMA, THF and toluene.
37
Therefore, we envisioned that p-cymene with
a high solubilizing property for conjugated polymers may allow an improvement in Mn for
the synthesis of PCDTBT via DArP. Using p-cymene as the solvent, PCDTBT was
afforded with Mn = 23.4 kg/mol using p-cymene as the solvent (Scheme 5.2), which is
much higher than those previously prepared via DArP (5.1-11.1 kg/mol).
37,38
Small amount
of premature precipitation was observed during this reaction, likely due to the fact that
PCDTBT is more inclined to aggregation as reported in the literature.
35-38
Suggested by
literature reports, a dichloromethane Soxhlet extraction has been added to the purification
166
of PCDTBT.
35,36
A slightly lower yield (55.8%) and a relatively lower polydispersity (Ð =
1.75) were obtained for this reaction, which can be attributed to a fraction of material being
removed in the dichloromethane fraction of the Soxhlet extraction.
5.3 Results and Discussion
Figure 5.2
1
H NMR analyses of P1 synthesized using p-cymene (Table 5.1, entry 3), CPME (Table
5.1, entry 4), and toluene (Table 5.1, entry 5) as the solvent. Major resonances (A-D’) and potential
resonances for end groups (a-e) and defects (δ, a) are denoted. Collected in CDCl 3 at 25 °C and
600 MHz.
Polymer structural characterization was performed by using
1
H NMR spectroscopy
and UV-vis absorbance spectroscopy (
1
H NMR and UV-vis spectra are included in
Appendix D). Major resonances in the
1
H NMR spectra collected for P1-P3, and PCDTBT,
were referenced and compared to those previously reported.
17,30,32,35,37,38
As shown in
Figure 5.2, for P1 prepared using p-cymene, CPME, and toluene (Table 5.1, entry 3-5),
major resonances (A-D’) in the
1
H NMR spectra were identical without any defects such
as donor-donor (δ) or acceptor-acceptor (a) homocoupling being detected. End-group
167
assignments were performed based on literature reports, and the intensities of end-groups
were minimized when p-cymene was used as the solvent to afford the highest Mn for P1
(33.8 kg/mol, entry 3).
17,30,32
UV-vis spectra of P1 from entry 3-5 were also identical with
λmax = 389 nm (Figure D.17). Similarly, major resonances of P2 (see Figure D.14)
synthesized from p-cymene and toluene (entry 6-7) matched without any δ or a
homocoupling defect. For the UV-vis spectra of P2, polymers obtained by using p-cymene
as the solvent (entry 6) with a higher Mn (51.3 kg/mol) displays a slight bathochromic shift
(λmax = 450 nm) compared to that by using toluene as the solvent (λmax = 445 nm) (Figure
D.18), which is consistent with the observation by Leclerc et al.
35
The
1
H NMR spectrum
of P3 was collected in C2D2Cl4 at 100 °C to be consistent with literature reports on the
same polymer and to allow better comparisons to be drawn.
9,35
For the
1
H NMR of P3, the
signals corresponding to the β-protons of the 2,2’-bithiophene (D, D’) were present and
integrated to the correct value (Figure D.12 and Figure D.15). This likely indicates the
exclusion of β-defects embedded in the polymer structure, which is also suggested by the
absence of insoluble materials as mentioned above. End-group assignments for P3 were
performed on the basis of the report by Leclerc et al., in which the same chemical shifts (at
7.25 ppm and 7.12 ppm) corresponding to the end-groups were depicted (See Figure D.15
for details).
35
Additionally, no δ or a homocoupling defect was identified for P3 (Figure
D.15). UV-vis spectroscopy shows a similar absorption profile for P3 (Figure D.19)
compared to that reported by Leclerc et al. with a similar λmax of 451 nm, which is found
to be the saturation of the absorption maximum with the increase of Mn (from 0.8 kg/mol
to 28.0 kg/mol) for P3.
35
168
As reported by Sommer et al., β-branching and δ homocoupling for the synthesis
of PCDTBT via DArP could be major sources of defects, however, no evidence of these
defects could be observed for PCDTBT provided using the conditions described in Scheme
5.2 (Figure D.16).
38
The UV-vis spectrum of PCDTBT is in agreement with previous
report with no deviation (Figure D.20).
37
Importantly, Sommer et al. reported that aromatic
solvents such as toluene exhibit high C-H reactivity, leading to end-capping events of
naphthalene diimide (NDI)-Br chain ends under DArP conditions.
26,39
However, all minor
resonances in the
1
H NMR spectra of the polymers synthesized by using p-cymene or
toluene as the solvent (P1-P3, PCDTBT) in this report can be adequately attributed to
potential end-groups from the monomers (see analysis above). Therefore, no end-groups
corresponding to potential solvent end-capping defects were identified for p-cymene or
toluene. This might be due to the use of different types of monomers (fluorene-based or
carbazole-based monomers, as opposed to NDI-based monomers) in this report. In
consistent with our findings, literature reports on DArP protocols using these type of
monomers did not evidence any end-capping defects with the use of aromatic solvents.
30,35
5.4 Conclusions
In conclusion, a novel, sustainable aromatic solvent for DArP has been presented
to replace traditional hazardous solvents such as DMA, THF, and toluene. As a constituent
of naturally-occurring essential oils produced from citrus fruit processing, p-cymene is
advantageous as a new green solvent for DArP relative to ethereal solvents such as CPME
because of its high boiling point and exceptional solubilizing property for conjugated
polymers. By comparing with CPME and toluene, we found that p-cymene provided the
best results for the synthesis of P1 and P2 using two different electron-deficient thiophene
substrates, affording polymers with Mn up to 51.3 kg/mol and yields up to 96.2%. P3, a
169
polymer previously found to crosslink due to significant β-defects when using DMA and
toluene, was synthesized using p-cymene as the solvent without forming any insoluble
materials, affording a high Mn of 42.0 kg/mol. Application of p-cymene towards the
synthesis of PCDTBT afforded a good Mn of 23.4 kg/mol and a yield of 55.8%, which is
significantly higher than the values previously reported via DArP (5.1-11.1 kg/mol) when
more hazardous solvents were employed.
1
H NMR spectroscopy confirmed the structures
of these polymers to be free of defects. Overall, this work reveals the significant potential
of p-cymene in DArP towards the synthesis of high-Mn, defect-free alternating copolymers
while aligning with the important principles of green chemistry.
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175
Chapter 6: “In-Water” Direct Arylation Polymerization (DArP) under Aerobic
Emulsion Conditions
6.1 Introduction
Conjugated polymers are inexpensive, lightweight, flexible semiconducting
materials that have drawn considerable attention owing to their excellent processability and
attractive optoelectrical properties.
1
Applications for these materials have expanded rapidly
in the past decades to which include organic photovoltaics (OPVs), organic light-emitting
diodes, organic field-effect transistors (OFETs), chemical sensors, electrochromic devices,
batteries, and biological sensors.
2–8
In recent years, a focus on the industrial-scale
production of conjugated polymers to meet the demands for roll-to-roll printed organic
electronic devices has grown, demonstrating the promising potential of these materials for
commercial applications.
9–11
However, the development of well-defined conjugated polymers relies on
conventional synthetic methods such as Stille and Suzuki polycondensations, which
require multistep preparations of organostannanes or organoboranes using hazardous,
pyrophoric reagents and challenging monomer purification processes.
12,13
Additionally, the
generation of stoichiometric quantities of toxic by-products (for instance, highly toxic tin
waste in the case of Stille) has led to some to doubt the industrial scalability of these
polymerization techniques.
14
In contrast, the emergence of Direct Arylation
Polymerization (DArP) has offered a robust, atom-economical synthetic approach for the
preparation of conjugated polymers by only generating environmentally-benign by-
products (for example, KBr).
15–19
By accessing directly through a C-H activation pathway,
176
DArP also alleviates the necessity of utilizing organolithium reagents involved in the
monomer-functionalization processes in Stille and Suzuki polymerizations.
Scheme 6.1 Development of DArP in water-compatible/sustainable conditions.
Despite being recognized as a “greener” approach, most DArP protocols are heavily reliant
on toxic, unsustainable organic solvents and therefore generate large quantities of
hazardous organic waste in the production of conjugated polymers.
20
Recently, Leclerc et
al. developed biphasic DArP conditions by introducing an equal-volume water/toluene
solvent system, which allows the use of easily-accessible “wet” solvents and reagents
(Scheme 6.1a).
21
While these DArP conditions are more user-friendly and cost-effective,
the sustainability of this protocol remains unimproved, since toxic organic solvents in these
reactions are employed in near the usual amount albeit with the addition of water to the
reaction mixture. Although recent advances in improving the sustainability of DArP have
focused on the employment of renewable, biocompatible green solvents such as
cyclopentyl methyl ether (CPME) and p-cymene (Scheme 6.1b),
22–26
a new DArP
methodology that involves a significant reduction in the required quantity of organic
solvents is highly attractive.
177
Recently, Beverina et al. introduced E-factor, a quantitative measure of the ratio of kg of
organic waste generated per kg of conjugated polymer synthesized using Suzuki
polymerization.
27
By utilizing water as the main reaction medium, along with the use of
minimal amount of organic solvent (9:1 water/toluene) and the presence of 2 wt% of
surfactant, efficient Suzuki polymerizations were executed in an emulsion under aerobic
conditions to afford conjugated polymers with Mn up to 26.9 kg/mol and yields up to 93%.
Specifically, in such micro-heterogeneous reaction environments, efficient
polymerizations occur in highly concentrated organic domains (known as the “hydrophobic
effect”), which allows a significant enhancement in the reaction rates while enabling a 10-
fold reduction of the E-factor.
28
Such “in-water” synthetic strategies have proven successful for the preparation of
various small-molecule organic semiconductors,
29–32
including a report by Beverina et al.
focused on Pd-catalyzed direct arylation reactions between brominated benzenes and
thiophene substates containing reactive C-H bonds.
33
Furthermore, the water-compatibility
of certain DArP conditions in aromatic solvent (toluene) is showcased in the
aforementioned report by Leclerc et al. (Scheme 6.1a).
21
Therefore, in this report, we aim
to extend these strategies to apply an “oil-in-water” emulsion polymerization method to
DArP for the first time (Scheme 6.1c). By using only 10 vol% of sustainable organic
solvent (p-cymene), efficient DArP protocols are carried out in water under aerobic
conditions to afford conjugated polymers with good Mn (up to 14.5 kg/mol) and yields (up
to 79.2%) with minimal structural defects detected.
178
Scheme 6.2 Synthesis of P1 via DArP using emulsion conditions listed in Table 6.1.
Herein, poly(9,9-dioctylfluorene-alt-bithiophene) (P1) was selected as the target
polymer for our model study (Scheme 6.2 and Table 6.1), since the synthesis of P1 has
been explored in analogous emulsion conditions using a Suzuki polymerization.
27
In
addition, P1 was afforded with high Mn (42.0 kg/mol) and yield (89.2%) via DArP using
the sustainable aromatic solvent, p-cymene, in our previous report.
23
Furthermore, the
potentially reactive C-H bonds on the b-positions of the 2,2’-bithiophene monomer pose a
significant challenge for the preparation of P1 via DArP, which makes this model study an
excellent examination of possible defect-formation using the novel DArP protocol in
emulsion conditions.
34,35
6.2 Experimental
We chose PdCl2(PPh3)2 (4 mol%) in combination with the ligand P(o-anisyl)3 (16
mol%) as the catalytic system for the optimization of the synthesis of P1 via DArP in
emulsion conditions, since this catalytic system has been demonstrated to be water-
compatible in biphasic-DArP conditions as well as being highly-efficient in the sustainable
aromatic solvent, p-cymene with DArP.
21,23
All polymerizations were performed using 48
hours of reaction time, which is the same reaction time reported for the Suzuki
polymerization in emulsion conditions.
27
Beverina et al. utilized Kolliphor EL (K-EL) as
179
the surfactant, which is a nonionic surfactant prepared by reacting castor oil with ethylene
oxide, to form and stabilize micellar solutions or emulsions in water. Specifically, the use
of 2 wt% of K-EL in deionized water allows the formation of micellar solutions, in which
effective Suzuki polymerization was carried out giving good Mn (15.9 kg/mol) and yield
(70%) of P1 at room temperature (25 °C) even without the use of organic solvent. However,
such micellar solutions are unstable at the high temperatures usually required for DArP
(100-120 °C), which disrupts the uniform dispersion of the colloids in water suspension
and results in thermally induced phase separation.
33
Presumably due to these reasons, DArP
attempted using H2O (with 2 wt% K-EL surfactant) as the reaction medium was
unsuccessful at 110 °C (Table 6.1, entry 1), in contrast to the result reported by Beverina
entry reaction medium
b
temperat
ure (°C)
base (equiv.) concentrati
on (M)
c
atm
Mn (kg/mol)
d
,
Ð
d
yield
e
(%)
1 H2O 110 Cs2CO3 (3.2) 0.25 air - -
2 H2O/toluene 9:1 110 Cs2CO3 (3.2) 0.25 air 8.8, 1.92 58.8
3 H2O/p-cymene 9:1 110 Cs2CO3 (3.2) 0.25 air 10.3, 2.01 63.3
4 H2O/p-cymene 9:1 110 Cs2CO3 (3.2) 0.05 air - 0
5 H2O/p-cymene 9:1 130 Cs2CO3 (3.2) 0.25 air 11.9, 3.17 75.4
6 H2O/p-cymene 9:1 130 NaO
t
Bu (3.2) 0.25 air - 0
7 H2O/p-cymene 9:1 130 K2CO3 (3.2) 0.25 air 13.6, 3.03 79.2
8 H2O/p-cymene 9:1 130 K2CO3 (3.2) 0.25 N2 12.9, 2.63 76.3
9
f
H2O/p-cymene 9:1 130 K2CO3 (40) 0.25 air - 0
10 H2O/p-cymene 9:1
(Aliquat 336)
130 K2CO3 (3.2) 0.25 air - 0
11 H2O/p-cymene 9:1
(TBAB)
130 K2CO3 (3.2) 0.25 air 7.0, 1.92 59.1
12
g
H2O/p-cymene 9:1 130 K2CO3 (3.2) 0.25 air 8.7, 2.52 69.1
a
All polymerizations were conducted using the general conditions shown in Scheme 6.2, unless otherwise
denoted.
b
2 wt% of K-EL (Kolliphor EL) was added to all reaction media as a surfactant. Additives in
parenthesis were used in the amount of 0.3 equiv. TBAB, tetrabutylammonium bromide.
c
Concentration
of each polymerization was calculated based on the total volume of the reaction medium added.
d
Estimated
by GPC.
e
Polymer products were purified via Soxhlet extraction.
f
This polymerization was not performed
due to the breakage of the emulsion.
g
1 mol% of Pd(PPh3)2Cl2 and 4 mol% of P(o-anisyl)3 were added.
Table 6.1 Optimization for the synthesis of P1 using emulsion conditions depicted in
Scheme 6.2.
a
180
et al. using Suzuki polymerization at 25 °C.
27
To overcome this challenge, a small amount of organic solvent, such as toluene (10
vol%), is often added to the reaction medium, which effectively converts the micellar
solutions into thermally-stable emulsions.
27–33
Dynamic Light Scattering (DLS)
measurements by Beverina et al. revealed that the addition of toluene increases the
diameter of the micelles from 12 nm to the 1 µm regime.
30
This improves the stability of
the medium during the polymerization by avoiding the phase separation of the organic
domain due to the low solubility of the synthesized polymer.
30
The effectiveness of this
strategy was further proven by our experiment using DArP, which afforded P1 with Mn of
8.8 kg/mol and yield of 58.8% (Table 6.1, entry 2) under aerobic conditions with 9:1
H2O/toluene emulsion. The lower Mn and yield compared to the Suzuki polymerization
counterpart (Mn = 20.7 kg/mol, yield = 72%) might be attributed to the necessity of direct
arylation to employ stronger inorganic carbonate bases (such as K2CO3 or Cs2CO3),
19
which in an emulsion reaction condition, are exclusively soluble in the aqueous domain
and are reliant on productive mass exchange to reach the catalytic site within the organic
micelles. Conversely, Suzuki polymerization allows the use of organic bases such as Et3N,
which promote the reactivity of the polymerization more efficiently within the lipophilic
pocket.
32
The impact of adopting such emulsion conditions on the sustainability of DArP is
significant as only 0.16 mL of organic solvent (toluene) is used to produce 164 mg of
conjugated polymer, compared to 0.75 mL of organic solvent required for the preparation
of 78 mg of P1 in conventional DArP methods, resulting in a 10-fold improvement of the
E-factor.
23
Previously, we disclosed p-cymene as a naturally-sourced, sustainable
181
alternative to toluene offering superior solubilizing ability for conjugated polymer
synthesis via DArP.
23
Therefore, we envisioned that by replacing the small amount (10
vol%) of toluene in the emulsion medium with p-cymene, a true “green” condition for
DArP could be realized. By mixing H2O/p-cymene 9:1 (v:v) with 2 wt% of K-EL, a stable
emulsion was obtained with an identical physical appearance compared to the H2O/toluene
9:1 (v:v) emulsion solution previously employed (see Figure E.1). This reaction medium
resulted in an improvement of the Mn for P1 to 10.3 kg/mol and the yield to 63.3% (Table
6.1, entry 3) (see Figure E.2 for the visualization of DArP in emulsion). Due to an observed
premature precipitation of P1 by the end of the polymerization, the monomer concentration
in the emulsion medium was decreased from 0.25 M to 0.05 M. Such a strategy has been
employed in Suzuki polymerization to afford fluorene-based conjugated polymers with
good Mn under mini-emulsion conditions albeit with a sacrifice of sustainability.
36
However, only oligomeric product of P1 was obtained when this strategy was applied to
DArP (entry 4), indicating the effectiveness of DArP in emulsion conditions relies on the
presence of highly-concentrated colloids.
Small-molecule direct arylation studies in emulsion conditions performed by
Beverina et al. demonstrated that an increased reaction temperature of 130 °C and the use
of NaO
t
Bu as the base afforded the desired coupling product with optimal yields.
33
Inspired
by this study, we were able to further improve the Mn of P1 to 11.9 kg/mol and the yield to
75.4% by increasing the reaction temperature from 110 °C to 130 °C (Table 6.1, entry 5).
However, changing the base from Cs2CO3 to NaO
t
Bu led to a suppression of the
polymerization (Table 6.1, entry 6), in contrast to the small-molecule study. On the other
hand, replacing Cs2CO3 with K2CO3 provided P1 with the optimal Mn of 13.6 kg/mol and
yield of 79.2% (entry 7). Significantly, consistent with the Suzuki polymerization
182
counterpart, these emulsion-DArP reactions were carried out under aerobic conditions,
which is highly attractive relative to conventional DArP conditions that require inert
atmosphere and stringent Schlenk-techniques.
21,37
As a control experiment, we attempted
the synthesis of P1 via DArP with emulsion conditions under N2 atmosphere (entry 8),
which did not result in an improvement of Mn or yield (12.9 kg/mol, 76.3%, respectively).
1
H NMR analysis reveals identical structures of P1 synthesized via emulsion-DArP under
aerobic and N2 atmosphere (Table 6.1, entry 7 and 8), which will be discussed below.
Scheme 6.3 Synthesis of PPDTBT via DArP using the optimal emulsion conditions listed in Table
6.1, entry 7.
Leclerc et al. reported that a saturated K2CO3 (40 equiv.) aqueous solution provided
the optimal results for DArP in biphasic conditions.
21
However, this strategy cannot be
applied to emulsion conditions because a high equivalence of K2CO3 drastically increases
the density of the aqueous solution, resulting in the breakage of the emulsion and the
separation of the 10 vol% of organic solvent (p-cymene) from water (see Figure E.3 for
visualization). With this approach, we found that the emulsion was turned into two separate
layers, which is against the intention of this study and therefore this polymerization was
not attempted (Table 6.1, entry 9). Interestingly, while Beverina et al. showcased that the
183
addition of Aliquat 336 as a phase transfer agent improved the yields of small-molecule
direct arylation couplings in emulsion conditions, such an approach was detrimental to the
DArP conditions developed in this study as no polymer product was obtained (Table 6.1,
entry 10).
33
Strategies that have been found to improve the performance of biphasic-DArP
protocols such as the addition of tetrabutylammonium bromide (TBAB) or the use of a
lower catalyst loading (1 mol%) did not enhance the outcome of DArP in the emulsion
medium (Table 6.1, entry 11 and 12).
21
With the optimal emulsion conditions for DArP in hand (entry 7), we explored the
compatibility of this novel, sustainable DArP method for the synthesis of poly[(2,5-bis(2-
hexyldecyloxy)phenylene)-alt-(4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)
(PPDTBT), a polymer that can be easily prepared and is promising for large-area, roll-to-
roll processed solar-cell devices with good power conversion efficiency (PCE).
14
By
applying the best conditions for the synthesis of P1 using a H2O/p-cymene 9:1 (v:v)
emulsion, PPDTBT was synthesized in air with Mn of 14.5 kg/mol and yield of 75.6%
(Scheme 6.3). Compared to PPDTBT prepared by conventional DArP protocol using the
unsustainable solvent THF, emulsion-DArP conditions afforded the polymer with a similar
Mn (14.5 kg/mol vs. 15 kg/mol) and yield (75.6% vs. 78%).
38
This indicates that the new
emulsion-DArP method is applicable for the preparation of conjugated polymers that are
suitable for large-area OPV devices and rivals the performance of traditional DArP
methods using toxic reaction media.
184
6.3 Results and Discussion
Figure 6.1
1
H NMR analyses of P1 synthesized using H 2O/p-cymene 9:1 (v:v) emulsion in air and
under N 2 atmosphere (Table 6.1, entry 7 and 8, respectively). Major resonances (A-D’), potential
resonances for end groups (*) and potential defects (δ, a) are denoted. Collected in C 2D 2Cl 4 at 100
°C and 600 MHz. The inset in the top spectrum shows the intensified region in which oxidative
end-capping event might appear (~6.95 ppm).
