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Rational design of simple and complex rare earth metal catalyst systems to enable reactive, controlled and selective block copolymerization of chemically dissimilar monomers
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Rational design of simple and complex rare earth metal catalyst systems to enable reactive, controlled and selective block copolymerization of chemically dissimilar monomers
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Content
RATIONAL DESIGN OF SIMPLE AND COMPLEX RARE EARTH METAL CATALYST SYSTEMS
TO ENABLE REACTIVE, CONTROLLED AND SELECTIVE BLOCK COPOLYMERIZATION OF
CHEMICALLY DISSIMILAR MONOMERS
by
Sophia C. Kosloski-Oh
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)
August 2023
Copyright 2023 Sophia C. Kosloski-Oh
ii
EPIGRAPH
Science knows no country, because knowledge belongs to humanity, and is the torch which illuminates the
world.
-Louis Pasteur
Thou hast only to follow the wall far enough and there will be a door in it.
-Marguerite de Angli
iii
DEDICATION
For my parents Raymond and Grace and my husband Chris
Without you the following pages would be blank
iv
ACKNOWLEDGMENTS
First and foremost, I would like to express my gratitude to my advisor, Megan E. Fieser.
Throughout the challenging Ph.D. “marathon”, you have remained a constant source of optimism and
encouragement during the low moments and have demonstrated how to celebrate the small victories. I am
so grateful for your unwavering support, direction, and for challenging me to grow as a scientist. You have
been the best mentor that I could have ever hoped for, and I will be channeling what you’ve taught me as I
move forward. I’m glad I was a part of the beginning of your lab and look forward to seeing how it will
flourish in the future.
I also would like to thank the members of my screening, quals, and defense committee for their
support and guidance throughout my time here at USC. My appreciation goes out to Barry Thompson, Mike
Inkpen, Richard Brutchey, Travis Williams, Noah Malmstadt, and Steve Nutt.
I feel so lucky to have been able to work with such wonderful labmates in the Fieser group. Mikiyas
Assefa, I am grateful for all the late-night talks in the lab…whether about science or not. Your insights
often sparked breakthroughs! Thank you to Yvonne Manjarrez, I enjoyed starting this lab with you and I’m
glad we went through this whole experience together. To Zach Wood, congratulations on the wedding and
I have enjoyed nerding out about video games with you. To Nancy Bush, thank you for being my hoodmate
(and tolerating my mess) and I’ll see you in Boston. To Kai Knight, thank you for bringing fun energy into
the lab (and for letting me borrow your NMR tubes). To Dana Cheng, thanks for sharing your drawings
with me and always having time to chat. To Eunice Castro, thank you for the wonderful flowers at my
graduation and I’ll see you online in Valorant. To Nicholas Roubineau, I’ve really enjoyed getting to know
you over this last year. Thank you for working with me and I hope the projects treat you well!
The Fieser group has also had many amazing undergrads over the years, and I would like to thank
them all for bringing in their unique perspectives and experiences. I would especially like to thank Alexei
Nosov, Collette Gordon, Galit Ashkenazi, and Taleen Boghossian who I had the chance to help mentor
while they were with the Fieser lab.
v
Finally, I’d like to thank my family. First to my Mom and Dad, thank you for being my first
teachers. You have demonstrated wisdom and instilled determination and hard work in me. Words really
cannot begin to describe how much you mean to me and how grateful I am for everything you have done
for me over the years. Last, to my husband and best friend Chris, thank you for putting up with all the drama
of the last five years and for reminding me that there is more to life than just the lab. I look forward to our
next chapter together.
vi
TABLE OF CONTENTS
EPIGRAPH ................................................................................................................................................. ii
DEDICATION............................................................................................................................................ iii
ACKNOWLEDGMENTS ......................................................................................................................... iv
LIST OF TABLES .................................................................................................................................. viii
LIST OF FIGURES .................................................................................................................................. ix
ABSTRACT ............................................................................................................................................. xiii
CHAPTER 1. General Introduction.......................................................................................................... 1
1.1 The World in a Plastic Disaster .......................................................................................................... 2
1.2 Polymer Source .................................................................................................................................. 6
1.3 End-of-life Considerations ................................................................................................................. 8
1.4 Block Copolymers ............................................................................................................................ 15
1.5 Block Copolymers Synthesis of Non-polar and Polar Monomers ................................................... 18
1.6 Dissertation Outline .......................................................................................................................... 31
1.7 References ........................................................................................................................................ 33
CHAPTER 2. Controlled, One-pot Synthesis of Recyclable Poly(1,3-diene)-polyester Block
Copolymers, Catalyzed by Yttrium β-diketiminate Complexes .......................................................... 37
2.1 Introduction ...................................................................................................................................... 38
2.2 Results and Discussion ...................................................................................................................... 40
2.2.1 Catalyst Section ........................................................................................................................ 40
2.2.2 Isoprene and Caprolactone ........................................................................................................ 41
2.2.3 PIP:PCL Block Copolymers ...................................................................................................... 43
2.2.4 Catalyst Efficiency ..................................................................................................................... 44
2.2.5 Monomer Scope ......................................................................................................................... 47
2.2.6 Recyclability .............................................................................................................................. 50
2.3 Conclusions ...................................................................................................................................... 52
2.4 Experimental Details and Additional Figures. ................................................................................. 53
2.4.1 General Considerations ............................................................................................................. 53
2.4.2 General Procedure for the Homopolymerization of 1,3-dienes or Cyclic Esters ...................... 54
2.4.3 General Procedure for the Block Copolymerization of 1,3-dienes and Cyclic Esters .............. 54
2.4.4 General Procedure for Degradation of PIP-b-PCL Diblock ..................................................... 54
2.4.5 Procedure for the Repolymerization of PIP-b-PCL Diblock .................................................... 55
2.4.6 Characterization Methods .......................................................................................................... 55
2.4.7 Formation of Alkoxide Macroinitiator with Recovered IP 50 and Y[N(SiMe 3) 2] 3 .................... 56
2.4.8 Additional Figures and Data ...................................................................................................... 58
2.5 References ........................................................................................................................................ 66
CHAPTER 3. Enhanced Control of Isoprene Polymerization with Trialkyl Rare Earth Metal
Complexes Through Neutral Donor Support ......................................................................................... 69
3.1 Introduction ...................................................................................................................................... 70
3.2 Results and Discussion ...................................................................................................................... 73
3.2.1 Yttrium Pre-catalyst Screening ................................................................................................. 73
3.2.2 Neutral Donor Scope ................................................................................................................. 76
3.2.3 Degree of Activation ................................................................................................................. 81
3.2.4 IP Polymerization with Ternary Systems................................................................................... 83
3.2.5 Extension to Other Rare Earth Metal Pre-catalysts ................................................................... 86
3.2.6 Activation Conditions ................................................................................................................ 88
3.2.7 Stability of the Pre-catalyst ....................................................................................................... 92
3.3 Conclusions ...................................................................................................................................... 93
3.4 Experimental Details and Additional Figures. ................................................................................. 94
vii
3.4.1 General Considerations ............................................................................................................. 94
3.4.2 Polymerization Methods ........................................................................................................... 95
3.4.3 Characterization Methods .......................................................................................................... 96
3.4.4 In situ synthesis of [Y(CH 2SiMe 3) 2(THF) 2]
+
[B(C 6F 5) 4]
-
and
[Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
................................................................................................... 97
3.4.5 GPC analysis of select polymerizations ................................................................................... 100
3.4.6 In situ NMR Studies with PPh 3 ................................................................................................ 101
3.4.7 GPC spectra of polymerizations with different equivalents of [Ph 3C][B(C 6F 5) 4] .................... 104
3.4.8 Reaction of Y(CH 2SiMe 3) 3(THF) 2 with 3 equiv. [Ph 3C][B(C 6F 5) 4] ......................................... 106
3.4.9 Reaction of Y(CH 2SiMe 3) 3(THF) 2 with 2 equiv. [Ph 3C][B(C 6F 5) 4] and 5 equiv. AlMe 3 ........ 107
3.4.10 Hammett Plot ......................................................................................................................... 111
3.4.11 Living Polymerization ........................................................................................................... 111
3.4.12 Reaction of Y(CH 2SiMe 3) 3(THF) 2 with 2 equiv. [Ph 3C][B(C 6F 5) 4] and 5 equiv. AlMe 3 ...... 101
3.5 References ...................................................................................................................................... 113
CHAPTER 4. Homopolymerizations and Block Polymerizations of Polar and Non-Polar
Monomers with Trialkyl Rare Earth Metal Complexes ...................................................................... 116
4.1 Introduction .................................................................................................................................... 117
4.2 Results and Discussion .................................................................................................................... 119
4.2.1 Homopolymerization of 1,3-Diene/Olefins ............................................................................. 119
4.2.2 Homopolymerization of Cyclic Ester Monomers .................................................................... 121
4.2.3 Block Copolymerization of 1,3-Diene/Olefins and Cyclic Esters ........................................... 123
4.3 Conclusions .................................................................................................................................... 125
4.4 Experimental Details and Additional Figures. ............................................................................... 125
4.4.1 General Considerations ........................................................................................................... 125
4.4.2 Polymerization Methods ......................................................................................................... 126
4.4.3 Characterization Methods ........................................................................................................ 128
4.5 References ...................................................................................................................................... 130
CHAPTER 5. Investigation into the Mechanism of the Block Copolymerization of Isoprene and
Caprolactone Using Pincer Complexes ................................................................................................. 132
5.1 Introduction ..................................................................................................................................... 133
5.2 Results and Discussion .................................................................................................................... 136
5.2.1 Pincer Ligand Design ............................................................................................................... 136
5.2.2 Rare Earth Metal Complex Design .......................................................................................... 137
5.2.3 Kinetics .................................................................................................................................... 138
5.2.4 Initial Polymerization Data ...................................................................................................... 140
5.3 Conclusions ..................................................................................................................................... 142
5.4 Experimental Details and Additional Figures ................................................................................. 142
5.4.1 General Considerations ............................................................................................................ 142
5.4.2 Characterization Methods ........................................................................................................ 143
5.4.3 Synthetic Procedures ................................................................................................................ 144
5.4.4 NMR Monitoring and Polymerization Kinetics Data .............................................................. 151
5.4.5 Bulk Polymerization Data ........................................................................................................ 157
5.5 References ...................................................................................................................................... 158
BIBLIOGRAPHY ................................................................................................................................... 160
APPENDIX A .......................................................................................................................................... 173
viii
LIST OF TABLES
Table 1.1 Select examples of linear block copolymerizations by different mechanisms and strategies .... 22
Table 2.1 Polymerization of IP or CL with pre-catalysts 6, 7, and 8 ......................................................... 42
Table 2.2 Block copolymerization of IP and CL with pre-catalyst 6 ......................................................... 44
Table 2.3 Investigating different activation conditions with pre-catalyst 6 ............................................... 46
Table 2.4 Homopolymerization of a range of olefin, 1,3-diene and cyclic ester monomers with pre-
catalysts 6 .................................................................................................................................................... 48
Table 2.5 Block copolymerization of 1,3-dienes and cyclic esters with pre-catalyst 6.............................. 49
Table 2.6 Homopolymerization of Different Feed Ratios of IP and CL with 6/[Ph 3C][B(C 6F 5) 4] ............ 58
Table 2.7 Homopolymerization of Olefins and Cyclic Esters with 8/[Ph 3C][B(C 6F 5) 4] ............................ 59
Table 3.1 Polymerization of IP with Y(CH 2SiMe 3) 3(THF) 2 pre-catalyst ................................................... 76
Table 3.2 Neutral donor additives in the homopolymerization of IP with Y(CH 2SiMe 3) 3(THF) 2 ............. 77
Table 3.3 Varying equivalents of [Ph 3C][B(C 6F 5) 4] in the homopolymerization of IP with
Y(CH 2SiMe 3) 3(THF) 2 ................................................................................................................................. 82
Table 3.4 Aluminum alkyl additives in the homopolymerization of IP with Y(CH 2SiMe 3) 3(THF) 2 ......... 84
Table 3.5 Polymerization of IP with RE(CH 2SiMe 3) 3(THF) n pre-catalysts with and without PPh 3 .......... 87
Table 3.6 Addition order in the homopolymerization of IP with RE(CH 2SiMe 3) 3(THF) n ......................... 89
Table 3.7 PPh 3 addition time variation in the homopolymerization of IP with RE(CH 2SiMe 3) 3(THF) n ... 90
Table 3.8 Sequential polymerization of IP using Y(CH 2SiMe 3) 3(THF) 2/[Ph 3C][B(C 6F 5) 4] both with
and without PPh 3 ......................................................................................................................................... 92
Table 3.9 IP polymerization with Y(CH 2SiMe 3) 3(THF) 2, 2 equiv. [Ph 3C][B(C 6F 5) 4], and different
para substituted donors ............................................................................................................................. 111
Table 3.10 Living plot homopolymerization of IP with Y(CH 2SiMe 3) 3(THF) 2 ....................................... 112
Table 3.11 Living plot homopolymerization of IP with Y(CH 2SiMe 3) 3(THF) 2 and PPh 3 ........................ 112
Table 3.12 Homopolymerization of IP with RE trialkyl pre-catalysts both with and without PPh 3 ........ 113
Table 4.1 Homopolymerization of 1,3-dienes and olefins with RE(CH 2SiMe 3) 3(THF) 2 ......................... 120
Table 4.2 Homopolymerization of cyclic esters with RE(CH 2SiMe 3) 3(THF) 2 pre-catalysts .................... 122
Table 4.3 Step-wise block copolymerization with RE(CH 2SiMe 3) 3(THF) 2 pre-catalysts ........................ 124
Table 5.1 Comparison of literature pre-catalyst polymerization conditions for the block
copolymerization of IP and CL ................................................................................................................ 134
Table 5.2 Polymerization isoprene with targeted pre-catalysts ................................................................ 141
ix
LIST OF FIGURES
Figure 1.1 Global production and waste generated during 2015 ................................................................. 4
Figure 1.2 Plastic productions and their ultimate fate .................................................................................. 5
Figure 1.3 How the lifecycle of polymer products has had numerous impacts on the environment and
human society ............................................................................................................................................... 6
Figure 1.4 Converting biomass into degradable thermosets ........................................................................ 8
Figure 1.5 Instilling degradability/recyclability into high-density polyethylene ......................................... 9
Figure 1.6 Adding chemical sophistication to mechanical recycling through compatibilization of
PET/PE ....................................................................................................................................................... 11
Figure 1.7 (a) Biocatalyst approach to PET recycling, (b) chemical catalysis for
cyclodepolymerization ................................................................................................................................ 12
Figure 1.8 Monomer design considering application and recycling .......................................................... 12
Figure 1.9 How different strategies fit together to impact the life cycle of polymers to achieve a
sustainable polymer economy ..................................................................................................................... 14
Figure 1.10 Phase separation behavior of triblock copolymers mimics natural rubber. ............................ 15
Figure 1.11 Synthetic rubber demand in 2020 and their major uses .......................................................... 17
Figure 1.12 Polymerization strategies for the preparation of linear block copolymers ............................. 18
Figure 1.13 General mechanism for chain growth polycondensation ........................................................ 27
Figure 1.14 Mechanism of block copolymerization of IP and CL with a RE metal catalyst ..................... 29
Figure 1.15 Different isomers for polyisoprene polymerizations .............................................................. 30
Figure 2.1 Five pre-catalysts reported for the block copolymerization of CL with S (1), IP (2, 3, 4, 5) ... 39
Figure 2.2 Representative diblock copolymerization of IP and CL with yttrium pre-catalyst 2 ................ 39
Figure 2.3 (a) Targeted yttrium BDI complexes for the block copolymerization of 1,3-dienes with
polar monomers. (b) Reported synthetic pathway to targeted yttrium BDI complexes .............................. 41
Figure 2.4 Comparison of activated complex 6 where BDI = {MeC(NDIPP)CHC(Me)[N(2-
OMeC 6H 4)]}Y(CH 2SiMe 3) 2 (DIPP = 2,6-
i
Pr 2C 6H 3) and counter anions are [B(C 6F 5) 4] ............................. 45
Figure 2.5 Proposed selective hydrolysis of CL block and repolymerization to IP:CL diblock
copolymer ................................................................................................................................................... 50
Figure 2.6 GPC traces of PIP (M n=42 kDa, Đ=1.15), recovered PIP (M n=43 kDa, Đ=1.15),
PIP-b-PCL (M n=55 kDa, Đ=1.09), and repolymerized PIP-b-PCL (M n= 56 kDa, Đ=1.16) ...................... 51
Figure 2.7
1
H NMR stack of Y[N(SiMe 3) 2] 3 before (bottom) and after recovered IP 50 addition (top)
in toluene-d 8 at 298 K ................................................................................................................................. 57
Figure 2.8
1
H NMR stack of PIP-b-PCL repolymerization with Y[N(SiMe 3) 2] 3 and PIP 50 in
toluene-d 8 at 298 K. After macroinitiator formation (bottom), 20 minutes after CL addition (middle),
and 6 h after CL addition (top).................................................................................................................... 57
Figure 2.9
19
F NMR of [Ph 3C][B(C 6F 5) 4] in C 6D 6 at 298 K ....................................................................... 60
Figure 2.10
13
C NMR spectrum of PS generated by 6/[Ph 3C][B(C 6F 5) 4] from Table 2.4, entry 4 in
CDCl 3 at 298 K (30 min). Representative peak assignment for PS ............................................................ 61
Figure 2.11
1
H NMR spectrum of PIP-b-PCL 50:50 equivalents generated by 6/[Ph 3C][B(C 6F 5) 4] in
CDCl 3 at 298 K ........................................................................................................................................... 61
Figure 2.12
13
C NMR spectrum of PIP-b-PCL 50:50 equivalents generated by 6/[Ph 3C][B(C 6F 5) 4] in
CDCl 3 at 298 K ........................................................................................................................................... 62
Figure 2.13 FT-IR spectrum of PIP-b-PCL 50:50 equivalents generated by 6/[Ph 3C][B(C 6F 5) 4] ............. 62
Figure 2.14 GPC spectrum of PIP-b-PCL 50:50 equivalents generated by 6/[Ph 3C][B(C 6F 5) 4]: (left)
LS; (right) RI............................................................................................................................................... 63
Figure 2.15 TGA curve of PIP-b-PCL 50:50 equivalents generated by 6/[Ph 3C][B(C 6F 5) 4] ..................... 63
Figure 2.16 DSC curve of PIP-b-PCL 50:50 equivalents generated by 6/[Ph 3C][B(C 6F 5) 4] ..................... 64
Figure 2.17
1
H NMR spectrum of recovered PIP 50 equivalents after hydrolysis of PIP-b-PCL 50:50
in CDCl 3 at 298 K (12 h). Alcohol Peak at 3.65 ppm ................................................................................. 64
Figure 2.18 FT-IR spectrum of recovered PIP 50 after hydrolysis of PIP-b-PCL 50:50 (12 h) ................ 65
x
Figure 2.19 DOSY NMR spectrum of PIP-b-PCL 800:300 equivalents generated by 6 and 0.5
equivalents of [Ph 3C][B(C 6F 5) 4] from Table 2.3, entry 3 in CDCl 3 at 298 K ............................................ 65
Figure 2.20 DOSY NMR spectrum of PIP-b-PCL 800:300 equivalents generated by 6 and 1.5
equivalents of [Ph 3C][B(C 6F 5) 4] from Table 2.3, entry 4 in CDCl 3 at 298 K ............................................. 66
Figure 3.1 Trialkyl RE metal complexes for the polymerization of ethylene or isoprene ......................... 66
Figure 3.2 Proposed yttrium pre-catalyst activation with 1 or 2 equivalents of [Ph 3C][B(C 6F 5) 4],
assuming no interaction with toluene or the borate anion........................................................................... 74
Figure 3.3 Hammett plot of log (K x/K H) versus the standard σ constants for the substituent. Reactions
run analogous to those in Table 2 ............................................................................................................... 79
Figure 3.4 Comparison of cis-1,4 and trans-1,4 selectivity versus the standard σ constants for the
different substituents. Reactions run analogous to those in Table 2. .......................................................... 79
Figure 3.5 M n vs conversion graphs for IP polymerization with Y(CH 2SiMe 3) 3(THF) 2 and 2 equiv.
[Ph 3C][B(C 6F 5) 4]; (a) without PPh 3; (b) with 1 equiv. PPh 3 ....................................................................... 80
Figure 3.6 Summary of major findings of this work .................................................................................. 93
Figure 3.7 In situ
1
H NMR spectrum of the monocationic active species
[Y(CH 2SiMe 3) 2(THF) 2]
+
[B(C 6F 5) 4]
-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 1 equiv.
[Ph 3C][B(C 6F 5) 4] in toluene-d 8 at room temperature .................................................................................. 97
Figure 3.8 In situ
1
H NMR spectrum of the monocationic active species
[Y(CH 2SiMe 3) 2(THF) 2]
+
[B(C 6F 5) 4]
-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 1 equiv.
[Ph 3C][B(C 6F 5) 4] after IP addition in toluene-d 8 at room temperature ....................................................... 98
Figure 3.9 In situ
19
F NMR spectrum of the monocationic active species
[Y(CH 2SiMe 3) 2(THF) 2]
+
[B(C 6F 5) 4]
-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 1 equiv.
[Ph 3C][B(C 6F 5) 4] in toluene-d 8 at room temperature .................................................................................. 98
Figure 3.10 In situ
1
H NMR spectrum of the dicationic active species
[Y(CH 2SiMe 3) 2(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2
equiv. [Ph 3C][B(C 6F 5) 4] in toluene-d 8 at room temperature ....................................................................... 99
Figure 3.11 In situ
19
F NMR spectrum of the dicationic active species
[Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 1 equiv.