In regard to polymer structural analysis,
1
H NMR spectroscopy and UV-vis
absorbance spectroscopy were performed for P1 and PPDTBT and compared to samples
previously prepared using conventional DArP methods. For P1, analysis was performed
for those with satisfactory Mn values (exceeding 10 kg/mol). Specifically,
1
H NMR and
UV-vis spectra of P1 in Table 6.1, entry 3, 5, 7 ,8, were collected and included in
Appendix E. Figure 6.1 shows a comparison of the
1
H NMR spectra of P1 synthesized
using the H2O/p-cymene 9:1 (v:v) emulsion in air and under N2 atmosphere (Table 6.1,
entry 7 and 8, respectively), illustrating identical major resonances (A-D’) without donor-
donor (d) or acceptor-acceptor (a) homocoupling defects being detected.
23
End-group
assignments were performed based on literature reports (7.25, 7.12 ppm).
23,35
Structures of
potential defects and end-groups are shown in Figure E.9. Importantly, resonances
corresponding to the b-protons of the 2,2’-bithiophene moiety (D,D’) were unaffected and
185
integrated to the correct values (see Figure D.4-D.7), which indicates a likely exclusion of
b-defects when using the novel emulsion conditions for DArP.
23,35
When emulsion
polymerization is used for Suzuki methods under aerobic conditions, OH-termination
signals of the fluorene unit (~6.95 ppm) were identified as a result of oxidative deborylation
events in the presence of oxygen in the reaction medium.
27
As expected, such termination
events did not occur when air was introduced to the emulsion-DArP conditions (Figure
6.1, top), which demonstrates another advantage of the C-H activation methodologies over
the Suzuki counterpart by alleviating the use of unstable organoboranes under aerobic
reaction conditions. UV-vis spectra of P1 samples (Figure E.10) show similar absorption
profiles compared to those reported with the same λmax of 451 nm.
23,35
It should be noted
that P1 synthesized using the conditions from Table 6.1, entry 5 displayed a vibronic
shoulder with a slightly higher intensity compared to the other polymer samples of P1,
although the reason for this result is unclear. The
1
H NMR spectrum of PPDTBT (Figure
E.8) synthesized using the emulsion-DArP protocol matched previous reports without
observable deviation.
22,38
The UV-vis spectrum of PPDTBT (Figure E.11) is similar to
samples synthesized using conventional methods with λmax = 655 nm, and an apparent
vibronic shoulder in the absorption profile suggesting a minimization, if not exclusion, of
b-defects of the synthesized polymer.
22,38
6.4 Conclusion
In summary, we presented a novel, sustainable method for DArP using a H2O/p-
cymene 9:1 (v:v) emulsion medium under aerobic conditions. With only 10 vol% of
organic solvent present in these emulsions, we were able to significantly decrease the
amount of organic waste generated per kg of conjugated polymer synthesized (E-factor) by
186
10 times compared to conventional DArP methods. As a model study for the optimization
of emulsion-DArP conditions, P1 was synthesized with Mn up to 13.6 kg/mol and yields
up to 79.2% without detectable structural defects. This sustainable DArP method is also
compatible with the preparation of a conjugated polymer suitable for roll-to-roll processed
OPV, PPDTBT, resulting in an Mn of 14.5 kg/mol and yield of 75.6%. With a growing
demand of large-scale preparation of semiconducting materials and roll-to-roll printed
organic electronic devices, we believe the “in-water” strategy for DArP under ambient
conditions presented in this study marks an important step towards enhancing the
sustainability of industrial-scale conjugated polymer synthesis. Future studies will focus
on the further optimization of reaction conditions to improve Mn and yield with the
emulsion-DArP protocol, such as utilizing organic bases that are more soluble in the
organic micelles, potentially enhancing reactivity under emulsion conditions.
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Chapter 7: Recycling Heterogenous Catalysts for Multi-Batch Conjugated Polymer
Synthesis via Direct Arylation Polymerization
7.1 Introduction
Conjugated polymers (CPs) are flexible low-cost materials that attract great interest
for a wide range of applications, such as organic photovoltaics (OPVs), organic light
emitting diodes (OLEDs), electrochromics, and bioelectronics.
1–5
Recently, with progress
towards commercialization and the emergence of large-area roll-to-roll processing, the
demand for industrial-scale CP synthesis has grown rapidly.
6,7
To synthesize CPs, the Stille
polycondensation is one of the most widely employed traditional cross-coupling methods,
which however has significant drawbacks. Specifically, the generation of stoichiometric,
highly toxic organotin byproducts and a multistep preparation of unstable stannylated
monomers, makes Stille impractical for the large-scale synthesis of CPs.
8
Scheme 7.1 Brief summary of Pd-catalyzed CP synthesis.
In recent years, Direct Arylation Polymerization (DArP) has emerged as a facile
synthetic tool for the preparation of high-quality CPs through an atom-economical C-H
activation pathway, alleviating the need for hazardous, pyrophoric reagents and reducing
toxic waste production (Scheme 7.1a).
8–10
Recent advances in replacing traditional,
hazardous solvents with bio-renewable green alternatives such as cyclopentyl methyl ether
194
(CPME) and p-cymene have further enhanced the sustainability of DArP protocols.
11,12
However, among the most crucial limitation of prevailing DArP methods is the catalyst,
which is almost exclusively based on the rare and expensive metal Pd.
13
While ongoing
efforts focus on developing earth-abundant metals, such as Cu, for DArP,
14,15
heterogenous
catalysts offer another avenue to potentially extend the lifetime of a Pd catalyst beyond a
single use, which could significantly reduce the cost of large-scale CP synthesis.
16
Although the utility of heterogenous catalysts in DArP has been demonstrated in a few
cases, the recyclability of these catalysts has never been tested beyond the initial batch of
CP.
17–20
Significantly, we address this important limitation of DArP here by disclosing the
first report on the recycling of heterogenous catalysts for the synthesis of up to 5 batches
of CPs via DArP (Scheme 7.1b), with minimization of batch-to-batch variation (Mn of
25±2.5 kg/mol across all 5 batches). The novel protocol developed herein marks a major
breakthrough for CP synthesis and marks an enabling step toward the inexpensive, scalable
preparation of semiconducting polymers.
Homogenous Pd catalysts on which most current DArP protocols are reliant
(Scheme 7.1a), such as Pd(OAc)2 and Pd2(dba)3, have several shortcomings, including the
lack of recyclability of the expensive catalysts and Pd contamination in the synthesized
CPs, which significantly hampers performance in organic electronics.
21,22
Conversely, we
identified SiliaCat® DPP-Pd, a diphenylphosphine-based Pd(II) species covalently bound
to an organosilica matrix, as a promising heterogenous catalyst due to its excellent
performance in molecular aryl-aryl cross-coupling reactions such as Suzuki and Heck with
low Pd-leaching.
23–26
Moreover, the recyclability of SiliaCat® DPP-Pd has previously
been showcased in various small-molecule Stille and Suzuki coupling reactions, and
further extended to large-scale synthesis in commercial flow reactors.
25–27
Although
195
Leclerc et al. have previously demonstrated the high reactivity of this heterogenous catalyst
in DArP using Ozawa-derived conditions (i.e., in non-polar solvents),
8,18
our preliminary
efforts revealed that the recovered SiliaCat® DPP-Pd was not reusable after the initial
polymerization under similar conditions (see Appendix F for details). This may be
attributed to the longer reaction time usually required for these polymerizations (24 hours
or more) compared to those for small-molecule cross-coupling reactions (~30 mins),
25,26
which likely results in significant leaching of Pd from the solid support suppressing the
catalytic activity of the recovered Pd species.
26,28
Furthermore, the use of an additional
ligand in Ozawa-derived DArP conditions (P(o-anisyl)3) may sequester Pd from the solid
support and contribute to the Pd-leaching.
24
Poly[(3,4-ethylenedioxythiophene-2,5-diyl)-(9,9-dioctylfluorene-2,7-diyl)]
(PEDOTF) is an important candidate for electrochromics and OLEDs with excellent
electroluminescent properties.
29
Moreover, Kanbara et al. have shown that short reaction
entry
[b]
number of cycle reaction time
(h)
catalyst recycled
(%)
[c]
Mn (kg/mol)
[d]
,
ᴆ
[d]
yields (%)
1 cycle 1 (initial) 0.5 - 29, 2.0 94
2 cycle 2 0.5 97 23, 1.8 89
3 cycle 3 0.5 92 6.1, 1.7 72
4 cycle 4 3 88 33, 1.9 93
5 cycle 1 (initial) 0.5 - 27, 2.0 96
6 cycle 2 0.5 98 21, 2.3 93
7 cycle 3 1.5 96 26, 2.0 90
8 cycle 4 3 91 63, 1.8 95
9 cycle 5 24 93 12, 1.9 89
[a] All polymerizations were conducted using the general conditions shown in Scheme 7.2. For Cycles 2-
4, the amounts of monomers were re-adjusted to keep the catalyst loading (5 mol%) and monomer
concentration (0.3 M) constant. See Appendix F for experimental details. [b] Entries 1-4, 5-9 are two
separate rounds of catalyst-recycling experiments. [c] Determined based on the mass of recovered catalyst
from the previous cycle. [d] Estimated by GPC (140 °C, 1,2,4-trichlorobenzene) calibrated with
polystyrene standards.
Table 7.1 Initial testing of the recyclability of SiliaCat® Pd-DPP using DArP
conditions depicted in Scheme 7.2.
[a]
196
times (30 mins for microwave heating or 3-6 hours for conventional heating) are viable for
the synthesis of PEDOTF via DArP using Fagnou-derived conditions, which lead to higher
reactivity due to the use of polar, coordinating solvents, making this polymer an ideal target
for testing the recyclability of heterogenous catalysts for CP synthesis.
30,31
These Fagnou-
derived conditions are highly efficient for the C-H activation of electron-rich thiophenes
such as 3,4-ethylenedioxythiophene (EDOT), but unsuitable for electron-deficient
thiophene monomers such as thieno[3,4-c]pyrrole-4,6-dione (TPD).
32
Welch et al. utilized
5 mol% of SiliaCat® DPP-Pd, along with K2CO3 as the base, pivalic acid (PivOH) as an
additive, and N,N-dimethylacetamide (DMA) as the solvent at 100 °C for small-molecule
direct arylation of thiophene-based substrate.
26
These conditions are highly similar to those
employed by Kanbara et al. for the preparation of PEDOTF using DArP.
30,31
Therefore, we
adopted these conditions for the synthesis of PEDOTF via DArP using SiliaCat® DPP-Pd
and the examination of its recyclability (Scheme 7.2).
Scheme 7.2 Synthesis of PEDOTF using SiliaCat® Pd-DPP as the heterogenous catalyst.
7.2 Experimental
The high reactivity of SiliaCat® DPP-Pd in this model system was first realized by
affording PEDOTF with Mn of 29 kg/mol and yield of 94% in only 0.5 h of reaction time
(Table 7.1, entry 1). A large scale polymerization (increased from 0.4 mmol to 2 mmol of
monomers) was performed using the same reaction conditions, which afforded gram-scale
production of PEDOTF (0.92 g) with a better Mn of 42 kg/mol (ᴆ = 2.5) and a good yield
of 87% (see Appendix F for details), demonstrating the industrial scalability of these DArP
conditions. These results demonstrate that SiliaCat® DPP-Pd is highly efficient with DArP
197
by enabling a shortened reaction time without microwave assistance, compared to
conventional homogenous Pd catalysts such as Pd(OAc)2 (affording PEDOTF with Mn of
14.9 kg/mol after 3 h of conventional heating under otherwise the same reaction conditions,
reported by Kanbara et al.).
30,31
The high reactivity of SiliaCat® DPP-Pd compared to
homogenous Pd(OAc)2 is also consistent with findings by Welch et al. in small-molecule
studies.
26
The heterogenous catalyst from this reaction was easily isolated from the
polymerization mixture through a simple filtration, washed and dried (see Appendix F for
experimental details), and subjected to the next cycle (Table 7.1, entry 2). This second
cycle of polymerization using the recycled SiliaCat DPP-Pd from the first run still
exhibited excellent catalytic activity by providing PEDOTF with good Mn of 23 kg/mol
and yield of 89% in only 0.5 h. Furthermore, we concluded that although a loss of catalytic
activity was observed during the third cycle (Table 7.1, entry 3) giving a lower Mn (6.1
kg/mol) and yield (72%), an extended reaction time from 0.5 h to 3 h effectively promoted
the reaction with high Mn (33 kg/mol) and yield (93%) for the fourth cycle (Table 7.1,
entry 4). Importantly, it is worth-noting that we were able to recycle large majorities (88.3-
96.6%) of the catalyst for the following cycles after each polymerization.
The effect of carboxylic acid additives was also investigated (Table F.1).
Interestingly, we found that replacing PivOH with the sterically bulkier neodecanoic acid
(NDA) resulted in lower Mn (19 kg/mol, 8.1 kg/mol, 4.1 kg/mol, respectively) for the first
three cycles with short reaction times (0.5 h) (Table F.1, entries 1-3). However, an
exceptional Mn of 82 kg/mol with 96% yield for PEDOTF was obtained during the fourth
cycle when the reaction time was extended to 3 h (Table F.1, entry 4). This might be
attributed to the steric bulkiness of NDA preventing aggregation of the Pd catalyst, which
leads to higher Mn given longer reaction times, yet results in a slower reaction rate in the
198
early stage.
30
Moreover, this result, along with the result from Table 7.1, Cycle 4, further
confirm the high catalytic performance of SiliaCat® DPP-Pd even after being recovered
and reused for 3 cycles.
Table 7.2 Further optimization of the recycling experiments using SiliaCat® Pd-DPP
using DArP conditions depicted in Scheme 7.2.
[a]
We have also found that through fine-tuning of reaction times for each cycle, batch-
to-batch variations of CP synthesis can potentially be minimized (Table 7.1, entries 3, 4,
7, 8). For further studies, the carboxylic acid additive was changed back to PivOH since
better results were obtained with PivOH for the first two cycles (Table 7.1, entry 1, 2).
Importantly, the second round of catalyst-recycling experiments showed great
reproducibility for the first two cycles with same reaction times (0.5 h) compared to those
of the first round (29 kg/mol, 23 kg/mol, respectively), offering highly similar Mn (27
kg/mol, 21 kg/mol, respectively for Table 7.1, entry 5 and 6). For the third cycle, the
reaction time was increased to 1.5 h, which afforded PEDOTF with Mn of 26 kg/mol and
yield of 90% with minimal variation compared to the first two cycles (Table 7.1, entry 7).
When the reaction time was further increased to 3 h for the fourth cycle, a much higher Mn
of 63 kg/mol was provided accompanied by a high yield (95%). Lastly, we tested the
recyclability of SiliaCat® DPP-Pd for a fifth cycle (Table 7.1, entry 9), which offered
[a] All polymerizations were conducted using the general conditions shown in Scheme 7.2.
Detailed experimental procedure can be found in Appendix F.3.
cycle 1 cycle 2 cycle 3 cycle 4 cycle 5
reaction time (h) 0.5 0.5 1.5 2 36
catalyst recycled (%) - 98 94 92 91
M n (kg/mol) 28 22 25 28 23
ᴆ 2.1 2.0 2.2 2.1 2.2
yields (%) 93 92 91 93 92
199
PEDOTF with a satisfactory Mn of 12 kg/mol and yield of 89% using a prolonged reaction
time (24 h).
Recycling SiliaCat® DPP-Pd for 5 batches of CP synthesis with good to high Mn
using DArP is an unprecedented result, since this heterogenous catalyst was only
previously recycled up to 3 times in Suzuki cross-coupling reactions with a considerable
loss observed in activity after the initial run.
25
However, we envisioned that the large
variations of DArP results from the last two cycles (Table 7.1, entry 8, 9) could be
minimized by the further optimization of reaction times. As shown in Table 7.2, cycle 1-3
of Table 7.2 employ the same reaction conditions used for Table 7.1, entries 5-7 with
highly similar results. Decreasing the reaction time for the fourth cycle from 3 h (Table
7.1, entry 8) to 2 h (Table 7.2, cycle 4) afforded PEDOTF with Mn of 28 kg/mol with little
variation from the previous 3 batches. Extending the reaction time for the fifth cycle from
24 h to 36 h improved the Mn to 23 kg/mol. The average Mn of the 5 batches of CPs from
Table 7.2 was calculated to be 25 kg/mol with a small standard deviation (±2.5 kg/mol),
which is promising for continuous flow methods of CP synthesis on industrial scales.
33
Cost analysis was performed (see Appendix F.4 for details), which showed a much
lower cost by employing the novel heterogenous approach (only costs $0.67 of SiliaCat®
DPP-Pd to synthesize a total of 1.67 mmol of PEDOTF across 5 batches) compared to the
conventional homogenous DArP method ($1.79 of Pd(OAc)2 would be required) to
synthesize the same total amount of polymer (assuming the same yields). Another
important advantage of recycling heterogenous catalysts for multi-batch CP synthesis is
the impact on sustainability by reducing the waste of precious Pd metal. The amount of Pd
required to prepare a total of 1.67 mmol of PEDOTF across 5 batches (Table 7.2) was
200
calculated to be 0.02 mmol (Appendix F.4), which was significantly lower compared to
the conventional homogenous approach (0.084 mmol of Pd). These advantages shed light
on the promising potential of extending the newly developed DArP protocol disclosed in
this study to continuous flow chemistry for CP synthesis. Although continuous flow
synthesis of CPs via DArP has been reported, expensive homogenous Pd catalysts were
pumped through the column reactors in the mobile phase, which resulted in significant
waste of the Pd species.
34,35
Conversely, the heterogenous approach for DArP developed
in this study offers great potential for truly low-cost flow chemistry by allowing the
expensive Pd catalyst to be packed in the stationary phase and used continuously
throughout the CP production process with a gradually decreasing flow rate.
27
The
introduction of a co-solvent in addition to DMA, such as sustainable non-polar solvent
CPME or p-cymene, will likely enhance the solubility of CPs during their industrial
production in flow. Such strategy (a DMA:CPME = 1:1 co-solvent system for DArP) has
been proven effective for the large-scale batch synthesis of a CP as conductive cathode
binder for lithium-ion batteries.
4
Other commercially-available heterogenous Pd catalysts besides SiliaCat® DPP-
Pd were also evaluated. The polypropylene fiber-supported Pd complex FibreCat® has
received attention for its good catalytic efficiency in Suzuki coupling protocols with good
recyclability (up to 5 times).
36,37
By replacing the Pd source with FibreCat® using reaction
conditions listed in Scheme 7.2 and Table 7.2, we found that recycling FibreCat® for 5
batches of PEDOTF was feasible, however, with much lower Mn (1.8-4.7 kg/mol) and
yields (Table F.2). This could be explained by the much lower reactivity of FibreCat®
under the DArP conditions utilized in this study, which was evidenced by control
experiments using 24 h of reaction time showing that lower Mn (7.7 kg/mol) of PEDOTF
201
was afforded using pristine FibreCat® compared to that by pristine SiliaCat® DPP-Pd (Mn
= 29 kg/mol) in only 0.5 h (Scheme F.3 and Table 7.1). Pd/C is one of the most widely
employed commercial solid-supported catalysts for cross-coupling reactions, and has been
successfully used in DArP protocols.
17,19
However, the effectiveness of carbon-supported
Pd catalysts is found to be reliant on the leaching of Pd into the solution, which significantly
limits the recyclability of the catalyst.
16,38,39
Indeed, although we found Pd/C to be effective
for the synthesis of PEDOTF via DArP (24 h of reaction time was needed to afford Mn of
40 kg/mol, see Scheme F.2, Table F.2), the catalyst was not reusable after recovering from
the initial run.
7.3 Results and Discussion
Figure 7.1
1
H NMR analyses of 5 batches of PEDOTF synthesized from the recycling of SiliaCat®
Pd-DPP (Table 7.2). Major resonances (A-H) are denoted. Collected in CDCl 3 at 25 °C and 600
MHz.
202
Polymer structural analysis was performed by using
1
H NMR spectroscopy. Major
resonances (Figure 7.1, A-H) for all polymers were identical to literature reports.
30,31
Detailed
1
H NMR analysis (Figure F.17) showed the absence of polymer structural defects
such as donor-donor homocouplings. Figure 7.1 demonstrates the identical
1
H NMR
spectra of 5 batches of CPs synthesized by recycling SiliaCat® Pd-DPP 5 times (Table
7.2), which confirms minimal batch-to-batch structural variation with this novel
heterogenous protocol for DArP. UV-vis absorption spectra of these 5 batches of CPs were
also identical with each other (Figure F.21) and consistent with reported spectra (Figure
F.18-23).
30
7.4 Conclusions
In conclusion, an approach for sustainable multi-batch CP synthesis is presented
for the first time by the recycling of heterogenous catalysts via DArP. The exceptional
reactivity and recyclability of SiliaCat® Pd-DPP for DArP was highlighted by affording
PEDOTF with Mn up to 82 kg/mol even after recycling of the catalyst 3 times. Through
optimization of reaction conditions, up to 5 batches of polymers were obtained by the
recycling of the same Pd catalyst, offering a minimization of batch-to-batch variations
across all batches (25±2.5 kg/mol). Though DArP is intrinsically the most sustainable
protocol to prepare CPs, this work addresses among the most significant limitations of
prevailing DArP methods by prolonging the lifetime of the expensive Pd species through
adapting a novel heterogeneous approach, which is highly promising for low-cost
continuous flow CP synthesis on industrial scales. As an initial demonstration of this new
concept for DArP, this report only focused on the synthesis of a representative polymer
(PEDOTF) with Fagnou-derived conditions using commercial heterogenous catalysts.
Future studies will focus on expanding the scope of the methodology by the further
203
optimization of reaction conditions, exploration of the employment of sustainable solvents
for CP synthesis, and design novel heterogenous catalysts that exhibit higher efficiency
and superior recycling properties.
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209
Biographical Sketch
Liwei Ye was born in Shantou, China. He received his Bachelor of Science degree in
Chemistry from the University of Oregon in 2016. During his undergraduate studies in
Oregon, he worked in Prof. Mark Lonergan's group focusing on organic synthesis and
device fabrication for semiconducting materials. Currently, he is performing his PhD
studies under the guidance of Prof. Barry C. Thompson at the University of Southern
California. His research is focused on developing novel synthetic method for organic
functional materials, polymer chemistry, and organic electronics.
210
Appendix A
Chapter 2: Green Solvent Processed Amide-Functionalized Conjugated Polymers
Prepared via Direct Arylation Polymerization (DArP)
A.1 General
Unless otherwise noted, all reagents were purchased and used as received from
commercial sources. All reactions were performed under dry N2 in oven dried glassware,
unless otherwise noted. Solvents and reagents were purchased from VWR and used without
purification, unless otherwise noted. Anhydrous tetrahydrofuran (EMD Millipore) and
cyclopentyl methyl ether (Acros Organics) were purchased and used as received. N-
hexylmethyl amine (Sigma-Aldrich), 2-ethylhexylamine (Alpha Aesar), oxalyl chloride
(Alpha Aesar), 3-thiophene carboxylic acid (Combi-Blocks), Pd(OAc)2 (Palldium(II)
acetate trimer ≥ 99.98%) (BeanTown Chemical), P(t-Bu)2Me-HBF4 (Sigma-Aldrich),
PdCl2(PPh3)2 (99.95%, BeanTown Chemical), tris(o-methoxyphenylphosphine) (Alpha
Aesar) were purchased and used as received. K2CO3, Cs2CO3 were ground into a fine
powder and dried at 120 °C in a vacuum oven before use.
All monomer NMR were recorded at 25 °C using CDCl3 on either a Varian Mercury
400 MHz, Varian VNMRS-500 MHz. Polymer NMR was obtained on a Varian VNMR-
500 MHz and VT-NMR was obtained on a Varian VNMR-500 MHz. All spectra were
referenced to CHCl3 (7.26 ppm) and C2D2Cl4 (6.00 ppm), unless otherwise noted.
Number average molecular weight (Mn) and polydispersity (Ð) were determined by
size exclusion chromatography (SEC) using a DAWN HELEOS-II GPC (Wyatt
Technology Corp.) WH2-13 model and a Optilab T-rEX Model WTREX-14 RI detector,
211
with 30 °C HPLC grade tetrahydrofuran (THF) as eluent at a flow rate of 0.3 mL/min on
one 250 × 4.6 mm Agilent Polypore Column. The instrument was calibrated vs. polystyrene
standards (1050−3,800,000 g/mol), and data were analysed using ASTRA V 7.1.4.8
software. Polymer samples were dissolved in HPLC grade THF at a concentration of 0.5
mg/mL, stirred until dissolved, and filtered through a 0.2 μm PTFE filter.
FT-IR: FT-IR spectra was acquired using a Bruker Vertex 80v spectrometer.
Polymer sample (2 mg) for analysis was mixed into a KBr (100 mg) matrix and pressed
into pellets. KBr was dried in vacuum oven at 120 °C prior of use. Sample compartment
was vacuum evacuated to 3 hPa prior of spectra collection to remove trace H2O.
Cyclic voltammetry (CV) was performed using a standard three-electrode cell
based on a Pt wire working electrode, a silver wire reference electrode, and a Pt wire
counter electrode was purged with nitrogen and maintained under a nitrogen atmosphere
during all measurements. Polymer films were made by drop-casting an o-DCB solution of
polymer (5 mg/mL) onto the Pt wire and dried under a nitrogen stream prior to
measurement. Anhydrous acetonitrile was used as the solvent and Bu4NPF6 (0.1 M) was
used as the supporting electrolyte.
UV−vis absorption spectra were obtained on a Perkin-Elmer Lambda 950
spectrophotometer. Polymer thin-films were prepared by spin-coating onto pre-cleaned
glass slides from different solutions with different concentrations (detailed preparation
procedures can be found in A.7). Optical bandgaps were determined from absorption band
edge in thin films, (Eg = 1240/ λedge). Thicknesses of the samples and were obtained via
XRR using Rigaku diffractometer Ultima IV using a Cu Kα radiation source (λ = 1.54 Å).
212
The grazing-incidence X-ray diffraction (GIXRD) measurements were obtained using the
same instrument in a grazing-incidence X-ray diffraction mode.
Differential Scanning Calorimetry (DSC) profiles were recorded on PerkinElmer
DSC 8000 under N2 with a scan rate of 10 °C/min. Sample size was ∼5 mg.
A.2 Synthesis
Scheme A.1. Monomer Synthesis.
Note regarding monomer design:
Traditional monomer design towards the synthesis of poly-thiophene derivatives via DArP
involves halogenation on 2-position of the thiophene ring, while C-H activation occurs on
5-position.
1
However, as depicted in Scheme 1 (or Scheme S1), the C-H functionality of
the monomer was reversed for synthetic simplicity, which started with bromination of 1
selectively at 5-position, followed by the generation of acyl chloride and amination
reactions using amines with desired R groups (R1, R2). It is worth-noting that monomers
3a and 3b were prepared in only two simple and scalable steps, and all starting materials
used are commercially available, making the synthesis of the corresponding synthesis of
the poly(alkylamidethiophenes) (P3AAT) very simple and easy.
213
Monomer Synthesis:
Synthesis of 5-bromothiophene-3-carboxylic acid (2):
To an Erlenmeyer flask equipped with a stir-bar, 3-thiophene carboxylic acid (20 g, 156
mmol, 1 equiv) was dissolved in glacial acetic acid (120 mL). To this, a solution of bromine
(11.22 g, 70.2 mmol, 0.9 equiv) in glacial acetic acid (60 mL) was added slowly. The
mixture was allowed to stir for 45 minutes and then it was poured in water (500 mL) and
stirred for 15 minutes. The solid was filtered off and washed with water. It was then
recrystallized from water (~500 mL), filtered, and dried in under vacuum (~100 mtorr)
overnight. 13.9 g, 43%.
1
H-NMR 400 MHz (CDCl3): δ (ppm) 8.11 (s, 1H), 7.51 (s, 1H).
Consistent with literature reports.
2
Synthesis of 5-bromo-N-hexyl-N-methylthiophene-3-carboxamide (3a)
In a 100-mL oven-dried 3-neck round bottom flask equipped with a N2 inlet and a stir-bar,
2 (3.5 g, 16.9 mmol, 1 equiv), were dissolved in anhydrous DCM (40 mL) and anhydrous
DMF (1.5 mL). This mixture was cooled down in an ice bath to 0 °C, followed by the
addition of oxalyl chloride (4.3 g, 33.8 mmol, 2.0 equiv) dropwise via syringe. After
completion, ice bath was removed and the mixture was allowed to stir overnight. The
S
N
O
Br
H
S
HO
O
Br
H
214
reaction mixture was then transferred into a single-neck round bottom flask in which DCM
and oxalyl chloride were removed under reduced pressure. The resulting yellow solid (acyl
chloride) was re-dissolved in 40 mL of fresh anhydrous DCM, cooled with an ice-bath, and
then N-Hexylmethylamine (4.87 g, 42.2 mmol, 2.5 equiv) followed by triethylamine (4.27
g, 42.2 mmol, 2.5 equiv) were added dropwise via syringe. The mixture was allowed to stir
overnight, and then it was poured in water and extracted with DCM three times. The
combined organic layer was washed with brine, dried with MgSO4, filtered, and
concentrated under reduced pressure. The crude product was further purified via column
chromatography using hexanes/ethyl acetate (9:1) to give a pale yellow oil (3.65 g, 71%).
1
H NMR (500 MHz, Chloroform-d) δ 7.35 (s, 1H), 7.13 (s, 1H), 3.42 (s, 1H), 3.36 (s, 1H),
3.03 (s, 3H), 1.66 – 1.51 (m, 3H), 1.33–1.18 (m, 5H), 0.94 – 0.80 (m, 3H).
13
C NMR (126
MHz, CDCl3) δ 137.71, 129.68, 127.66, 126.74, 112.76, 51.24, 48.02, 37.30, 33.04, 31.52,
28.45, 26.57, 22.61, 14.08.
Synthesis of 5-bromo-N-(2-ethylhexyl)thiophene-3-carboxamide (3b):
In a 100-mL oven-dried 3-neck round bottom flask equipped with a N2 inlet and a stir-bar,
2 (2 g, 9.66 mmol, 1 equiv), were dissolved in anhydrous DCM (30 mL) and anhydrous
DMF (1 mL). This mixture was cooled down in an ice bath to 0 °C, followed by the addition
of oxalyl chloride (2.45 g, 19.3 mmol, 2.0 equiv) dropwise via syringe. After completion,
ice bath was removed and the mixture was allowed to stir overnight. The reaction mixture
S
N
O
H
Br
H
215
was then transferred into a single-neck round bottom flask in which DCM and oxalyl
chloride were removed under reduced pressure. The resulting yellow solid (acyl chloride)
was re-dissolved in 30 mL of fresh anhydrous DCM, cooled with an ice-bath, and then 2-
Ethyl-1-hexylamine (3.12 g, 24.2 mmol, 2.5 equiv) followed by triethylamine (2.45 g, 24.2
mmol, 2.5 equiv) were added dropwise. The mixture was allowed to stir overnight, and
then it was poured in water and extracted with DCM three times. The combined organic
layer was washed with brine, dried with MgSO4, filtered, and concentrated under reduced
pressure. The crude product was further purified via column chromatography using
hexanes/ethyl acetate (8:2) to give a pale yellow viscous oil (1.17 g, 38%).
1
H NMR (500
MHz, Chloroform-d) δ 7.73 (s, 1H), 7.30 (s, 1H), 5.84 (s, 1H), 3.39 – 3.30 (m, 2H), 1.52
(m, 1H), 1.33 (m, 8H), 0.93 – 0.86 (m, 6H).
13
C NMR (126 MHz, CDCl3) δ 162.06, 138.15,
129.24, 128.53, 113.47, 42.87, 39.60, 31.18, 29.01, 24.42, 23.11, 14.17, 11.00.
Representative polymerization procedure via DArP (P1) (Entry 5 of Table 2.1):
To an oven dried, high-pressure vessel (15 mL), compound 3a (91.28 mg, 0.3 mmol, 1
equiv.), neodecanoic acid (25.8 mg, 0.15 mmol, 0.5 equiv), tris(o-
methoxyphenylphosphine) (25.4 mg, 0.072 mmol, 0.24 equiv), and Cs2CO3 (146.6 mg,
0.45 mmol, 1.5 equiv) was added. The vessel was then sparged with a stream of nitrogen
for 15 minutes. CPME (1.5 mL), which was degassed prior with N2 for 15 minutes, was
added through the rubber septum to give a 0.2 M concentration. PdCl2(PPh3)2 (7.16 mg,
0.0102 mmol, 0.06 equiv) was added quickly and the rubber septum was replaced with a
Teflon screwcap equipped with a rubber o-ring. The sealed vessel was then placed into a
preheated oil bath (110 °C) and stirred for 72 hours. The vial was then removed from heat,
the mixture was precipitated into a chilled 50%: 50% MeOH/water solution with high-
216
stirring. The solids were then filtered into a Soxhlet thimble dried under nitrogen for 30
minutes, and purified via Soxhlet extraction (hexanes, and CHCl3). The CHCl3 fraction
was concentrated to ~0.5 mL, and re-precipitated into cold hexanes with vigorous stirring.
The dark red fluffy polymer was then filtered and further dried overnight under vacuum
(~100 mtorr).
poly(N-hexyl-N-methylthiophene-3-carboxamide-2,5-diyl) (P1).
1
H-NMR (500 MHz,
CDCl3): δ 7.10 (s, 1H), 3.52 (s, 1H), 3.15 (s, 1H), 3.09 (s, 1H), 2.84 (s, 1H), 1.66 (s, 1H),
1.39 (m, 5H), 1.16 (m, 4H), 0.86 (m, 4H).
Representative polymerization procedure via DArP (P2) (Entry 9 of Table 2.1):
To an oven dried, high-pressure vessel (15 mL), compound 3b (159.14 mg, 0.5 mmol, 1
equiv.), neodecanoic acid (43 mg, 0.25 mmol, 0.5 equiv), P(t-Bu)2Me-HBF4 (14.3 mg, 0.05
mmol, 0.1 equiv), and K2CO3 (103.65 mg, 0.75 mmol, 1.5 equiv) was added. The vessel
was then sparged with a stream of nitrogen for 15 minutes. CPME (2.5 mL), which was
degassed prior with N2 for 15 minutes, was added through the rubber septum to give a 0.2
M concentration. Pd(OAc)2 (5.6 mg, 0.025 mmol, 0.05 equiv) was added quickly and the
rubber septum was replaced with a Teflon screwcap equipped with a rubber o-ring. The
sealed vessel was then placed into a preheated oil bath (110 °C) and stirred for 24 hours.
The vial was then removed from heat, the mixture was precipitated into a chilled 50%: 50%
MeOH/water solution with high-stirring. The solids were then filtered into a Soxhlet
thimble dried under nitrogen for 30 minutes, and purified via Soxhlet extraction (hexanes,
and CHCl3). The CHCl3 fraction was concentrated to ~0.5 mL, and re-precipitated into
cold hexanes with vigorous stirring. The dark red /black polymer was then filtered and
further dried overnight under vacuum (~100 mtorr).
217
poly(N-(2-ethylhexyl)thiophene-3-carboxamide-2,5-diyl) (P2).
1
H-NMR (500 MHz,
CDCl3) δ 7.29 (br, 1H), 5.82 (br, 1H), 3.33 (br, 2H), 1.60 (br, 1H), 1.29 (br, 8H), 0.88 (br,
6H).
A.3
1
H-NMR and
13
C-NMR for monomers 2, 3a, 3b.
Figure A.1
1
H NMR of compound 2 in CDCl3 at 25 °C and 500 MHz.
218
Figure A.2
1
H NMR of compound 3a in CDCl3 at 25 °C and 500 MHz.
Figure A.3
13
C-NMR of Compound 3a in CDCl3 at 25 °C and 500 MHz.
219
Figure A.4
1
H-NMR of Compound 3b in CDCl3 at 25 °C and 500 MHz.
220
Figure A.5
13
C-NMR of Compound 3b in CDCl3 at 25 °C and 500 MHz.
221
A.4 Polymer NMR
Figure A.6
1
H NMR of P1 (Table 2.1, Entry 2) in CDCl3 at 25 °C and 500 MHz.
Figure A.7
1
H-NMR of P1 (Table 2.1, Entry 3) in CDCl3 at 25 °C and 500 MHz.
222
Figure A.8
1
H-NMR of P1 (Table 2.1, Entry 4) in CDCl3 at 25 °C and 500 MHz.
Figure A.9
1
H-NMR of P1 (Table 2.1, Entry 5) in CDCl3 at 25 °C and 500 MHz.
223
Figure A.10
1
H-NMR of P1 (Table 2.1, Entry 6) in CDCl3 at 25 °C and 500 MHz.
Figure A.11
1
H-NMR of P1 (Table 2.1, Entry 7) in CDCl3 at 25 °C and 500 MHz.
224
Figure A.12
1
H-NMR of P2 (Table 2.1, Entry 8) in CDCl3 at 25 °C and 500 MHz.
Figure A.13
1
H-NMR of P2 (Table 2.1, Entry 9) in CDCl3 at 25 °C and 500 MHz.
225
Figure A.14
1
H-NMR of P2 (Table 2.1, Entry 9) in C2D2Cl4 at 25 °C and 600 MHz.
Figure A.15
1
H-NMR of P2 (Table 2.1, Entry 9) in C2D2Cl4 at 75 °C (348 K) and 600
MHz.
226
Figure A.16
1
H-NMR of P2 (Table 2.1, Entry 9) in C2D2Cl4 at 100 °C (373 K) and 600
MHz.
Figure A.17
1
H-NMR of P2 (Table 2.1, Entry 9) in C2D2Cl4 at 125 °C (398 K) and 600
MHz.
227
Figure A.18 Expanded region of
1
H-NMR of P2 (Table 2.1, Entry 9) in C2D2Cl4 at 75 °C
(348 K), 100 °C (373 K) and 125 °C (398 K) at 600 MHz.
228
A.5 FTIR spectra
Figure A.19 FTIR spectra of P2 (Table 2.1, Entry 9).
229
A.6 Polymer Solubility Tests
Initial results indicated good solubility of P1 in a variety of solvents, while P2 was
found to only be soluble in halogenated solvents. To probe the solubility of P1 further, we
performed solubility tests on P1 with the highest Mn (Entry 5 of Table 2.1), which have
Mn of 15.4 kDa. P1 (Table 2.1, Entry 5) was found to be soluble in ethanol, 1-butanol,
anisole, and dichlorobenzene (maximum solubility of P1 (Table 2.1, Entry 5, highest Mn
of 15.4 kDa) in green solvents are summarized in Table A.1). With lower molecular weight
sample (Table 2.1, Entry 3, Mn = 8.1 kDa), P1 was also found to be soluble in ethanol/water
(88%:12%) at concentration of 5 mg/mL.
Table A.1 P1 (Table 2.1, Entry 5, Mn = 15.4 kDa) maximum solubilities in different green
solvents.
Green solvents Ethanol 1-butanol Anisole
P1 (Table 2.1, Entry 5) 9 mg/mL 18 mg/mL 14 mg/mL
230
A.7 Detailed Procedure for UV-Vis Film Preparations
For polymer thin-film measurements, solutions of P1 (Table 2.1, Entry 2, 3)
(Figure A.20) were spin-coated onto pre-cleaned glass slides from different solutions with
different concentrations (Figure A.21):
For ethanol processed film, polymer (Table 2.1, Entry 2) was dissolved at
concentration of 15 mg/mL, stirred with 45°C heating overnight, and spin-coated onto pre-
cleaned glass slide (without solution cooling) with spin speed of 2000 rpm for 60s, which
gives film thickness of 34.2 nm.
For anisole processed film, polymer (Table 2.1, Entry 2) was dissolved at
concentration of 7 mg/mL, spin-coated onto pre-cleaned glass slide with spin speed of 700
rpm for 60 s. The film was found to be non-uniform (see Figure A.21), and therefore was
not used for UV-vis absorbance and SCLC mobility measurements.
For 1-butanol processed film, polymer (Table 2.1, Entry 2) was dissolved at
concentration of 7 mg/mL, spin-coated onto pre-cleaned glass slide with spin speed of 700
rpm for 60 s, which gives a film thickness of 41.2 nm.
For ethanol/water (88%:12%) processed film, polymer (Table 2.1, Entry 3) was
dissolved at concentration of 5 mg/mL, spin-coated onto pre-cleaned glass slide with spin
speed of 2000 rpm for 60 s. The film was found to be non-uniform (see Figure A.21) and
so this solvent was not used for UV-vis absorbance and SCLC mobility measurements.
For DCB processed film, polymers (both P1 and P2) (for P1, Entry 2, Table 2.1
was used; for P2, Entry 9, Table 2.1 was used) were dissolved at concentration of 7 mg/mL,
spin-coated onto pre-cleaned glass slide with spin speed of 700 rpm for 60 s, which gives
film thickness of 30.1 nm.
231
Figure A.20 Solution of P1 (Table 2.1, Entry 2, Mn = 10.4 kDa) in green solvents (ethanol,
1-butanol and anisole) and DCB used for spin-coating.
Figure A.21. Films of P1 (Table 2.1, Entry 2 or 3) processed by green solvents (EtOH,
anisole, 1-butanol, and EtOH:H2O (88:12)) on glass slides.
232
A.8 UV-VIS Data and Spectra in Solutions
UV-VIS data and spectra of P1 and P2 were collected by dissolving P1 and P2 at a
concentration of 50 μg/mL in various of solvents.
Figure A.22 UV-VIS spectra of solutions of P1 (in DCB, EtOH, 1-Butanol), and P2 (in
DCB).
233
Figure A.23 Comparison of UV-VIS spectra of P1 both in solutions (in DCB) (black) and
as cast on film (with DCB) (red).
Figure A.24 Comparison of UV-VIS spectra of P1 both in solutions (in 1-Butanol) (black)
and as cast on film (with 1-Butanol) (red).
234
Figure A.25 Comparison of UV-VIS spectra of P1 both in solutions (in EtOH) (black) and
as cast on film (with EtOH) (red).
Figure A.26 Comparison of UV-VIS spectra of P2 both in solutions (in DCB) (black) and
as cast on film (with DCB) (red).
235
Table A.2 Summary of optical bandgaps, λmax values in various solvents.
P1 in DCB P1 in 1-Butanol P1 in EtOH P2 in DCB
Polymers on
Film
λmax = 468 nm λmax = 471 nm λmax = 468 nm λmax = 475 nm
Polymers in
solutions
λmax = 471 nm λmax = 467 nm λmax = 468 nm λmax = 471 nm
optical
bandgap (Eg)
2.18 eV 2.18 eV 2.16 eV 1.89 eV
A.9 Space-charge Limited Current (SCLC) Mobility Measurements.
SCLC Measurements: Hole mobility was measured using a hole-only device
configuration of ITO/PEDOT:PSS(0.1% GOPS)/polymer/Al in the space charge limited
current regime (SCLC) as described in literature.
3
Specific details on device fabrication
are described below. The dark current was measured under ambient conditions (T = 27 °C
and RH = 35%) from a range of -1 to 4 V, and the results fitted to the Mott-Gurney
equation (equation 1):
JSCLC =
!
"
𝜀𝜀
#
𝜇
$
!
%
"
(1)
where JSCLC is the current density, ε0 is the permittivity of free space (8.85×10
-14
F
cm
-1
), ε is the dielectric constant of the active layer, μ is the charge carrier mobility, V is
the potential across the device (V = Vapplied–Vbi–Vr), corrected for potential loss due to built-
in potential (Vbi) and series resistance (Vr), and L is the polymer layer thickness.
Fabrication of Devices for SCLC Measurements: All processes were performed
under ambient conditions outside of glovebox (T = 27 °C and RH = 35%). ITO-coated
glass substrates were cleaned sequentially with sonication (10 minutes each at 70 °C) using
a 1% Tergitol aqueous (v:v) solution, water, tetrachloroethylene, acetone, isopropanol, and
dried under a stream of nitrogen. A PEDOT:PSS (Clevios) 0.1% (v:v) solution of 3‐
glycidyloxypropyl)trimethoxysilane (GOPS) was prepared by adding the GOPS to the
236
PEDOT:PSS solution via microsyringe and sonicating for 3 minutes at room temperature
to ensure a uniform dispersion, as described in the literature.