[Ph 3C][B(C 6F 5) 4] in toluene-d 8 at room temperature .................................................................................. 99
Figure 3.12 GPC spectrum of PIP 500 equivalents generated by Y(CH 2SiMe 3) 3(THF) 2 and 1
equivalent [Ph 3C][B(C 6F 5) 4] from Table 3.1, entry 1 (30 min): (left) LS; (right) RI ............................... 100
Figure 3.13 GPC spectrum of PIP 500 equivalents generated by Y(CH 2SiMe 3) 3(THF) 2 and 1
equivalent [Ph 3C][B(C 6F 5) 4] from Table 3.3, entry 1 (7 h): (left) LS; (right) RI ..................................... 100
Figure 3.14 GPC spectrum of PIP 500 equivalents generated by Y(CH 2SiMe 3) 3(THF) 2, 2 equivalents
[Ph 3C][B(C 6F 5) 4], and 1 equivalent PPh 3 from Table 3.2, entry 8 (30 min): (left) LS; (right) RI ........... 101
Figure 3.15 In situ
31
P NMR spectrum of the dicationic active species
[Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2
equiv. [Ph 3C][B(C 6F 5) 4] and 1 equiv. PPh 3 added 10 min after activation in toluene-d 8 at room
temperature. Reaction monitored overtime,
31
P NMR taken in 5 minute intervals .................................. 101
Figure 3.16 In situ
31
P NMR spectrum of 1 equiv. PPh 3 with 2 equiv. [Ph 3C][B(C 6F 5) 4] in toluene-d 8
at room temperature. Reaction monitored overtime,
31
P NMR taken in 5 minute intervals .................... 102
Figure 3.17 In situ
31
P NMR spectrum of the dicationic active species
[Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2
equiv. [Ph 3C][B(C 6F 5) 4] and 1 equiv. PPh 3 in toluene-d 8 at -80 ºC .......................................................... 103
Figure 3.18 DOSY NMR spectrum of the dicationic active species
[Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2
equiv. [Ph 3C][B(C 6F 5) 4] in toluene-d 8 at room temperature .................................................................... 103
Figure 3.19 DOSY NMR spectrum of the dicationic active species
[Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2
equiv. [Ph 3C][B(C 6F 5) 4] and 1 equiv. PPh 3 in toluene-d 8 at room temperature ........................................ 104
xi
Figure 3.20 GPC spectrum of PIP 500 equivalents generated by Y(CH 2SiMe 3) 3(THF) 2 and 1.5
equivalents [Ph 3C][B(C 6F 5) 4] from Table 3.3, entry 3 (7 h): (left) LS; (right) RI .................................... 104
Figure 3.21 GPC spectrum of PIP 500 equivalents generated by Y(CH 2SiMe 3) 3(THF) 2 and 2
equivalent [Ph 3C][B(C 6F 5) 4] from Table 3.3, entry 5 (7 h): (left) LS; (right) RI ...................................... 105
Figure 3.22 GPC spectrum of PIP 500 equivalents generated by Y(CH 2SiMe 3) 3(THF) 2 and 2.5
equivalents [Ph 3C][B(C 6F 5) 4] from Table 3.3, entry 7 (7 h): (left) LS; (right) RI .................................... 105
Figure 3.23 In situ
1
H NMR spectrum of the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 3 equiv.
[Ph 3C][B(C 6F 5) 4] in toluene-d 8 at room temperature. NMR taken 10 minutes after catalyst addition ..... 106
Figure 3.24 In situ
1
H NMR spectrum of the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2 equiv.
[Ph 3C][B(C 6F 5) 4] and 5 equiv. AlMe 3 in toluene-d 8 at room temperature ................................................ 107
Figure 3.25 In situ
27
Al NMR spectrum of the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2
equiv. [Ph 3C][B(C 6F 5) 4] and 5 equiv. AlMe 3 in toluene-d 8 at room temperature ..................................... 107
Figure 3.26 In situ
1
H NMR spectrum of the dicationic active species
[Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2
equiv. [Ph 3C][B(C 6F 5) 4] and PPh 3 added at time 0 min in toluene-d 8 at room temperature ..................... 108
Figure 3.27 In situ
1
H NMR spectrum of the dicationic active species
[Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2
equiv. [Ph 3C][B(C 6F 5) 4] and PPh 3 added at time 10 min in toluene-d 8 at room temperature ................... 109
Figure 3.28 In situ
1
H NMR spectrum of the dicationic active species
[Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2 equiv.
[Ph 3C][B(C 6F 5) 4] and PPh 3 added at time 30 min in toluene-d 8 at room temperature .............................. 110
Figure 4.1 Trialkyl rare earth metal pre-catalysts .................................................................................... 119
Figure 4.2 DOSY NMR spectrum of Myr-b-CL block copolymer generated by
Gd(CH 2SiMe 3) 3(THF) 2 with 2 equiv. [Ph 3C][B(C 6F 5) 4] in CDCl 3 at room temperature .......................... 129
Figure 4.3 DOSY NMR spectrum of Myr-b-CL block copolymer generated by
Gd(CH 2SiMe 3) 3(THF) 2 with 2 equiv. [Ph 3C][B(C 6F 5) 4] in CDCl 3 at room temperature after acetone
wash .......................................................................................................................................................... 130
Figure 5.1 PNP pincer pre-catalysts for 1,3-diene polymerization ......................................................... 135
Figure 5.2 Synthesized pincer ligands as ancillary supports for rare earth metal complexes .................. 136
Figure 5.3 Synthesized rare earth metal complexes ................................................................................ 137
Figure 5.4 Crystal structure of 2
tbu
open crystallized from THF/pentane solution at -35 °C .................... 137
Figure 5.5 VT-
31
P NMR of polymerization of ε-caprolactone in toluene-d 8 at -30 ºC ........................... 139
Figure 5.6
31
P NMR spectrum of
tbu
open in CDCl 3 at 25 ºC ................................................................... 145
Figure 5.7
1
H NMR spectrum of 2
tbu
closed in C 6D 6 at 25 ºC. relative integrations consistent with two
benzyl groups and one THF bound to the metal center ............................................................................ 147
Figure 5.8
31
P NMR spectrum of 2
tbu
closed in C 6D 6 at 25 ºC. Doublet at -4.8 ppm indicates ligand
bound to an Y metal center ...................................................................................................................... 147
Figure 5.9
1
H NMR spectrum of 2
tbu
open in C 6D 6 at 25 ºC. Relative integration of the
t
Bu on the
ligand to the CH 2 of the benzyl group indicates two alkyls bound to the metal center ........................... 148
Figure 5.10
31
P NMR spectrum of 2
tbu
open in C 6D 6 at 25 ºC. Doublet at -5.5 suggests ligand is a
cleanly bound to the yttrium metal center ................................................................................................ 148
Figure 5.11
31
P NMR spectrum of 1
tbu
closed in C 6D 6 at 25 ºC ............................................................... 150
Figure 5.12
31
P NMR spectrum of 1
tbu
open in C 6D 6 at 25 ºC .................................................................. 150
Figure 5.13 In situ
31
P NMR spectrum cationic active species synthesized by reacting 1 equivalent
1
H
open with 1 equiv. [Ph 3C][B(C 6F 5) 4] in Toluene-d 8 at room temperature ........................................... 151
Figure 5.14 In situ
19
F NMR spectrum of cationic active species synthesized by reacting 1 equivalent
1
H
open with 1 equiv. [Ph 3C][B(C 6F 5) 4] in toluene-d 8 at room temperature ............................................ 151
Figure 5.15 Representative
1
H NMR of the polymerization of IP with 1
H
open activated by 1 equiv.
[Ph 3C][B(C 6F 5) 4] in Toluene-d 8 at -30 ºC ................................................................................................ 152
xii
Figure 5.16 COPASI fit for IP polymerization (concentration = 0.408 M) with 1
H
open. Filled in
colored circles represent experimental data. (Blue = PIP, Red = IP). Lines represent the fit data for
their respective species. The open circles represent how good the fit was to the experimental data ........ 153
Figure 5.17 Representative
1
H NMR of the polymerization of CL with 1
H
open activated by 1 equiv.
[Ph 3C][B(C 6F 5) 4] in Toluene-d 8 at -30 ºC ................................................................................................ 154
Figure 5.18 COPASI fit for CL polymerization (concentration = 0.059 M) with 1
H
open. Filled in
colored circles represent experimental data. (Blue = PCL, Red = CL). Lines represent the fit data for
their respective species. The open circles represent how good the fit was to the experimental data ........ 155
Figure 5.19 COPASI fit for CL polymerization (concentration = 0.059 M) with 1
tBu
closed. Filled in
colored circles represent experimental data. (Blue = PCL, Red = CL). Lines represent the fit data for
their respective species. The open circles represent how good the fit was to the experimental data ........ 156
Figure 5.20
13
C NMR spectrum of polyisoprene-b-caprolactone diblock copolymer generated by
1
tbu
open (500 MHz, CDCl 3) ...................................................................................................................... 157
Figure 5.21 Thermal gravimetric analysis of polyisoprene-b-caprolactone diblock copolymer
generated by 1
tbu
open ............................................................................................................................... 158
xiii
ABSTRACT
The development of catalysts for the stereospecific living polymerization of 1,3-dienes and olefins
is an area of intense interest for both academic and industrial laboratories. This is attributed to the
exceptional physical and mechanical properties exhibited by these polymers, as well as the versatility of
poly(1,3-diene) materials in numerous applications. Although Ziegler-Natta catalysts have been
traditionally associated with this type of polymerization, their effectiveness is hindered by their limited
tolerance towards polar functional groups. On the other hand, alkyl rare earth metal complexes supported
by a diverse range of ligand frameworks have demonstrated promising selectivity for the polymerization of
1,3-dienes, while maintaining stability in the presence of polar monomers. Rare earth metals present an
intriguing avenue for studying 1,3-diene polymerization and copolymerizations, as they possess a unique
gradual change in ionic radii and Lewis acidity unmatched by other elements in the periodic table.
Furthermore, the steric and electronic properties of the supporting ligand frameworks can be carefully
tailored to influence the rate and selectivity of these polymerization reactions. However, it is worth noting
that ligand frameworks often play a crucial role in providing stability, yet their synthesis can be both costly
and time-consuming. Regrettably, only a limited number of studies in the literature have delved into
understanding the diverse trends that lead to the development of fast and selective catalysts, especially for
copolymerizing multiple monomers. This work aims to identify optimal rare earth metal centers, catalyst
ligand environments, activation conditions, and reaction parameters for achieving rapid, controlled, and
selective polymerization of 1,3-dienes, as well as copolymerization of 1,3-dienes with cyclic esters. The
insights gained from these investigations will hopefully pave the way for the optimization of other similar
systems.
1
CHAPTER 1
General Introduction
A portion of this chapter has appeared in print:
Kosloski-Oh, S. C.; Fieser, M. E. One Earth 2023, 6, 587-590.
2
1.1 The World in a Plastic Disaster
Our modern lifestyle has undoubtedly benefited from polymers, but their durability and persistence
have created a major environmental challenge: plastic pollution. This raises the question of how we can
make commercial polymers a sustainable option without compromising their useful functionality and
ensuring global economic viability. Finding sustainable solutions for all types of commercial polymers is a
daunting task that requires a multi-prong approach. Commodity synthetic polymers, commonly the main
component of what is termed “plastics”, are long-chain molecules composed of repeating chemical units,
or monomers, that have had a profound positive impact on modern life.
Since mass production of polymers began to accelerate in the wake of World War II, global demand
has skyrocketed as materials that used to make our products (e.g., wood, metal, ivory) are being superseded
by polymers.
1
This increase is evident as between the years 1950 and 2019 the global annual production of
polymers has risen exponentially from 2 million tonnes to 460 million tonnes per year.
2
It is unsurprising
that polymers have integrated themselves into our lives as they have exceptional versatility in their thermal
and mechanical properties, are cheap and easy to manufacture, have an abundance of source material, and
are highly durable and stable. Because of these attributes they have boosted food preservation, improved
medical safety while lowering costs, and increased transportation efficiency by reducing vehicle weight.
3
Moreover, their integration has yielded positive environmental outcomes, such as reducing metal mining
and CO 2 emissions across industries, such as manufacturing and transportation. In most cases, other
alternatives to commodity polymers like glass, aluminum, steel, and paper produce more greenhouse gas
(GHG) emissions while also introducing other environmental concerns such as energy consumption,
toxicity, water depletion, tree farming, and overuse of fertilizer.
4
Polymers also play a critical role in the
production of renewable energy through wind turbines and solar cells.
5
However, the very same qualities that make polymers valuable also pose a significant problem.
Their exceptional stability means they persist in the environment for extended periods, often taking
hundreds or even thousands of years to degrade.
6
Instead of biodegrading, most commodity polymers are
3
weathered by UV radiation, sea water, and are fragmented by mechanical forces such as waves into small
pieces or microplastics. Microplastic is defined as polymer fragments less than 5 mm in size or more
recently as <2mm in size at least in one dimension and are further categorized into primary and secondary
classifications.
7,8
Primary microplastics refers to polymers that were originally manufactured to be small
and are subsequently released into the environment, while secondary microplastics refers to polymers that
were once part of a larger item that has been weathered down into smaller pieces. No matter their origin,
virtually no place on earth has not been affected by microplastic pollution, from being found in the smallest
organisms to having been detected in Antarctic snow.
9,10
They have completely permeated our society and
ecosystems, including human blood and breastmilk, and estimates have shown that on average each human
will consume about 44 pounds of plastic throughout their life.
11,12
While the long-term health implications
are still under investigation, microplastics have been linked to inflammation, oxidative stress, and metabolic
homeostasis in humans.
13,14
Marine life has already suffered fatal consequences from ingestion of non-
digestible plastics as it is estimated that about 100 thousand marine animals die as a result of entanglement
or ingestion of plastic.
15
With the ever-increasing demand for plastic items, accumulation in our ocean and
environment will soon reach critical levels unless we embrace a system change to improve the sustainability
of our commercial plastics.
There are two broad types of polymers manufactured on a large commercial scale: thermosets and
thermoplastics. Thermosets are cured to form irreversible chemical cross-links, resulting in a permanent
insoluble molecular structure that is resistant to high temperatures, chemicals, and deformation.
16
They are
mostly used for long-term applications such as automotive, aerospace, electronics, construction, and
electrical insulation because of their high durability. Common examples of thermosets include
polyurethanes (PUR), epoxy resins, phenolic resins, and unsaturated polyester resins. Thermoplastics,
unlike thermosets, have weak intermolecular forces between polymer chains, and maintain their molecular
structure even after multiple heating and cooling cycles. This allows them to be molded and reshaped
multiple times without undergoing significant chemical change. Due to their processability, thermoplastics
4
can be formed into various shapes using techniques such as injection molding, extrusion, blow molding,
and thermoforming.
17
They are commonly used in a wide range of applications, including packaging,
automotive parts, consumer goods,
medical devices, pipes, and
electrical insulation. There are six
major types of thermoplastics: low-
density polyethylene (LDPE), high-
density polyethylene (HDPE),
polypropylene (PP), polyvinyl
chloride (PVC), polystyrene (PS),
and polyethylene terephthalate (PET) which makes up about 87% of the global plastics market.
18
These
non-fiber plastics are predominantly used for single use packaging (approximately 42%), meaning that the
plastics we produced the most of are the ones that have the shortest period of use. Thermosets only account
for around 10% of the market, and the top contributor is PUR. Figure 1.1 presents the top nine contributors
of global plastic production and waste generated in 2015. Polyester, polyamide, and acrylic fibers (PP&A
fibers) mostly consists of PET when it is used in textiles. Additives, which are important to enhance polymer
properties such as plasticizers, fillers, and flame retardants, also contribute to plastic pollution. In total
during 2015, 407 million metric tonnes (Mt) of plastic were produced and 302 Mt was thrown out. Between
1950 to 2015, a staggering 8300 Mt of plastic material has been made, with 30% of it still in use (Figure
1.2).
19
A concerning 55% of all plastic ever made has been discarded while another 9% has been incinerated
either with or without energy recovery. Only a mere 6% of plastic has been recycled once and only 1% has
been recycled more than once.
This data shows that current recycling strategies are insufficient to make the current plastic use
sustainable, indicating the need to rethink the current model. When considering various avenues to improve
Figure 1.1. Global production and waste generated during 2015.
18
57
40
55
17 15
32
16
42
17
64
52
68
25
38
33
27
59
25
2015 Primary Production (Mt) 2015 Primary Waste Generation (Mt)
5
the sustainability of polymers, two
predominant themes emerge:
improving the sustainability of either
the material’s source or its end-of-
life management. Irrespective of the
chosen approach, it is essential that
the processing and use of polymer
items remain uninterrupted. This
implies that the polymers used to
address sustainability concerns must retain the physical properties required for their intended functions.
These two themes often center around the sourcing for biofeedstocks and recycling/upcycling for materials
at the end of their lifecycle. Most strategies primarily target disposable polymer applications such as single
use items, as these materials contribute most significantly to environmental accumulation and offer the
greatest potential for life-cycle enhancements.
20
The truth is, no single solution exists to combat plastic pollution, primarily because of the vast
number of plastics developed for different applications. Improving the sustainability of polymers requires
considering the entire life cycle of the material to ensure that it is more sustainable than current commercial
products. We need to consider the source of the chemicals, polymer manufacturing processes, product
engineering, product use and lifespan, and finally end-of-life disposal (Figure 1.3). Achieving this
necessitates collaboration between experts from diverse fields including chemistry, engineering,
environmental science, and economics. The solutions should be affordable for consumers, profitable for
manufacturers, and should not require dramatic lifestyle changes or compromise product effectiveness.
Figure 1.2. Plastic productions and their ultimate fate.
19
Discarded: 55%
1950-2015
8300 Mt of
plastic
Produced
Incinerated: 9%
Recycled: 6%
Being used: 30%
Recycled more then once: 1%
1
6
1.2 Polymer Source
Considering the beginning of a polymer’s life can be just as important as considering its end-of-
life in respect to sustainability. Most polymers produced are sourced from fossil resources, and in 2019 it
is estimated to have consumed about 5-7% of the global oil supply.
21
This has released >850 million tonnes
of CO 2 into the atmosphere which is about 2% of all CO 2 emitted that year. Alternatives to petroleum, such
as biomass, are often costly and only account for <1% of global polymer production.
22,23
A study modeling
GHG emissions of petroleum-based plastics vs sugarcane-based plastics showed that the major difference
in CO 2 emissions was in the production step when considering the entire lifespan of a plastic item.
Switching 100% of monomer source to sugarcane could reduce GHG emissions by 25%.
24
However, they
did note that based on a production rate of about 250 million tonnes of bio-based polymers per year, it
would take as much as 5% of all arable land, which would negate the carbon benefits of moving away from
fossil fuels.
25
Additionally, if 100% recycling could be achieved for all fossil fuel-based polymers, GHG
Figure 1.3. How the lifecycle of polymer products has had numerous impacts on the environment
and human society.
7
emissions could be equivalent or lower compared to bio-based polymers. This means that development of
chemical recycling could be just as effective at mitigating GHG emissions as switching to bio-sources.
Biopolymers, or bioplastics, have gained significant attention and are touted as the potential
solution to plastic sustainability. Biopolymers encompass a broad range of materials that share one or more
characteristic such as being derived from biofeedstocks, being biodegradable, or being produced via
biological processes.
26
Currently, only 4 Mt of bioplastics are produced annually although this could be
changing soon.
27
Recently, current United States President Joe Biden announced a target of achieving 90%
bioplastics in the next 20 years.
28
While this would reduce the use of petroleum resources, it does nothing
to address plastics that are already in our environment. To identify whether complete replacement of
polymer items with biopolymers is the most sustainable solution, it is important to understand what
biopolymers are and to assess their benefits and drawbacks.
There are several approaches researchers take when using biopolymers. One approach involves
using biofeedstocks to produce the same monomers as those derived from petroleum, which are referred to
as ‘drop-in’ polymers.
29
This includes the synthesis of monomers for PET, poly(isoprene) (PI), and
polyolefins, although these biosourced polymers are neither more degradable nor more recyclable than their
petroleum-based counterparts. While this avoids the issue of using and refining petrochemicals, it does not
address the end-of-life of these polymers. Nonetheless, this strategy can prove particularly valuable for
polymers that already have established end-of-life strategies, such as PET, where mechanical and chemical
recycling are viable options.
30
Another strategy involves creating new polymers from biodegradable or compostable
biofeedstocks. This leads to polymers that range dramatically in how much the biofeedstock is processed,
but generally result in polymers with oxygen in the backbone which imparts degradability. These polymers
can have useful and applicable properties for societal use, but can exhibit thermal, environmental, or
mechanical limitations that may require additives for improvement or a narrowed scope in application. For
example, synthesizing degradable thermoset polymers from biomass presents challenges as a delicate
8
balance must be found between degradability and mechanical properties. Biomass-derived materials can
form highly crosslinked networks, that are resistant to degradation.
31
Additionally, many biomass-derived
materials have limited thermal stability, which restricts their usefulness as thermosetting materials.
Recently, Ellison and Reineke developed the use of levoglucosan as a biobased feedstock for 3D printable
thermoset materials (Figure 1.4).
32
Importantly, they address the end-of-life of these materials by
demonstrating their complete hydrolytic degradability.
Other important factors to
consider when using biomass
feedstocks is where and how the
polymers break down. The
environmental and health effects of
the molecular products released
from the polymer during
degradation makes them not always ideal for their intended applications. The use of robust, scalable
biofeedstocks that do not impact food needs is also critical. While biopolymers are undoubtedly an
important component of the solution, they cannot be the sole answer.
1.3 End-of-life Considerations
Most commercial synthetic polymers produced are derived from petroleum resources and often are
non-degradable due to strong carbon-carbon bonds in the backbone. Polymers can also be biodegradable or
compostable. Biodegradable polymers can be broken down into simple compounds (CO 2, H 2O, and biomass)
through natural biological processes (e.g., microorganisms) but may leave behind toxic residues.
33
Biodegradable polymers generally have functional groups in the backbone such as esters, amides, or
carbohydrates which make them more susceptible to enzymatic or microbial attack. The rate and extent of
biodegradation can vary depending on factors such as environmental conditions (temperature, moisture, pH,
Figure 1.4. Converting biomass into degradable thermosets.
32
Ellison and Reineke 2023
9
and oxygen), the presence of specific microorganisms capable of metabolizing the compound, and the
chemical properties of the polymer itself.
Compostable polymers are a subset category of biodegradable polymers but have a more restrictive
definition. They will completely break down into CO 2, H 2O, and biomass and not release harmful by-
products.