4
PEDOT:PSS (0.1% GOPS)
was then spin-coated onto the ITO-substrates at 4000 rpm for 40 seconds, and it was
annealed on a hot-plate at 150 °C for 40 min under air to facilitate cross-linking. After
cooling the substrates to room temperature in a nitrogen dry-box, the active layers were
spin-coated on. For all dichlorobenzene (DCB) solutions, the polymer solutions were
stirred at 65 °C overnight (before using) and kept at 65 °C immediately before spin-coating.
The polymer-DCB solutions were spin-coated at 700 rpm for 60 seconds, followed by a
wicking-step of 3000 rpm for 5 seconds. For the 1-butanol solutions, the polymer solutions
were stirred at 65 °C overnight (before using) and kept at 65 °C immediately before spin-
coating. The polymer-butanol solutions were spin-coated at 700 rpm for 60 seconds,
followed by a wicking-step of 3000 rpm for 5 seconds. For ethanol (EtOH) solutions, the
polymer solution was stirred at 45 °C overnight and before spin-coating. The ethanol
solutions were spin-coated at a rate of 2000 rpm for 60 seconds. Thicknesses for the
polymer layers are provided below in Table A.3. All polymer solutions were filtered
through a 0.45 μm PTFE filter before spin-coating, and were placed in a nitrogen dry-box
for 25 minutes after spin-coating to ensure drying. Evaporation of aluminum was
performed under high vacuum (2×10
-6
torr) at a rate of 3-5 Å per second to provide a 100
nm thick Al-layer.
237
Table A.3 SCLC-Hole Mobility Data
Entr
y
Poly
mer
a
Solvent
b
Thickness
(nm)
μh, avg (cm
2
/(V·s)) μh, max
(cm
2
/(V·s))
1
c
P1 DCB 30.1 4.74 × 10
-6
± 2.81 × 10
-6
1.03 × 10
-5
2
d
P2 DCB 29.0 5.42 × 10
-6
± 1.64 × 10
-6
8.82 × 10
-6
3
e
P1 1-Butanol 53.6 8.48 × 10
-6
± 8.18 × 10
-6
2.74 × 10
-5
4 P1 EtOH 20.0 - -
5 P1 EtOH
f
34.2 - -
a
P1 was Entry 5 and P2 was Entry 9 of Table 2.1.
b
Concentration was 7 mg/mL, unless
otherwise noted.
c
Average of 7 pixels.
d
Average of 10 pixels.
e
Average of 17 pixels.
f
Concentration of 15 mg/mL.
A.10 Cyclic Voltammetry (CV) measurements.
Figure A.27 Cyclic Voltammetry (CV) measurement of P1 (Table 2.1, Entry 5).
Determined by cyclic voltammetry (vs. Fc/Fc
+
) in 0.1 M TBAPF5 in acetonitrile solution.
238
Figure A.28 Cyclic Voltammetry (CV) measurement of P2 (Table 2.1, Entry 9).
Determined by cyclic voltammetry (vs. Fc/Fc
+
) in 0.1 M TBAPF5 in acetonitrile solution.
239
A.11 GPC Traces
Figure A.29 Gel Permeation Chromatography (GPC) trace of P1 (Table 2.1, Entry 2).
Determined as Mn = 10.4 kDa, PDI = 1.7.
Figure A.30 Gel Permeation Chromatography (GPC) trace of P1 (Table 2.1, Entry 3).
Determined as Mn = 8.1 kDa, PDI = 1.6.
240
Figure A.31 Gel Permeation Chromatography (GPC) trace of P1 (Table 2.1, Entry 4).
Determined as Mn = 8.3 kDa, PDI = 1.5.
Figure A.32 Gel Permeation Chromatography (GPC) trace of P1 (Table 2.1, Entry 5).
Determined as Mn = 15.4 kDa, PDI = 1.5.
241
Figure A.33 Gel Permeation Chromatography (GPC) trace of P1 (Table 2.1, Entry 6).
Determined as Mn = 10.8 kDa, PDI = 2.0.
Figure A.34 Gel Permeation Chromatography (GPC) trace of P1 (Table 2.1, Entry 7).
Determined as Mn = 13.5 kDa, PDI = 1.5.
242
Figure A.35 Gel Permeation Chromatography (GPC) trace of P2 (Table 2.1, Entry 8).
Determined as Mn = 7.9 kDa, PDI = 1.5.
Figure A.36 Gel Permeation Chromatography (GPC) trace of P2 (Table 2.1, Entry 9).
Determined as Mn = 11.6 kDa, PDI = 1.9.
243
A.12 DSC Traces
Figure A.37 DSC trace of P1 (Table 2.1, Entry 5). No Tm, Tc were found.
Figure A.38 DSC trace of P2 (Table 2.1, Entry 9). No Tm, Tc were found.
244
A.13 GIXRD Spectra
Figure A.39 GIXRD diffraction pattern of P1 (Table 2.1, Entry 5). No diffraction peak
found.
Figure A.40 GIXRD diffraction pattern of P2 (Table 2.1, Entry 9). No diffraction peak
found.
245
A.14 References
(1) A. E. Rudenko and B. C. Thompson, Journal of Polymer Science Part A: Polymer
Chemistry, 2015, 53, 2494–2500.
(2) R. Heuvel, F. J. M. Colberts, M. M. Wienk and R. A. J. Janssen, J. Mater. Chem. C,
2018, 6, 3731–3742.
(3) A. Kokil, K. Yang and J. Kumar, J. Polym. Sci. B Polym. Phys., 2012, 50, 1130–
1144.
(4) A. Håkansson, S. Han, S. Wang, J. Lu, S. Braun, M. Fahlman, M. Berggren, X.
Crispin and S. Fabiano, Journal of Polymer Science Part B: Polymer Physics, 2017,
55, 814–820.
246
Appendix B
Chapter 3: Synthesis of Conjugated Polymers using Aryl-Bromides via Cu-
Catalyzed Direct Arylation Polymerization (Cu-DArP)
B.1. General
All reactions were performed under dry N2 in oven dried glassware, unless
otherwise noted. Unless otherwise noted, all reagents were purchased and used as received
from commercial sources through VWR. Solvents were purchased from VWR and used
without purification, unless otherwise noted. Cu(I) iodide (99.999%-Cu) PURATREM was
purchased from Strem Chemicals and used as received. 2,2',3,3',5,5',6,6'-
Octafluorobiphenyl (2) was purchased from TCI and used as received. 5-(2-ethylhexyl)-
4H-Thieno[3,4-c]pyrrole-4,6(5H)-dione and 5-(2-decyltetradecyl)-4H-Thieno[3,4-
c]pyrrole-4,6(5H)-dione, and 5,5'-dibromo-2,2'-bithiophene (5) were prepared previously
following reported procedures.
1,2
K3PO4 and K2CO3 was ground into a fine powder and
dried at 120 °C in a vacuum oven before use. Anhydrous N,N-dimethylacetamide (DMA)
was purchased from Acros Organics and used as received. m-xylene and o-xylene were
dried over CaH2 and distilled onto activated molecular sieves (4 Å) prior to use.
All
1
H NMR were recorded at 25 °C using CDCl3 on either a Varian Mercury 400
MHz, Varian VNMRS-500 MHz, or a Varian VNMR-600 MHz. All spectra were
referenced to CHCl3 (7.26 ppm), unless otherwise noted. Number average molecular
weight (Mn) and polydispersity (Ð) were determined by size exclusion chromatography
247
(SEC) using a Viscotek GPC Max VE 2001 separation module and a Viscotek Model 2501
UV detector, with 70 °C HPLC grade 1,2-dichlorobenzene (o-DCB) as eluent at a flow rate
of 0.6 mL/min on one 300 × 7.8 mm TSK-Gel GMHHR-H column (Tosoh Corp). The
instrument was calibrated vs. polystyrene standards (1050−3,800,000 g/mol), and data
were analysed using OmniSec 4.6.0 software. Polymer samples were dissolved in HPLC
grade o-dichlorobenzene at a concentration of 0.5 mg ml
−1
, stirred at 110-120 °C until
dissolved, cooled to room temperature, and filtered through a 0.2 μm PTFE filter.
B.2 Additional optimization of Cu-DArP conditions and polymerization results
Table B.1 Additional polymerization results for the synthesis of P1 (PDOF-OD) using Cu-
DArP.
a
99.999%-Puratrem Cu(I) iodide was used as the copper source.
b
Ligand loadings were 1:1 ratio
to Cu(I)
c
All polymerizations were conducted using 4 equivalence of base.
d
N,N-
dimethylacetamide = DMA. All polymerizations were conducted in 0.5 M concentration at 140 °C.
e
Determined for polymer products after purification.
Entry Cat.
Mol
a
%
Ligand
b
Base
c
Solvent
d
Time
(hr.)
M n (kDa)
e
,
Ð
e
Yield
e
(%)
1 50 1, 10-
Phananthroline
K 3PO 4 DMA/m-
xylene (1:1)
16 22.0, 2.1 67
2 5 1, 10-
Phananthroline
K 3PO 4 DMA/m-
xylene (1:1)
72 - 0
248
Table B.2 Optimization of Cu-DArP conditions for the synthesis of TPD-copolymers.
a
Abbreviation of polymers are referred to Scheme 3.3.
b
99.999%-Puratrem Cu(I) iodide was used
as the copper source. Ligand (Phanthroline) loadings were 1:1 ratio to Cu(I) iodide
c
All
polymerizations were conducted at 140 °C.
d
All polymerizations were conducted using 4
equivalence of base.
e
Determined for polymer products after purification.
Detailed optimization process for the synthesis of TPD copolymers (Table B.2):
As shown in Table B.2 and Scheme 3.3, we initially focused on extending the
optimized polymerization condition from our first report to this study with aryl-bromide 1.
This did not provide any reaction (Table B.2, Entry 1), which indicates a much lower
reactivity of aryl-bromides in Cu-DArP of TPD-copolymers, and is consistent with our
observation from the synthesis of P1 (Table 3.1, Entry 1). Based on our study on the
synthesis of P1 (PDOF-OD) (Table 3.1), we concluded that the DMA/m-xylene (1:1) co-
solvent system is critical for Cu-DArP of aryl-bromides, therefore, we applied this
methodology to the Cu-DArP of TPD-copolymers (Table B.2, Entry 2), which led to P2
of Mn = 2.6 kDa, yield = 32%. Although an improved reaction outcome was obtained, the
Entry Polymer
a
Cat. Mol
b
% Solvent
c
Concentr
ation (M)
Base
d
Mn (kDa)
e
,
Ð
e
Yield
e
(%)
1 P2 50 DMA 0.1 K2CO3 - 0
2 P2 50 DMA/m-
xylene (1:1)
0.1 K2CO3 2.6, 1.6 32
3 P2 50 DMA/m-
xylene (1:1)
0.4 K2CO3 5.1, 1.6 46
4 P2 50 DMA/m-
xylene (1:1)
0.4 K3PO4 - 0
5 P3 50 DMA/m-
xylene (1:1)
0.4 K2CO3 8.5, 1.8 61
6 P4 50 DMA/m-
xylene (1:1)
0.4 K2CO3 10.4, 1.7 72
249
low Mn and yield suggested that further optimization was required. Our previous study
employed a diluted concentration (0.1 M) due to low solubilities of TPD-copolymers in
polar amide solvent (DMA), however, conditions with higher concentration (>0.1 M) are
much more favorable in step-growth polymerizations and are often utilized in DArP
protocols.
2
Traditional DArP or Stille methodologies for the synthesis of TPD-copolymers
are often conducted in non-polar solvents such as toluene, and with the introduction of a
non-polar solvent (m-xylene), a much higher concentration (0.4 M) was employed for the
synthesis of P2 (Table B.2, Entry 3), which improved the Mn (5.1 kDa) and yield (46%).
Replacement of K2CO3 with a stronger base K3PO4 (Table B.2, Entry 4) completely
prohibited the reaction.
Replacing octyl chains of the aryl-bromide 1 with hexyl substituted fluorene 3
effectively improved Mn (8.5 kDa) and yield (61%) (Table B.2, Entry 5), and we postulate
that hexyl side chains less sterically-hinder the copper catalyst center as a similar trend was
found in our previous report.
1
To increase the solubility of polymer in the more
concentrated solution, the 2-decyltetradecyl side chain was chosen to replace the 2-
ethylhexyl chain attached to the TPD monomer, while keeping hexyl substituted fluorene
3 as the aryl-halide. This provides P4 with high Mn of 10.4 kDa and good yield 72% (Table
B.2, Entry 6), which is the highest Mn reported for TPD co-polymers synthesized via Cu-
DArP.
250
B.3 Monomer Synthesis
9,9-Bis(octyl)-2,7-dibromofluorene (1). To a 100 mL 3-neck round-bottomed flask
equipped with a stir bar under N2 atmosphere, 2,7-dibromofluorene (1.0 equiv., 12.35
mmol, 4 g), KOH (6.0 equiv., 74.1 mmol, 4.16 g) were added and the 3-neck flask was
vacuum backfilled three times, followed by addition of DMSO (30 mL) and n-
octylbromide (3 equiv., 37.04 mmol, 7.15 g). The reaction was allowed to stir overnight at
80 °C. The reaction mixture was cooled down to room temperature, H2O was then added
(30 mL) and the mixture was extracted with hexanes three times. The organic layers were
then washed with water, brine, and dried with MgSO4. Purification was performed by
column chromatography using hexanes as the eluent, and the solid was further purified by
recrystallization using EtOH to obtain as a white solid
(6.19 g, 87%).
1
H NMR (500 MHz,
Chloroform-d) δ 7.52 (d, J = 7.9 Hz, 2H), 7.45 (d, J = 8.0 Hz, 4H), 1.93 – 1.87 (m, 4H),
1.25 – 1.04 (m, 20H), 0.87 – 0.80 (m, 6H), 0.63 – 0.55 (m, 4H). Consistent with literature
report.
3
Br
Br
Oct Oct
Br
Br
KOH, DMSO
80 °C
n-octylbromide
1
Br
Br
Hex Hex
Br
Br
KOH, DMSO
80 °C
n-hexylbromide
3
251
9,9-Bis(hexyl)-2,7-dibromofluorene (3). To a 50 mL 3-neck round-bottomed flask
equipped with a stir bar under N2 atmosphere, 2,7-dibromofluorene (1.0 equiv., 6.18 mmol,
2 g), KOH (6.0 equiv., 37.1 mmol, 2.08 g) were added and the 3-neck flask was vacuum
backfilled three times, followed by addition of DMSO (20 mL) and n-hexylbromide (3
equiv., 18.52 mmol, 3.06 g). The reaction was allowed to stir overnight at 80 °C. The
reaction mixture was cooled down to room temperature, H2O was then added (20 mL) and
the mixture was extracted with hexanes three times. The organic layers were then washed
with water, brine, and dried with MgSO4. Purification was performed by column
chromatography using hexanes as the eluent, and the solid was further purified by
recrystallization using EtOH to obtain as a white solid (2.89 g, 90%).
1
H NMR (500 MHz,
Chloroform-d) δ 7.52 (d, J = 7.9 Hz, 2H), 7.46 (d, J = 8.0 Hz, 4H), 1.91 (m, 4H), 1.05 (m,
12H), 0.78 (m, 6H), 0.58 (m, 4H). Consistent with literature report.
4
2,5-dibromothiophene (4). To a 50 mL 3-neck round-bottomed flask equipped with a stir
bar under under N2 atmosphere, thiophene (1.0 equiv., 17.8 mmol, 1.5 g) and anhydrous
DMF (20 mL) was added, followed by addition of N-bromosuccinimide (2.0 equiv., 36.5
mmol, 6.5 g) under protection from light. The mixture was allowed to stir at room
temperature for 16 hours, and was poured in H2O and extracted with Et2O three times. The
organic layers were then washed with water, brine, and dried with MgSO4. The solvent was
S
Br
Br
S NBS, DMF
RT, 16 h
4
252
then removed under reduced pressured, and the crude product was purified by column
chromatography using hexanes as the eluent. The product was then further purified via
vacuum distillation to afford a colorless liquid (3 g, 70%).
1
H NMR (500 MHz,
Chloroform-d) δ 6.84 (s, 2H). Consistent with literature report.
5
B.4 Representative polymerization procedures
For P1 (PDOF-OD) (Table 3.1, Entry 6):
An oven-dried 15 mL high-pressure vessel equipped with a stir-bar was stoppered
with a rubber-septum and cooled under a flow N2. A stock solution of N,N-
dimethylacetamide (DMA)/m-xylene (1:1) (0.25 mL DMA + 0.25 m-xylene) was mixed
in a 10 mL 1-neck round-bottom-flask and degassed for 10 minutes. 9,9-bis(octyl)-2,7-
dibromofluorene (1) (0.125 mmol, 1 equiv.), 2,2',3,3',5,5',6,6'-Octafluorobiphenyl (0.125
mmol, 1 equiv.), K3PO4 (4 equiv.), and phenanthroline (0.15 equiv) were added. The vessel
was further sparged with N2 (5 min.) followed by addition of DMA/m-xylene (1:1) stock
solution (0.5 mL). CuI (0.15 equiv) was quickly added and the vessel sealed with a Teflon
screw-cap. The vessel was then stirred at room temperature for 5 minutes. It was then
submerged in a pre-heated oil bath with moderate stirring for 16 hours. The reaction was
then cooled to room temperature, solids were dissolved in hot dichlorobenzene (3-5 mL),
and the mixture was precipitated into a cold 10% (v:v) NH4OH/methanol solution with
high-stirring (100 mL). The grey precipitate was then collected via filtration, and it was
washed with water, methanol, acetone, and hexanes several times yielding a brown/tan
solid. The polymer was collected and dissolved in hot 1,2-dichlorobenzene (3-5 mL),
253
precipitated into cold MeOH (100 mL), and filtered to yield an off-white/grey solid. It was
then dried under high-vacuum overnight.
poly[(9,9-dioctylfluorene-2,7-diyl)-(2,2′,3,3′,5,5′,6,6′-octafluoro-4,4′-diphenylene)]
(PDOF-OD) (P1).
1
HNMR 500 MHz (CDCl3): δ (ppm) 7.94 (br, 2H), 7.59 (br, 4H), 2.06
(br, 4H), 1.15 (br, 20H), 0.83 (br, 10H). Consistent with literature report.
6
For TPD-copolymers (P3) (Table B.2, Entry 5):
An oven-dried 15 mL high-pressure vessel equipped with a stir-bar was stoppered
with a rubber-septum and cooled under a flow N2. A stock solution of N,N-
dimethylacetamide (DMA)/m-xylene (1:1) (0.32 mL DMA + 0.32 m-xylene) was mixed
in a 10 mL 1-neck round-bottom-flask and degassed for 10 minutes. 9,9-bis(hexyl)-2,7-
dibromofluorene (0.125 mmol, 1 equiv.), 5-(2-ethylhexyl)-4H-Thieno[3,4-c]pyrrole-
4,6(5H)-dione (0.125 mmol, 1 equiv.), K2CO3 (4 equiv.), and phenanthroline (0.5 equiv)
were added. The vessel was further sparged with N2 (5 min.) followed by addition of
DMA/m-xylene (1:1) stock solution (0.63 mL) to yield a 0.4 M concentration solution. CuI
(0.5 equiv) was quickly added and the vessel sealed with a Teflon screw-cap. The vessel
was then stirred at room temperature for 5 minutes. It was then submerged in a pre-heated
oil bath with moderate stirring for 72 hours. The reaction was then cooled to room
temperature, and was precipitated into a cold 10% (v:v) NH4OH/methanol solution with
high-stirring (100 mL). The yellow precipitate was then collected via filtration, and it was
washed with water, methanol, acetone, and hexanes several times yielding a yellow solid.
254
The polymer was collected and dissolved in chloroform, concentrated down to ~1 mL, and
re-precipitated into cold MeOH (100 mL), and filtered to yield a yellow solid. It was then
dried under high-vacuum overnight.
For TPD-copolymers (P4) (Table 3.1, Entry 8):
An oven-dried 15 mL high-pressure vessel equipped with a stir-bar was stoppered
with a rubber-septum and cooled under a flow N2. A stock solution of N,N-
dimethylacetamide (DMA)/m-xylene (1:1) (0.32 mL DMA + 0.32 m-xylene) was mixed
in a 10 mL 1-neck round-bottom-flask and degassed for 10 minutes. 9,9-bis(octyl)-2,7-
dibromofluorene (0.125 mmol, 1 equiv.), 5-(2-decyltetradecyl)-4H-Thieno[3,4-c]pyrrole-
4,6(5H)-dione (0.125 mmol, 1 equiv.), K2CO3 (4 equiv.), and phenanthroline (0.15 equiv)
were added. The vessel was further sparged with N2 (5 min.) followed by addition of
DMA/m-xylene (1:1) stock solution (0.63 mL) to yield a 0.4 M concentration solution. CuI
(0.15 equiv) was quickly added and the vessel sealed with a Teflon screw-cap. The vessel
was then stirred at room temperature for 5 minutes. It was then submerged in a pre-heated
oil bath with moderate stirring for 72 hours. The reaction was then cooled to room
temperature, and was precipitated into a cold 10% (v:v) NH4OH/methanol solution with
high-stirring (100 mL). The yellow precipitate was then collected via filtration, and it was
washed with water, methanol, and acetone several times yielding a yellow solid. The
polymer was collected and dissolved in chloroform, concentrated down to ~1 mL, and re-
precipitated into cold MeOH (100 mL), and filtered to yield a yellow solid. It was then
dried under high-vacuum overnight.
255
(Note: For P4, hexanes was not used for polymer washing because of high
solubility of P4 in hexanes, even when Mn is high (presumably due to 2-
decyltetradecyl chain on TPD monomer). P2, P3, P5, P6, however, were all collected
after washing with hexanes during purifications.)
poly[(5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione-1,3-diyl)-alt-(9,9-
dioctylfluorene2,7-diyl)] (P2)
Yellow solid.