34
Also, it's important to note that compostable materials are often designed to break down in
industrial composting facilities or specific composting systems that provide the necessary conditions, such
as temperature, moisture, oxygen, and microbial activity, to facilitate efficient decomposition. Compostable
materials may not break down as intended in other environments, such as landfills or natural settings so it
is important that the consumer place the product in the appropriate garbage bin. These terms are often
misidentified by consumers, so clearer definitions and regulations should be developed surrounding the
difference between biodegradable or compostable. In a psychological study by Miodownik et al. on the
general UK population they found that many people misunderstood the difference between these two terms,
and this caused a substantial barrier to people buying products labeled as such.
35
Strategies have recently
been emerging to add degradability/recyclability by incorporating functional groups into previously non-
degradable polymers while minimizing the impact on their properties and application. This is especially
important for polymers without heteroatoms in the backbone where chemically recycling is difficult.
Adding degradability is often accomplished through the incorporation of functional groups along a
polyolefin chain. A notable example of this strategy was presented by Coates et al. where they envisioned
a pathway for the chemical recycling of high-density polyethylene (HDPE), a polymer found in milk jugs
(Figure 1.5).
36
This
strategy involved the
dehydrogenation of
HDPE, to install
double bonds followed by metathesis and hydrogenation to create low molecular weight fragments. With
Figure 1.5. Instilling degradability/recyclability into high-density polyethylene.
36
LaPointe, Delferro, and Coates 2022
10
the inclusion of diethanolamine, a polyethylene-like polymer could be synthesized with comparable thermal
and mechanical properties to virgin polyethylene, with pathways for selective depolymerization.
There are four classifications given for polymer recycling: primary and secondary (mechanical
recycling), tertiary (chemical recycling), and quaternary (incineration).
37
Both primary and secondary
recycling are methods of mechanical recycling but differ in that primary recycling refers to post-industrial
polymer or uncontaminated post-consumer polymers while secondary recycling refers to post-consumer
waste. Mechanical recycling is a physical method of recycling, meaning that the material will not undergo
significant chemical changes. It involves sorting, cleaning, grinding, and melting plastic waste to produce
recycled plastic materials that can be used as raw materials in the manufacturing of new products. This is a
desirable method as it helps conserve natural resources by reducing the need for virgin plastic and reducing
the demand for petroleum or natural gas. It also has many environmental benefits compared to making
virgin plastics, such as generally requiring less energy, emitting less greenhouse gas, and diverting plastic
waste from landfills or incineration. However, exploring the economic viability of this strategy is difficult,
as most data is proprietary. A study by Faraca et al. comparing pyrolysis to mechanical recycling schemes
determined that the latter method provided economic benefits.
38
Other studies have shown that mechanical
recycling of PS could be profitable and give a return rate of 14% but that other polyolefin-based polymers,
especially mixed polyolefins, would have their profitability dependent on oil prices.
39
They concluded that
government intervention was needed to insure profitability. Secondary recycling is the most common
strategy employed to recycle post-consumer polymers, but the resulting products are generally different
from the original product. Additionally, the material’s mechanical properties are often degraded, leading to
only limited reprocessability (generally 2-3 times). Many polymers cannot be continuously mechanically
recycled due to chain scission events that disrupt the polymer properties, which is exacerbated by foreign
substances recycled with the polymer. Improvements in separating and cleaning plastic waste will allow
for more consistent methods of removing foreign matter from the polymer, minimizing this issue. When
separation is challenging, compatibilizers are being developed to mechanically recycle immiscible polymer
11
mixtures to higher performing materials. In this case, the compatibilizers will allow the combined
mechanical recycling of two or more incompatible polymers to create a new polymer blend with advanced
properties. In a recent publication, Ellison et al. presented a strategy to compatibilize PET/LLDPE blends
by dosing in hydroxy-telechelic polyethylene (HO-PE-OH) which forms PET-b-PE-b-PET triblock
copolymers during the melt processing (Figure 1.6).
40
This strategy is particularly attractive as PET and
PE are among the most widely produced plastics and are also used in tandem for layered packaging.
41
Furthermore, additional sorting is required as mixing
materials that have undergone different amounts of
mechanical recycling leads to even further depreciated
mechanical properties and eliminates any further
reprocessability. It is also limited in applicability as only
thermoplastics can undergo this labor-intensive process and
necessitates quality control of plastic waste streams.
Quaternary recycling or direct incineration of polymers has been the fate of about 12% of all
commodity polymers made and can be coupled with an energy recovery process to convert heat to steam
or electricity, although it only recovers half of the energy that is required to heat the material (16 kJ/kg vs.
36 kJ/kg).
42
Additionally, many toxic by-products and GHG are released during this process. This is a
particular problem in rural and developing areas where unregulated and unfiltered burning of waste is
common.
43
Tertiary recycling or chemical recycling presents an attractive alternative when mechanical
recycling cannot be used effectively such as in the case of layered materials or mixtures of immiscible
polymers. In these cases, polymers can be depolymerized back into their constituent monomers, and then
used to make the same product without the need for virgin material. A recent review highlights advances
in synthesizing new polymers that can depolymerize back into their monomers, thus promoting a circular
economy.
44
The use of a catalyst, whether it is chemical or biological based, can be important for many
Figure 1.6. Adding chemical sophistication
to mechanical recycling through
compatibilization of PET/PE.
40
Ellison 2022
12
polymers to improve the efficiency and selectivity of the products.
30
Solvolysis of commercial polymers
such as PET (a plastic frequently found in water bottles) has been a long-standing field, especially given
how ubiquitous PET waste is. In addition to traditional solvolysis strategies to tackle PET waste, Beckham,
McGeehan, and Pickford have been demonstrating the
use of the enzyme, IsPETase, as an effective method to
break PET into monomeric units (Figure 1.7, part a).
45
Other examples of different strategies to convert
polymers back to monomers, such as
cyclodepolymerization, are still emerging. Byers et al.
have shown the selective cyclodepolymerization of
multiple industrial polyesters including polylactic acid
and polycaprolactone using a simple earth abundant
Lewis acid catalyst (Figure 1.7, part b).
46
One way to promote depolymerization is to subtly modify the chemical structure of a well-known
monomer to find a balance between the thermodynamics of polymerization/depolymerization. Chen et al.
have contributed much to this strategy and have recently demonstrated the use of α,α-disubstituted
polyhydroxyalkanoates (PHA)
synthesized either from monomers
derived from the step-growth
polymerization of acetaldehyde, or the
lactonization of hydroxy acid followed by
ring-opening polymerization (ROP)
(Figure 1.8).
47
This polymer
demonstrated robust thermal stability and tough mechanical properties. A closed-loop cycle could be
Figure 1.8. Monomer design considering application and
recycling.
47
Chen 2023
Figure 1.7. (a) biocatalyst approach to PET
recycling,
45
(b) chemical catalysis for
cyclodepolymerization.
46
b. Byers 2023
a. Beckham, McGeehan, Pickford 2023
13
realized by undergoing a chain-unzipping process back to the lactone or hydrolytic depolymerization into
hydroxy acid.
Chemical recycling prevents waste from entering landfills or the environment and can be used on
a wide range of chemical functional groups especially those which can’t be recycled mechanically. Also,
because it eliminates the need for additional feedstock, it would reduce GHG as the extraction and
purification process tends to be the highest polluting steps. However, many new technologies are still
emerging and are not yet completely commercially viable or scalable. Additionally, chemical recycling can
be energy-intensive and currently still costly.
Another chemical recycling strategy, upcycling, centers around converting post-consumer
polymers into a new product that serve a different purpose than the original polymer.
48
The term ‘upcycling’
is not well-defined, meaning there is no standard for what the value of the product is being compared to.
For example, the cost/value of upcycled products can be compared to the value of virgin polymer products,
waste polymer products, mechanically recycled polymer products, or the cost of recycling the waste
polymer products back to virgin polymer. Nonetheless, the upcycled product should be commercially more
valuable than the original polymer and ideally have a sustainable end-of-life. This strategy is particularly
valuable when the commercial polymers are anchored by an all-carbon backbone, making chemical
recycling unfavorable and/or inefficient. Amongst numerous emerging ideas, our group has been interested
in repurposing PVC waste through catalytic methods into useful polymer and chlorine-containing products,
while reducing the environmental hazards of this chlorinated polymer. This method generated various
poly(ethylene-co-styrene) copolymers through the tandem hydrodechlorination/Friedel-Crafts alkylation
using a silylium ion catalyst generated in aromatic solvents.
49
The reality is that there is no singular solution to improve polymer sustainability, primarily because
of the extensive range of polymers developed for diverse applications. Improving the sustainability of
polymers necessitates a comprehensive assessment of the entire material’s life cycle, ensuring it surpasses
the sustainability of current commercial products. As previously discussed, solutions must consider
14
chemical sourcing, polymer
manufacturing, product
engineering, product use and
lifespan, and finally end-of-life
disposal, while also providing
practical and consumer-friendly
approaches (Figure 1.9). Most
importantly, these solutions need
to consider how they will
integrate into society long-term
and how they might impact other
strategies. The readiness for
industrial implementation may
vary among the solutions
discussed, depending on the
specific application, but the field continues to witness a proliferation of ongoing innovations. Another
barrier is the slow translation of new innovations into a practical reality, which may require more effective
communication between academic and industrial sectors.
The success of various solutions that hold promise for enhancing sustainability of polymers
ultimately hinges upon the public’s proper use of these initiatives and their willingness to integrate them
into their life. With numerous economic articles suggesting that sending all polymers to landfills is the best
option, the public’s trust in recycling tends to wane.
50
Greenwashing practices within the industry further
erode consumer confidence, leading them to question whether paying a premium for sustainable polymer
items truly benefits society. Also, confusion over sustainable terms such as “compostable” and “degradable”
are causing consumers to avoid buying products with these labels.
Figure 1.9. How different strategies fit together to impact the life cycle of
polymers to achieve a sustainable polymer economy.
Raw
Material
Acquisition
Manufacturing
Polymer Product
Use and Lifetime
of Product
-Instilling degradability
End-of-life
-Polymer upcycling
-Developing compatibilizers
for mixed plastics recycling
-Biomass to
new degradable
polymers
-Monomer design
considering
application and
recycling
-Recycling through
biocatalyst
-and chemical
catalysis
Monomer
Processing
and Polymer
Synthesis
15
With these things in mind, we need to organize and design a communication scheme and collection
strategy that effectively encourages the proper disposal of polymer items. This communication should aim
for national and/or global consistency to minimize confusion when individuals relocate or travel. In the
case of compostable polymers, clear guidelines must be provided regarding the appropriate composting
facilities for these items. Similarly, for items like films and bags that require separate handling to prevent
clogging of recycling center conveyor belts, consumers should be informed of the reasons behind such
requirements. Therefore, it is crucial to implement consistent labeling on the polymer items themselves and
provide clear instructions on collection locations. Additionally, comprehensive education on proper
disposal practices should be provided at all stages of life. By adopting these measures, we can fully realize
the potential of these solutions.
1.4 Block Copolymers
Linear block copolymers are composed of two or more different types of monomers covalently
bound together with at least one junction between blocks. They have become indispensable for a diverse
range of applications including thermoplastic elastomers (TPEs), drug delivery, adhesives, electronics, soft
lithography, and construction.
51
One of the most common structures is an ABA type triblock copolymer
(Figure 1.10). These copolymers are made from discrete regions of two different types of polymers
covalently bound
together. They are
composed of a soft
(low T g) center block B
capped on either end
by hard (high T g) A
blocks. The difference
between the two T g of the blocks form the upper and lower limit of the “service temperature”, which is the
range at which the block copolymer can be useful as a TPE. TPEs rely on phase separation to achieve their
Figure 1.10. Phase separation behavior of triblock copolymers mimics natural
rubber.
Phase separation
ABA type block
copolymer
Hard domain
(High T
g
)
Soft flexible segment
(Low T
g
)
16
superior mechanical properties. Due to the inherent immiscibility of the chemically distinct monomers, in
the solid or liquid state, these block copolymers will phase separate, forming spherical domains of the hard
block connected to the soft block. The morphology of the domains are controlled by the number of repeating
units and the Flory-Huggins interaction parameter, forming spheres (S), cylinders (C), gyroids (G), and
lamellae (L). The hard domain of the TPE is a physical mimic of the chemical crosslinks of vulcanized
rubber.
52,53
This is why TPEs are called synthetic rubber. When heated past the hard block’s T g (for an
amorphous polymer) or T m (for a crystalline polymer), the system will become a homogeneous liquid. Upon
cooling, phase separation will occur once more, allowing them to be reprocessed as a thermoplastic. This
directly contrasts with vulcanized rubber, which, when heated, will decompose. There are numerous
advantages to TPEs when compared to vulcanized rubber, such as streamlined processing, cost efficiency,
lower energy consumption, and reuse of scraps generated in manufacturing.
54
The origin of synthetic rubber in industry dates to the 1930s when there was great interest
surrounding flexible plastics and in particular the plasticization of polyvinyl chloride (PVC) which was
achieved at the B. F. Goodrich Company. The later development of PVC/butadiene-b-acrylonitrile rubber
(NBR) formed material that exhibited rubber-like behavior and was a forerunner to modern thermoplastic
elastomers. To date, the most commercially successful block copolymers are linear styrene-diene-based
referred to as styrenic block copolymers (SBC). SBC were first synthesized by Shell in the 1960’s using
anionic copolymerization to make polystyrene-b-polybutadiene-b-polystyrene (SBS) and polystyrene-b-
polyisoprene-b-polystyrene (SIS). The first product that was released was Kraton®, entering the market in
1966. These consist of PS-b-polybutadiene (SB diblocks), PS-b-polyisoprene (SI), PS-b-PB-b-PS (SBS
triblocks), and PS-b-PI-b-PS (SIS triblock). Today over 15 Mt of different synthetic rubber products are
produced each year, with most of these composed of butadiene and styrene-b-butadiene block copolymers
(Figure 1.11).
55,56
The share of the market is projected to grow as SBC is likely to replace PVC as stricter
government regulations seek to reduce the amount of PVC in the market.
17
From the 1960s to today SBCs are almost exclusively made industrially using anionic
polymerization.
57
SBCs have superb chemical and physical properties such as weather resistance and
chemical stability and are low cost due to being made from olefin-based monomers.
58
However, polyolefins
have limitations due to an absence of reactive functional groups which makes them inferior for applications
in dyeability, adhesion, and miscibility. A prominent strategy is to add a polar block into the non-polar
polyolefin which would address these deficiencies as well as increase the overall toughness. To date, most
polar and non-polar block copolymers studied incorporate a non-degradable flexible center block, such as
acrylates or functionalized vinyl monomers.
59,60
Using cyclic ester monomers in the center block would be
an attractive alternative target because they generate aliphatic polyester polymers that are biocompatible
and can be degraded thermally at low temperatures with the aid of simple metal chlorides or through
enzymatic hydrolysis.
61,62
Nevertheless, identifying a catalyst that can tolerate both a polar and non-polar
monomer is challenging, owing to the need for the catalyst to switch between two different mechanisms.
Most catalysts that are facile for the homopolymerization of olefins or dienes are oxophilic, so they are
easily poisoned by cyclic ester monomers. While anionic polymerization of SBC can produce very well-
defined and controlled block copolymers under living conditions, stringent purification to eliminate
contaminates and limited functional group tolerance remain obstacles that warrant the examination of
different synthetic strategies.
Figure 1.11. Synthetic rubber demand in 2020 and their major uses.
56
BR 27.70%
E-SBR 26.70%
S-SBR
11.90%
EPDM
11.90%
IIR
9.90%
IR & CR
6.90%
NBR
5%
Synthetic Rubber Demand
BR
E-SBR
S-SBR
EPDM
IIR
IR & CR
NBR
Butadiene Rubber (BR)
Emulsion Polymerized Styrene Butadiene Rubber (E-SBR)
Solution Polymerized Styrene Butadiene Rubber (S-SBR)
Ethylene Propylene Diene Monomer (EPDM)
Isobutylene-Isoprene Rubber/Butyl Rubber (IIR)
Polyisoprene Rubber & Polychloroprene Rubber (IR & CR)
Nitrile Butadiene Rubber (NBR)
Tire, footwear, construction,
polymer modification, adhesives
Automotive, constructions, plastic
modifications, tires & tubes, wires &
cables, lubricant additives
Automobile (tires, seals), medical
stoppers, protective clothing, sporting
goods, wire & cable sheath, hoses
Tire, coating, adhesives, sealants, textile,
footwear, industrial rubber products,
electrical and electronics
Automotive, transportation & oil, gas (O-
rings, seals, hoses, belts, cables), medical
& industrial (gloves), adhesives, molded &
extruded products
Tire, polymer modification, industrial
rubber manufacturing, chemicals,
sporting goods, footwear
18
1.5 Block Copolymers Synthesis of Non-polar and Polar Monomers
Linear block copolymers are one of the most common structures studied. An important component
of block copolymer design hinges on the precise control over the length of the individual blocks and a
clearly delineated region between them.
63
There are two different tactics that are generally employed: chain
growth polymerization and polycondensation, which are further subdivided into different strategies.
Broadly, in chain growth polymerization an initiator is used to create a reactive center which could be a
radical, cation, or anion depending on the initiator. Then propagation only occurs at this reactive center.
There are four different chain growth
polymerization strategies that lead to block
copolymer formation.
64
The first method is a
sequential monomer addition where block
copolymers are made through a stepwise
process. The initial monomer of one type is
added to the catalyst/initiator then a second
type of monomer is added once all monomers
of the first type is consumed (Figure 1.12, A).
In this polymerization the active chain end
after consumption of the first monomer can
polymerize the second; however, often the
reverse sequence of monomer addition is
impossible due to reactivity of the end groups.
This strategy can further be extended to the
formation of triblock copolymers through the
addition of a telechelic initiator. The second method involves a dual initiator which is a single molecule
with two different functional groups suited to polymerize monomers using two distinct mechanisms in
Figure 1.12. Polymerization strategies for the preparation
of linear block copolymers.
64
Chain Growth Polymerization
Polycondensation
A. Sequential monomer addition
B. Dual initiator
C. End-group modification
D. Coupling of end-groups
I
1
I
1
I
1
I
1
I
1
I
2
I
2
EG
1 +
EG
1
EG
2 EG
2
I
2
I
1
I
2
I
1
I
2
I
1
I
2
E. Telechelic macroinitiator from step-growth polycondensation
End group
modification
+
19
opposite directions (Figure 1.12, B).
65
Dual initiators would allow combined polymerizations of different
polymerization types such as living cationic and living radical polymerization. This can either be performed
as a one-pot or two separate polymerizations depending on the conditions such as the monomers, solvent,
or temperature. The third method is chain end-modification which involves the polymerization of two types
of monomers in different steps (Figure 1.12, C). In this method, the functionality of the end group is
modified after the first polymerization step to fit the polymerization mechanism of the second monomer
type. This strategy is often used in the case where the end group of the first block will not polymerize the
second. The fourth method involves polymerization of the two monomer types independent of each other
and then coupling their end groups together to form a block copolymer (Figure 1.12, D). This method
creates good separation between the two blocks and minimizes the possibility of homopolymer side
products. Traditionally, this is done using azide-alkyne cycloaddition or thiol-ene click chemistry.
The other major tactic used to form block copolymers is polycondensation. In general,
polycondensation polymerization is a step-growth process which means that reactions occur between the
functional groups on the monomers themselves leading to the slow buildup of molecular weight. In
polycondensation reactions the reactive functional groups on the monomers release small molecule side
products such as H 2O and HCl. In general, step-growth polycondensation polymerizations are favored when
it is desirable to insert heteroatoms in the backbone of the polymer. Overall, this mechanism produces
polymers with poorly controlled molecular weights and broad molecular weight distributions. A
polycondensation method used to synthesize non-conjugated polar and non-polar block copolymers is the
formation of telechelic macroinitiators using step-growth polycondensation (Figure 1.12, E).
66
In this case,
a traditional polycondensation step-growth reaction yields a polymer with functional terminal groups such
as an alcohol. This can be used as a macroinitiator to polymerize a different type of monomer using a
separate mechanism and can either take place by direct addition of monomer or by modifying end groups
prior to monomer addition depending on the functionality of the desired polymer.
20
Of these six strategies presented, the most pervasive methods are sequential monomer addition and
end group modification. However, all these strategies are used in different polymerization types to produce
block copolymer morphologies. A recent review by Harth et al. provides a detailed discussion on the major
strategies for synthesis of block copolymers.
67
Presented in the next sections will be discussions on common
polymerization mechanisms and how they can be used alone or in tandem to produce block copolymers of
non-polar and polar monomers.
Living Radical Polymerization
Traditional free radical polymerization (FRP) allows access to high molecular weight polymers of
vinyl monomers using a range of different conditions.
68
However, this polymerization is extremely limited
due to poor control of molecular weight, limited end group functionality and architecture, and high
molecular weight distributions, which prevents the necessary control needed to achieve block copolymers
with valuable thermal and mechanical properties. This has given rise to the rapid development of living
radical polymerization (LRP). Many LRP methods have been generated including atom transfer radical
polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT), nitroxide mediated
polymerization (NMP), and organometallic-mediated radical polymerization (OMRP). These different
methods are unified in that a dynamic equilibrium is achieved between a high concentration of dormant
chains unable to propagate and a low concentration of active propagating chain ends. This equilibrium
allows for the suppression of side reaction that leads to deactivation of polymer. Generally, these methods
have only been used to polymerize vinyl monomers and they are limited as they cannot polymerize
monomers that lead to heteroatoms in the backbone.
NMP is a metal free polymerization excellent for producing colorless and odorless polymers
without the need for purification.
69
Side reactions are minimized by the presence of the “persistent radical
effect”. The persistent radical in NMP is a nitroxide who’s role in the polymerization is to react with
transient propagating radicals thus minimizing termination events that could lead to dead polymer.
Monomers most often polymerized using this method are styrene, functionalized styrenics, acrylates, and
21
1,3-dienes such as isoprene, myrcene, and farnesene. This polymerization is still limited in its ability to
polymerize acrylate monomers, often requiring comonomer addition with a more controlled monomer such
as styrene. Métafiot et al. recently were able to make TPE triblock copolymers with either isoprene or
myrcene as the flexible center block and either styrene or isobornyl methacrylate (IBOMA) as the outer
blocks (Table 1.1, entry 1).