1
HNMR 500 MHz (CDCl3): δ (ppm) 8.26-8.24 (m, 4H), 7.86 (d, 2H, J = 7.8
Hz), 3.65 (b, 2H), 2.17 (b, 4H), 1.93 (b, 1H), 1.56-1.34 (b, 8H), 1.20-1.12 (b, 20H), 0.98-
0.92 (b, 6H), 0.82-0.68 (b, 10H).
poly[(5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione-1,3-diyl)-alt-(9,9-
dihexylfluorene02,7-diyl)] (P3).
Oct Oct
N
S
O
O
n
C
2
H
5
C
4
H
9
Hex Hex
N
S
O
O
n
C
2
H
5
C
4
H
9
256
Yellow solid.
1
HNMR 500 MHz (CDCl3): δ (ppm) 8.27-8.23 (m, 4 H), 7.86 (d, 2H, J = 7.8
Hz), 3.65 (b, 2H), 2.17 (b, 4H), 1.92 (b, 1H), 1.54-1.34 (b, 8H), 1.14 (b, 12H), 0.98-0.93
(b, 6H), 0.78 (b, 10H).
poly[(5-(2-decyltetradecyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione-1,3-diyl)-alt-(9,9-
dihexylfluorene02,7-diyl)] (P4).
Dark yellow solid.
1
HNMR 500 MHz (CDCl3): δ (ppm) 8.28-8.21 (b, 4H), 7.786 (b, 2H),
3.62 (b, 2H), 2.16 (b, 4H), 1.97 (b, 1H), 1.41-1.10 (b, 52H), 0.87 (b, 6H), 0.79-0.69 (b,
10H).
poly[(5-(2-decyltetradecyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione-1,3-diyl)-alt-
(thiphene)] (P5)
Hex Hex
N
S
O
O
n
C
12
H
25
C
10
H
21
S
N
S
O
O
n
C
12
H
25
C
10
H
21
257
Dark purple solid.
1
HNMR 500 MHz (CDCl3): δ (ppm) 8.03 (b, 2H), 3.56 (b, 2H), 1.91 (b,
1H), 1.25 (b, 40 H), 0.86 (b, 6H).
poly[(5-(2-decyltetradecyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione-1,3-diyl)-alt-(5,5'-
dibromo-2,2'-bithiophene)] (P6)
Dark purple solid.
1
HNMR 500 MHz (CDCl3): δ (ppm) 7.94 (b, 2H), 7.05 (b, 2H), 3.31 (b,
2H), 1.55 (b, 1H), 1.25 (b, 40 H), 0.86 (b, 6H)
N
S
O
O
S
S
n
C
12
H
25
C
10
H
21
258
B.5 NMR of Monomers
Figure B.1
1
H NMR of 9,9-bis(octyl)-2,7-dibromofluorene (1). Collected in CDCl3 at 25
°C and 500 MHz. Referenced to previous reports.
3
259
Figure B.2
1
H NMR of 9,9-bis(hexyl)-2,7-dibromofluorene (3). Collected in CDCl3 at 25
°C and 500 MHz. Referenced to previous reports.
4
Figure B.3
1
H NMR of 2,5-dibromothiophene (4). Collected in CDCl3 at 25 °C and 500
MHz. Referenced to previous reports.
5
260
B.6 Polymer NMR
Figure B.4
1
H NMR of P1 (PDOF-OD) synthesized using the conditions in Table 3.1
(Entry 6). Collected in CDCl3 at 25 °C and 500 MHz. Referenced to previous reports.
6
Figure B.5
1
H NMR of P1 (PDOF-OD) synthesized using the conditions in Table B.1
(Entry 1). Collected in CDCl3 at 25 °C and 500 MHz. Referenced to previous reports.
6
261
Figure B.6
1
HNMR (500 MHz) of P2 (Table B.2, Entry 2) in CDCl3 at 25 °C. End groups
denoted with *.
Figure B.7
1
HNMR (500 MHz) of P2 (Table B.2, Entry 3) in CDCl3 at 25 °C. End groups
denoted with *.
262
Figure B.8
1
HNMR (500 MHz) of P3 (Table B.2, Entry 5) in CDCl3 at 25 °C. End groups
denoted with *.
Figure B.9
1
HNMR (500 MHz) of P4 (Table B.2, Entry 6) in CDCl3 at 25 °C. End groups
denoted with *.
263
Figure B.10
1
HNMR (500 MHz) of P4 (Table 3.1, Entry 8) in CDCl3 at 25 °C. End groups
denoted with *.
Figure B.11
1
HNMR (500 MHz) of P5 (Table 3.1, Entry 9) in CDCl3 at 25 °C. End groups
denoted with *.
264
Figure B.12
1
HNMR (500 MHz) of P6 (Table 3.1, Entry 10) in CDCl3 at 25 °C. End
groups denoted with *.
B.7 References
(1) Pankow, R. M.; Ye, L.; Thompson, B. C. Copper Catalyzed Synthesis of Conjugated
Copolymers Using Direct Arylation Polymerization. Polym. Chem. 2018, 9 (30),
4120–4124.
(2) Pankow, R. M.; Ye, L.; Thompson, B. C. Influence of an Ester Directing-Group on
Defect Formation in the Synthesis of Conjugated Polymers via Direct Arylation
265
Polymerization (DArP) Using Sustainable Solvents. Polym. Chem. 2019, 10 (33),
4561–4572.
(3) Peng, F.; Zhong, Z.; Ma, Y.; Huang, Z.; Ying, L.; Xiong, J.; Wang, S.; Li, X.; Peng,
J.; Cao, Y. Achieving Highly Efficient Blue Light-Emitting Polymers by
Incorporating a Styrylarylene Amine Unit. J. Mater. Chem. C 2018, 6 (45), 12355–
12363. https://doi.org/10.1039/C8TC04411B.
(4) Voortman, T. P.; Chiechi, R. C. Thin Films Formed from Conjugated Polymers with
Ionic, Water-Soluble Backbones. ACS Appl. Mater. Interfaces 2015, 7 (51), 28006–
28012.
(5) Ozdemir, M.; Choi, D.; Kwon, G.; Zorlu, Y.; Cosut, B.; Kim, H.; Facchetti, A.; Kim,
C.; Usta, H. Solution-Processable BODIPY-Based Small Molecules for
Semiconducting Microfibers in Organic Thin-Film Transistors. ACS Appl. Mater.
Interfaces 2016, 8 (22), 14077–14087.
(6) Pankow, R. M.; Ye, L.; Thompson, B. C. Sustainable Synthesis of a Fluorinated
Arylene Conjugated Polymer via Cu-Catalyzed Direct Arylation Polymerization
(DArP). ACS Macro Lett. 2018, 7 (10), 1232–1236.
266
Appendix C
Chapter 4: An Efficient Precatalyst Approach for the Synthesis of Thiazole-
Containing Conjugated Polymers via Cu-Catalyzed Direct Arylation Polymerization
(Cu-DArP)
C.1 General
All reactions were performed under dry N2 in oven dried glassware, unless
otherwise noted. Unless otherwise noted, all reagents were purchased and used as received
from commercial sources through VWR. Solvents were purchased from VWR and used
without purification, unless otherwise noted. Bromotris(triphenylphosphine)copper(I)
(Cu(PPh3)3Br) (98%) and 1,10-Phenanthroline (phen) (>99%) were purchased from
Sigma-Aldrich and used as received. 5-Bromothiazole and Thiazole were purchased from
ChemScene and used as received. 2,2’-Bithiazole (2-BTz) and 2,7-Dibromo-9-(9-
heptadecyl)carbazole were prepared previously following reported procedures.
1,2
K2CO3
was ground into a fine powder and dried at 120 °C in a vacuum oven before use. Anhydrous
N,N-dimethylacetamide (DMA), anhydrous cyclopentyl methyl ether (CPME) and
anhydrous m-xylene was purchased from Acros Organics and used as received. Anhydrous
tetrahydrofuran (THF) was purchased from EMD Millipore and used as received.
All monomer and Cu-precatalyst NMR were recorded at 25 °C using CDCl3 on
either a Varian Mercury 400 MHz, Varian VNMRS-500 MHz. Polymer NMR was
obtained on a Varian VNMR-500 MHz. All spectra were referenced to CHCl3 (7.26 ppm),
unless otherwise noted.
267
Number average molecular weight (Mn) and polydispersity (Ð) were determined by
two GPC instruments: (1) Size exclusion chromatography (SEC) using a DAWN
HELEOS-II GPC (Wyatt Technology Corp.) WH2-13 model and a Optilab T-rEX Model
WTREX-14 RI detector, with 30 °C HPLC grade tetrahydrofuran (THF) as eluent at a flow
rate of 0.3 mL/min on one 250 × 4.6 mm Agilent Polypore Column. The instrument was
calibrated vs. polystyrene standards (1050−3,800,000 g/mol), and data were analysed using
ASTRA V 7.1.4.8 software. Polymer samples were dissolved in HPLC grade THF at a
concentration of 0.5 mg/mL, stirred until dissolved, and filtered through a 0.2 μm PTFE
filter. (2) Waters Alliance HPLC system with an e2695 separation module using an Agilent
PLgel 5 μm MiniMIX-D column at 35 °C with THF as the eluent. Refractive index traces
from a Waters 2414 differential refractive index detector were used for molecular weight
determination using polystyrene calibration standards (Agilent Technologies).
C.2 Monomer Synthesis
9,9-Bis(2-ethylhexyl)-2,7-dibromofluorene. To a 100 mL 3-neck round-bottomed flask
equipped with a stir bar under N2 atmosphere, KOH (5.19 g, 92.56 mmol), KI (153.70 mg,
0.926 mmol), 2,7-dibromofluorene (3.00 g, 9.26 mmol), and 2-ethylhexylbromide (7.15 g,
37.04 mmol) were added. The reaction was allowed to stir overnight at 40 °C. The reaction
mixture was cooled down to room temperature, cold H2O was then added (250 mL) and
the mixture was extracted with EtOAc. The organic layers were then washed with water,
Br
Br
EH EH
Br
Br
KOH, KI, DMSO
40 °C
2-ethylhexylbromide
268
brine, and dried with MgSO4. Purification was performed by column chromatography with
hexanes as the eluent affording the desired product as a light yellow oil in 83% yield (4.24
g).
1
H-NMR (CDCl3, 25 °C, 500 MHz): δ 7.51 (d, 2H), 7.49 (m, 2H), 7.45 (dd, 2H), 1.94
(m, 4H), 1.25 (m, 2H), 0.88 (m, 22H), 0.52 (m, 6H). Consistent with literature report.
3
5-Tributylstannylthiazole. Procedure was adapted from the literature.
4
Lithium
diisopropylamine (LDA) solution was prepared by adding n-butyllithium (1.6 M in
hexanes, 37.5 mL, 60 mmol, 2.2 eqv.) dropwise to a solution of distilled diisopropylamine
(9.1 mL, 65 mmol) dissolved in 120 mL of dry THF at 0 °C under a nitrogen atmosphere.
The reaction was stirred at 0 °C for 15 min and was then cooled to -78 °C. Tributyltin
chloride (9.8 g, 30 mmol, 1.1 eqv.) dissolved in 20 mL of dry THF was slowly added to
the solution, and the reaction was stirred for another 15 min. Thiazole (2.3 g, 27 mmol, 1
eqv.) dissolved in 20 mL of dry THF was then added dropwise at -78 °C. The resulting
solution was allowed to stir overnight at room temperature. Water was then added and the
mixture was extracted with ethyl acetate. The organic phase was washed with water and
dried over MgSO4. Ethyl acetate was removed under reduced pressure, and the crude
product (dark orange/red oil) was used for the next step without further purification.
1
H
NMR (CDCl3, 25 °C, 500 MHz) δ = 9.09 (s, 1H), 7.88 (s, 1H), 1.60-1.49 (m, 6H), 1.40-
1.26 (m, 6H), 1.18-1.10 (m, 6H), 0.93-0.85 (m, 9H). Consistent with literature report.
4
N
S LDA (2.2 eqv.), THF
-78 °C
SnBu
3
Cl (1.1 eqv.) N
S
SnBu
3
1 eqv.
269
5,5’-Bithiazole (5-BTz). To a 100 mL 3-neck round-bottomed flask equipped with a stir
bar under N2 atmosphere, 5-Bromothiazole (1.0 equiv., 6.7 mmol, 1.1 g), 5-
Tributylstannylthiazole (1.0 equiv., 6.7 mmol, 2.5 g) and anhydrous toluene (30 mL) was
added. The mixture was degassed with a flow of nitrogen for 15 minutes, followed by the
addition of Pd(PPh3)4 (0.05 equiv., 0.31 mmol, 0.35 g). The mixture was further degassed
for 10 minutes, and was heated to reflux for overnight. After cooling to room temperature,
toluene was removed under reduced pressure. The mixture was purified by column
chromatography using ethyl acetate as the eluent to afford a yellow solid (1.03 g, 91.4%).
1
H NMR (CDCl3, 25 °C, 500 MHz) δ 8.79 (s, 1H), 8.01 (s, 1H). Consistent with literature
report.
5
C.3 Cu-precatalyst Synthesis
Cu(phen)(PPh3)Br. Procedure was adapted from the literature.
6
In an Erlenmeyer flask
equipped with a stir-bar, tris(triphenylphosphine)copper(I)bromide (Cu(PPh3)3Br) (1.40 g,
l.5 mmol) was added to chloroform (100 ml). After complete dissolution, 1,10-
phenanthroline (0.27 g, 1.5 mmol) was then added. The colorless solution immediately
changed to orange. The mixture was allowed to stir for 30 minutes at room temperature.
The solvent was then removed under reduced pressure (rotovap) to afford an orange-yellow
N
S
SnBu
3
N
S
Br +
reflux
Toluene
Pd(PPh
3
)
4
N
S
N
S
N N
Cu
Ph
3
P Br
1,10-Phenanthroline
Chloroform
Cu(PPh
3
)
3
Br
RT
270
solid. The crude product was recrystallized by dissolving the solid in 60 ml of
dichloromethane and layered with 20 ml of diethyl ether. After overnight of stirring under
nitrogen, yellow precipitate was collected by vacuum filtration (667 mg, 76.0%).
1
H NMR
(500 MHz, Chloroform-d): δ 7.67 (m, 2H), 7.47 (m, 11H), 7.33 (m, 5H), 7.28 (m, 3H), 7.25
(m, 2H).
Cu(neocup)(PPh3)Br. Similar to the synthesis of Cu(phen)(PPh3)Br, but with
Neocuproine instead of 1,10-phenanthroline. Yield: 59%.
1
H NMR (500 MHz,
Chloroform-d): δ 8.34 (br, 2H), 7.65 (s, 1H), 7.49 (d, J = 62.0 Hz, 8H), 7.30 (m, 2H), 7.23
(m, 6H), 1.76 (s, 6H).
Cu(Bipy)(PPh3)Br. Similar to the synthesis of Cu(phen)(PPh3)Br, but with 2,2’-bipyridine
instead of 1,10-phenanthroline. Yield: 67%.
1
H NMR (500 MHz, Chloroform-d): δ 7.70 –
7.65 (m, 1H), 7.44 (m, 9H), 7.34 (m, J = 7.4 Hz, 5H), 7.28 (d, J = 7.6 Hz, 8H).
Cu(phen)(PPh3)I. To a 50 mL 3-neck round-bottomed flask equipped with a stir bar under
N2 atmosphere, CuI (200 mg, l.05 mmol, 1.0 equiv.) was dissolved in 20 mL of CH3CN.
PPh3 (826.21mg, 3.15 mmol, 3.0 equiv.) was added to the solution and the mixture was
N N
Cu
Ph
3
P Br
Neocuproine
Chloroform
Cu(PPh
3
)
3
Br
RT
N N
Cu
Ph
3
P Br
2,2’-bipyridine
Chloroform
Cu(PPh
3
)
3
Br
RT
N N
Cu
Ph
3
P I
1,10-Phenanthroline
Cu(PPh
3
)
3
I
RT
PPh3 (3 eqv.)
RT
MeCN
CuI
271
allowed to stir at room temperature for 10 minutes. 1,10-phenanthroline (189.2 mg, 1.05
mmol, 1.0 equiv.) was then added. The colorless solution immediately changed to red. The
mixture was allowed to stir for 30 minutes at room temperature. Diethyl ether (20 mL) was
added and precipitate was collected via vacuum filtration. The crude product was
recrystallized by dissolving the solid in 60 ml of dichloromethane and layered with 20 ml
of diethyl ether. After overnight of stirring under nitrogen, orange/red precipitate was
collected by vacuum filtration (427.9 mg, 41.7%).
1
H NMR (500 MHz, Chloroform-d) δ
9.02 (s, 1H), 8.33 (s, 1H), 7.89 (s, 1H), 7.71 – 7.64 (m, 2H), 7.48 (m, 6H), 7.30 – 7.23 (m,
12H).
C.4 Representative polymerization procedures
For the synthesis of PF-5BTz using CuI, phen (Table 4.1, entry 3):
An oven-dried 15 mL high-pressure vessel equipped with a stir-bar was stoppered
with a rubber-septum and cooled under a flow N2. A stock solution of N,N-
dimethylacetamide (DMA)/m-xylene (1:1) (0.31 mL DMA + 0.31 m-xylene) was mixed
in a 10 mL 1-neck round-bottom-flask and degassed for 10 minutes. 9,9-bis(2-ethylhexyl)-
2,7-dibromofluorene (1) (0.125 mmol, 1 equiv.), 5-BTz (0.125 mmol, 1 equiv.), K2CO3 (4
equiv.), and phenanthroline (0.15 equiv.) were added. The vessel was further sparged with
N2 (5 min.) followed by addition of DMA/m-xylene (1:1) stock solution (0.625 mL). CuI
(0.15 equiv) was quickly added and the vessel sealed with a Teflon screw-cap. The vessel
was then stirred at room temperature for 5 minutes. It was then submerged in a pre-heated
oil bath with moderate stirring for 72 hours. The reaction was then cooled to room
temperature, solids were dissolved in chloroform (3-5 mL), and the mixture was
precipitated into a cold 10% (v:v) NH4OH/methanol solution with high-stirring (100 mL).
The yellow precipitate was then collected via filtration, and it was washed with water,
272
methanol, acetone, several times yielding a yellow solid. The polymer was then dried under
high-vacuum overnight.
For the synthesis of PF-5BTz using Cu-precatalyst (Cu(phen)(PPh3)Br) (Table 4.1,
entry 4, 9):
An oven-dried 15 mL high-pressure vessel equipped with a stir-bar was stoppered
with a rubber-septum and cooled under a flow N2. A stock solution of N,N-
dimethylacetamide (DMA)/m-xylene (1:1) (0.31 mL DMA + 0.31 m-xylene) was mixed
in a 10 mL 1-neck round-bottom-flask and degassed for 10 minutes. 9,9-bis(2-ethylhexyl)-
2,7-dibromofluorene (1) (0.125 mmol, 1 equiv.), 5-BTz (0.125 mmol, 1 equiv.), K2CO3 (4
equiv.), and Cu(phen)(PPh3)Br (0.15 or 0.05 equiv.) were added. The vessel was further
sparged with N2 (5 min.) followed by addition of DMA/m-xylene (1:1) stock solution
(0.625 mL). The vessel was quickly sealed with a Teflon screw-cap. It was then submerged
in a pre-heated oil bath with moderate stirring for 16 hours. The reaction was then cooled
to room temperature, solids were dissolved in chloroform (3-5 mL), and the mixture was
precipitated into a cold 10% (v:v) NH4OH/methanol solution with high-stirring (100 mL).
The yellow precipitate was then collected via filtration, and it was washed with water,
methanol, acetone, and hexanes several times yielding a yellow solid. The polymer was
then dried under high-vacuum overnight.
N
S
N
S
n
EH
EH
273
poly[(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl)-alt-(5,5’-bithiazole)] (PF-5BTz).
1
H
NMR (500 MHz, Chloroform-d): δ 8.03 (br, 6H), 7.84 (d, 2H), 2.15 (d, 4H), 0.88 (br, 18H),
0.60 (br, 12H).
7
poly[(N-9’-heptadecanyl-2,7-carbazole)-alt-(5,5’-bithiazole)] (PC-5BTz).
1
H
NMR (500 MHz, Chloroform-d): δ 8.07 (br, 4H), 7.74 (br, 4H), 2.39 (br, 1H), 2.07 (br,
2H), 1.25 (br, 23H), 0.84 (br, 9H).
For the failed attempt of the synthesis of PF-5BTz using Pd(OAc)2 in DMA (Table
4.1, entry 12):
To an oven dried 3-neck round bottom flask (15 mL) equipped with a stir-bar were
added 5,5’-bithiazole (16.82 mg, 0.10 mmol) and K2CO3 (41.46 mg, 0.30 mmol) and the
flask subsequently vacuum-backfilled three times. Neodecanoic acid (5.17 mg, 0.03
mmol), 9,9-bis(2-ethylhexyl)-2,7-dibromofluorene (54.84 mg, 0.10 mmol) and DMA (5.00
mL) were added and the resulting mixture was degassed (20 mins with nitrogen). Pd(OAc)2
(0.449 mg, 2.00 μmol) were added and degassing was continued for another 20 mins. The
rubber septum was then quickly replaced with a glass stopper and the mixture was placed
in a pre-heated oil bath (70 °C) for 16 hours. After cooling to room temperature, the
reaction mixture was precipitated into a cooled 10% NH4OH/MeOH solution with rapid
stirring but no precipitation was observed.
N
S
N
S
n
N
Oct
Oct
274
For the failed attempt of the synthesis of PF-5BTz using Pd2(dba)3 in THF (Table 4.1,
entry 13):
An oven dried high-pressure vessel (15 mL) equipped with a stir-bar was capped
with an inverted rubber septum and cooled under a stream of nitrogen. To this was added
5,5’-bithiazole (16.82 mg, 0.10 mmol), 9,9-bis(2-ethylhexyl)-2,7-dibromofluorene (54.84
mg, 0.10 mmol), neodecanoic acid (17.23 mg, 0.10 mmol), P(o-anisyl)3 (12.17 mg, 4.00
μmol), Pd2dba2 (0.914 mg, 1.00 μmol), and Cs2CO3 (97.75 mg, 0.30 mmol). The vessel
was then purged with a stream of nitrogen for 10 minutes, and then degassed (15 minutes
with nitrogen) THF (1.00 mL) was added via syringe. The rubber septum was then quickly
replaced with a Teflon screwcap equipped with a rubber o-ring, and the vessel was placed
in a pre-heated oil bath (120 °C) for 16 hours. After cooling to room temperature, the
reaction mixture was precipitated into a cooled 10% NH4OH/MeOH solution with rapid
stirring but no precipitation was observed.
For the failed attempt of the synthesis of PF-5BTz using PdCl2(PPh3)2 in CPME
(Table 4.1, entry 14):
An oven dried high-pressure vessel (15 mL) equipped with a stir-bar was capped
with an inverted rubber septum and cooled under a stream of nitrogen. To this was added
5,5’-bithiazole (16.82 mg, 0.10 mmol), 9,9-bis(2-ethylhexyl)-2,7-dibromofluorene (54.84
mg, 0.10 mmol), neodecanoic acid (17.23 mg, 0.10 mmol), P(o-anisyl)3 (12.17 mg, 4.00
μmol), PdCl2(PPh3)3 (1.40 mg, 2.00 μmol), and Cs2CO3 (97.75 mg, 0.30 mmol). The vessel
was then purged with a stream of nitrogen for 10 minutes, and then degassed (15 minutes
275
with nitrogen) CPME (1.00 mL) was added via syringe. The rubber septum was then
quickly replaced with a Teflon screwcap equipped with a rubber o-ring, and the vessel was
placed in a pre-heated oil bath (110 °C) for 16 hours. After cooling to room temperature,
the reaction mixture was precipitated into a cooled 10% NH4OH/MeOH solution with rapid
stirring but no precipitation was observed.
C.5 Monomer NMR
Figure C.1
1
H NMR of 9,9-bis(2-ethylhexyl)-2,7-dibromofluorene. Collected in CDCl3 at
25 °C and 500 MHz. Referenced to previous reports.
3
276
Figure C.2
1
H NMR of 5-Tributylstannylthiazole. Collected in CDCl3 at 25 °C and 500
MHz. Referenced to previous reports.
4
Figure C.3
1
H NMR of 5,5’-Bithiazole (5-BTz). Collected in CDCl3 at 25 °C and 500
MHz. Referenced to previous reports.
5
277
C.6 NMR of Cu-precatalysts
Figure C.4
1
H NMR of Cu(phen)(PPh3)Br. Collected in CDCl3 at 25 °C and 500 MHz.
Figure C.5
1
H NMR of Cu(neocup)(PPh3)Br. Collected in CDCl3 at 25 °C and 500 MHz.
278
Figure C.6
1
H NMR of Cu(bipy)(PPh3)Br. Collected in CDCl3 at 25 °C and 500 MHz.
Figure C.7
1
H NMR of Cu(phen)(PPh3)I. Collected in CDCl3 at 25 °C and 500 MHz.
279
C.7 Polymer NMR
Figure C.8
1
H NMR of PF-5BTz synthesized using the conditions in Table 4.1 (entry 3).
Collected in CDCl3 at 25 °C and 500 MHz. Referenced to previous reports.
7
Figure C.9
1
H NMR of PF-5BTz synthesized using the conditions in Table 4.1 (entry 4).
Collected in CDCl3 at 25 °C and 500 MHz. Referenced to previous reports.
7
280
Figure C.10
1
H NMR of PF-5BTz synthesized using the conditions in Table 4.1 (entry 5).
Collected in CDCl3 at 25 °C and 500 MHz. Referenced to previous reports.
7
Figure C.11
1
H NMR of PF-5BTz synthesized using the conditions in Table 4.1 (entry 9).
Collected in CDCl3 at 25 °C and 500 MHz. Referenced to previous reports.
7
281
Figure C.12
1
H NMR of PC-5BTz synthesized using the conditions in Scheme 4.2.
Collected in CDCl3 at 25 °C and 500 MHz.
282
C.8 Proposed end-group assignments
Figure C.13 Proposed end-group assignments based on the model compounds S1,
8
S2,
9
S3,
10
the
1
H NMR spectrum is that of Table 4.1 (entry 4), and was collected in CDCl3 at
500 MHz and 25 °C.
Note: All end groups or homo-coupling defects can be approximately assigned except for
end-groups a, b, c, which were shifted slightly upfield (from 7.36-7.35 ppm to 7.47-7.56
ppm). The reason can likely be attributed to less thiazole unit of S1 compared to the end
group of PF-5BTz, which has one more thiazole unit. The extra electron-deficient thiazole
unit likely enhances the deshielding effect relative to S1 with regards to end-groups a, b, c.
C.9 References
(1) Pankow, R. M.; Ye, L.; Thompson, B. C. Influence of an Ester Directing-Group on
Defect Formation in the Synthesis of Conjugated Polymers via Direct Arylation
283
Polymerization (DArP) Using Sustainable Solvents. Polymer Chemistry 2019, 10
(33), 4561–4572. https://doi.org/10.1039/C9PY00815B.
(2) Saeki, A.; Yoshikawa, S.; Tsuji, M.; Koizumi, Y.; Ide, M.; Vijayakumar, C.; Seki, S.
A Versatile Approach to Organic Photovoltaics Evaluation Using White Light Pulse
and Microwave Conductivity. Journal of the American Chemical Society 2012, 134
(46), 19035–19042. https://doi.org/10.1021/ja309524f.
(3) Kuwabara, J.; Tsuchida, W.; Guo, S.; Hu, Z.; Yasuda, T.; Kanbara, T. Synthesis of
Conjugated Polymers via Direct C–H/C–Cl Coupling Reactions Using a Pd/Cu
Binary Catalytic System. Polymer Chemistry 2019, 10 (18), 2298–2304.
https://doi.org/10.1039/C9PY00232D.
(4) Chávez, P.; Ngov, C.; Frémont, P. de; Lévêque, P.; Leclerc, N. Synthesis by Direct
Arylation of Thiazole–Derivatives: Regioisomer Configurations–Optical Properties
Relationship Investigation. The Journal of Organic Chemistry 2014, 79 (21), 10179–
10188.
(5) Usta, H.; Sheets, W. C.; Denti, M.; Generali, G.; Capelli, R.; Lu, S.; Yu, X.; Muccini,
M.; Facchetti, A. Perfluoroalkyl-Functionalized Thiazole–Thiophene Oligomers as
N-Channel Semiconductors in Organic Field-Effect and Light-Emitting Transistors.
Chemistry of Materials 2014, 26 (22), 6542–6556.
https://doi.org/10.1021/cm503203w.
(6) Gujadhur, R. K.; Bates, C. G.; Venkataraman, D. Formation of Aryl−Nitrogen,
Aryl−Oxygen, and Aryl−Carbon Bonds Using Well-Defined Copper(I)-Based
Catalysts. Org. Lett. 2001, 3 (26), 4315–4317.
284
(7) Guo, Q.; Wu, D.; You, J. Oxidative Direct Arylation Polymerization Using Oxygen
as the Sole Oxidant: Facile, Green Access to Bithiazole-Based Polymers.
ChemSusChem 2016, 9 (19), 2765–2768. https://doi.org/10.1002/cssc.201600827.
(8) Chau, N.-Y.; Ho, P.-Y.; Ho, C.-L.; Ma, D.; Wong, W.-Y. Color-Tunable Thiazole-
Based Iridium(III) Complexes: Synthesis, Characterization and Their OLED
Applications. Journal of Organometallic Chemistry 2017, 829, 92–100.
https://doi.org/10.1016/j.jorganchem.2016.11.018.
(9) Faradhiyani, A.; Zhang, Q.; Maruyama, K.; Kuwabara, J.; Yasuda, T.; Kanbara, T.
Synthesis of Bithiazole-Based Semiconducting Polymers via Cu-Catalysed Aerobic
Oxidative Coupling. Materials Chemistry Frontiers. 2018, 2 (7), 1306–1309.
https://doi.org/10.1039/C7QM00584A.
(10) Kuwabara, J.; Kuramochi, M.; Liu, S.; Yasuda, T.; Kanbara, T. Direct Arylation
Polycondensation for the Synthesis of Bithiazole-Based Conjugated Polymers and
Their Physical Properties. Polymer Journal. 2017, 49 (1), 123–131.
https://doi.org/10.1038/pj.2016.75.
285
Appendix D
Chapter 5: p-Cymene: A Sustainable Solvent that is Highly Compatible with Direct
Arylation Polymerization (DArP)
D.1 General
p-Cymene was purchased from Alfa Aesar (97%) and dried from CaH2 followed
by vacuum distillation to 3 Å sieves. Anhydrous cyclopentyl methyl ether (CPME) was
purchased from Acros Organics and used as received. Anhydrous toluene was purchased
from EMD Millipore and used as received. Pd(OAc)2 (Palldium(II) acetate trimer ≥
99.98%) (BeanTown Chemical), PdCl2(PPh3)2 (99.95%, BeanTown Chemical), tris(o-
methoxyphenylphosphine) (Alpha Aesar) were purchased and used as received. Cs2CO3
were ground into a fine powder and dried at 120 °C in a vacuum oven before use.
9,9-Dioctyl-2,7-dibromofluorene was purchased from Combi-Blocks (98%) and
recrystallized from EtOH to form a white solid prior to use. 2,2’-Bithiophene was
purchased from Matrix Scientific (97%) and subjected to a short column chromatography
followed by recrystallization from MeOH prior to use. 4,7-di-2-thienyl-2,1,3-
benzothiadiazole,
1
5-(2-ethylhexyl)-thieno-[3,4-c]-pyrrole-4,6-dione,
2
and 2,7-dibromo-9-
(9-heptadecyl)carbazole
3
were prepared previously following reported procedures.
Monomer NMR were recorded at 25 °C using CDCl3 on a Varian Mercury Varian
VNMRS-500 MHz. Polymer NMR was obtained on a Varian VNMR-600 MHz. All
286
spectra were referenced to CHCl3 (7.26 ppm) and C2D2Cl4 (6.03 ppm), unless otherwise
noted.
Number average molecular weight (Mn) and polydispersity (Ð) were determined by
size exclusion chromatography (SEC) using a Agilent 1260 Infinity II High Temperature
GPC and a Differential Refractive Index (DRI) detector, with 80 °C HPLC grade 1,2,4-
trichlorobenzene (TCB) as eluent at a flow rate of 1.0 mL/min. The instrument was
calibrated vs. polystyrene standards (1050−3,800,000 g/mol). Polymer samples were
dissolved in HPLC grade TCB at a concentration of 0.5 mg/mL, stirred until dissolved, and
filtered through a 0.2 μm PTFE filter.
For polymer thin-film measurements, solutions were spin-coated onto pre-cleaned
glass slides from o-dichlorobenzene (o-DCB) solutions at 7 mg/mL, which were then
annealed at 150 °C for 30 minutes under N2. UV−vis absorption spectra were obtained on
a Perkin-Elmer Lambda 950 spectrophotometer. Thicknesses of the samples and were
obtained via XRR using Rigaku diffractometer Ultima IV using a Cu Kα radiation source
(λ = 1.54 Å).
D.2 Monomer Synthesis and NMR
Dimethyl 3,4-thiophenedicarboxylate. To a 100 mL 1-neck round-bottomed flask
equipped with a stir bar, 3,4-thiophenedicarboxylic acid (1 g, 5.8 mmol), MeOH (30 mL)
S
O O
OH HO
H H
S
O O
O O
H H
H
2
SO
4
, MeOH
reflux
287
and H2SO4 (1 mL) were added. The mixture was allowed to reflux overnight under N2. The
reaction mixture was cooled down to room temperature, and the solvent was removed
under reduced pressure. Saturated NaHCO3 solution was slowly added, and was then
extracted with DCM three times. The combined organic layer was washed with water and
brine, dried with MgSO4, filtered and concentrated. Purification was performed by column
chromatography with hexanes/DCM (70:30) as the eluent affording the desired product as
a colorless oil in 80% yield (929 mg). The product slowly crystallized at room temperature
to become a white solid.
1
H-NMR (CDCl3, 25 °C, 500 MHz): δ 7.86 (s, 2H), 3.88 (s, 6H).
Consistent with literature report.
4
Figure D.1
1
H NMR of dimethyl 3,4-thiophenedicarboxylate. Collected in CDCl3 at 25 °C
and 500 MHz. Referenced to previous reports.
4
288
D.3 General Polymerization Procedure
For the synthesis of P1-P3 of Scheme 5.1:
An oven-dried 15 mL reaction vessel equipped with a stir-bar was stoppered with
a rubber-septum and cooled under a flow of N2. In a separate 25 mL 1-neck round-bottom-
flask, the appropriate solvent was added and degassed for 15 minutes. 9,9-dioctyl-2,7-
dibromofluorene (0.15 mmol, 1.0 equiv.), thiophene-based monomer (0.15 mmol, 1.0
equiv.), Cs2CO3 (0.48 mmol, 3.2 equiv.), neodecanoic acid (0.15 mmol, 1.0 equiv.), tris(o-
methoxyphenylphosphine) (0.024 mmol, 0.16 equiv), PdCl2(PPh3)2 (0.006 mmol, 0.04
equiv), were added. The degassed appropriate solvent was added through the rubber
septum to give the desired concentration. The vessel was further sparged with N2 (5 min.)
and was then quickly sealed with a Teflon screw-cap with a rubber o-ring. The vessel was
then submerged in a pre-heated oil bath at 110 °C for 24 hours. The reaction was then
cooled to room temperature and the mixture was diluted with 2 mL of CHCl3 and then
precipitated into a cold 10% (v:v) NH4OH/methanol solution with high-stirring (100 mL).
The solids were then filtered into a Soxhlet thimble and purified via Soxhlet extraction
(methanol, hexanes, and CHCl3). The CHCl3 fraction was concentrated to ~1-2 mL, and
re-precipitated into cold methanol with vigorous stirring. The polymer was then filtered
and further dried overnight under vacuum (~100 mtorr).
289
P1 of Scheme 5.1
poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(dimethyl-3,4-thiophenedicarboxylate)] (P1, entry
1-5).
1
H NMR (600 MHz, CDCl3, 25 °C) δ 7.80 – 7.73 (m, 2H), 7.55 – 7.50 (m, 4H), 3.78
(s, 6H), 2.01 (s, 4H), 1.19 – 1.07 (m, 20H), 0.81 – 0.78 (m, 6H), 0.73 – 0.65 (m, 4H).
Consistent with literature report.
4
Figure D.2 P1 synthesized using p-cymene (Table 5.1, entry 3), CPME (Table 5.1, entry
4) and toluene (Table 5.1, entry 5).
Oct
Oct
S
O
O
O
O
n
290
P2 of Scheme 5.1
poly-(9,9-dioctylfluorene-2,7-diyl)-alt-[(5-(2-ethylhexyl)-thieno-[3,4-c]pyrrole4,6-dione-
1,3-diyl)] (P2).
1
H NMR (600 MHz, CDCl3, 25 °C) δ 8.29 – 8.20 (m, 4H), 7.92 – 7.79 (m,
2H), 3.66 (br, 2H), 2.17 (br, 4H), 1.93 (br, 1H), 1.44 – 1.34 (m, 8H), 1.19 – 1.09 (m, 20H),
0.98 – 0.92 (m, 6H), 0.84 – 0.73 (m, 10H). Consistent with literature report.
5
Figure D.3 P2 synthesized using p-cymene (Table 5.1, entry 6) and toluene (Table 5.1,
entry 7).
Oct
Oct
S
n
N
O
O
EH
291
P3 of Scheme 5.1
Poly(9,9-dioctyl-2,7-fluorene-alt-2,2′-bithiophene) (P3).
1
H NMR (600 MHz, C2D2Cl4,
100 °C) δ 7.89 – 7.57 (m, 6H), 7.48 – 7.26 (m, 4H), 2.21-2.03 (br, 4H), 1.40-1.09 (m,
20H), 1.02-0.82 (m, 10H). Consistent with literature report.
6
Figure D.4 P3 synthesized using p-cymene (Table 5.1, entry 8).
For the synthesis of PCDTBT:
An oven-dried 15 mL reaction vessel equipped with a stir-bar was stoppered with a rubber-
septum and cooled under a flow of N2. In a separate 25 mL 1-neck round-bottom-flask, p-
Oct
Oct
S
S
n
292
cymene was added and degassed for 15 minutes. 2,7-dibromo-9-(9-heptadecyl)carbazole
(0.15 mmol, 1.0 equiv.), 4,7-di-2-thienyl-2,1,3-benzothiadiazole (0.15 mmol, 1.0 equiv.),
Cs2CO3 (0.48 mmol, 3.2 equiv.), neodecanoic acid (0.15 mmol, 1.0 equiv.), tris(o-
methoxyphenylphosphine) (0.024 mmol, 0.16 equiv), PdCl2(PPh3)2 (0.006 mmol, 0.04
equiv), were added. The degassed p-cymene (0.75 mL) was added through the rubber
septum to give a concentration of 0.2 M. The vessel was further sparged with N2 (5 min.)
and was then quickly sealed with a Teflon screw-cap with a rubber o-ring. The vessel was
then submerged in a pre-heated oil bath at 110 °C for 24 hours. The reaction was then
cooled to room temperature and the mixture was diluted with 2 mL of CHCl3 and then
precipitated into a cold 10% (v:v) NH4OH/methanol solution with high-stirring (100 mL).
The solids were then filtered into a Soxhlet thimble and purified via Soxhlet extraction
(methanol, hexanes, dichloromethane and CHCl3). The CHCl3 fraction was concentrated
to ~1-2 mL, and re-precipitated into cold methanol with vigorous stirring. The polymer
was then filtered and further dried overnight under vacuum (~100 mtorr).
(PCDTBT)
poly[(9-(heptadecan-9-yl)-9H-carbazole)-alt-(4,7-di(thiophen-2-yl)
benzo[c][1,2,5]thiadiazole)] (PCDTBT).
1
H NMR (600 MHz, C2D2Cl4, 100 °C) δ 8.38 –
8.14 (m, 4H), 8.10 – 7.85 (m, 4H), 7.74 – 7.57 (m, 4H), 4.78 (br, 1H), 2.48 (br, 2H), 2.18
(br, 2H), 1.55 – 1.32 (m, 24H), 1.06-0.78 (m, 6H). Consistent with literature report.
3
N
Oct Oct
S
S
N
S
N
n
293
D.4 Polymer NMR
Figure D.5
1
H NMR of P1 synthesized using the conditions in Table 5.1 (entry 1).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
294
Figure D.6
1
H NMR of P1 synthesized using the conditions in Table 5.1 (entry 2).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
Figure D.7
1
H NMR of P1 synthesized using the conditions in Table 5.1 (entry 3).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
295
Figure D.8
1
H NMR of P1 synthesized using the conditions in Table 5.1 (entry 4).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
Figure D.9
1
H NMR of P1 synthesized using the conditions in Table 5.1 (entry 5).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
296
Figure D.10
1
H NMR of P2 synthesized using the conditions in Table 5.1 (entry 6).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
5
Figure D.11
1
H NMR of P2 synthesized using the conditions in Table 5.1 (entry 7).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
5
297
Figure D.12
1
H NMR of P3 synthesized using the conditions in Table 5.1 (entry 8).
Collected in C2D2Cl4 at 100 °C and 600 MHz. Referenced to previous reports.
6
Figure D.13
1
H NMR of PCDTBT synthesized using the conditions in Scheme 5.2.
Collected in C2D2Cl4 at 100 °C and 600 MHz. Referenced to previous reports.
3
298
D.5 Detailed NMR analysis for P2, P3, and PCDTBT
Figure D.14
1
H NMR analyses of P2 synthesized using p-cymene (Table 5.1, entry 6), and
toluene (Table 5.1, entry 7) as the solvent. Major resonances (A-C’) and potential
resonances for end groups (a-e) and defects (δ, a) are denoted and referenced based on
literature report.
5
Collected in CDCl3 at 25 °C and 600 MHz.
299
Figure D.15
1
H NMR analyses of P3 synthesized using p-cymene (Table 5.1, entry 8) as
the solvent. Major resonances (A-D’) and potential resonances for end groups (*) and
defects (δ, a) are denoted and referenced based on literature report.
6
Collected in C2D2Cl4
at 100 °C and 600 MHz.
300
Figure D.16
1
H NMR analyses of PCDTBT synthesized using p-cymene (Scheme 5.2) as
the solvent. Major resonances (A-F’) and potential and defects (δ) are denoted and
referenced based on literature report.
3
Collected in C2D2Cl4 at 100 °C and 600 MHz.
301
D.6 UV-Vis Spectra
Figure D.17 UV-vis spectra of P1 (Table 5.1, entry 3-5) synthesized by using p-cymene
(Table 5.1, entry 3, black), CPME (Table 5.1, entry 4, red), and toluene (Table 5.1, entry
5, blue).
302
Figure D.18 UV-vis spectra of P2 (Table 5.1, entry 6-7) synthesized by using p-cymene
(Table 5.1, entry 6, black), and toluene (Table 5.1, entry 7, red).
Figure D.19 UV-vis spectra of P3 (Table 5.1, entry 8) synthesized by using p-cymene.
303
Figure D.20 UV-vis spectra of PCDTBT (Scheme 5.2) synthesized by using p-cymene.
D.7 References
(1) Ye, L.; Pankow, R. M.; Schmitt, A.; Thompson, B. C. Synthesis of Conjugated
Polymers Using Aryl-Bromides via Cu-Catalyzed Direct Arylation Polymerization
(Cu-DArP). Polym. Chem. 2019, 10 (48), 6545–6550.
https://doi.org/10.1039/C9PY01478K.
(2) Pankow, R. M.; Ye, L.; Thompson, B. C. Copper Catalyzed Synthesis of Conjugated
Copolymers Using Direct Arylation Polymerization. Polym. Chem. 2018, 9 (30),
4120–4124. https://doi.org/10.1039/C8PY00913A.
304
(3) Gobalasingham, N. S.; Ekiz, S.; Pankow, R. M.; Livi, F.; Bundgaard, E.; Thompson,
B. C. Carbazole-Based Copolymers via Direct Arylation Polymerization (DArP) for
Suzuki-Convergent Polymer Solar Cell Performance. Polym. Chem. 2017, 8 (30),
4393–4402. https://doi.org/10.1039/C7PY00859G.