70
This was done using a telechelic poly(ethylene-stat-butylene) macro-initiator
terminated by N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide (SG1). The
polymerization was carried out in a sequential manner by first polymerizing the center 1,3-diene block and
leaving nitroxide groups on either side. Upon workup the outer blocks were then installed by adding the
desired monomer. This produced only mild cis-1,4 selectivity (68-72%) polyisoprene. Interestingly, the
IBOMA based triblock showed improved thermal and mechanical properties compared to the styrene based
triblock, including a higher service temperature and improved toughness.
One advantage of using NMP is that functional groups such as alcohols which are frequently used
in the ROP of lactides or cyclic esters do not disrupt the polymerization. Waymouth and Hedrick et al.
developed a method by which block copolymers of PS and PLA could be polymerized by using a dual
functional initiator (Table 1.1, entry 2).
71
This initiator had both a nitroxide group that could first
polymerize styrene and an alcohol end group. Then, rac-lactic acid was polymerized using the PS-OH
macro-initiator catalyzed by a bifunctional thiourea-amine catalyst. These diblock copolymers exhibited
very low molecular weight distributions (1.04-1.07); however, reactions were slow, taking 72 hrs to reach
high conversion. Additionally, they also explored RAFT-ROP polymerization by using a different dual
functional initiator amenable to the RAFT method. The monomers used for the RAFT method were tert-
butyl acrylate, methyl methacrylate, and 2-vinyl pyridine followed by ROP of LA which showed similar
dispersity control compared to the NMP case.
Atom transfer radical polymerization (ATRP) is a technique that was discovered by Mitsuo
Sawamoto and Krzysztof Matyjaszewski in the 1990s.
72
It is useful for the synthesis of vinyl polymers. As
the name would imply, the key step for this polymerization is an equilibrium between a halide abstraction
22
by the metal center to leave a carbon centered radical on the alkyl fragment.
73
In this way, there is an
equilibrium between a dormant species (i.e., the alkyl halide) and the active propagating radical, which
controls the molecular weight by minimizing termination events. Generally, there are five main components
that control the polymerization: catalyst, solvent, monomer, initiator, and ligand. In terms of block
copolymerization synthesis, an important component is to ensure that the end groups are active. This is
Table 1.1. Select examples of linear block copolymerizations by different mechanisms and strategies.
Entry 1
st
Block
Mechanism
2
nd
Block
Mechanism
Catalyst (1
st
Block/2
nd
Block)
Strategy Composition Mn
(KDa)
Đ Ref.
1 NMP NMP PEB-(SG1)2
Telechelic
sequential
monomer
addition
PIBOMA-b-
PIP-b-
PIBOMA
94 1.76 70
2 NMP ROP
Bifunctional
nitroxide/Thiourea
-containing
bifunctional
organocatalyst
Dual initiator PS-b-PLA 12 1.07 71
3 ATRP RAFT CuBr/AMBN
Coupling of
end groups
PS-b-P(tBA) 11 1.29 75
4 RAFT ROP
Benzyl
(diethoxyphosphor
yl)dithioformate/tr
ans,trans-2,4-
hexadien-1-ol
Coupling of
end groups
PS-b-PCL 5.5 1.28 77
5 RAFT ROP
2,2′-
azobis(isobutyroni
trile)/(1,5,7-
triazabicyclo[4.4.0
]dec-5-ene (TBD))
Chain end
modification
PS-b-PCL-b-
PS
32 1.2 78
6 OMRP OMRP MeTe-CH(CH3)Ph
Sequential
monomer
addition
PS-b-PMMA 14 1.25 80
7 CIP OMRP
Co
III
(η
5
-
C5H5)P(OMe)3I2)
and
methylaluminoxan
e (MMAO)
Sequential
monomer
addition
PE-b-PMA 14 2.74 81
8 CP RAFT TiCl4/AIBN
Chain end
modification
PIB-b-(Boc-L-
Ala-HEMA)
14 1.39 83
9 AP RAFT
sec-
butyllithium/AIB
N
Chain end
modification
PS-b-PMA 37 1.31 88
10 Step-growth ATRP
La(acac)3/CuBr/bp
y
Telechelic
macroinitiator
from step-
growth
PS-b-PC-b-PS 95 1.36 91
11
Metallocene
polymerization
ROP
[(COD)RhCl]2 and
NBu3/Sn(Oct)2
Chain end
modification
aPP-b-PLA 17 1.08 94
23
often called “end group fidelity” which ensures efficient transition from the first block to the second block.
Two methods can be used to synthesize block copolymers using ATRP: sequential monomer addition and
end group modification. Good molecular wight distribution is generally considered to be between 1.1-1.5
for ATRP. The sequential monomer addition is the preferred technique, as it is a one-pot method; however,
radical termination events make it challenging to perform. Drawbacks to ATRP include slow reactivity with
vinyl aromatics, no control over diene polymerization, and sensitivity to oxidants.
74
Commonly, a Cu metal
catalyst is needed which poses concerns over separation and toxicity issues with the polymer. Webster et
al. synthesized 24 different diblock copolymers using a combined ATRP/RAFT approach (Table 1.1, entry
3).
75
Each block was synthesized independently using either ATRP or RAFT polymerization, then they
were linked together using azide click chemistry. Monomers that were studied included styrene, butyl
acrylate (BA), tertbutyl acrylate (tBA), and methoxy poly(ethylene glycol) monoacrylate (PEG). These
polymerizations were carried out in a high throughput manner and demonstrated a dispersity range between
1.14 to 1.96. This demonstrated the versatility and the promise of combining these two polymerization
techniques.
Reversible addition-fragmentation chain transfer (RAFT) polymerization was first published a few
years after ATRP.
76
It is excellent at polymerizing vinyl monomers with high molecular weight control and
dispersity. RAFT polymerization has slow reactivity with vinyl aromatic monomers, and while it can be
used to polymerize diene monomers in a controlled way, higher temperatures are needed (>120 °C) which
can produce undesirable crosslinking side reactions. Diblock copolymers of polystyrene-b-
polycaprolactone have been synthesized using the approach of coupling two homopolymers together
(Figure 1.12, D). Barner-Kowollik et al. modified styrene with an electron-deficient dithioester prior to
RAFT polymerization. They independently synthesized polycaprolactone using a hydroxyl functionalized
dieneophile. They combined the two blocks through hetero Diels-Alder (HDA) cycloaddition (Table 1.1,
entry 4).
77
This method only produced diblock copolymers, but to access more sophisticated block
copolymers they switched to modifying the end groups instead of coupling two homopolymers together. In
24
this way, PCL-b-PS-b-PCL triblock copolymers were able to be synthesized (Table 1.1, entry 5). This was
accomplished by first synthesizing PS through RAFT polymerization. The end groups were then
transformed from dithioester groups into hydroxy groups and then this was used as a telechelic
macroinitiator to facilitate the ROP of ε-caprolactone.
78
Another living radical polymerization mechanism that has been useful for the block
copolymerization of non-polar and polar monomers is the organometallic-mediated radical polymerization
(OMRP).
79
Two different mechanisms have been proposed. The first is reversible deactivation (RD) where
the metal can reversibly cap the growing radical polymer chain and functions as a persistent radical. The
second mechanism is a degenerative transfer where the metal complex can switch between polymer chains.
Vinyl monomers such as vinyl esters, acrylic acid, acrylonitrile, and acrylic esters can be polymerized using
this method. The most common catalyst studied is Co(acac) 2 alongside other main group metal catalysts.
Yoshida et al. synthesized diblocks and triblocks of styrene and methyl methacrylate using an
organotellurium based catalyst using a sequential monomer addition strategy (Table 1.1, entry 6).
80
However, this was not a one-pot synthesis, as workup was needed between each step to remove remaining
monomer. This method has certain challenges such as slow polymerization, limited monomer scope, and
the possibility of toxic catalyst left in the polymer. OMRP has also been used in combination with other
mechanisms. Recently, Nozaki et al. designed a system that can produce up to tetrablock copolymers of
polyethylene (PE) and polymethyl acrylate (PMA) (Table 1.1, entry 7).
81
The key to achieving this polymer
composition was the catalyst systems (petamethylcyclopentadienyl cobalt complex/isobutyl modified
methylaluminoxane (MAO)) which could reversibly switch between OMRP of MA and coordination-
insertion polymerization (CIP) of PE. Diblock copolymers (PE-b-PMA) could also be made using the
sequential monomer addition method.
Living Ionic Polymerization
There are two types of ionic polymerization: cationic and anionic. Cationic polymerization is one
of the oldest methods researched, with some records dating back to the 18
th
century.
82
The typical initiators
25
used in a cationic system are Lewis acids (AlCl 3, TiCl 4) and mineral acids (H 2SO 4, H 3PO 4) where the more
acidic an initiator is the more effective it is at initiating polymerization. Vinyl monomers can undergo
cationic polymerization but are often plagued by side reactions such as chain transfer to monomer,
backbiting, and spontaneous ejection of HX. This results in it being difficult to achieve high molecular
weight polymers. Cooling the reaction can minimize these side reactions; however, living cationic
polymerization is still a difficult challenge. The monomers that have been studied the most are vinyl ethers,
and isobutene because of the stability of the reactive species. Styrene and methyl styrene have also been
studied, but the lack of activating polar functional groups and the steric hinderance due to the methyl group
make them more challenging. Block copolymerization using cationic polymerization is best carried out in
tandem with another method due to the very limited monomer scope. Faust and De et al. synthesized diblock
copolymers using a combined cationic RAFT approach (Table 1.1, entry 8).
83
First, isobutylene was
polymerized by living cationic polymerization with a TiCl 4 catalyst and quenched with allyltrimethylsilane
(ATMS) to introduce alkyl groups to the end of the polymer. The allyl group was converted to an alcohol
using hydroboration oxidation. Then the alcohol was reacted with the chain transfer agent 4-cyano-4-
(dodecylsulfanylthiocarbonyl)sulfanyl pentoic acid (CDP). RAFT polymerization was then carried out
using 2,2’-azobis(isobutyronitrile) (AIBN) with Boc- L-alanine methacryloyloxyethyl ester (Boc- L-Ala-
HEMA). Polymers exhibited moderate to good dispersity (1.54–1.15) depending on conditions used.
However, the RAFT polymerization was sluggish, reaching only 60% conversion in 11 hrs.
Anionic polymerization was first reported by Ziegler in 1936 when he observed no chain transfer
or termination events in the polymerization of 1,3-butadiene by organometallic initiators.
84
Medvedev later
elucidated the kinetics of the ionic polymerization of 1,3-butadienes.
85
Anionic polymerization is frequently
activated by the addition of a negative ion or by a Lewis base. Common examples of activation by addition
of a negative ion are simple organometallic compounds of alkali metals (e.g., isobutyllithium) due to their
solubility in organic compounds.
86
Then propagation proceeds through the nucleophilic attack of the
carbanions on a monomer. Because these sites are so reactive it is important that reactions be free of
26
contaminants to avoid chain termination events. Common industrial synthetic rubbers such as SBS, SIS, PI,
and PB are all synthesized via a sequential anionic polymerization. Other monomers including vinyl
aromatics, and some cyclic esters are also efficiently polymerized in this way, while other monomers with
highly reactive electrophilic groups often compete with propagation. For example, methacrylic monomers
need to be cooled to achieve high molecular weight. Additionally, to transfer to a different monomer, the
propagating anion must be nucleophilic enough to attack the new monomer. The ascending order of
increasing electron affinity is styrene < butadiene ≈ isoprene < vinyl pyridine < methyl acrylate < ethylene
oxide. Hadjichristidis et al. exemplified this order of addition by synthesizing a pentablock copolymer using
this sequence of monomers which exhibited exceptionally low dispersity of 1.04.
87
Using a combined
approach would be highly beneficial when targeting these types of monomers. RAFT polymerization is
complementary to anionic polymerization as each strategy excels at the monomers that are troublesome to
the other. In particular, anionic polymerization is excellent at vinyl aromatics and diene polymerization
while RAFT provides better control for vinyl based monomers.
Cochran et al. recently used a combined anionic/RAFT polymerization to produce block
copolymers of styrene and methyl acrylate or methyl methacrylate (Table 1.1, entry 9).
88
Their method
involved first polymerizing styrene using anionic polymerization using sec-butyllithium. This
polymerization was quenched by the addition of a hydroxyl functionality. Traditionally this has been done
with ethylene oxide which is highly toxic. To circumvent this, they tested other functionalized capping
agents such as halogenated acetal or halogenated silane-protected alcohol. The protecting groups on these
quenching agents were removed to provide an alcohol. Further transformation of the alcohol into a bromide
followed by addition of bis(thiobenzoyl)disulfide (TBDS) produced the desired chain transfer agent for
RAFT polymerization. Then the MA block was installed using this macro-chain transfer agent then capped
with various chain transfer groups that are compatible with RAFT polymerization.
27
Polycondensation Polymerization
Step-growth polycondensation
polymerization is often used with
conjugated polymers. In these reactions,
high temperatures (>100 ºC) and long
reaction times are required and produces
polymers with poor molecular weight
control. A metamorphosis came in the form of the discovery of chain growth polycondensation which was
pioneered by Yokozawa and Yokoyama in the 2000s.
89
In this mechanism the typical step-growth
polycondensation reaction can be transformed into a chain growth process where growth of the polymer
only occurs on the active chain ends (Figure 1.13). Monomer is deactivated by strong electron donating
groups on one side of the monomer, and an initiator is added which has an electron withdrawing group
which activates the functionality on the opposite side. The reaction between these two forms a weak electron
donating group which eliminates the strong electron donating group thereby activating the chain end.
Traditional step-growth polycondensation is now most often used in preparing polyamide-based copolymer
such as PEG-polyamide-PEG or in the preparation of aramides (Kevlar).
90
Höcker et al. reported the triblock formation of polystyrene-b-poly(bisphenol A carbonate)-b-
polystyrene and poly(methyl methacrylate)-b-poly(bisphenol A carbonate)-b-poly(methyl methacrylate)
using a combined polymerization approach of step-growth polycondensation and ATRP (Table 1.1, entry
10).
91
The block copolymers were made by first synthesizing the center block through step-growth
polymerization of bisphenol A (BPA) and diphenyl carbonate (DPC) using a rare earth metal catalyst,
La(acac) 3. The end groups after this reaction were alcohols, which were further modified using α-
chlorophenylacetyl chloride. This bifunctional chloro-telechelic PC macroinitiator could then undergo
RAFT polymerization by adding either methyl methacrylate or PS catalyzed by a Cu
1
catalyst.
Figure 1.13. General mechanism for chain growth
polycondensation.
89
Chain growth polycondensation
inactive
reactive reactive
I
1
I
1
I
1
I
1
28
Coordination Insertion Polymerization
Since Ziegler and Natta first discovered polyolefin polymerization with metal catalysts, it has been
the inspiration for many other systems.
92
Chief among the catalysts that were inspired by Ziegler-Natta are
the metallocene complexes which were introduced by Fisher and Wilkinson in the 1950s. Metallocene
complexes undergo a coordination-insertion polymerization mechanism. In the first step, an Al cocatalyst
is typically activated with MAO which abstracts a Cl from the metal center. Then the other Cl atom on the
metal center is alkylated using the Al species. Afterwards, the olefin monomer coordinates to the empty
coordination site and then it inserts in between the metal-carbon bond. Propagation continues from the
newly formed metal-carbon bond. These catalysts are generally stable, cheap, and highly active and easy
to synthesize. However, because of the strong Lewis acidity of the metal center, they are easily poisoned
by polar functional groups making access to non-polar and polar block copolymers challenging.
93
Guironnet
et al. has recently developed a strategy to make polyolefin-polyester diblock copolymers based on the end-
group modification method (Table 1.1, entry 11).
94
In this one-pot synthesis a metallocene zirconium
complex activated with B(C 6F 5) 3 was used to polymerize either the ethylene or propylene to make the first
block, then the end group of the block was modified using a hydroformylation/hydrogenation to install an
alcohol end group using a [(COD)RhCl] 2/NBu 3 catalyst system. Finally, the second block was installed via
ring-opening polymerization (ROP) using a Sn(Oct) 2.
Despite there being many examples of metallocene polymerization, there are many drawbacks that
make them undesirable for industrial applications. These include their high cost, and that an Al cocatalyst
is required which is often difficult to work with. They are also not able to tolerate polar functional groups.
These concerns have led to the expansion of new catalysts called “post-metallocene” complexes which
feature a wide range of different ligand designs and a range of different metals. A recent review spotlights
how different synergistic effects between transition metals and other components (chemicals, light, and
solid supports) aid in the copolymerization between olefin and polar monomers.
95
One category of catalyst
that has quickly become prominent in catalysis is rare earth (RE) metals complexes, which include the
29
lanthanide series (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) along with Sc and Y.
The term “RE metals” is a misnomer, as many of the larger metals are on the same order of abundance as
popular industrial metal such as Ni and Cu.
96
They were the last naturally occurring metals to be discovered
so they were mistakenly thought to be rare. RE metals are generally nontoxic which makes them useful for
polymer applications in industries such as the medical field without stringent purification practices or
toxicity concerns. RE metal catalysts are also able to polymerize a diverse range of monomers including
non-polar and polar monomers. The polymerization of non-polar and polar monomers has generally relied
on polymeric backbones featuring only C-C bonds. A switch to monomers which feature heteroatoms such
as O or N in the backbone could add a degree of degradability into the polymer backbone, making them
more amenable to chemical recycling practices. This makes RE metal catalysts potential candidates for the
efficient and selective polymerization of olefin/1,3-diene and cyclic ester block copolymers.
Understanding the key steps in the polymerization mechanism would lead to better catalyst design.
A common type of RE metal catalyst features bis-alkyl ligands, which have been particularly active and
selective as polymerization catalysts. This is traditionally done in a one-pot reaction where the monomers
are added using the sequential monomer addition method. The polydiene block is synthesized by first
activating the RE metal center by abstracting an alkyl to form a cationic species (Figure 1.14, a). 1,3-diene
monomer is added and binds to this open coordination site, making the monomer more receptive to
nucleophilic
attack of the
remaining
alkyl on the
RE metal
center. The
metal center
can either bind
Figure 1.14. Mechanism of block copolymerization of IP and CL with a RE metal catalyst.
30
η
1
or η
3
to the monomer depending on the steric crowding around the metal center. Then propagation will
continue at the RE metal center. The initial binding of the monomer to the metal center is essential for
effective initiation. RE metal centers need to be coordinatively unsaturated enough to allow for this binding;
however, providing too much or too little steric bulk could lead to unselective polymerization. One
important component to consider when polymerizing 1,3-dienes is the selectivity of the final polymer
product. There are different stereo- and regio- selectivities in polydiene products, including cis-1,4, trans-
1,4, 3,4, and 1,2 (Figure 1.15). The regioselectivity is ultimately dictated by many different factors such as
the binding mode of the growing polymer chain and the approach of the next monomer. Other factors
include the Lewis acidity of the metal, ligand effects, concentration, solvent, and temperature of the reaction.
Nonetheless, these different microstructures still have a dramatic impact over the thermal, mechanical, and
crystallinity properties of the polymer. For example, 100% cis-1,4 polymers have a T g around -60 °C and
are amorphous. These polymers are used in applications such as footwear and adhesives while in contrast
polymers with trans-1,4 have a slightly elevated T g but are much more crystalline. Trans-1,4 polymers are
used in applications separate from cis-1,4 polymers such as golf balls and underwater cables. Achieving
100% cis-1,4 has historically been challenging and is currently only attainable using natural rubber derived
from the rubber tree (Hevea brasiliensis). Industrially synthesized rubber using either anionic
polymerization or Ziegler-Natta catalysts achieve only 98% cis-1,4 selectivity. However, designing
catalysts that can polymerize high content of these
different isomers could still be useful for many different
applications. This would be especially true when
designing block copolymers for specialized applications.
Once the 1,3-diene monomer is consumed, the
cyclic ester monomer is added to the reaction and a
coordination insertion mechanism of cyclic esters is performed using RE metal complexes (Figure 1.14,
b). Once the cyclic ester is added, it binds to the carbonyl oxygen of the RE metal center, which often
Figure 1.15. Different isomers for
polyisoprene polymerizations.
31
dictates the rate of the reaction.
97
Then there is a nucleophilic attack of the growing polymer chain on the
carbonyl carbon. Then the reaction undergoes a 4-membered transition state which results in transfer of the
polymer chain onto the carbonyl carbon and the coordination of the oxygen atom in the ring of the monomer
to the metal center. Finally, the C-O bond of the monomer is broken, leading to a metal alkoxide species.
Different characteristics of the metal can affect the rate of polymerization. Larger metals generally lead to
faster polymerization rates because it facilitates improved binding of the monomer. However, smaller
metals are more electrophilic which also improves binding of the monomer. These two characteristics
conflict with each other so finding a balance between both properties is important. The identity of the
solvent also impacts this polymerization as a more polar solvent can compete with monomer binding thus
slowing polymerization. The final point to consider is ligand design. A larger ligand can make the metal
more sterically crowded and can change the electronic properties of the metal. Overall, block copolymers
with both 1,3-dienes and cyclic esters could be promising for creating a circular economy, but a careful
balance of both polymerizations must be considered. Development of catalysts that are controlled for both
polymerizations will be further discussed in later chapters.
1.6 Dissertation Outline
The research presented in this dissertation aims to improve the sustainability of polymer products
by applying sustainable methods to block copolymers while also developing catalyst design principles to
synthesize these targets.
In chapter 2, a sequence of yttrium β-diketiminate alkyl complexes were synthesized featuring one
or two pendant donor motifs ranging in donor strength. It was uncovered that a single weaker -OMe donor
had the fastest rate and higher cis-1,4 selectivity. Additionally, five new block copolymer combinations
were synthesized from two 1,3-dienes and three cyclic ester monomers. Finally, a poly(isoprene-b-
caprolactone) block copolymer was recycled by selectively degrading the polyester and using the recovered
polyisoprene as a macroinitiator to facilitate the polymerization of new ε-caprolactone to produce a
32
poly(isoprene-b-caprolactone) block copolymer with similar thermal and spectroscopic signatures to the
virgin material.
In chapter 3, simple homoleptic trialkyl rare earth metal complexes were synthesized and tested for
their efficacy as pre-catalysts for isoprene polymerization using 1 or 2 equivalents of [Ph 3C][B(C 6F 5) 4]
activator. We investigated the addition of commercially available in situ donors, leading to the identification
of triphenylphosphine as an ideal support to enhance the dispersity control and prevent loss of catalyst
activity. We demonstrated how the activation and reaction conditions, including the order/time of reagent
addition and donor electronics, had a major impact on the rapid, controlled, and selective polymerization
of 1,3-dienes. Further interrogation of the catalyst system identifies the crucial role of triphenylphosphine
in providing enhanced stability and control in this living catalyst system.
In chapter 4, simple homoleptic trialkyl rare earth metal complexes were tested for their ability to
polymerize a range of different 1,3-diene/olefin and cyclic ester monomers. These studies suggest that
conditions must be optimized for individual monomers achieve increased control. Larger metals, such as
Gd, appear promising for the controlled polymerization of polymyrcene (PMyr) with polar monomers.
In chapter 5, bisalkyl yttrium complexes were synthesized supported by a pincer ancillary ligand.
Changes to the steric and electronic properties of this ligand impacted the rate of IP polymerization. It was
seen that a more flexible ligand framework and the more nucleophilic bisalkyl initiator enhanced the rate
of IP polymerization.
Appendix A includes a few side stories, including isoprene polymerization with divalent samarium
complexes, the perfectly alternating copolymerization of epoxides and cyclic and anhydrides with pincer
supported Yttrium complexes, and the synthesis of a new NNN pincer ligand using Buchwald-Hartwig
cross-coupling reactions.
33
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Chen, C. Designing catalysts for olefin polymerization and copolymerization: Beyond electronic and steric tuning.
Nat. Rev. Chem. 2018, 2, 6–14.
(94) Yan, T.; Walsh, D. J.; Qiu, C.; Guironnet, D. One-Pot Synthesis of Block Copolymers Containing a Polyolefin
Block. Macromolecules 2018, 51, 10167-10173.
(95) Tan, C.; Chen, M.; Chen, C. ‘Catalyst + X’ strategies for transition metal-catalyzed olefin-polar monomer
copolymerization. Trends Chem. 2023, 5(2), 147-159.
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polymerization of cyclic esters. Coord. Chem. Rev. 2019, 392, 83-145.
37
CHAPTER 2
Controlled, One-pot Synthesis of Recyclable Poly(1,3-diene)-polyester Block
Copolymers, Catalyzed by Yttrium β-diketiminate Complexes
A portion of this chapter has appeared in print:
Kosloski-Oh, S, C.; Manjarrez, Y.; Boghossian, T. J.; Fieser, M. E. Chem. Sci. 2022, 13, 9515-9524.
38
2.1 Introduction
In the last few decades, block copolymers have become an indispensable class of soft materials
with an expanding range of applications including drug delivery, adhesives, electronics, and
construction.
1
Poly(1,3-diene)-based block copolymers have been extensively commercialized due to their
low cost, light weight, durability, wide service temperature range, and resistance to chemical reactivity.
2
Block copolymers of poly(1,3-dienes) and polar polymers often exhibit improved adhesive, dyeing, and
moisture absorption properties, making them suitable for a broader spectrum of applications.
3
Current
efforts in synthesizing olefin/polar block copolymers have focused on incorporating olefins with polar
functional groups such as alkyl acrylates.
4–9
This produces block copolymers with strong aliphatic carbon–
carbon bonds in the backbone, making these materials difficult to chemically recycle. In addition, block
copolymerization of these two dissimilar monomers is challenging because catalysts that excel at olefin or
1,3-diene polymerization often struggle with polar monomers. Furthermore, these catalysts tend to be
highly oxophilic and can be easily poisoned by polar functional groups.
6
Alternative polar monomers, such
as cyclic esters, could maintain valuable physical properties while providing a degradable polymer block.
Additionally, cyclic esters can be sourced from biomass; therefore, incorporation of more sustainable
monomers in polymers derived from petroleum sources would alleviate depletion of petroleum
feedstocks.
10,11
Yet, switching from olefin/1,3-diene polymerization to cyclic ester polymerization can also
poison the polymerization catalyst.
12
Cationic rare earth metal alkyl catalysts are highly efficient at the
homopolymerization of olefins and 1,3-dienes.
13
These complexes usually bear an outer-sphere borate
anion generated in situ and have shown resistance to poisoning. These complexes can also lead to polymers
with different stereoselectivity depending on the monomer and catalyst structure.
To date, only five rare earth metal catalysts have been identified as efficient catalysts for the block
copolymerization of olefins or 1,3-dienes and cyclic esters (Figure 2.1).
14–19
Hou and coworkers reported
the first block copolymerization of styrene (S) and ε-caprolactone (CL) with a scandium alkyl half-
39
sandwich pre-catalyst (1).
14
Cheng and coworkers later extended this work to include a bifunctional
initiating alkyl to form triblock copolymers of polystyrene (PS) and polycaprolactone (PCL), with PS in
the middle block.
18
Hou and coworkers also developed a PNP carbazolide bis(alkyl) rare earth metal
catalyst capable of producing block copolymers consisting of cis-1,4-polyisoprene (PIP) and PCL in a
living manner (2), Figure 2.2.
15
In 2014, Cui and coworkers reported an amidino N-heterocyclic carbene-
supported lutetium
bis(alkyl) complex
for the block
copolymerization of
isoprene (IP) and CL
(4).
16
In contrast with
the Hou pre-catalyst (2), this catalyst produced 3,4-regulated polyisoprene (PIP). The change in selectivity
could be due to the reduced ionic radii of the metal as well as the greater steric crowding of the carbene
ligand in the latter complex. However, due to the vast differences between the ligands in 2 and 4, it is
difficult to designate the exact factors that led to the disparate catalyst selectivity.
13
Pan and coworkers
synthesized lutetium and yttrium bis(alkyl) complexes supported by an anilido-oxazoline ligand (3) for the
block copolymerization of IP with CL.
17
More recently, Shi and coworkers altered the pendant carbene arm
of the amidinate ligand in 4 to a pyridine donor (5), and tested these new complexes for the block
Figure 2.1. Five pre-catalysts reported for the block copolymerization of CL with S (1)
14
, IP (2
15
, 3
17
, 4
16
, 5
19
).
Figure 2.2. Representative diblock copolymerization of IP and CL with yttrium pre-
catalyst 2.
15
40
copolymerization of IP and CL.
19
This subtle change dramatically altered the selectivity of IP
polymerization to trans-1,4 polymerization. Changing the pendant arm length of the pyridine donor also
made a large impact on the rate of polymerization and selectivity of IP polymerization.
Since these previous reports have generally polymerized the monomers to full conversion, there
have been no studies identifying how ligand structure impacts the polymerization rate and control for the
block copolymerization of olefins/1,3-dienes and cyclic esters. The monomer scope has also been limited
to one olefin, one 1,3-diene, and only one cyclic ester. Finally, no efforts have been directed towards
addressing the recyclability of these block copolymers.
Herein, we report the use of β-diketiminate (BDI) supported yttrium complexes for the catalytic
block copolymerization of several 1,3-dienes and cyclic esters, expanding the literature monomer scope
with the addition of one bioderived 1,3-diene ( β-myrcene (Myr)) and two cyclic esters ( δ-valerolactone
(VL), and ε-decalactone (DL)). Subtle changes in pendant neutral donors on the BDI ligands show a large
impact on the polymerization of both monomer types. Additionally, we demonstrate that the poly(1,3-diene)
block can be recycled to remake the same block copolymers.
2.2 Results and Discussion
2.2.1 Catalyst Section
BDI ligands have been utilized to support rare earth metal catalysts for the polymerization of
various polyesters.
20
In particular, complexes 6–8 were found to be highly active for the ring-opening
polymerization (ROP) of cyclic esters; however, reaction conditions did not reveal any trends on how the
ligand structure impacts the rate of polymerization. These complexes were extended to the perfectly
alternating copolymerization of epoxides and cyclic anhydrides, particularly butylene oxide with phthalic
anhydride, where complex 6 was the slowest catalyst and 8 was the fastest.
21
This suggested that strong
field donors enhanced the rate of polymerization. We sought to use complexes 6–8 to identify if related
trends could be discovered for the block copolymerization of IP and CL. All three complexes were
synthesized according to literature methods by reacting the protonated BDI ligand with one equivalent of
Y(CH 2SiMe 3) 3(THF) 2 (Figure 2.3).
22–24
41
2.2.2 Isoprene and
Caprolactone
Initially, 6–8 were tested
as pre-catalysts for the
homopolymerization of IP at
room temperature for 12 hours,
with 10 μmol (0.125 mol%) of
catalyst and 10 μmol of [Ph 3C][B(C 6F 5) 4] (Table 2.1, entries 1–3). Complex 7 showed no polymerization
of IP within 12 hours and was not pursued further (Table 2.1, entry 2). This inactivity is likely due to
inhibiting steric or coordinative saturation from the two pendant donors. Complex 8 showed slow
polymerization of IP, achieving only 85% conversion after 12 hours (Table 2.1, entry 3), and had a
preferred selectivity (62%) for trans-1,4 polymerization. Surprisingly, complex 6 showed full conversion
to PIP with 98% selectivity for 1,4 polymerization, with a slight preference for cis-1,4 over trans-1,4
selectivity (Table 2.1, entry 1). Even when reactions are run past full conversion, a dispersity below 1.10
is maintained, suggestive of excellent polymerization control (Table 2.1, entry 4).
Contrary to the perfectly alternating copolymerization of epoxides and cyclic anhydrides, the
superior rate of complex 6 suggests that a weak field donor leads to the fastest polymerization of IP.
21
We
attribute this to the more electron-deficient yttrium center in complex 6 compared to complex 8, which
would likely lead to better activation of IP towards polymerization. The change in selectivity is interesting,
as the slightly bulkier –NMe 2 donor in 8 shows a higher preference for trans-1,4 and 3,4-polymerization,
while the –OMe donor in 6 shows almost exclusive preference for 1,4-polymerization. This is consistent
with what is found in the literature where a bulkier ligand leads to increased 3,4-selectivity.
16,25,26
While it
is also true that bulkier ligands have been shown to promote cis-1,4 over trans-1,4 polymerization, we
speculate 8's selectivity for trans-1,4 might arise from the decreased Lewis acidity of its yttrium center.
27
Figure 2.3. (a) Targeted yttrium BDI complexes for the block
copolymerization of 1,3-dienes with polar monomers. (b) Reported
synthetic pathway to targeted yttrium BDI complexes.
22–24
42
Shortening the reaction time revealed that 64% conversion of IP is already achieved within 30
minutes for complex 6, while 8 shows only 22% conversion within this timeframe (Table 2.1, entries 5 and
6, respectively). Interestingly, both shortened reactions show more cis-1,4 selectivity than their respective
12 hours reactions. Previous reports have indicated that higher concentrations of IP can lead to a preference
for cis-1,4, which explains the higher cis-1,4 selectivity at shorter reaction times.
28
It is interesting that the
longer reaction time with 8 (Table 2.1, entry 3) has no presence of cis-1,4 selectivity, while the shorter time
has 8% cis-1,4 selectivity (Table 2.1, entry 6). Since cis-1,4 selectivity seems to drop with IP concentration,
Table 2.1. Polymerization of IP or CL with pre-catalysts 6, 7, and 8.
a
Entry Cat. Monomer Time
Conv.
(%)
b
Mn
(kDa)
c
Đ
c
Microstructure
d
Cis-1,4/Trans-
1,4/3,4
Tg
(ºC)
e
Tm
(ºC)
e
Eff
(%)
f
1 6 IP 12 h >99 84(9) 1.04(1) 52/46/2
g
-67
h
– 67(6)
2 7 IP 12 h 0 0 0 – – – –
3 8 IP 12 h 85 51 1.10 0/62/38 -53 – 91
4 6 IP 24 h >99 82(11) 1.03(1) 58/40/2
g
-65
h
– 67(9)
5 6 IP 30 min 64 76 1.03 67/32/1 -65 – 54
6 8 IP 30 min 22 15 1.10 8/65/27 -53 – 80
7 6 CL 10 min 89 40 1.13 – – 55 75
8 8 CL 10 min >99 37 1.34 – – 55 93
9
6 CL 2 h >99 66(5) 1.2(1) – – 55
h
52(3)
a
Conditions: [Y] 10 μmol; [Ph3C][B(C6F5)4] 10 μmol; IP 0.80 M; CL 0.30M; [IP]/Y = 800; [CL]/Y = 300; toluene 10 mL;
room temperature, entries 1,4 and 9 are done in triplicate.
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures,
comparing monomer peaks to polymer.
c
Determined by gel permeation chromatography (GPC) in THF using a Wyatt DAWN
HELEOS II MALS detector.
d
1,4 and 3,4 selectivity determined by
1
H NMR. Cis-1,4 and trans-1,4 selectivity determined by
13
C NMR.
e
Determined by low temperature differential scanning calorimetry (DSC).
f
Catalyst efficiency, calculated by
Mn(theor.)/Mn(exp.).
g
Average of the triplicate runs.
h
Data for one of the individual runs.
43
this could be due to the cis-1,4 getting buried in the baseline of the NMR spectrum for the long reaction
times. Alternatively, this could be due to variability in selectivity between separate reactions.
Complexes 6 and 8, activated with [Ph 3C][B(C 6F 5) 4], were also used for the homopolymerization
of CL at room temperature (Table 2.1, entries 7 and 8, respectively). While complex 8 showed faster
polymerization, the higher dispersity of the resulting polymer (1.34) suggested either lack of polymerization
control or the presence of transesterification reactions when polymerization is complete. Activated
complex 6 showed high conversion (89%) of CL after just 10 minutes while maintaining an excellent
dispersity (1.13). Leaving the reaction well past full conversion showed no evidence of transesterification,
with dispersity remaining low (Table 2.1, entry 9). It is worth noting that the activated complex 6 also
showed better molecular weight control and/or less transesterification than the non-activated
complex 6 (Table 2.6, entry 7). Considering the necessity of a cationic catalyst species for olefin or 1,3-
diene polymerization, this result presents the activated complex as better suited for further study. While
several reports have demonstrated the efficacy of neutral non-activated catalysts with cyclic ester
polymerizations, using the activated cationic complex would provide a better representative understanding
of the transition between the IP and CL polymerizations.
29,30
The results found herein suggest that although 8 was the fastest catalyst for CL polymerization, its
rate for IP polymerization was significantly lower in comparison to 6. Since the target block
copolymerization requires an efficient catalyst that can provide control for both olefin or 1,3-diene and
cyclic ester monomers, 6 was used as the ideal candidate for further studies in synthesizing block
copolymers.
2.2.3 PIP:PCL Block Copolymers
Stepwise block copolymerization of IP and CL was conducted in triplicate using organoborate
activated complex 6 (Table 2.2). Longer reaction times (12 h and 2 h, respectively) were chosen to identify
any side reactions present after full conversion of each monomer. Different ratios of IP and CL could be
polymerized effectively and reproducibly, in which the polymerization control of both steps was maintained
well past full conversion, as evidenced by the low dispersities of the resulting block copolymers. Selectivity
44
for cis-1,4 PIP was highest for the 800:300 IP:CL combination (Table 2.2, entry 1). As the IP amount was
lowered, the ratio of trans-1,4 to cis-1,4 selectivity increased, as was discussed previously in IP
homopolymerizations. For all feed ratio combinations, narrow dispersities were seen. The molecular weight
of the 800:300 IP:CL combination (Table 2.2, entry 1) indicates catalyst activation to be much higher (81–
88%) than that of other catalysts in the literature for the block copolymerization of both monomers, which
has been reported to be between 24 and 56%.
19
2.2.4 Catalyst Efficiency
It was identified that the molecular weights of most polymerizations with activated 6 were often
inconsistent and higher than expected. While the literature often describes this as catalyst efficiency, i.e.,
the degree of catalyst activation by the organoborate, we aimed to probe this further.
It is presumed that the reaction of complex 6 with one equivalent of [Ph 3C][B(C 6F 5) 4] leads to the
abstraction of one alkyl to form 6a, which serves as the active catalyst (Figure 2.4). Overactivation
Table 2.2. Block copolymerization of IP and CL with pre-catalyst 6.
a
Entry
Feed ratio
(IP:CL)
Conv.
(%)
b
Mn
(kDa)
c
Đ
c
Poly(1,3-
diene):polyester (%)
d,e
Microstructure
e,f
Cis-1,4/Trans-
1,4/3,4
Tg
(ºC)
g,h
Tm
(ºC)
g,h
1 800:300 >99 110(21) 1.10(2) 68:32 51/48/1 -66 51
2 550:550 >99 90.9(17) 1.12(1) 47:53 44/5/2 -67 55
3 300:800 >99 101(20) 1.23(3) 26:74 28/70/2 -64 54
a
Conditions: 6, 10 μmol; [Ph3C][B(C6F5)4], 10 μmol; toluene, 10 mL; room temperature; IP 12 h; CL 2 h; all entries are done in
triplicate.
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures, comparing monomer peaks to polymer.
c
Determined by gel permeation chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
d
Determined
by
1
H NMR spectroscopy of the isolated polymer.
e
Average of the triplicate runs.
f
1,4 and 3,4 selectivity determined by
1
H
NMR. Cis-1,4 and trans-1,4 selectivity determined by
13
C NMR.
g
Determined by low temperature differential scanning
calorimetry (DSC).
h
Data for one of the individual runs.
45
of 6 with two equivalents of
[Ph 3C][B(C 6F 5) 4] would lead to the
abstraction of both alkyls,
generating 6b, a species with no bound
initiators which is likely inactive for
polymerization of IP. The higher-than-expected molecular weights would indicate that there is less active
catalyst in solution than anticipated, implying that there is incomplete activation to 6a.
We aimed to execute control reactions in triplicate to allow us to better understand this activation
(Table 2.3). First, the polymerization activity of unactivated 6 (with no [Ph 3C][B(C 6F 5) 4]) and 6b (with 3
equivalents of [Ph 3C][B(C 6F 5) 4]) were tested. Neither condition showed any polymerization of IP, further
validating the active catalyst as 6a.
To test whether catalyst activation is being disrupted by preemptive monomer addition, we adjusted
the time between the addition of [Ph 3C][B(C 6F 5) 4] and monomer. In prior experiments, 6 and the
[Ph 3C][B(C 6F 5) 4] are mixed for 10 minutes before exposure to monomers. A reaction in which 6 and the
[Ph 3C][B(C 6F 5) 4] are mixed quickly, followed by immediate IP addition gave a similar molecular weight
(M n) of 84 kDa to that shown in Table 2.1, entry 1 (Table 2.3, entry 1). Additionally, mixing 6 and the
[Ph 3C][B(C 6F 5) 4] for 30 minutes prior to IP addition also led to indistinguishable M n of 83 kDa (Table 2.3,
entry 2). These studies indicate that catalyst activation is not being interrupted by monomer addition.
1
H
NMR spectra of 6 activated with one equivalent of [Ph 3C][B(C 6F 5) 4] maintains a clean ligand environment,
with no evidence of protonated ligand. This result is in contrast to a report by Li and coworkers, where a
BDIYCl 2(THF) 2 complex was activated with [PhNMe 2H][B(C 6F 5) 4].
31
This reaction led to the protonation
of the ligand and the formation of a proposed ion pair [YCl 2(THF) 2][B(C 6F 5) 4]. We rationalize the absence
of an analogous protonation reaction here, as our activating agent does not have an available proton, and
the ligand has an added chelate that would likely make dissociation of the ligand more difficult.
Figure 2.4. Comparison of activated complex 6 where BDI =
{MeC(NDIPP)CHC(Me)[N(2-OMeC 6H 4)]}Y(CH 2SiMe 3) 2 (DIPP
= 2,6-
i
Pr 2C 6H 3) and counter anions are [B(C 6F 5) 4].
46
A
19
F NMR spectrum (Figure 2.9) of [Ph 3C][B(C 6F 5) 4] revealed the presence of a minor impurity
(98% purity). This suggests the amount of [Ph 3C][B(C 6F 5) 4] added would need to be tuned to maximize the
catalyst efficiency. Thus, the addition of 1.5 equivalents of [Ph 3C][B(C 6F 5) 4], relative to 6, showed a drop
in molecular weight of the 800:300 PIP:PCL block copolymers to 93 kDa (Table 2.3, entry 4), in good
agreement with the theoretical M n of 89 kDa, thereby increasing the catalyst efficiency to 96%. In contrast,
the use of only 0.5 equivalents of [Ph 3C][B(C 6F 5) 4], relative to 6, showed a large increase in the block
copolymer molecular weight to 188 kDa (Table 2.3, entry 3) with a much lower catalyst efficiency of 48%.
Additionally, while DOSY NMR experiments (Figures 2.19-2.20) do not identify a mixture of two
polymers, GPC analysis of the resulting polymer showed a slightly bimodal appearance. With only 0.5
equivalents of [Ph 3C][B(C 6F 5) 4], we expect a mixture of 6 and 6a. While only 6 does not initiate IP, it can
Table 2.3. Investigating different activation conditions with pre-catalyst 6.
a
Entry
Activator
equiv.
IP
Addition
Monomer
(M1:M2)
Conv.
(%)
b
Mn
(kDa)
c
Đ
c
Microstructure
d
Cis-1,4/Trans-
1,4/3,4
Eff (%)
e
1 1 0 IP >99 83.5(4) 1.04(2) 57/41/3 65(3)
2 1 30 min IP >99 82.6(3) 1.04(2) 58/40/2 66(2)
3 0.5 10 min IP:CL >99 188(30) 1.41(3) 43/55/2 48(8)
4 1.5 10 min IP:CL >99 93.1(13) 1.15(1) 57/40/2 96(14)
a
Conditions: 6, 10 μmol; [Ph3C][B(C6F5)4], 5-30 μmol; [IP]/6 = 800; [CL]/6 = 300; toluene, 10 mL; room temperature; IP 12
h; CL 2 h; all entries are done in triplicate; at full conversion, PIP:PCL is 800:300 for entries 3 and 4.
b
Determined by
1
H
NMR spectroscopy of crude reaction mixtures, comparing monomer peaks to polymer.
c
Determined by gel permeation
chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
d
1,4 and 3,4 selectivity determined by
1
H
NMR. Cis-1,4 and trans-1,4 selectivity determined by
13
C NMR.
e
Catalyst efficiency, calculated by Mn(theor.)/Mn(exp.).