(4) Sun, M.; Wang, W.; Liang, L.; Yan, S.; Zhou, M.; Ling, Q. Substituent Effects on
Direct Arylation Polycondensation and Optical Properties of Alternating Fluorene-
Thiophene Copolymers. Chin J Polym Sci 2015, 33 (5), 783–791.
https://doi.org/10.1007/s10118-015-1555-9.
(5) Kuwabara, J.; Yamazaki, K.; Yamagata, T.; Tsuchida, W.; Kanbara, T. The Effect of
a Solvent on Direct Arylation Polycondensation of Substituted Thiophenes. Polym.
Chem. 2015, 6 (6), 891–895. https://doi.org/10.1039/C4PY01387E.
(6) Morin, P.-O.; Bura, T.; Sun, B.; Gorelsky, S. I.; Li, Y.; Leclerc, M. Conjugated
Polymers à La Carte from Time-Controlled Direct (Hetero)Arylation Polymerization.
ACS Macro Lett. 2015, 4 (1), 21–24. https://doi.org/10.1021/mz500656g.
305
Appendix E
Chapter 6: “In-Water” Direct Arylation Polymerization (DArP) under Aerobic
Emulsion Conditions
E.1 General
p-Cymene was purchased from Alfa Aesar (97%) and dried from CaH2 followed
by vacuum distillation to 3 Å sieves. Pd(OAc)2 (Palldium(II) acetate trimer ≥ 99.98%)
(BeanTown Chemical), PdCl2(PPh3)2 (99.95%, BeanTown Chemical), tris(o-
methoxyphenylphosphine) (Alpha Aesar) were purchased and used as received.
Preparation of the emulsions: (adapted from Beverina et al.
1
) 2 wt% aqueous
dispersion of K-EL was prepared by mixing 1.8 g of Kolliphor EL in 88.2 mL of deionized
water. 100 mL of K-EL 2 wt% H2O:toluene (9:1 v/v) or K-EL 2 wt% H2O:p-cymene (9:1
v/v) emulsion was then prepared by stirring the 2 wt% K-EL aqueous solution with 10 mL
of toluene/p-cymene overnight until a stable, milky dispersion is obtained.
9,9-Dioctyl-2,7-dibromofluorene was purchased from Combi-Blocks (98%) and
recrystallized from EtOH to form a white solid prior to use. 2,2’-Bithiophene was
purchased from Matrix Scientific (97%) and subjected to a short column chromatography
followed by recrystallization from MeOH prior to use. 1,4-dibromo-2,5-bis[(2-
hexyldecyl)oxy]-benzene,
2
4,7-di-2-thienyl-2,1,3-benzothiadiazole,
2
were prepared
previously following reported procedures.
306
Monomer NMR were recorded at 25 °C using CDCl3 on a Varian Mercury Varian
VNMRS-500 MHz. Polymer NMR was obtained on a Varian VNMR-600 MHz. All
spectra were referenced to CHCl3 (7.26 ppm) and C2D2Cl4 (6.03 ppm), unless otherwise
noted.
Number average molecular weight (Mn) and polydispersity (Ð) were determined by
size exclusion chromatography (SEC) using a Agilent 1260 Infinity II High Temperature
GPC and a Differential Refractive Index (DRI) detector, with 80 °C HPLC grade 1,2,4-
trichlorobenzene (TCB) as eluent at a flow rate of 1.0 mL/min. The instrument was
calibrated vs. polystyrene standards (1050−3,800,000 g/mol). Polymer samples were
dissolved in HPLC grade TCB at a concentration of 0.5 mg/mL, stirred until dissolved, and
filtered through a 0.2 μm PTFE filter.
For polymer thin-film measurements, solutions were spin-coated onto pre-cleaned
glass slides from o-dichlorobenzene (o-DCB) solutions at 7 mg/mL. UV−vis absorption
spectra were obtained on a Perkin-Elmer Lambda 950 spectrophotometer.
307
E.2 Visualization of emulsions and DArP in emulsion conditions
Figure E.1 Comparison between K-EL 2 wt% H2O:toluene (9:1 v/v) emulsion (left) and
K-EL 2 wt% H2O:p-cymene (9:1 v/v) emulsion (right).
Figure E.2 DArP conducted using K-EL 2 wt% H2O:p-cymene (9:1 v/v) emulsion.
308
Figure E.3 The breakage of emulsion (separated into two layers) as a result of increasing
the loading of K2CO3 to 40 equiv., which increases the density of the aqueous solution
(Table 6.1, entry 9).
E.3 General polymerization procedures
For the synthesis of P1 under emulsion, aerobic conditions (Table 6.1, entry 7 as an
example):
An 15 mL pressure vessel equipped with a stir-bar was added 9,9-dioctyl-2,7-
dibromofluorene (0.4 mmol, 1.0 equiv.), 2,2’-bithiophene (0.4 mmol, 1.0 equiv.), K2CO3
(1.28 mmol, 3.2 equiv.), neodecanoic acid (0.4 mmol, 1.0 equiv.), tris(o-
methoxyphenylphosphine) (0.064 mmol, 0.16 equiv), PdCl2(PPh3)2 (0.016 mmol, 0.04
equiv), were added in air. 1.6 mL of the prepared K-EL 2 wt% H2O:p-cymene (9:1 v/v)
emulsion was added in air to give the desired concentration of 0.25 M. The vessel was then
sealed with a Teflon screw-cap with a rubber o-ring and submerged in a pre-heated oil bath
at 130 °C for 48 hours. The reaction was then cooled to room temperature and the mixture
309
was diluted with 2 mL of CHCl3 and then precipitated into a cold 10% (v:v)
NH4OH/methanol solution with high-stirring (100 mL). The solids were then filtered into
a Soxhlet thimble and purified via Soxhlet extraction (methanol, hexanes, and CHCl3). The
CHCl3 fraction was concentrated to ~1-2 mL, and re-precipitated into cold methanol with
vigorous stirring. The polymer was then filtered and further dried overnight under vacuum
(~100 mtorr).
For the synthesis of P1 under emulsion, N2 conditions (Table 6.1, entry 8):
An oven-dried 15 mL pressure vessel equipped with a stir-bar was stoppered with a rubber-
septum and cooled under a flow of N2. 9,9-dioctyl-2,7-dibromofluorene (0.4 mmol, 1.0
equiv.), 2,2’-bithiophene (0.4 mmol, 1.0 equiv.), K2CO3 (1.28 mmol, 3.2 equiv.),
neodecanoic acid (0.4 mmol, 1.0 equiv.), tris(o-methoxyphenylphosphine) (0.064 mmol,
0.16 equiv), PdCl2(PPh3)2 (0.016 mmol, 0.04 equiv), were added under N2. 1.6 mL of the
prepared K-EL 2 wt% H2O:p-cymene (9:1 v/v) emulsion through the rubber septum to give
the desired concentration of 0.25 M. The vessel was further sparged with N2 (5 min.) and
was then quickly sealed with a Teflon screw-cap with a rubber o-ring. The vessel was then
submerged in a pre-heated oil bath at 130 °C for 48 hours. The reaction was then cooled to
room temperature and the mixture was diluted with 2 mL of CHCl3 and then precipitated
into a cold 10% (v:v) NH4OH/methanol solution with high-stirring (100 mL). The solids
were then filtered into a Soxhlet thimble and purified via Soxhlet extraction (methanol,
hexanes, and CHCl3). The CHCl3 fraction was concentrated to ~1-2 mL, and re-precipitated
into cold methanol with vigorous stirring. The polymer was then filtered and further dried
overnight under vacuum (~100 mtorr).
310
P1 of Scheme 6.2
Poly(9,9-dioctyl-2,7-fluorene-alt-2,2′-bithiophene) (P1).
1
H NMR (600 MHz, C2D2Cl4,
100 °C) δ 7.89 – 7.57 (m, 6H), 7.48 – 7.26 (m, 4H), 2.21-2.03 (br, 4H), 1.40-1.09 (m, 20H),
1.02-0.82 (m, 10H). Consistent with literature report.
3
For the synthesis of PPDTBT under emulsion, aerobic conditions (Scheme 6.3):
An 15 mL pressure vessel equipped with a stir-bar was added 1,4-dibromo-2,5-bis[(2-
hexyldecyl)oxy]-benzene (0.4 mmol, 1.0 equiv.), 4,7-di-2-thienyl-2,1,3-benzothiadiazole
(0.4 mmol, 1.0 equiv.), K2CO3 (1.28 mmol, 3.2 equiv.), neodecanoic acid (0.4 mmol, 1.0
equiv.), tris(o-methoxyphenylphosphine) (0.064 mmol, 0.16 equiv), PdCl2(PPh3)2 (0.016
mmol, 0.04 equiv), were added in air. 1.6 mL of the prepared K-EL 2 wt% H2O:p-cymene
(9:1 v/v) emulsion was added in air to give the desired concentration of 0.25 M. The vessel
was then sealed with a Teflon screw-cap with a rubber o-ring and submerged in a pre-
heated oil bath at 130 °C for 48 hours. The reaction was then cooled to room temperature
and the mixture was diluted with 2 mL of CHCl3 and then precipitated into a cold 10%
(v:v) NH4OH/methanol solution with high-stirring (100 mL). The solids were then filtered
into a Soxhlet thimble and purified via Soxhlet extraction (methanol, hexanes, and CHCl3).
The CHCl3 fraction was concentrated to ~1-2 mL, and re-precipitated into cold methanol
with vigorous stirring. The polymer was then filtered and further dried overnight under
vacuum (~100 mtorr).
Oct
Oct
S
S
n
311
Poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(4,7-di(thiophen-2-
yl)benzo[c][1,2,5]thiadiazole)](PPDTBT). 1 HNMR (600 MHz, CDCl3, 25 °C): δ ppm
8.18 (br, 2H), 7.93 (br, 2H), 7.69 (br, 2H), 7.37 (br, 2H), 4.10 (br, 4H), 2.01 (br, 2H), 1.68
(br, 4H), 1.44−1.23 (m, 44H), 0.86−0.82 (br, 12H). Consistent with literature report.
E.4 Polymer NMR
Figure E.4
1
H NMR of P1 synthesized using the conditions in Table 6.1 (entry 3).
Collected in C2D2Cl4 at 100 °C and 600 MHz. Referenced to previous reports.
3,4
O
O
C
6
H
13
C
8
H
17
C
6
H
13
C
8
H
17
N
S
N
S
S
n
PPDTBT
312
Figure E.5
1
H NMR of P1 synthesized using the conditions in Table 6.1 (entry 5).
Collected in C2D2Cl4 at 100 °C and 600 MHz. Referenced to previous reports.
3,4
Figure E.6
1
H NMR of P1 synthesized using the conditions in Table 6.1 (entry 7).
Collected in C2D2Cl4 at 100 °C and 600 MHz. Referenced to previous reports.
3,4
313
Figure E.7
1
H NMR of P1 synthesized using the conditions in Table 6.1 (entry 8).
Collected in C2D2Cl4 at 100 °C and 600 MHz. Referenced to previous reports.
3,4
Figure E.8
1
H NMR of P1 synthesized using the conditions in Scheme 6.3. Collected in
CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
2,5
314
Figure E.9
1
H NMR analyses of P1 synthesized using H2O/p-cymene 9:1 (v:v) emulsion
in air and under N2 atmosphere (Table 6.1, entry 7 and 8, respectively). Potential
resonances and corresponding structures for end groups (*) and potential defects (δ, a) are
denoted. Collected in C2D2Cl4 at 100 °C and 600 MHz.
315
E.5 UV-Vis Spectra
Figure E.10 UV-vis spectra of P1 (Table 6.1, entry 3, 5, 7, 8) synthesized by using
emulsion conditions.
Figure E.11 UV-vis spectra of PPDTBT (Scheme 6.3) synthesized by emulsion
conditions.
316
E.6 References
(1) Calascibetta, A. M.; Mattiello, S.; Sanzone, A.; Facchinetti, I.; Sassi, M.; Beverina,
L. Sustainable Access to π-Conjugated Molecular Materials via Direct
(Hetero)Arylation Reactions in Water and under Air. Molecules 2020, 25 (16), 3717.
https://doi.org/10.3390/molecules25163717.
(2) Pankow, R.; Ye, L.; Thompson, B. Investigation of Green and Sustainable Solvents
for Direct Arylation Polymerization (DArP). Polym. Chem. 2018, 9, 3885–3892.
https://doi.org/10.1039/C8PY00749G.
(3) Morin, P.-O.; Bura, T.; Sun, B.; Gorelsky, S. I.; Li, Y.; Leclerc, M. Conjugated
Polymers à La Carte from Time-Controlled Direct (Hetero)Arylation Polymerization.
ACS Macro Lett. 2015, 4 (1), 21–24. https://doi.org/10.1021/mz500656g.
(4) Ye, L.; Thompson, B. C. P-Cymene: A Sustainable Solvent That Is Highly
Compatible with Direct Arylation Polymerization (DArP). ACS Macro Lett. 2021, 10
(6), 714–719. https://doi.org/10.1021/acsmacrolett.1c00274.
(5) Livi, F.; Gobalasingham, N. S.; Thompson, B. C.; Bundgaard, E. Analysis of Diverse
Direct Arylation Polymerization (DArP) Conditions toward the Efficient Synthesis of
Polymers Converging with Stille Polymers in Organic Solar Cells. J. Polym. Sci. Part
A: Polym. Chem. 2016, 54 (18), 2907–2918. https://doi.org/10.1002/pola.28176.
317
Appendix F
Chapter 7: Recycling Heterogenous Catalysts for Multi-Batch Conjugated Polymer
Synthesis via Direct Arylation Polymerization
F.1 General
Anhydrous N,N’-dimethylacetamide (DMA) was purchased from Acros Organics
and used as received. SiliaCat DPP-Pd (0.3 mmol/g) was purchased from SiliCycle Inc.
FibreCat (0.5 mmol/g) and Pd/C (palladium on carbon, 10 wt% of Pd content) were
purchased from Sigma-Aldrich. Tris(o-methoxyphenylphosphine) (Alpha Aesar) was
purchased and used as received. Cs2CO3, K2CO3 were ground into a fine powder and dried
at 120 °C in a vacuum oven before use.
9,9-Dioctyl-2,7-dibromofluorene was purchased from Combi-Blocks (98%) and
recrystallized from EtOH to form a white solid prior to use. 3,4-ethylenedioxythiophene
(EDOT) was purchased from TCI (97%) and distilled prior to use. 5-(2-ethylhexyl)-thieno-
[3,4-c]-pyrrole-4,6-dione
1
was prepared following previously reported procedures.
Polymer NMR was obtained on a Varian VNMR-600 MHz. All spectra were
referenced to CHCl3 (7.26 ppm).
Number average molecular weight (Mn) and polydispersity (Ð) were determined by
size exclusion chromatography (SEC) using a Agilent 1260 Infinity II High Temperature
GPC and a Differential Refractive Index (DRI) detector, with 140 °C HPLC grade 1,2,4-
trichlorobenzene (TCB) as eluent at a flow rate of 1.0 mL/min. The instrument was
318
calibrated vs. polystyrene standards (1050−3,800,000 g/mol). Polymer samples were
dissolved in HPLC grade TCB at a concentration of 0.5 mg/mL, stirred until dissolved, and
filtered through a 0.2 μm PTFE filter.
For polymer thin-film measurements, solutions were spin-coated onto pre-cleaned
glass slides from o-dichlorobenzene (o-DCB) solutions at 7 mg/mL, which were then
annealed at 150 °C for 30 minutes under N2. UV−vis absorption spectra were obtained on
a Perkin-Elmer Lambda 950 spectrophotometer.
F.2 Preliminary results for catalyst-recycling experiments
Our preliminary efforts focused on extending our previously reported DArP
conditions using sustainable aromatic solvent, p-cymene, to the heterogenous method in
this study.
2
With homogenous Pd catalyst, Pd(PPh3)Cl2, fluorene-thieno[3,4-c]pyrrole-4,6-
dione-based copolymer (PTPDF) was afforded in p-cymene with optimal Mn (51.3 kg/mol)
in 24 hours.
2
These conditions are highly similar compared to those utilized by Leclerc et
al. with the heterogenous catalyst SiliaCat DPP-Pd, which include aromatic solvent,
toluene, with Ozawa-derived DArP conditions (rely on less polar aryl or ether solvents and
highly tailored phosphine ligands that control the reactivity in non-coordinating solvents).
3
As shown in Scheme S1, replacing Pd(PPh3)Cl2 with SiliaCat DPP-Pd also provided
PTPDF with high Mn (39.5 kg/mol) and yield (73%). However, we were unable to reuse
the catalyst after recovering it through filtration. To avoid high leaching of Pd resulting
from long reaction time, we also attempted the reaction with shorter reaction time (6 hours),
which provided the desired polymer with lower Mn (7.8 kg/mol) and yield (34%). This
shows that, DArP using Ozawa conditions usually requires longer reaction time, which
might be unsuitable for this study using SiliaCat Pd-DPP as the heterogenous catalyst.
319
Research on further optimizing Ozawa conditions using SiliaCat Pd-DPP to allow the
reduction of reaction time and recycling of the catalyst for multi-batch conjugated polymer
synthesis is ongoing. For the purpose of this study, Fagnou-derived DArP conditions (rely
on the critical use of coordinating amide solvents, as the reaction is likely based on the
amide coordinated Pd species [(DMA)Pd(Ar)(O2CR)].) with superior reactivity is therefore
chosen.
Scheme F.1 Preliminary results for the catalyst-recycling experiments using Ozawa-
derived conditions.
F.3 General polymerization procedures
Representative catalyst-recycling experiments for the 5 batches of conjugated
polymer synthesis listed in Table 7.2 (Cycle 1-5):
Cycle 1:
An oven-dried 15 mL Schlenk tube equipped with a stir-bar was cooled under N2.
In a separate 25 mL 1-neck round-bottom-flask, anhydrous DMA was added and degassed
for 20 minutes. The Schlenk tube was purged with N2 with vacuum backfill for three times.
9,9-dioctyl-2,7-dibromofluorene (219.4 mg, 0.4 mmol, 1.0 equiv.), 3,4-
320
ethylenedioxythiophene (EDOT) (56.9 mg, 0.4 mmol, 1.0 equiv.), K2CO3 (138.2 mg, 1.0
mmol, 2.5 equiv.), PivOH (12.3 mg, 0.12 mmol, 0.3 equiv.), SiliaCat Pd-DPP (0.02 mmol,
66.7 mg, 0.05 equiv.), were subsequently added. The degassed DMA (1.33 mL) was added
to give the desired concentration of 0.3 M. The Schlenk tube was then submerged in a pre-
heated oil bath at 100 °C for 0.5 hours (30 minutes). The reaction was then cooled to room
temperature and the mixture was diluted with 20-30 mL of CHCl3. The reaction mixture
was subjected to a vacuum filtration with a 1.2 μm filter to collect the heterogenous
catalyst, SiliaCat Pd-DPP. The solid was further washed with ~50 mL of CHCl3 until the
disappearance of the green polymer color. The filtrate (polymer in CHCl3) was transferred
to a separate round bottom flask for further workup. The recovered catalyst was then
washed with 10% HCl solution (to remove the inorganic base), DI water, and finally
acetone. The catalyst was then stored under vacuum (~100 mtorr) for the next cycle of
polymerization experiment. The polymer solution in the round bottom flask was
concentrated to ~1-2 mL, and then precipitated into a cold methanol solution with high-
stirring (100 mL). The polymer was then filtered, washed with methanol and hexanes
several times, and further dried overnight under vacuum (~100 mtorr). GPC (1,2,4-
trichlorobenzene, 140 °C): Mn = 27.6 kg/mol, ᴆ = 2.13. Yield = 93.4%.
The recovered SiliaCat Pd-DPP from Cycle 1 was calculated to be 65.4 mg, which
corresponds to 98.1% of recovery. After drying under vacuum for several hours, it was
then used for Cycle 2.
321
Cycle 2:
(note: the amounts of all reagents, including monomers, K2CO3, PivOH, and
DMA were re-adjusted to keep the monomer concentration (0.3 M), catalyst loading (5
mol%), base loading (2.5 equiv.), and carboxylic acid loading (0.3 equiv.) constant
compared to the previous cycle. This will allow us to draw better comparisons between
each cycle to observe the change of reactivity of the catalyst.)
An oven-dried 15 mL Schlenk tube equipped with a stir-bar was cooled under N2.
In a separate 25 mL 1-neck round-bottom-flask, anhydrous DMA was added and degassed
for 20 minutes. The Schlenk tube was purged with N2 with vacuum backfill for three times.
9,9-dioctyl-2,7-dibromofluorene (215.3 mg, 0.393 mmol, 1.0 equiv.), 3,4-
ethylenedioxythiophene (EDOT) (55.8 mg, 0.393 mmol, 1.0 equiv.), K2CO3 (135.6 mg,
0.98 mmol, 2.5 equiv.), PivOH (12.03 mg, 0.118 mmol, 0.3 equiv.), recycled SiliaCat Pd-
DPP (0.0196 mmol, 65.4 mg, 0.05 equiv.), were subsequently added. The degassed DMA
(1.30 mL) was added to give the desired concentration of 0.3 M. The Schlenk tube was
then submerged in a pre-heated oil bath at 100 °C for 0.5 hours (30 minutes). The reaction
was then cooled to room temperature and the mixture was diluted with 20-30 mL of CHCl3.
The reaction mixture was subjected to a vacuum filtration with a 1.2 μm filter to collect the
heterogenous catalyst, SiliaCat Pd-DPP. The solid was further washed with ~50 mL of
CHCl3 until the disappearance of the green polymer color. The filtrate (polymer in CHCl3)
was transferred to a separate round bottom flask for further workup. The recovered catalyst
was then washed with 10% HCl solution (to remove the inorganic base), DI water, and
finally acetone. The catalyst was then stored under vacuum (~100 mtorr) for the next cycle
of polymerization experiment. The polymer solution in the round bottom flask was
322
concentrated to ~1-2 mL, and then precipitated into a cold methanol solution with high-
stirring (100 mL). The polymer was then filtered, washed with methanol and hexanes
several times, and further dried overnight under vacuum (~100 mtorr). GPC (1,2,4-
trichlorobenzene, 140 °C): Mn = 22.1 kg/mol, ᴆ = 1.96. Yield = 91.6%.