47
initiate CL polymerization, suggesting a possible small impurity of PCL homopolymer in the isolated
sample. This highlights the importance of high catalyst efficiencies. Notably, molecular weight control was
maintained for both reactions, with dispersities remaining below 1.4. Higher ratios of [Ph 3C][B(C 6F 5) 4]
slightly increased selectivity for cis-1,4 vs. trans-1,4 IP polymerization, as would be expected for higher
active catalyst concentrations.
29
Since the reactions were done in triplicate, we identified that activation of
the catalyst was variable under the same conditions. This suggests that individual runs may not be entirely
representative of the average efficiency for a particular condition.
2.2.5 Monomer Scope
After complex 6 was identified as an active and controlled pre-catalyst for the block
copolymerization of IP and CL, extension to other monomers of interest was pursued. First, 6 (activated
with one equivalent of [Ph 3C][B(C 6F 5) 4]) was tested for the homopolymerization of several olefin, 1,3-
diene, and cyclic ester monomers (Table 2.4). Since 8 showed higher activity for CL polymerization, it was
also evaluated for these different monomers. However, there were no cases in
which 8 outperformed 6 (Table 2.7).
Activated 6 was found to be active for the homopolymerization of Myr at room temperature, in
which full conversion of 800 equivalents of monomer could be achieved within 3 hours (Table 2.4, entries
1–3). While dispersity remained low at conversions of 44% (1.13) and 72% (1.15), upon reaching full
conversion the dispersity broadened slightly (1.59). Complex 6 showed excellent selectivity towards cis-
1,4 over trans-1,4 or 3,4 Myr polymerization.
32
Interestingly, Myr polymerization with complex 8 (Table
2.7, entry 1) demonstrated a preference for trans-1,4 selectivity resembling the selectivity found for its
polymerization of IP. Additionally, activated 6 was able to polymerize S (Table 2.4, entry 4) albeit at slow
rates. Even at a long reaction time of 20 hours (Table 2.4, entry 5) only 23% conversion of S was
reached.
13
C NMR analysis (Figure 2.10) showed only atactic PS was synthesized
using 6.
33
Activated 6 was also able to polymerize cyclic esters that are often difficult to ring open, such as
VL and DL. In particular, high conversion of VL (81%) could be achieved within 10 minutes at room
temperature, with a low dispersity of 1.24 (Table 2.4, entry 6). Polymerization of DL to high conversions
48
could also be achieved with low dispersities, but a higher reaction temperature (60 °C) and longer reaction
times (6 h) were needed (Table 2.4, entry 7). It is worth noting that complex 8 also demonstrated
polymerization of VL and DL with comparable rates and marginally broader dispersities (Table 2.7, entries
3 and 4, respectively).
Extensions of these studies to the synthesis of block copolymers was conducted for all monomers
except S due to the incomplete conversion with 6. Combinations of 1,3-dienes (IP, Myr) with cyclic esters
(CL, VL, and DL) has led to five more block copolymerization morphologies, all of which are new polymers
Table 2.4. Homopolymerization of a range of olefin, 1,3-diene and cyclic ester monomers with pre-catalysts 6.
a
Entry Monomer Time
Temp.
(ºC)
Conv.
(%)
b
Mn
(kDa)
c
Đ
c
Microstructure
d
Cis-1,4/Trans-
1,4/3,4
Tg (ºC)
e
Tm (ºC)
e
1 Myr 30 min rt 44 121 1.13 97/2/1 -64 –
2 Myr 90 min rt 72 293 1.15 97/2/1 -64 –
3 Myr 3 h rt >99 388 1.59 85/14/1 -64 –
4 S 30 min rt 18 9.4 2.03 – 96 –
5 S 20 h rt 23 21.2 2.17 – 99 –
6 VL 10 min
rt
81 26.2 1.24
– – 53
7 DL 6 h 60 84 24.9 1.14 – -50 –
a
Conditions: 6,10 μmol; [Ph3C][B(C6F5)4], 10 μmol; toluene, 10 mL; [olefin or 1,3-diene]/6 = 800; [cyclic ester]/ 6 = 300.
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures, comparing monomer peaks to polymer.
c
Determined by gel
permeation chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
d
1,4 and 3,4 selectivity
determined by
1
H NMR. Cis-1,4 and trans-1,4 selectivity determined by
13
C NMR.
e
Determined by low temperature
differential scanning calorimetry (DSC).
49
never reported in prior literature (Table 2.5). A consistent 800:300 1,3-diene:cyclic ester ratio was used for
all combinations. Block copolymerization of IP with either VL or DL (Table 2.5, entries 1 and 2,
respectively) reached full conversion for both monomers, producing high molecular weight polymers with
narrow dispersities comparable to those of their respective homopolymers. IP selectivity was akin to the
IP:CL combination (Table 2.2, entry 1). Myr block copolymerization was next explored with CL, VL, and
DL (Table 2.5, entries 3, 4, and 5, respectively). In all three cases, full conversion of Myr was achieved,
while incomplete conversion of the cyclic ester was observed. Incomplete enchainment of the cyclic ester
could be a result of increased viscosity in the reaction medium or due to the bulky high molecular weight
polymyrcene (PMyr) blocking access to the active metal center. Additionally, high dispersities (2.07–2.41)
and low solubility were seen, indicating the presence of side reactions, such as transesterification and/or
Table 2.5. Block copolymerization of 1,3-dienes and cyclic esters with pre-catalyst 6.
a
Entry
Monomer
(M1:M2)
Time
(h)
Temp.
(ºC)
Conv.
(%)
b
Mn
(kDa)
c
Đ
c
Poly(diene:
ester) (%)
d
Microstructure
e
Cis-1,4/ Trans-
1,4/3,4
Tg
(ºC)
(1
st
/2
nd
)
f
Tm
(ºC)
f
1 IP:VL 12:2 rt:rt >(99:99) 141 1.04 66:34 58/40/2 -64/– 50
2 IP:DL 12:12 rt:60 >(99:99) 75.1 1.27 60:30 46/52/2 -65/-48 –
3 Myr:CL 3:2 rt:rt >99:58 482 2.41 89:11 93/5/2 -69/– 51
4 Myr:VL 3:2 rt:rt >99:67 277 2.27 43:57 90/8/2 -63/– 48
5 Myr:DL 3:12 rt:60 >99:56 387 2.07 88:12 87/12/1 -64/-53 –
a
Conditions: 6, 10 μmol; [Ph3C][B(C6F5)4], 10 μmol; toluene, 10 mL; [1,3-diene]/6 = 800; [cyclic ester]/6 = 300.
b
Determined
by
1
H NMR spectroscopy of crude reaction mixtures, comparing monomer peaks to polymer.
c
Determined by gel permeation
chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
d
Determined by
1
H NMR spectroscopy of
the isolated polymer.
e
1,4 and 3,4 selectivity determined by
1
H NMR. Cis-1,4 and trans-1,4 selectivity determined by
13
C
NMR.
f
Determined by low temperature differential scanning calorimetry (DSC).
50
cross-linking.
34
The lower conversions and broader dispersities of the Myr copolymers highlight the need
for a better understanding of catalyst design principles to encourage efficient and controlled polymerization
of a range of olefin/1,3-dienes and cyclic ester monomers, as well as seamless transfer from one monomer
to the next.
These results show the versatility of pre-catalyst 6 and introduce two new cyclic esters (VL and
DL) and a bio-derived 1,3-diene (Myr) to the literature monomer scope for this block copolymerization.
Additionally, these polymerizations represent the first examples of block copolymerization of Myr with
cyclic esters. With new polymers now available, the testing and further understanding of polymer physical
properties due to the variations of monomers and ratios of each block are currently underway.
2.2.6 Recyclability
One of the main motivations for the block copolymerization of 1,3-dienes with cyclic esters is the
ability to recycle the poly(1,3-diene) block. Therefore, it was of interest to show proof of concept that the
polyester block from poly(isoprene-block-caprolactone) (PIP-b-PCL) copolymers could be selectively
degraded, leaving the PIP block intact to be used again to reform the desired block copolymer (Figure 2.5).
Towards this
end, a 50:50
PIP:PCL
block
copolymer
was synthesized in a stepwise fashion using activated 6. This reaction was done using 20 μmol of catalyst,
in which half of the reaction mixture after the first step was used to synthesize the PIP-b-PCL copolymer
with 50 equivalents of CL added, while the other half was used to characterize the original PIP block
(Figure 2.6). The synthesized 50:50 PIP-b-PCL were fully characterized by NMR and IR spectroscopy,
GPC, and TGA/DSC (Figures 2.11, 2.12, 2.13, 2.14, 2.15, and 2.16, respectively). The molecular weight
of the block copolymer was 55 kDa, which was much higher than expected with the catalyst efficiency only
Figure 2.5. Proposed selective hydrolysis of CL block and repolymerization to IP:CL diblock
copolymer.
51
being 17%. We anticipated this was due to the low
concentration of IP in the solution since the same volume of
solvent was used for this reaction as for all other reactions.
A reaction done at a much higher concentration lowered the
molecular weight of the PIP-b-PCL block copolymer to 21
kDa, improving the overall catalyst efficiency (43%) and
getting much closer to the expected molecular weight for
the ratio of monomers. Selective degradation of the CL
block in the 50:50 PIP-b-PCL copolymer was achieved
through alkaline hydrolysis. The copolymer was solubilized
by the addition of minimal THF and heated in a 2 M aqueous NaOH solution at 100 °C.
35
The degraded
polymer was characterized by NMR spectroscopy, GPC, and FT-IR and compared to the original PIP block,
which showed nearly identical molecular weights and dispersities, as well as similar NMR, and FT-IR
spectral features (Figures 2.6, 2.17 and 2.18, respectively).
Yttrium tris[N,N-bis(trimethylsilyl)amide] (Y[N(SiMe 3) 2] 3) was chosen as the repolymerization
catalyst as it is commercially available. It is also well known to readily exchange with an alcohol to form
an yttrium tris-alkoxide species that is active in the polymerization of CL.
36,37
It was reasoned that the
recovered 50 PIP block would terminate with an alcohol if complete hydrolysis of the ester bonds was
achieved, and could exchange with Y[N(SiMe 3) 2] 3 to form a macroinitiator that could polymerize CL.
Indeed, NMR and FT-IR spectroscopy of recovered 50 PIP confirmed the presence of a hydroxyl functional
group (Figures 2.17 and 2.18, respectively). Thus, the recovered 50 PIP block (2.5 equiv.) was combined
with Y[N(SiMe 3) 2] 3 (1 equiv.) and
1
H NMR monitoring revealed the growth of a hexamethyldisilazane
(HMDS) peak (Figure 2.7) consistent with an alkoxide exchange. CL (125 equiv.) was subsequently added,
and complete consumption of CL was achieved within 6 hours (Figure 2.8). The dispersity of the
repolymerized PIP-b-PCL was comparable to the virgin PIP-b-PCL (1.09 and 1.16, respectively), while
their M n values were essentially identical. These findings demonstrate the potential of 1,3-diene and cyclic
Figure 2.6. GPC traces of PIP (M n=42 kDa,
Đ=1.15), recovered PIP (M n=43 kDa,
Đ=1.15), PIP-b-PCL (M n=55 kDa, Đ=1.09),
and repolymerized PIP-b-PCL (M n=56 kDa,
Đ=1.16).
0
0.2
0.4
0.6
0.8
1
5 6 7 8 9
LS Response
Retention Time (min)
PIP
Recovered PIP
PIP-b-PCL
Repolymerized IP-b-CL
0.9
0.95
1
6.4 6.9
LS Response
Retention Time (min)
52
ester block copolymers to be efficiently recycled, warranting further studies into diversifying the polymer
structure and exploring their future applications.
2.3 Conclusions
Herein, we identified cationic alkyl yttrium β-diketiminate complexes as active catalysts for the
homopolymerization and block copolymerization of two 1,3-dienes and three cyclic esters. This study
demonstrated that the number of pendant donors on the ancillary ligand had a dramatic impact on the target
polymerization. Ancillary BDI ligands bearing two donors shut down 1,3-diene polymerization, while
ligands with a single pendant donor could be tuned to affect the selectivity and rate of 1,3-diene and cyclic
ester polymerization. Overall, the rate of 1,3-diene polymerization was faster with a weaker field donor.
The –OMe weak field donor promoted 1,4 selectivity over 3,4 selectivity in IP polymerization with a slight
preference for cis-1,4 over trans-1,4, while the strong field –NMe 2 donor produced 3,4 and trans–1,4
selectivity. Comparable with results observed with IP, the catalyst with the weak field donor favored 1,4
over 3,4 Myr polymerization, with a strong preference for cis-1,4 over trans-1,4 selectivity. High molecular
weight polymers could be achieved with moderate dispersities of 1.13–1.59. Overall, for cyclic ester
polymerization, both catalysts with one neutral donor demonstrated fast polymerization and narrow
dispersities. One-pot block copolymerizations led to a total of 6 diblock morphologies, 5 of which are
entirely new materials. Rigorous inquiry into the thermal and mechanical properties of these new materials
is currently underway.
Investigating the activation of the pre-catalyst demonstrated that monomer addition did not inhibit
the formation of the active catalyst. Also, super stoichiometric ratios of [Ph 3C][B(C 6F 5) 4] to pre-catalyst
(1.5 equiv.) led to experimental molecular weights in better agreement with theoretical molecular weights,
suggesting that stoichiometric addition of [Ph 3C][B(C 6F 5) 4] to pre-catalyst is insufficient for complete
catalyst activation. On the other hand, a vast excess of [Ph 3C][B(C 6F 5) 4] (3 equiv.) completely shuts down
catalyst activity. For PIP-b-PCL copolymers, selective degradation of the PCL block can be achieved
through simple alkaline catalyzed hydrolysis of ester bonds recovering the PIP block with identical
molecular weight and dispersities to virgin PIP. Subsequent repolymerization with CL using a
53
commercially available yttrium catalyst reproduced the PIP-b-PCL copolymers with high molecular
weights and narrow dispersities, both of which are analogous to those of virgin block copolymers. For the
first time, this study introduces a possible recycling scheme for 1,3-diene and cyclic ester block copolymers.
2.4 Experimental Details and Additional Figures.
2.4.1 General Considerations
All reactions involving air and moisture sensitive compounds were carried out using Schlenk line
techniques or in a Vacuum Atmospheres OMNI-LAB glovebox under an oxygen free, N 2 atmosphere.
Solvents used in air free reactions (toluene, hexanes, pentane, and tetrahydrofuran) were purchased from
Fisher, sparged under ultrahigh purity (UHP) grade argon and passed through two columns of drying agent
in a JC Meyer solvent purification system and dispensed directly into the glovebox. All other solvents were
used without further purification. Deuterated NMR solvents, C 6D 6, CDCl 3, and toluene-d 8 were purchased
from Cambridge Isotope Laboratories and were used as received. Toluene-d 8 and C 6D 6 suitable for air
sensitive compounds were dried by stirring over Na/benzophenone for two days, followed by three freeze-
pump-thaw cycles and vacuum transferred into a flame-dried Straus flask and stored in a glovebox under a
N 2 atmosphere. Complexes {MeC(2,6-
i
Pr) 2C 6H 3NH)CHC(Me[N(2-OMeC 6H 4)]}Y(CH 2SiMe 3) 2 (6)
38
,
{CH(CHC(Me[N(2-OMeC 6H 4) 2]}Y(CH 2SiMe 3) 2 (7)
39
, and {CH 3C(2,6-(
i
Pr) 2C 6H 3NH)CHC(CH 3)(NCH 2-
NMe 2)}Y(CH 2SiMe 3) 2 (8)
40
and Y[N(SiMe 3) 2] 3
41
were synthesized following literature procedure. Isoprene,
purchased from Sigma-Aldrich, was dried over 4Å molecular sieves for 7 days, followed by three freeze-
pump-thaw cycles and a vacuum transfer into a flame-dried Straus flask and stored in a glovebox at -35 ºC
under a N 2 atmosphere. ε-Caprolactone, ε-decalactone, δ-valerolactone, and β-myrcene were purchased
from Sigma-Aldrich and were dried over CaH 2 for 3 days followed by three freeze-pump-thaw cycles and
a vacuum transfer to a flame-dried Straus flask and stored in a glovebox at -35 ºC under a N 2 atmosphere.
All other reagents and chemicals were obtained from commercial vendors (Sigma-Aldrich, TCI, Alfa Aesar,
and VWR) and were used without further purification.
54
2.4.2 General Procedure for the Homopolymerization of 1,3-dienes or Cyclic Esters
In a glovebox, catalyst 6-8 (10 μmol, 200 μL of a 0.5 M stock solution) was placed in a stir bar charged 20
mL vial and diluted in toluene (7 mL). Trityl (tetrakis(pentafluorophenyl)borate [Ph 3C][B(C 6F 5) 4] (10 μmol,
1 mL of a 0.01 M stock solution) was add to the vial and the mixture was stirred for 10 minutes. 1,3-diene
(300-800 equiv.) was added by micro syringe in one portion, and the polymerization was carried out for the
designated time with constant stirring during which the reaction turned from orange to gold. The reaction
mixture was poured into a large quantity of ethanol (100 mL) to give colorless copolymer that was dried in
a vacuum oven at 40 ºC for 12 h to a constant weight (0.90 g, 100%).
2.4.3 General Procedure for the Block Copolymerization of 1,3-dienes and Cyclic Esters
In a glovebox, catalyst 6-8 (10 μmol, 200 μL of a 0.5 M stock solution) was placed in a stir bar charged 20
mL vial and diluted in toluene (7 mL). Trityl (tetrakis(pentafluorophenyl)borate [Ph 3C][B(C 6F 5) 4] (10 μmol,
1 mL of a 0.01 M stock solution) was add to the vial and reaction was stirred for 10 minutes. 1,3-diene
(300-800 equiv.) was added by micro syringe in one portion, and the polymerization was carried out for the
designated time with constant stirring during which the reaction turned from orange to gold. Then cyclic
ester (300-800 equiv.) was added by micro syringe to the above system and polymerization was continued
for the designated time. The gel like reaction mixture was poured into a large quantity of ethanol (100 mL)
to give colorless copolymer that was dried in a vacuum oven at 40 ºC for 12 h to a constant weight (0.90 g,
100%). Note: All conditions that deviate from this general procedure are clearly indicated in each table.
2.4.4 General Procedure for Degradation of PIP-b-PCL Diblock
In a 100 mL round bottom flask, charged with a stir bar, PIP-b-PCL 50:50 (0.2 g, 55 kDa) was dissolved in
THF (~25 mL). A 2 M NaOH aqueous solution (50 mL) was then added, and the reaction flask was fitted
with a reflux condenser and heated to 100 ºC for 12 h. After cooling, the solution was neutralized with
acetic acid. The organic fraction was separated, and the aqueous fraction was washed with THF (25 mL)
three times. Organic fractions were combined and washed twice with 2 M NaHCO 3 (50 mL) followed by a
brine wash (40 mL), dried over MgSO 4, and filtered. The volatiles were evaporated in vacuo to afford a
colorless gel. The residue was redissolved in toluene (5 mL) and colorless polymer was crashed out using
55
cooled EtOH. Polymer was dried on a vacuum line at 45 ºC for 3 days, then transferred to a high vacuum
line for an additional day before being stored at -35 ºC in a glovebox (75 mg, 63% mass loss).
2.4.5 Procedure for the Repolymerization of PIP-b-PCL Diblock
In a glovebox, yttrium tris[N,N-bis(trimethylsilyl)amide] (Y[N(SiMe 2) 2] 3) (1 equiv., 47 μL of a 0.01 M
stock solution, 0.47 μmol) was dissolved in toluene (2 mL) and transferred to a stir bar charged Schlenk
flask. PIP-OH (2.5 equiv., 0.05 g, 43 kDa) was dissolved in toluene (5 mL) and slowly added. After stirring
for 10 minutes, ε-caprolactone (125 equiv., 8 μL, 58 μmol) was added to the reaction. Reaction was removed
from the glovebox and placed on a Schlenk line. The side arm of the Schlenk flask was evacuated and
backfilled with dry N 2 three times and the reaction mixture was heated to 70 ºC for 12 h under N 2 atmosphere.
The colorless reaction mixture was poured into a large quantity of ethanol (50 mL) to quench the
polymerization. The collected colorless polymer was dried in a vacuum oven at 40 ºC for 12 h to a constant
weight of 0.152 g.
2.4.6 Characterization Methods
NMR Spectroscopy.
1
H,
13
C, and
19
F, NMR spectra were recorded using a Varian Mercury 400 MHz, Varian
500 MHz, or Varian 600 MHz spectrometers. Chemical shifts are referenced to residual protons in the
deuterated solvent or the deuterated solvent itself for
1
H (7.26 ppm for CDCl 3, 2.08 for toluene-d 8) or
13
C
(77.16 ppm for CDCl 3) NMR spectra. All NMR spectra were recorded at room temperature in specified
deuterated solvents. Representative peak assignments are shown for each polystyrene.
42
Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR spectra were recorded using an Agilent Cary 630
FT-IR equipped with a Diamond ATR sampling accessory. Accompanying MicroLab FT-IR software was
used to acquire 72 scans at 4 cm
−1
resolution with a spectral range of 400-4000 cm
−1
.
Gel Permeation Chromatography (GPC). GPC analyses were conducted using an Agilent 1260 Infinity II
GPC System equipped with a Wyatt DAWN HELEOS-II and a Wyatt Optilab T-rEX as well as an Agilent
1260 Infinity autosampler and UV-detector. The GPC system was equipped with an Agilent PolyPore
column (5 micron, 4.6 mmID) which was calibrated using monodisperse polystyrene standards, eluted with
THF at 30 ºC at 0.3 mL/min. The number average molar mass and dispersity values were determined from
56
multi-angle light scattering (MALS) using dn/dc values calculated by 100% mass recovery method from
the refractive index (RI) signal.
Thermal Gravimetric Analysis (TGA). All TGA traces were recorded using Mettler-Toledo STARe System
TGA/DSC 3+ equipped with STARe software, a TA SDTA Sensor LF, XP1 Balance, and a sample robot.
Data from these four samples were collected on a TGA Q50 instrument. Sample weight of purified polymer
between 5-15 mg was sealed in a 40 μL aluminum crucible fitted with a pierceable lid. General method
involves heating from 25 ºC to 500 ºC at a scan rate of 10 ºC/min under a constant flow of N 2 (15 mL/min).