The recovered SiliaCat Pd-DPP from Cycle 2 was calculated to be 61.4 mg, which
corresponds to 93.9% of recovery. After drying under vacuum for several hours, it was
then used for Cycle 3.
Cycle 3:
An oven-dried 15 mL Schlenk tube equipped with a stir-bar was cooled under N2.
In a separate 25 mL 1-neck round-bottom-flask, anhydrous DMA was added and degassed
for 20 minutes. The Schlenk tube was purged with N2 with vacuum backfill for three times.
9,9-dioctyl-2,7-dibromofluorene (202.1 mg, 0.368 mmol, 1.0 equiv.), 3,4-
ethylenedioxythiophene (EDOT) (52.4 mg, 0.368 mmol, 1.0 equiv.), K2CO3 (127.3 mg,
0.92 mmol, 2.5 equiv.), PivOH (11.3 mg, 0.11 mmol, 0.3 equiv.), recycled SiliaCat Pd-
DPP (0.0184 mmol, 61.4 mg, 0.05 equiv.), were subsequently added. The degassed DMA
(1.22 mL) was added to give the desired concentration of 0.3 M. The Schlenk tube was
then submerged in a pre-heated oil bath at 100 °C for 1.5 hours (90 minutes). The reaction
was then cooled to room temperature and the mixture was diluted with 20-30 mL of CHCl3.
The reaction mixture was subjected to a vacuum filtration with a 1.2 μm filter to collect the
heterogenous catalyst, SiliaCat Pd-DPP. The solid was further washed with ~50 mL of
CHCl3 until the disappearance of the green polymer color. The filtrate (polymer in CHCl3)
was transferred to a separate round bottom flask for further workup. The recovered catalyst
was then washed with 10% HCl solution (to remove the inorganic base), DI water, and
323
finally acetone. The catalyst was then stored under vacuum (~100 mtorr) for the next cycle
of polymerization experiment. The polymer solution in the round bottom flask was
concentrated to ~1-2 mL, and then precipitated into a cold methanol solution with high-
stirring (100 mL). The polymer was then filtered, washed with methanol and hexanes
several times, and further dried overnight under vacuum (~100 mtorr). GPC (1,2,4-
trichlorobenzene, 140 °C): Mn = 24.5 kg/mol, ᴆ = 2.21. Yield = 90.9%.
The recovered SiliaCat Pd-DPP from Cycle 3 was calculated to be 56.7 mg, which
corresponds to 92.4% of recovery. After drying under vacuum for several hours, it was
then used for Cycle 4.
Cycle 4:
An oven-dried 15 mL Schlenk tube equipped with a stir-bar was cooled under N2.
In a separate 25 mL 1-neck round-bottom-flask, anhydrous DMA was added and degassed
for 20 minutes. The Schlenk tube was purged with N2 with vacuum backfill for three times.
9,9-dioctyl-2,7-dibromofluorene (186.6 mg, 0.340 mmol, 1.0 equiv.), 3,4-
ethylenedioxythiophene (EDOT) (48.3 mg, 0.340 mmol, 1.0 equiv.), K2CO3 (117.5 mg,
0.85 mmol, 2.5 equiv.), PivOH (10.4 mg, 0.102 mmol, 0.3 equiv.), recycled SiliaCat Pd-
DPP (0.0170 mmol, 56.7 mg, 0.05 equiv.), were subsequently added. The degassed DMA
(1.13 mL) was added to give the desired concentration of 0.3 M. The Schlenk tube was
then submerged in a pre-heated oil bath at 100 °C for 2 hours (120 minutes). The reaction
was then cooled to room temperature and the mixture was diluted with 20-30 mL of CHCl3.
The reaction mixture was subjected to a vacuum filtration with a 1.2 μm filter to collect the
heterogenous catalyst, SiliaCat Pd-DPP. The solid was further washed with ~50 mL of
CHCl3 until the disappearance of the green polymer color. The filtrate (polymer in CHCl3)
324
was transferred to a separate round bottom flask for further workup. The recovered catalyst
was then washed with 10% HCl solution (to remove the inorganic base), DI water, and
finally acetone. The catalyst was then stored under vacuum (~100 mtorr) for the next cycle
of polymerization experiment. The polymer solution in the round bottom flask was
concentrated to ~1-2 mL, and then precipitated into a cold methanol solution with high-
stirring (100 mL). The polymer was then filtered, washed with methanol and hexanes
several times, and further dried overnight under vacuum (~100 mtorr). GPC (1,2,4-
trichlorobenzene, 140 °C): Mn = 27.8 kg/mol, ᴆ = 2.05. Yield = 92.7%.
The recovered SiliaCat Pd-DPP from Cycle 3 was calculated to be 51.7 mg, which
corresponds to 91.1% of recovery. After drying under vacuum for several hours, it was
then used for Cycle 5.
Cycle 5:
An oven-dried 15 mL Schlenk tube equipped with a stir-bar was cooled under N2.
In a separate 25 mL 1-neck round-bottom-flask, anhydrous DMA was added and degassed
for 20 minutes. The Schlenk tube was purged with N2 with vacuum backfill for three times.
9,9-dioctyl-2,7-dibromofluorene (170.1 mg, 0.310 mmol, 1.0 equiv.), 3,4-
ethylenedioxythiophene (EDOT) (44.1 mg, 0.310 mmol, 1.0 equiv.), K2CO3 (107.1 mg,
0.775 mmol, 2.5 equiv.), PivOH (9.5 mg, 0.093 mmol, 0.3 equiv.), recycled SiliaCat Pd-
DPP (0.0155 mmol, 51.7 mg, 0.05 equiv.), were subsequently added. The degassed DMA
(1.03 mL) was added to give the desired concentration of 0.3 M. The Schlenk tube was
then submerged in a pre-heated oil bath at 100 °C for 36 hours. The reaction was then
cooled to room temperature and the mixture was diluted with 20-30 mL of CHCl3. The
reaction mixture was subjected to a vacuum filtration with a 1.2 μm filter to collect the
325
heterogenous catalyst, SiliaCat Pd-DPP. The solid was further washed with ~50 mL of
CHCl3 until the disappearance of the green polymer color. The filtrate (polymer in CHCl3)
was transferred to a separate round bottom flask and was concentrated to ~1-2 mL, and
then precipitated into a cold methanol solution with high-stirring (100 mL). The polymer
was then filtered, washed with methanol and hexanes several times, and further dried
overnight under vacuum (~100 mtorr). GPC (1,2,4-trichlorobenzene, 140 °C): Mn = 22.5
kg/mol, ᴆ = 2.15. Yield = 91.7%.
PEDOTF
poly[(3,4-ethylenedioxythiophene)-alt-(9,9-dioctylfluorene-2,7-diyl)] (PEDOTF).
1
H
NMR (600 MHz, CDCl3) δ 7.82 (d, J = 8.1 Hz, 2H), 7.73 – 7.67 (m, 4H), 4.45 (s, 4H), 2.06
(br, 4H), 1.10 (br, 20H), 0.82 (br, J = 7.0 Hz, 6H). Consistent with literature report.
4
C
8
H
17
C
8
H
17
S
O
O
n
326
F.4 Additional Recycling Experiments using SiliaCat Pd-DPP
Table F.1 Testing of recyclability of SiliaCat Pd-DPP using DArP conditions with the use
of neodecanoic acid (NDA) in place of PivOH
.[a]
[a] All polymerizations were conducted using the general conditions shown in Scheme 7.2, except
with the use of NDA in place of PivOH. For polymerizations using recycled catalyst from their
previous cycles, the amounts of monomers were re-adjusted to keep the catalyst loading (5 mol%)
and monomer concentration (0.3 M) constant. See the above section for experimental details. [b]
Determined based on the mass of recovered catalyst from the previous cycle. [c] Estimated by GPC
(140 °C, 1,2,4-trichlorobenzene) calibrated with polystyrene standards.
Entry Number
of Cycle
Reaction
time (h)
Carboxylic
acid additive
Catalyst recycled
(%)
[b]
Mn (kg/mol)
[c]
, ᴆ
[c]
Yields
(%)
1 Cycle 1
(initial)
0.5
NDA - 19.3, 1.90 88.0
2 Cycle 2 0.5 NDA 98.1 8.1, 2.06 81.0
3 Cycle 3 0.5 NDA 98.7 4.1, 1.61 6.21
4 Cycle 4 3 NDA 81.1 82.3, 2.16 96.2
327
F.5 The testing of other commercially-available heterogenous Pd catalysts
Scheme F.2 Synthesis of PEDOTF using FibreCat or Pd/C as the heterogenous catalyst.
Table F.2 Testing of recyclability of FibreCat and Pd/C using DArP conditions shown in
Scheme F.2.
[a]
Number of Cycle Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5
Pd
Fibre
Cat
Reaction time 30 mins 30 mins 90 mins 3 h 24 h
M n (kDa)
[b]
, ᴆ
[b]
3.5, 1.8 1.8, 1.5 4.2, 1.9 4.4, 1.5 4.7, 1.5
Yields (%) 73 24 69 37 38
Pd/C Reaction time 24 h 24 h
M n (kDa)
[b]
, ᴆ
[b]
39.5, 2.43 -
Yields (%) 92.3 -
[a] All polymerizations were conducted using the general conditions shown in Scheme F.2. For
polymerizations using recycled catalyst from their previous cycles, the amounts of monomers were
re-adjusted to keep the catalyst loading (5 mol%) and monomer concentration (0.3 M) constant.
See the above section for experimental details. [b] Estimated by GPC (140 °C, 1,2,4-
trichlorobenzene) calibrated with polystyrene standards.
Scheme F.3 Synthesis of PEDOTF using pristine FibreCat as the heterogenous catalyst for
24 hours of reaction time.
Br
Br
C
8
H
17
C
8
H
17
S
O O
H H
+
C
8
H
17
C
8
H
17
S
O
O
n
FibreCat
or Pd/C (5 mol%)
PivOH (30 mol%)
DMA (0.3 M), 100 °C
Reaction Time
PEDOTF
Br
Br
C
8
H
17
C
8
H
17
S
O O
H H
+
C
8
H
17
C
8
H
17
S
O
O
n
FibreCat (5 mol%)
(pristine)
PivOH (30 mol%)
DMA (0.3 M), 100 °C
24 h
PEDOTF
M
n
= 7.7 kg/mol, Ð = 1.79
yield = 65%
328
F.6 Polymer NMR
Figure F.1
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 1).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
329
Figure F.2
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 2).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
Figure F.3
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 4).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
330
Figure F.4
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 5).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
Figure F.5
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 6).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
331
Figure F.6
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 7).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
Figure F.7
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 8).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
332
Figure F.8
1
H NMR of PEDOTF synthesized using the conditions in Table 7.1 (entry 9).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
Figure F.9
1
H NMR of PEDOTF synthesized using the conditions in Table F.1 (entry 1).
Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
333
Figure F.10
1
H NMR of PEDOTF synthesized using the conditions in Table F.1 (Cycle
1). Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
Figure F.11
1
H NMR of PEDOTF synthesized using the conditions in Table F.1 (Cycle
2). Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
334
Figure F.12
1
H NMR of PEDOTF synthesized using the conditions in Table F.1 (Cycle
3). Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
Figure F.13
1
H NMR of PEDOTF synthesized using the conditions in Table F.1 (Cycle
4). Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
335
Figure F.14
1
H NMR of PEDOTF synthesized using the conditions in Table F.1 (Cycle
5). Collected in CDCl3 at 25 °C and 600 MHz. Referenced to previous reports.
4
Figure F.15
1
H NMR of PEDOTF synthesized using the conditions in Scheme F.3 (using
FibreCat as the catalyst for 24 hours of reaction time). Collected in CDCl3 at 25 °C and
600 MHz. Referenced to previous reports.
4
336
Figure F.16
1
H NMR of PEDOTF synthesized using the conditions in Table F.2 using
Pd/C as the catalyst (Cycle 1). Collected in CDCl3 at 25 °C and 600 MHz. Referenced to
previous reports.
4
337
F.7 Detailed NMR analysis for representative PEDOTF
Figure F.17
1
H NMR analyses of PEDOTF synthesized using the reaction conditions listed
in Table 7.1, entry 7 (third cycle of a round of catalyst-recycling experiments). Major
resonances (A-C’) and potential resonances for end groups (a-e) and homocoupling defects
(δ) are denoted and referenced based on literature report, which show the exclusion of any
observable defect.
4
Collected in CDCl3 at 25 °C and 600 MHz.
338
F.8 UV-vis spectra
Figure F.18 UV-vis spectra of PEDOTF (Table 7.1, entry 1, 2, 4).
Table 7.1, entry 1
Table 7.1, entry 2
Table 7.1, entry 4
339
Figure F.19 UV-vis spectra of PEDOTF (Table 7.1, entry 5-9).
Table 7.1,
entry 5
Table 7.1,
entry 6
Table 7.1,
entry 7
Table 7.1,
entry 8
Table 7.1,
entry 9
340
Figure F.20 UV-vis spectra of PEDOTF (Table F.1, entry 1, 4).
Figure F.21 UV-vis spectra of UV-vis spectra of PEDOTF (Table 7.2, Cycle 1-5).
Table F.1,
entry 1
Table F.1,
entry 4
341
Figure F.22 UV-vis spectra of UV-vis spectra of PEDOTF (Table F.2, using Pd/C).
Figure F.23 UV-vis spectra of UV-vis spectra of PEDOTF (Scheme F.3, using pristine
FibreCat).
342
F.9 References
(1) Pankow, R. M.; Ye, L.; Thompson, B. C. Copper Catalyzed Synthesis of Conjugated
Copolymers Using Direct Arylation Polymerization. Polym. Chem. 2018, 9 (30),
4120–4124. https://doi.org/10.1039/C8PY00913A.
(2) Ye, L.; Thompson, B. C. P-Cymene: A Sustainable Solvent That Is Highly
Compatible with Direct Arylation Polymerization (DArP). ACS Macro Lett. 2021, 10
(6), 714–719. https://doi.org/10.1021/acsmacrolett.1c00274.
(3) Grenier, F.; Goudreau, K.; Leclerc, M. Robust Direct (Hetero)Arylation
Polymerization in Biphasic Conditions. J. Am. Chem. Soc. 2017, 139 (7), 2816–2824.
https://doi.org/10.1021/jacs.6b12955.
(4) Yamazaki, K.; Kuwabara, J.; Kanbara, T. Detailed Optimization of Polycondensation
Reaction via Direct C–H Arylation of Ethylenedioxythiophene. Macromolecular
Rapid Communications 2013, 34 (1), 69–73.
https://doi.org/10.1002/marc.201200550.
Abstract (if available)
Abstract
Conjugated polymers are attractive semiconducting materials for applications such as organic photovoltaics (OPVs), organic light emitting diodes (OLEDs), organic transistors, batteries, and bioelectronics. Traditional synthetic methodologies such as Stille-Migita and Suzuki-Miyaura polymerization are often utilized to prepare these important materials, which require the use of toxic, pyrophoric reagents and challenging (if not impossible) monomer purifications. In recent years, Direct Arylation Polymerization (DArP) has emerged as a facile, eco-friendly pathway for the synthesis of conjugated polymers by reducing the number of synthetic steps and highly toxic organotin or organoboron by-products. By circumventing monomer functionalization with toxic transmetallating reagents such as organostannane and organoboron required for Stille-Migita and Suzuki-Miyaura polymerization methods, DArP proceeds through a metal-catalyzed C-H activation pathway for the preparation of high performance conjugated polymer materials. The rapid development of DArP protocols has allowed the preparation of well-defined conjugated polymers via a sustainable, atom-economical pathway. ❧ However, despite its inherent sustainability, there are many problematic aspects of DArP that seek to be addressed to further improve both its efficiency and sustainability. For instance, as the major component in these reactions, the solvents most prevalently employed for DArP are hazardous and produced from unsustainable sources. However, little development of employing sustainable solvents or “greener” synthetic methods for DArP has been reported. Furthermore, sustainable processing of conjugated polymers synthesized using DArP requires attention. Moreover, DArP relies on the expensive, rare metal, Pd, as the catalyst. With the growing demand for industrial-scale conjugated polymer synthesis and the development of continuous flow methods, the need for less-expensive metal catalysts or recyclable heterogenous catalysts for DArP has become urgent. In this dissertation, strategies for improving the efficiency and sustainablity of DArP by addressing all the problems enlisted above are presented to synthesize functional, well-defined conjugated polymers with the development of several novel DArP protocols. ❧ Chapter 1 evaluates the development of several classes of efficient catalysts/catalytic systems from small-molecule studies to polymerizations, including the mechanisms involved in these transformations and how they inspire catalyst and monomer design for defect-free conjugated polymer synthesis. Recent advances in developing more sustainable first-row transition metal catalysts for DArP are also highlighted, and the fundamental understanding of these efficient and sustainable catalysts motivates our pursuit for the next generation of novel reaction protocol design to enable more effective and environmentally-friendly conjugated polymer synthesis detailed in Chapters 2-7. ❧ In Chapter 2, the first class of poly (3-alkylamidethiophenes) (P3AAT) is prepared via the sustainable method of DArP that can be processed using green, sustainable solvents. The unprecedented synthesis of P3AAT reveals the superiority of DArP, as P3AAT can be readily prepared in only three simple steps with molecular weights (Mn) up to 15.4 kg/mol and yields up to 90% exclusively with this methodology. The tertiary amide, poly(N-hexyl-N-methylthiophene-3-carboxamide-2,5-diyl) (P1) has excellent solubility in the green solvents ethanol, 1-butanol, and anisole. Processing of P1 in 1-butanol is shown to provide comparable SCLC hole mobility versus dichlorobenzene and commensurate photophysical properties. Also, the secondary amide, poly(N-(2-ethylhexyl)thiophene-3-carboxamide-2,5-diyl) (P2) was successfully synthesized, demonstrating excellent functional-group-tolerance for DArP, while showing hydrogen bonding features and similar SCLC hole mobility as P1. This study provides a facile synthetic strategy for a novel structural motif that can be processed in sustainable solvents without a compromise in performance. ❧ In Chapter 3, the first synthesis of conjugated polymers via Cu-catalyzed DArP using aryl-bromides is reported. This work has significantly advanced the development of Cu-DArP, enabling the synthesis of a broad scope of alternating donor-acceptor copolymers using aryl-bromides with catalytic copper, completely replacing expensive, unstable aryl-iodides previously employed. Through optimization of Cu-DArP conditions, less-reactive aryl bromides can be successfully polymerized with different coupling partners using catalytic quantities of Cu (15 mol%), allowing the preparation of perfectly-alternating conjugated copolymers with Mn up to 17.3 kg/mol using a co-solvent approach. ❧ In Chapter 4, we explore the utility of a well-defined, easy-to-prepare, highly-soluble and stable precatalyst, Cu(phen)(PPh3)Br, as an alternative to the CuI, 1,10-phenanthroline catalytic system previously used for Cu-DArP. We report a drastic improvement of Cu-DArP methodology for the synthesis of 5,5’-bithiazole (5-BTz)-based conjugated polymers enabled by an efficient precatalyst approach, affording polymers with good Mn (up to 16.5 kg/mol) and excellent yields (up to 79%). 1HNMR studies reveal the exclusion of homo-coupling defects, which further verifies the excellent stability of Cu(phen)(PPh3)Br compared to CuI. Furthermore, the Cu catalyst loading is decreased from 15 mol% to only 5 mol% (Mn of 11.8 kg/mol, 64% yield), which is unprecedented when aryl-bromides are employed for Cu-DArP. Significantly, 5-BTz is shown to be inactive under various of Pd-DArP conditions, which demonstrates the high compatibility of Cu-DArP as the only pathway for the C-H activation of the 5-BTz unit and a clear case demonstrating an advantage of Cu-DArP relative to Pd-DArP. ❧ In Chapter 5, we report the application of a sustainable, naturally-sourced, high-boiling aromatic solvent, p-cymene, to DArP for the first time. p-Cymene was found to display excellent solubilizing ability in the synthesis of a broad scope of alternating copolymers with Mn up to 51.3 kg/mol and yields up to 96.2%, outperforming those prepared using cyclopentyl methyl ether (CPME) and toluene. Structural analysis revealed the exclusion of defects in these polymers prepared by using p-cymene as the solvent, which in the case of a 2,2’-bithiophene monomer, challenging to access through the use of conventional solvents for DArP, such as DMA and toluene. This report demonstrates the significant potential of p-cymene as a green solvent to replace traditional hazardous, unsustainable solvents for DArP. ❧ In Chapter 6, we report the first synthesis of conjugated polymers via DArP using emulsion reaction media. Importantly, these polymerizations were found suitable under aerobic conditions. By using an “oil-in-water” strategy with a H2O/p-cymene 9:1 (v:v) emulsion medium stabilized by a surfactant, we were able to significantly decrease the amount of organic waste generated per kg of conjugated polymer synthesized (E-factor) by 10 times compared to conventional DArP methods, preparing well-defined conjugated polymers with Mn up to 14.5 kg/mol. This work demonstrates a significant enhancement in the sustainability aspect of DArP, and the novel emulsion-DArP method presented is promising towards environmental-compatible, low-cost industrial-scale conjugated polymer synthesis. ❧ In Chapter 7, the first report on the recycling of heterogenous catalysts for CP synthesis using DArP is presented. We found SiliaCat® Pd-DPP to be a highly efficient and recyclable catalyst for multi-batch CP synthesis providing CPs with Mn up to 82 kg/mol even after being recycled 3 times. Batch-to-batch variations were further optimized to afford up to 5 batches of polymers with Mn of 25±2.5 kg/mol without structural disparity. Significantly, this work discloses among the most sustainable CP synthesis protocols to date and presents the critical concept of catalyst-recycling to the important field of organic semiconducting polymers, which potentially enables access to truly low-cost flow chemistry for industrial-scale CP synthesis.
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Ye, Liwei
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Core Title
Improving the efficiency and sustainability of direct arylation polymerization for conjugated polymer synthesis
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College of Letters, Arts and Sciences
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Doctor of Philosophy
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Chemistry
Degree Conferral Date
2022-05
Publication Date
02/16/2022
Defense Date
02/08/2022
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catalysis
conjugated polymers
polymer synthesis