Differential Scanning Calorimetry (DSC). DSC traces were recorded using a Perkin-Elmer DSC 8000 and
processed with Pyris software. Sample weight of purified polymer was between 7-30 mg measured out on
a microbalance and sealed in standard Perkin-Elmer Aluminum Pans. Data was collected at a consistent
scan rate of 10 ºC/min under a N 2 flow of 20 mL/min. General collection method involved 1) 5 minutes
isothermal step at -80 ºC, 2) heating step from -80 ºC to 100 ºC, 3) 5 minutes isothermal step, 4) cooling
step from 100 ºC to -80 ºC. These 4 steps were repeated through two additional heating and cooling cycles.
All T g and T m values reported are from the 3rd heating scan. Any deviations from this method involved a
temperature range shift.
2.4.7 Formation of Alkoxide Macroinitiator with Recovered IP 50 and Y[N(SiMe 3) 2] 3.
NMR scale PIP-b-PCL repolymerization was carried out in a J-young tube in toluene-d 8. Initial
1
H NMR
of the Y[N(SiMe 3) 2] 3 showed a singlet at 0.23 ppm corresponding to yttrium bound N(SiMe 3) 2 (Figure 2.8,
bottom). Upon addition of PIP-OH 50 an immediate loss of the 0.23 ppm peak as well as formation of a
new downfield HN(SiMe 3) 2 peak was seen (Figure 2.8, top). Subsequent addition of CL lead to immediate
appearance of a peak a 3.92 ppm which corresponds to PCL (Figure 2.11, middle). Full conversion of PCL
was seen after 6 hours (Figure 2.11, top).
57
Figure 2.7.
1
H NMR stack of Y[N(SiMe 3) 2] 3 before (bottom) and after recovered IP 50 addition (top) in toluene-d 8 at
298 K.
Figure 2.8.
1
H NMR stack of PIP-b-PCL repolymerization with Y[N(SiMe 3) 2] 3 and PIP 50 in toluene-d 8 at 298 K.
After macroinitiator formation (bottom), 20 minutes after CL addition (middle), and 6 h after CL addition (top).
58
2.4.8 Additional Figures and Data
Table 2.6. Homopolymerization of Different Feed Ratios of IP and CL with 6/[Ph 3C][B(C 6F 5) 4].
a
Entry Monomer Feed Ratio Mn (kDa)
b
Đ
b
Microstructure
c
Cis-1,4/ Trans-
1,4/3,4
Tg (ºC)
d
Tm (ºC)
d
1 IP 300 50.8(13.1) 1.11(1 19/79/2 -68 ––
2 IP 550 67.4(13.3) 1.05(1) 46/52/2 -65 ––
3 IP 800 84.2(4.53) 1.04(1) 52/46/2 -66 ––
4 CL 300 66.3(4.58) 1.23(1) –– –– 54
5 CL 550 103(4.71) 1.33(1) –– –– 54
6 CL 800 112(3.62) 1.36(1) –– –– 58
7
e
CL 300 51 1.26 –– –– 55
a
Conditions: 6, 10 μmol; [Ph3C][B(C6F5)4], 10 μmol; toluene, 10 mL; room temperature; IP 12 h; CL 2 h; entries 1-6 are done in
triplicate; full conversion was seen in all cases.
b
Determined by gel permeation chromatography (GPC) in THF using a Wyatt
DAWN HELEOS II MALS detector.
c
1,4 and 3,4 selectivity determined by
1
H NMR. Cis-1,4 and trans-1,4 selectivity determined
by
13
C NMR.
d
Determined by low temperature differential scanning calorimetry (DSC).
e
No [Ph3C][B(C6F5)4] added.
59
Table 2.7. Homopolymerization of 1,3-Dienes/Olefins and Cyclic Esters with 8/[Ph 3C][B(C 6F 5) 4].
a
Entry Monomer Time Temp(ºC)
Conv.
(%)
b
Mn
(kDa)
c
Đ
c
Microstructure
d
Cis-1,4/ Trans-
1,4/3,4
Tg (ºC)
e
Tm (ºC)
e
1 Myr 30 min rt 19 12 1.14 4/93/3 -65 ––
2 S 30 min rt 13 4.9 2.03 –– 87 ––
3 VL 10 min rt 82 26 1.27 –– –– 54
4 DL 6 h 60 69 42 1.07 –– -51 ––
a
Conditions: 8, 10 μmol; [Ph3C][B(C6F5)4], 10 μmol; toluene, 10 mL; [1,3-diene or olefin]/8 = 800; [cyclic ester]/ 8 = 300.
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures, comparing monomer peaks to polymer.
c
Determined by gel
permeation chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
d
1,4 and 3,4 selectivity determined
by
1
H NMR. Cis-1,4 and trans-1,4 selectivity determined by
13
C NMR.
e
Determined by low temperature differential scanning
calorimetry (DSC).
60
Impurity in [Ph 3C][B(C 6F 5) 4]
Figure 2.9.
19
F NMR of [Ph 3C][B(C 6F 5) 4] in C 6D 6 at 298 K.
61
Figure 2.10.
13
C NMR spectrum of PS generated by 6/[Ph 3C][B(C 6F 5) 4] from Table 2.4, entry 4 in CDCl 3 at 298 K
(30 min). Representative peak assignment for PS.
42
Figure 2.11.
1
H NMR spectrum of PIP-b-PCL 50:50 equivalents generated by 6/[Ph 3C][B(C 6F 5) 4] in CDCl 3 at 298
K.
62
Figure 2.12.
13
C NMR spectrum of PIP-b-PCL 50:50 equivalents generated by 6/[Ph 3C][B(C 6F 5) 4] in CDCl 3 at 298
K.
Figure 2.13. FT-IR spectrum of PIP-b-PCL 50:50 equivalents generated by 6/[Ph 3C][B(C 6F 5) 4].
500 1000 1500 2000 2500 3000
Wavenumber (cm
-1
)
Wavenumber (cm
−1
)
63
Figure 2.15. TGA curve of PIP-b-PCL 50:50 equivalents generated by 6/[Ph 3C][B(C 6F 5) 4].
Figure 2.14. GPC spectrum of PIP-b-PCL 50:50 equivalents generated by 6/[Ph 3C][B(C 6F 5) 4]: (left) LS; (right)
RI.
5% mass loss at 262 ºC
Weight (%)
64
Figure 2.16. DSC curve of PIP-b-PCL 50:50 equivalents generated by 6/[Ph 3C][B(C 6F 5) 4].
Figure 2.17.
1
H NMR spectrum of recovered PIP 50 equivalents after hydrolysis of PIP-b-PCL 50:50 in CDCl 3 at
298 K (12 h). Alcohol Peak at 3.65 ppm.
Exo
Heating
Cooling
T
g
= -65.44 ºC
Onset T
m
= 50.70 ºC
1
1
65
Figure 2.18. FT-IR spectrum of recovered PIP 50 after hydrolysis of PIP-b-PCL 50:50 (12 h).
Figure 2.19.
DOSY NMR spectrum of PIP-b-PCL 800:300 equivalents generated by 6 and 0.5 equivalents of
[Ph 3C][B(C 6F 5) 4] from Table 2.3, entry 3 in CDCl 3 at 298 K.
500 1000 1500 2000 2500 3000 3500
Wavenumber (cm-1)
O-H stretch seen after hydrolysis
Wavenumber (cm
−1
)
CDCl 3
66
Figure 2.20.
DOSY NMR spectrum of PIP-b-PCL 800:300 equivalents generated by 6 and 1.5 equivalents of
[Ph 3C][B(C 6F 5) 4] from Table 2.3, entry 4 in CDCl 3 at 298 K.
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Synthesis and catalytic activity towards ring-opening polymerization of L-lactide. Polym. Sci. A: Polym. Chem.
2007, 45, 5662-5672.
(39) Liu, X.; Shang, X.; Tang, T.; Hu, N.; Pei, F.; Cui, D.; Chen, X.; Jing, X. Achiral Lanthanide Alkyl Complexes
Bearing N,O Multidentate Ligands. Synthesis and Catalysis of Highly Heteroselective Ring-Opening Polymerization
of rac-Lactide. Organometallics 2007, 26, 2747-2757.
(40) Xu, X.; Xu, X.; Chen, Y.; Sun, J. Dialkyllanthanide Complexes Containing New Tridentate Monoanionic
Ligands with Nitrogen Donors. Organometallics 2008, 27, 758-763.
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69
CHAPTER 3
Enhanced Control of Isoprene Polymerization with Trialkyl Rare Earth Metal
Complexes Through Neutral Donor Support
A portion of this chapter has appeared in print:
Kosloski-Oh, S, C.; Knight, K.; Fieser, M. E. 2023, Submitted.
70
3.1 Introduction
Conjugated diolefin polymers, such as those made from isoprene (IP) and butadiene monomers,
have been extensively studied for their high-value commercial application in the automotive and footwear
industries.
1,2
Selective and living polymerization of these monomers has led to polymers with enhanced
thermal and mechanical properties. However, achieving such polymerization control often requires
elaborate metal catalyst designs that are not always economically feasible for many commercial
applications. For example, rare earth (RE) metal (scandium, yttrium, and the lanthanide series) dialkyl
complexes have emerged as excellent but expensive catalysts for the stereoselective polymerization of 1,3-
dienes.
3,4
Many RE metal complexes have been strategically designed to achieve excellent stereoregularity
and dispersity control by way of elaborate ancillary ligand supports, including pincer, β-diketiminate,
cyclopentadienyl, amidinate, and many others.
5-10
High cis-1,4, trans-1,4, or 3,4 selectivities can also be
realized on demand by control of the size and Lewis acidity of the metal as well as the sterics and electronics
of the supporting ancillary ligand. Supporting ligands not directly involved in the polymerization still play
a vital catalytic role by providing stability, solubility, and control. Recently, we reported a family of yttrium
β-diketiminate dialkyl complexes featuring only subtle variances in the ancillary ligand donor strength,
which, in turn, displayed a dramatic difference in the rate and selectivity for IP polymerization.
11
However,
such designer ligands can be costly to synthesize and can require many intensive synthetic steps. Without
these supporting ligands, bare metal alkyls can be used as rapid polymerization catalysts, but they often
suffer from poor selectivity and poor dispersity control (vide infra). We questioned whether we could
extend simple metal alkyl catalyst lifetimes and improve their control without lengthy ligand and catalyst
syntheses.
Several examples of simplified catalyst systems exist in both industry and academia. Some of these
RE metals have already been employed by industry with excellent efficiency. For example, Goodyear Tire
Company uses a ternary catalyst system composed of a Nd complex, a co-catalyst, and a halide donor (e.g.,
Nd carboxylates/triisobutylaluminum (Al
i
Bu 3)/diethyl aluminum chloride (DEAC)) to prepare high
71
molecular weight poly(isoprene) (PIP) and co-
polymers of polyisoprene-b-polybutadiene.
12 , 13
However, these systems are not living, and precise
control over molecular weight and dispersity has
not been achieved.
14,15
Another set of examples
involves RE trialkyl complexes, which are often
used as convenient metal precursors for the
synthesis of many RE polymerization catalysts in
the literature but are seldom treated as catalysts
themselves. These examples include Hessen’s
lanthanum tribenzyl, La(CH 2Bn) 3(THF) 3,
complex, which exhibited only mild activity
towards ethylene polymerization (Figure 3.1, part
A), and Boisson’s yttrium triallyl and trialkyl
complexes, Y[1,3-(SiMe 3) 2C 3H 3] 3,
Y(CH 2SiMe 2Ph) 3(THF) 2, and Y(CH 2C 6H 4NMe 2) 3,
and their ability to polymerize IP and ethylene (Figure 3.1, part B).
16,17
Boisson and co-workers found that
the latter yttrium complexes needed activation with the cationizing agent [Ph 3C][B(C 6F 5) 4] and the addition
of a Al
i
Bu 3 co-catalyst for polymerization to occur. These simplified catalyst systems could achieve 70%
cis-1,4 selectivity and demonstrated living character.
Despite having simplified catalyst systems, the aforementioned examples still require the use of an
additional co-catalyst and lack stability and polymerization control. Trialkyl RE metal complexes
RE(CH 2SiMe 3) 3(THF) n (RE= Sc, Lu, Tm, Yb, Y, Er, Ho, Dy, Tb, Gd, n=2; RE= Sm, La, n=3) are prevalent
in the RE metal literature, with established synthetic routes readily available for many of the metals in the
series, offering promising opportunities for studying the effects of Lewis acidity in catalysis while also
Figure 3.1. Trialkyl RE metal complexes for the
polymerization of ethylene or isoprene.
16,17,23,26
72
being relatively temperature stable.
18
These catalysts have even been used as neutral catalysts to polymerize
several aromatic vinyl polar monomers. Xu et al. used RE(CH 2SiMe 3) 3(THF) 2 (RE= La, Sc, Y, Dy, Lu) and
Lu(CH 2SiMe 3) 3(Pyridine) 2 to polymerize 2,5-divinylpyridine at room temperature.
19
They found that the
use of a Lewis base such as THF or pyridine could change the stereoselectivity of the polymer. They noted
that the use of a co-catalyst [B(C 6F 5) 3] with Lu(CH 2SiMe 3) 3(pyridine) 2 led to the undesired formation of
pyridine·B(C 6F 5) 3, which prevented pyridine from aiding in the stereoselectivity of the polymerization as
exemplified by the switch from syndiotactic to perfect isotacticity of the polymer with a broad dispersity
of 3.21. Recently, Li and co-workers showed that these RE metal trialkyl complexes, upon activation with
[Ph 3C][B(C 6F 5) 4], were effective catalysts for the polymerization of polar functionalized olefins such as
isocyanides.
20
The RE(CH 2SiMe 3) 3(THF) 2 (RE= Sc, Y, Lu) complexes particularly demonstrated
exceptional tolerance to polar functionalities. Remarkably, the groups of Okuda and Gade identified the
ability to abstract one or two alkyls from this precursor to form mono- and dicationic rare earth metal
complexes, respectively.
21 –23
Okuda and coworkers activated several RE(CH 2SiMe 3) 3(THF) 2 complexes
(RE= Tb, Dy, Ho, Y, Er, Tm, Yb, Lu, and Sc) with 5 equivalents of [NMe 2HPh][B(C 6F 5) 4] activator in the
presence of Al
i
Bu 3 (Figure 3.1, part C).
24
These activated species were found to polymerize ethylene at
room temperature with dispersities ranging from 1.7 to 5.3. In this case, larger metals showed faster
conversion of ethylene, while smaller metals produced longer polymer chains. They hypothesized that a
dicationic active catalyst ([M(CH 2SiMe 3)(solvent) x]
2+
[B(C 6F 5) 4]
+2
) was responsible for the polymerization
activity. Dicationic yttrium alkyl (with methyl and trimethylsilylmethyl alkyls) complexes have been
crystallographically characterized, but only when [B(C 6F 5) 4] was exchanged for the [BPh 4] anion, and the
complex was crystallized in the presence of THF and/or crown ethers.
24–26
Gade and co-workers were able
to coordinate a trisoxazoline ligand to Sc(CH 2SiMe 3)(THF) 2, from which two alkyl groups could be
abstracted with [Ph 3C][B(C 6F 5) 4] (Figure 3.1, part C).
27
The resulting dicationic Sc species was found to
be active for polymerizing 1-hexene at low temperatures. The fastest polymerization occurred at 21 °C,
while the most controlled dispersity was achieved at -30 °C.
73
In a similar case, Okuda and co-workers found that Y[(μ-Me 2) 2-(AlMe 2)] 3 could be combined with
[NEt 3H][BPh 4] to form a dicationic methyl complex, [YMe(THF) 6]
2+
[BPh 4]
+2
.
24
When activated without
the presence of THF, these complexes were active for isoprene and 1,3-butadiene polymerization, with
dicationic species showing higher polymerization rates than the analogous monocationic species. The
presence of trialkylaluminum reagents had a greater impact on the isoprene polymerization rate and
selectivity.
28
Once again, in these cases, dispersities ranged from 1.5 to 4.4. High selectivity for cis-1,4
polymerization of 1,3-dienes with a simple ligand system is promising, particularly if the dispersity can be
controlled.
While these examples show that active cationic and dicationic complexes can be formed by
abstracting alkyls from simple rare earth metal trialkyl species, limited information exists regarding their
polymerization behavior. Many of the studies focus on different monomers or involve only the use of
trialkylaluminum additives. Additionally, all the catalysts investigated have shown poor dispersity control.
In this work, we report that RE(CH 2SiMe 3) 3(THF) n (RE= Sm, n=3; RE= Gd, Y, Tm, n=2) can be activated
to achieve rapid and selective polymerization of isoprene, with exceptional molecular weight control, in the
presence of a commercially available, soft, neutral donor (Figure 3.1, part D). These results present an
unexpected and attractive strategy for d
0
metal catalysis, as the use of a soft donor to support the reactivity
and control of rare earth metal ions is not typically considered due to the binding mismatch between the
soft donor and hard metal ion.
We also elucidate how the observed catalyst activity and polymerization
control are dictated by the order and timing of addition, the stoichiometry of activating agent and donor,
and the size of the metal ion.
3.2 Results and Discussion
3.2.1 Yttrium Pre-catalyst Screening
The yttrium trialkyl complex, Y(CH 2SiMe 3) 3(THF) 2, was initially selected to test for polymerization
activity, as it is a mid-sized metal, and the complex is diamagnetic. This complex was synthesized following
literature procedure by reacting YCl 3 with 2.9 equivalents of (trimethylsilyl)methyllithium (LiCH 2SiMe 3)
74
at reduced temperatures.
29
Initially, the pre-catalyst showed no IP polymerization activity without the
addition of [Ph 3C][B(C 6F 5) 4]. The yttrium pre-catalyst was subsequently screened with 1 or 2 equivalents
of [Ph 3C][B(C 6F 5) 4], leaving 2 or 1 alkyls to enact polymerizations, respectively (Figure 3.2). This
activation step serves the dual purpose of increasing the Lewis acidity of the metal center and opening a
metal binding pocket to allow enough physical space for the crucial binding of IP. As previously discussed,
Boisson and co-workers activated Y(CH 2SiMe 2Ph) 3(THF) 2 with 1 equivalent of [Ph 3C][B(C 6F 5) 4] in THF,
which led to an isolable cationic dialkyl species with three THF molecules coordinated. This species was
inactive for IP polymerization without Al
i
Bu 3. When Okuda and co-workers generated dicationic
monoalkyl species for polymerizations, they did so in toluene, presumably to avoid additional THF
binding.
24,28
In this report, activation is done in toluene to prevent THF from occupying the IP coordination
site.
Reaction of Y(CH 2SiMe 3) 3(THF) 2 with 1 equivalent of [Ph 3C][B(C 6F 5) 4] in C 7D 8 reveals indication of
the formation of an monocationic species in the NMR. The
1
H NMR shows two broad peaks (δ = -0.68 and
-0.61). These peaks are in the region that had been previously assigned to analogous monocationic bisalkyl
yttrium complexes (Figure 3.7).
26
Upon addition of IP, the peaks resolved to a single doublet at -0.69 ppm
giving clear indication of Y-CH 2 bond (Figure 3.8). In this case, the
19
F NMR shows one set of C 6F 5
resonances, which suggests the [B(C 6F 5) 4] anion is either not coordinating to the Y or is fluxional (Figure
3.9). Reaction of Y(CH 2SiMe 3) 3(THF) 2 with 2 equivalents of [Ph 3C][B(C 6F 5) 4] shows two broad Y-CH 2
signals in the
1
H NMR (δ= -1.10 and δ= -1.22). The chemical shift is consistent with other dicationic species
(Figure 3.10), however, the fact that they are broad singlets suggest that the complex is highly fluxional.
26
Indeed, contrary to the monocationic case, the
19
F NMR shows two sets of C 6F 5 resonances with one being
Figure 3.2. Proposed yttrium pre-catalyst activation with 1 or 2 equivalents of [Ph 3C][B(C 6F 5) 4], assuming no
interaction with toluene or the borate anion.
75
broad and having a lower intensity which suggests a minor species with coordination of the [B(C 6F 5) 4]
anion to the Y metal center (Figure 3.11). These interactions with the borate anion being fluxional have
been identified for other complexes where the crystal structure shows coordination of the fluorine to a rare
earth metal, while the NMR shows one set of
19
F resonance.
30
IP was chosen as the monomer to be studied because of its industrial value, and since it is well-
established as a benchmark for general 1,3-diene polymerizations. IP polymerization using 1 or 2
equivalents of [Ph 3C][B(C 6F 5) 4] was performed to determine the efficacy of the monocationic and dicationic
active catalyst species (Table 3.1, entries 1 and 2). Polymerizations were conducted in toluene with the IP
monomer added 10 mins after the pre-catalyst was activated with [Ph 3C][B(C 6F 5) 4]. With 1 equivalent of
[Ph 3C][B(C 6F 5) 4], the polymerization only reached 20% conversion after 30 mins, but with 2 equivalents
of [Ph 3C][B(C 6F 5) 4], full conversion was reached within the same timeframe. These results are in accord
with the increased Lewis acidity and reduced steric crowding around the yttrium metal center creating a
more reactive catalyst species. There are two alkyls present when one equivalent of [Ph 3C][B(C 6F 5) 4] is
used. At the early conversion shown in entry 1 in Table 3.1 the experimental molecular weight matches
reasonably well for either one or two active alkyl initiators although it agrees more with one active alkyl
initiator. This is likely due to slow initiation of the second alkyl initiator at the monocationic yttrium metal
center. The molecular weight is shown to better match the two alkyl initiators at higher IP conversion,
discussed below (Table 3.3, entry 1). The GPC data is also consistent with this as the 30 minute reaction
in Table 3.1, entry 1 shows a monomodal distribution while the longer reaction in Table 3.3, entry 1 shows
a bimodal distribution in the light-scattering trace (Figures 3.12 and 3.13, respectively). This behavior was
also shown for other yttrium trialkyl pre-catalysts when only one alkyl was abstracted.
16
The molecular weight of the polymer from entry 2 in Table 3.1 matches the expected value for a single
alkyl initiator on the proposed dicationic active catalyst. Similar to the trend seen with rate, the dispersity
was much higher when 2 equivalents of [Ph 3C][B(C 6F 5) 4] were used and narrower with only 1 equivalent,
a further indication of the highly reactive nature of the former active catalyst. The high dispersity of the
dicationic monoalkyl rare earth complex is consistent with prior literature.
24
76
With 1 equivalent of [Ph 3C][B(C 6F 5) 4] the microstructure shows cis-1,4 and trans-1,4 PIP, however,
with 2 equivalents of [Ph 3C][B(C 6F 5) 4] no trans-1,4 selectivity is observed. This is consistent with the trend
seen for butadiene polymerization, where an increased Lewis acidic metal center encourages the production
of cis vs. trans selectivity.
31,32
Interestingly, when using a Y[(μ-Me 2) 2-(AlMe 2)] 3 precatalyst with 1 or 2
equivalents of [PhNHMe 2][B(C 6F 5) 4] showed the same 3,4 selectivity, yet slightly different 1,4 isoprene
selectivity.
28
In that case, the cationic dialkyl species showed a higher cis-1,4 than this study, while the
dicationic monoalkyl species showed trans-1,4 selectivity. It is unclear if these differences in selectivity
are due to the pre-catalyst, activating agent or differed reaction conditions.
Table 3.1. Polymerization of IP with Y(CH 2SiMe3) 3(THF) 2 pre-catalyst.
a
Entry
Borate (B)
(equiv.)
Conv. (%)
b Theor
Mn (kDa)
c Exp
Mn (kDa)
d
Đ
d
Microstructure
e
Cis-1,4/Trans-
1,4/3,4
1 1 20(1) 7 5(1) 1.44(5) 41/41/18
2 2 >99 34 40(2) 2.04(2) 83/0/17
a
Conditions: Y(CH2SiMe3)3(THF)2, 10 μmol; [Ph3C][B(C6F5)4] (B), 10-20 μmol; toluene, 10 mL; [IP]/Y=500; all entries are done
in duplicate.
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures, comparing monomer peaks to polymer.
c
Calculated
based on one alkyl initiator, [IP mol/Y mol] x IP molecular weight x Conversion.
d
Determined by gel permeation chromatography
(GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
e
All selectivity data is an average of duplicate runs. 1,4 and 3,4
selectivity determined by
1
H NMR. Cis-1,4 and trans-1,4 selectivity determined by
13
C NMR.
3.2.2 Neutral Donor Scope
While the yttrium pre-catalyst demonstrates a fast rate and excellent cis-1,4 versus trans-1,4
selectivity with 2 equivalents of [Ph 3C][B(C 6F 5) 4], the poor dispersity exemplifies a need to modulate the
catalysis. It was hypothesized that addition of a neutral donor in situ might provide an exciting avenue to
give the catalyst support without intensive ligand synthesis. A previous study from Xu et al. showed how
adding Lewis bases such as (THF, pyridine, styrene, triphenyl phosphine, triethyl phosphine and trimethyl
phosphine) to cationic RE alkyl complexes effected the stereoselectivity of 2-vinylpyridine
polymerization.
33
In these reactions, the Y(CH 2SiMe 3) 3(THF) 2 was first activated with 2 equivalents of
[Ph 3C][B(C 6F 5) 4] for 10 mins, followed by reaction with neutral donor for another 10 mins, prior to IP
77
addition. Since most ancillary ligands for IP polymerization catalysts are multidentate, bidentate ligands
with harder nitrogen donors and softer phosphorus donors were tested (Table 3.2). In the cases of bidentate
phosphines 1,2-bis(diphenylphosphino)ethane (dppe) and Bis[(2-diphenylphosphino)phenyl] ether
(DPEphos), polymerization activity was halted completely (Table 3.2, entries 1 and 2). On the other hand,
when a harder bidentate nitrogen donor, bipyridine (bipy) was added, polymerization activity was
significantly diminished but not halted (Table 3.2, entry 3). Given the harder nitrogen donors in bipy would
lead to a comparatively weaker yttrium Lewis acid, its greater reactivity may be explained by a reduced
steric crowding compared to the two phosphine donors.
Because of the poor activity with bidentate neutral donors, several monodentate nitrogen and
phosphine donors were tested. Two monodentate nitrogen donors, pyridine, and acetonitrile, showed
comparable conversions to those seen with bipy (Table 3.2, entries 4, and 5), suggesting the denticity of
these donors was not crucial. Yttrium dicationic complexes have been known to activate C-H bonds of
pyridine, although this is extremely slow at room temperature in neat pyridine so it is unlikely to occur in
Table 3.2. Neutral donor additives in the homopolymerization of IP with Y(CH2SiMe3)3(THF)2.
a
Entry Cat. Donor (D)
Borate (B)
(equiv.)
Conv. (%)
b
Theor
Mn
(kDa)
c
Exp
Mn
(kDa)
d
Đ
d
Microstructure
e
Cis-1,4/ Trans-
1,4/3,4
1 Y Dppe 2 0 –– –– –– ––
2 Y DPEphos 2 0 –– –– –– ––
3 Y Bipy 2 14(1) 5 30(3) 1.37(2) 66/8/26
4
f
Y Pyridine 2 4(1) 1 –– –– ––
5 Y Acetonitrile 2 20(1) 7 35(5) 1.99(7) 84/0/16
6 Y P(o-tolyl)3 2 60(4) 20 37(5) 2.06(2) 83/0/17
7 Y PCy3 2 36(1) 12 7.9(1) 1.41(5) 66/8/26
8 Y PPh3 2 76(3) 26 33(2) 1.16(4) 61/13/26
a
Conditions: Y(CH2SiMe3)3(THF)2, 10 μmol; [Ph3C][B(C6F5)4] (B), 20 μmol; toluene, 10 mL; 10 μmol, Donor; [IP]/Y= 500;
all entries are done in duplicate.
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures, comparing monomer peaks
to polymer.
c
Calculated based on one alkyl initiator, [IP mol/Y mol] x IP molecular weight x Conversion.
d
Determined by gel
permeation chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
e
All selectivity data is an
average of duplicate runs. 1,4 and 3,4 selectivity determined by
1
H NMR. Cis-1,4 and trans-1,4 selectivity determined by
13
C
NMR.
f
Yield was insufficient for characterization.
78
this case with catalytic quantities.
34
Although, it also may not be likely in this case as Okuda and co-workers
used pyridine-d 5 to characterize similar dicationic species.
26
Next, three monodentate phosphine ligands,
tri(ortho-tolyl)phosphine (P(o-tolyl) 3), tricyclohexylphosphine (PCy 3), and triphenylphosphine (PPh 3),
showed higher activity compared to all the nitrogen donors and the bidentate phosphine donors (Table 3.2,
entries 6, 7, and 8, respectively). Gratifyingly, a much lower dispersity of 1.16 was achieved when PPh 3
was used while not dramatically lowering the rate compared to the reactions without a neutral donor. In
comparison, there was only a slight improvement to the polymerization when 1 equivalent of PPh 3 was
added to the monocationic catalyst (Table 3.5, entry 13). When comparing the three monodentate
phosphine donors, an important question arises about whether the excellent control demonstrated by the
PPh 3 is caused by a steric profile that may help deter aggregation of the dicationic catalyst, or by the
electronics of the donor. In the case of P(o-tolyl) 3, which would have a similar donor strength as PPh 3, the
steric profile appears to be too large (Tolman cone angle 194º) for the donor to stabilize the catalyst as
evidenced by the fact that the dispersity and microstructure match the data where a donor was not added
(Table 3.1, entry 2), and the discrepancy between the theoretical M n and the experimental M n is large.
35
It
was hypothesized that PPh 3 was providing adequate steric/coordinative support to the catalyst without
disrupting the Lewis acidity of the metal center or blocking access to the IP monomer. The Tolman cone
angle PCy 3 (170º) represents a middle ground in size. The decrease in conversion and lowered dispersity
indicates that PCy 3 is directly impacting the catalyst activity. However, it is unclear if the lowered
conversion, in comparison to PPh 3, is due to the difference in steric crowding or donor strength of the
ligands. Additionally, comparing the GPC traces of the three monodentate phosphine donors, when PPh 3 is
used the light scattering trace appears more unimodal (Figure 3.14).
In order to isolate the electronic effects of the PPh 3 donor, a Hammett plot of log (k X/k H) versus the
standard σ constants for para-substituents on the phenyl rings was constructed (Figure 3.3).
36
Three
commercially available para-X-substituted PPh 3 (X = OMe, CH 3, F) were tested for their efficacy as a donor
support for the polymerization of IP with Y(CH 2SiMe 3) 3(THF) 2/[Ph 3C][B(C 6F 5) 4] catalyst system, using
reaction conditions identical to those used for Table 3.2. As hypothesized, electron-withdrawing groups
79
enhanced the rate of polymerization as the weaker
donors would be expected to not lower the Lewis
acidity of the metal ion. A linear relationship was
extrapolated from the Hammett plot with a positive
slope of 3.11, indicating that the reaction is sensitive
to electronic changes i.e., negative charge is built, or
positive charge is lost during the reaction.
Information on the conversion and number-average
molecular weight (M n) can be found in Table 3.9.
The effect of the donor substituents on the selectivity
of the PIP microstructure was also examined (Figure
3.4). It was found that the 3,4 selectivity remained
unchanged for each donor, which is expected as 3,4
selectivity is generally dependent on the steric
crowding around the metal center. However, higher
cis-1,4 content was seen with more electron-
withdrawing substituents where full cis-1,4 versus
trans-1,4 selectivity was achieved with the para-
fluorinated substituent. Importantly, while the p-F substituent enhanced both the rate and the cis-1,4
selectivity, PPh 3 still demonstrated superior M n and dispersity control (Table 3.9, entry 4 and Table 3.2,
entry 8, respectively). This might be because the donor could have a weaker bond to the metal center, thus
losing some of the stabilizing effect of the donor. This highlights the need to design ligand frameworks that
are modulated to provide sufficient support to stabilize the complex without sacrificing the Lewis acidity
of the metal center.
In order to further examine the effect of the PPh 3 on the polymerization of IP, a dynamic study of
the Y(CH 2SiMe 3) 3(THF) 2 activated by 2 equivalents [Ph 3C][B(C 6F 5) 4] was carried out both with and
Figure 3.4. Comparison of cis-1,4 and trans-1,4
selectivity versus the standard σ constants for the
different substituents. Reactions run analogous to
those in Table 2.
0
10
20
30
40
50
60
70
80
0
10
20
30
40
50
60
70
80
-0.27 -0.17 0 0.06
Trans-1,4 (%)
Cis-1,4 (%)
σ
p
p-OMe
p-Me
H
p-F
p-F
H
p-Me
p-OMe
Figure 3.3. Hammett plot of log (k x/k H) versus the
standard σ constants for the substituent. Reactions
run analogous to those in Table 2.
OMe
EW
F Me H
ED
X =
y = 3.1092x - 0.0174
R² = 0.9966
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
-0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1
log(k
X
/k
H
)
σ
p
p-F
H
p-Me
p-OMe
80
without inclusion of PPh 3 (Figure 3.5 a and b, respectively). Gratifyingly, in the case where PPh 3 was added
the M n increased linearly with conversion while the dispersity remained unchanged ( Đ≈1), indicating that
the polymerization had living character. However, in the reaction without a donor, the dispersity was broad
(>1.5) and increased early in the reaction. This further highlights the stability and control that the PPh 3 adds
to the polymerization. Information on the selectivity of the microstructure can be found in Tables 3.10 and
3.11.
We hypothesized that the phosphine is loosely binding to the metal center, which is causing the
added dispersity control. Since the electron-withdrawing groups increases the polymerization rate, it
appears that the loose binding is important to maintain catalytic activity. NMR studies were conducted to
characterize the active catalyst with PPh 3. Therefore, we activated Y(CH 2SiMe 3) 3(THF) 2 with 2 equivalents
of [Ph 3C][B(C 6F 5) 4] for 0, 10, and 30 minutes before adding one equivalent of PPh 3, and collected
31
P NMR
spectra over time. In all three cases, two
31
P signals show up at 24.18 and 23.84. Over time the growth of a
signal at -5.28 ppm is observed. As this solution is left over time, solids form and the signals around 24
ppm disappear (Figure 3.15). A control reaction of [Ph 3C][B(C 6F 5) 4] and PPh 3 reveals the same two
31
P
signal around 24.18 and 23.84 as well as several small upfield peaks between (6.50-5.50). Over time a
single peak around -6.48 is observed and the formation of solids in the NMR tube is seen (Figure 3.16). As
the signal at -5.28 is very close to free PPh 3 and the lack of a doublet suggests weak, fluxional, or no
Figure 3.5. Comparison of cis-1,4 and trans-1,4 selectivity versus the standard σ constants for the different
substituents. Reactions run analogous to those in Table 2.
y = 0.4277x + 14.008
R² = 0.9943
0
1
2
3
4
5
0
10
20
30
40
50
60
0 30 60 90
Đ
M
n
(kDa)
Conversion (%)
M
n
Đ
y = 0.5914x - 12.908
R² = 0.9892
0
1
2
3
4
5
0
10
20
30
40
50
40 60 80 100
Conversion (%)
Đ
M
n
(kDa)
M
n
Đ
a) b)
81
interaction with yttrium, we reasoned that by cooling the reaction we could resolve the peaks. Variable
temperature (VT) NMR was conducted on Y(CH 2SiMe 3) 3(THF) 2 with 2 equivalents of [Ph 3C][B(C 6F 5) 4]
for 10 minutes before adding one equivalent of PPh 3; however, even at a temperature as low as -80 °C, the
spectrum only showed a singlet
31
P signal (Figure 3.17). This might not be entirely surprising due to the
mismatch in polarizability between the hard Lewis acid Y center and the soft Lewis base PPh 3. Indeed,
Anwander et al. crystallized the neutral complex Y(CH 2SiMe 3) 3THF(dmpe) (dmpe = 1,2-
bis(dimethylphosphino)ethane); however, they did not observe a doublet in the
31
P NMR at room temperate
or at -80 °C.
37
Therefore, while PPh 3 clearly impacts the polymerization by significantly lowering the
dispersity, it cannot be conclusively confirmed to be binding to the metal ion. In another attempt to confirm
if the PPh 3 was bound to the yttrium metal center, DOSY NMR was taken before and after PPh 3 addition.
In both cases, a single diffusion coefficient is observed, indicating a single species in solution (Figure 3.18
and 3.19). This provides support that the PPh 3 is interacting the yttrium active species.
3.2.3 Degree of Activation
We next investigated the effect of varying the equivalents of [Ph 3C][B(C 6F 5) 4] on IP
polymerization, both with and without PPh 3. First, with only 0.5 equivalents of [Ph 3C][B(C 6F 5) 4],
Y(CH 2SiMe 3) 3(THF) 2 was inactive towards IP polymerization. We then gradually increased the amount of
[Ph 3C][B(C 6F 5) 4] in 0.5 equivalent increments up to 3 equivalents for separate IP polymerizations (Table
3.3). With 1 equivalent of [Ph 3C][B(C 6F 5) 4], both with and without PPh 3, we observed only about 70%
conversion within 7 hrs, whereas higher equivalents reached full conversion within the same time frame.
Additionally, with 1 equivalent of [Ph 3C][B(C 6F 5) 4], we observed that the experimental M n was consistent
with two active alkyl initiators, now that higher conversion had been reached. As the amount of
[Ph 3C][B(C 6F 5) 4] was increased, the molecular weight increased relative to the theoretical molecular weight
values for either one or two active alkyl initiators. This is consistent with the idea that, with fewer alkyls to
stabilize the metal center, some of the catalysts would begin to decompose before polymerization. The
discrepancy between the experimental M n and the theoretical M n was decreased in the cases where PPh 3
was added, indicating that PPh 3 helps to stabilize the active catalyst. In addition to improving the molecular
82
weight, PPh 3 significantly reduced the dispersity. As the amount of [Ph 3C][B(C 6F 5) 4] is increased the more
the light scattering GPC traces deviate from a unimodal distribution indicating multiple catalyst
environments (Figures 3.20, 3.21, and 3.22). In each case, with the addition of PPh 3 the light scattering
GPC traces appear more unimodal.
Surprisingly, we observe polymerization activity even with 3 equivalents of [Ph 3C][B(C 6F 5) 4]. We
have noted in a previous study that an impurity is present in the [Ph 3C][B(C 6F 5) 4] which could mean that
we could be adding less [Ph 3C][B(C 6F 5) 4] than we are expecting. Additionally, we only react the
[Ph 3C][B(C 6F 5) 4] with Y(CH 2SiMe 3) 3(THF) 2 for 10 mins prior to IP addition, which could lead to
incomplete activation. However, Okuda and co-workers were not able to abstract the third alkyl of
Y(CH 2SiMe 3) 3(THF) 2, even with 5 equivalents of [NMe 2HPh][B(C 6F 5) 4].
24
In a later study, Okuda and co-
workers were able to see NMR evidence of an alkyl free complex by reacting Y(CH 2SiMe 3) 3(12-crown-4),
3 equivalents of [NMe 2HPh][B(C 6F 5) 4], and 1 equivalent 12-crown-4 for 1 h.
26
Even though in our study it
Table 3.3. Varying equivalents of [Ph3C][B(C6F5)4] in the homopolymerization of IP with
Y(CH2SiMe3)3(THF)2.
a
Entry
PPh3
(P)
(equiv.)
Borate
(B)
(equiv.)
Order of
Addition
Conv.
(%)
b
Theor
Mn (KDa)
(2 Inr/1 Inr)
c
Exp
Mn
(KDa)
d
Đ
d
Microstructure
e
Cis-1,4/ Trans-
1,4/3,4
1 –– 1 –– 72(2) 12/24 18(1) 1.29(7) 13/74/13
2 1 1 B →P 70(3) 12/24 15(4) 1.21(4) 12/75/13
3 –– 1.5 –– >99 17/34 49(8) 1.6(10) 70/6/24
4 1 1.5 B →P >99 17/34 25(5) 1.33(8) 66/7/27
5 –– 2 –– >99 17/34 57(6) 1.98(6) 83/0/17
6 1 2 B →P >99 17/34 46(5) 1.18(1) 61/13/26
7 –– 2.5 –– >99 17/34 56(3) 1.96(7) 84/0/16
8 1 2.5 B →P >99 17/34 44(2) 1.26(3) 64/11/25
9 –– 3 –– >99 17/34 148(7) 3.89(10) 86/0/14
10 1 3 B →P >99 17/34 67(4) 1.41(8) 82/0/18
a
Conditions: Y(CH2SiMe3)3(THF)2, 10 μmol; [Ph3C][B(C6F5)4] (B), 10-30 μmol; toluene, 10 mL; 0-10 μmol PPh3 (P);
[IP]/Y=500; all entries are done in duplicate.
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures, comparing
monomer peaks to polymer.
c
Calculated based on 1 or 2 alkyl initiators, [IP mol/RE mol] x IP molecular weight x Conversion.
Inr = alkyl initiator.
d
Determined by gel permeation chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS
detector.
e
All selectivity data is an average of duplicate runs. 1,4 and 3,4 selectivity determined by
1
H NMR. Cis-1,4 and trans-
1,4 selectivity determined by
13
C NMR.
83
is difficult to discern if the third alkyl is being abstracted or initiator/precatalyst deactivation occurs before
isoprene addition, we saw that the molecular weight continues to increase as [Ph 3C][B(C 6F 5) 4] equivalence
is increased. An NMR scale reaction between Y(CH 2SiMe 3) 3(THF) 2 with 3 equivalents of [Ph 3C][B(C 6F 5) 4]
was conducted to identify if the third alky is being abstracted. The
1
H spectra showed resonances due to Y-
CH 2 methylene groups (δ = -0.86, -1.05(d, J YH= 3.4), -1.16). (Figure 3.23). Only one of the signals showed
a clear doublet at a chemical shift value consistent with the Y-CH 2 peak of the dicationic species. But, it is
worth noting that the neutral complex in toluene-d 8 appears as a broad singlet, so the other two upfield
signals might also be attributable to Y-CH 2 interactions. This indicates that within the time scale of these
polymerizations not all the alkyls are being abstracted.
3.2.4 IP Polymerization with Ternary Systems
The addition of a co-catalyst is often used to promote better selectivity with RE metal alkyl
catalysts. We tested the influence of three commonly used aluminum alkylating agents as co-catalysts for
IP polymerization: AlMe 3, AlEt 3, and Al
i
Bu 3 (Table 3.4). These reactions were carried out by combining
Y(CH 2SiMe 3) 3(THF) 2 with 2 equivalents of [Ph 3C][B(C 6F 5) 4] for 10 mins, followed by the addition of the
desired aluminum alkylating agent. After 10 mins of mixing, 500 equivalents of IP were added. The
aluminum alkyls form varied steric environments and different conformations around the Y metal center,
which will impact the rate and selectivity of isoprene polymerization. The bulkier alkylating agents allowed
for faster IP polymerization rates (Al
i
Bu 3 > AlEt 3 > AlMe 3). When Al
i
Bu 3 was used, full conversion was
reached within 30 mins. The enhanced rate with the bulkier co-catalysts parallels other reports, where the
aluminum co-catalysts with shorter alkyl chains can form heterobimetallic RE-Al alkyl species that slow
down activity.
38,39
As the ratio between the alkylating agent and catalyst increased, a drop in M n and a
significant broadening of the dispersity (Table 3.4, entries 7–9) were observed, indicating that Al
i
Bu 3 was
acting as a chain transfer agent. As the equivalents of alkylating agent increased, the conversion decreased,
suggesting that chain transfer was competitive with polymer propagation. Dispersity also increased with
higher alkylating equivalents, further indicating competition between chain transfer and propagation. After
Al
i
Bu 3 demonstrated the fastest rate, we wanted to optimize the reaction and speculated that adding PPh 3
84
could aid in lowering the dispersity, just as it did without an alkylating agent. PPh 3 was incorporated into
the reaction after 10 mins of activation of Y(CH 2SiMe 3) 3(THF) 2 with 2 equivalents of [Ph 3C][B(C 6F 5) 4],
and 5–15 equivalents of Al
i
Bu 3 were added after 10 mins of stirring. With 5 and 10 equivalents of Al
i
Bu 3,
the reaction reached full conversion, but at 15 equivalents, the rate decreased. This could be due to the
increased steric crowding around the metal center. Overall, reactions with PPh 3 lowered the M n and slightly
decreased the dispersity (Table 3.4, entries 10-12). The cis-1,4 selectivity was enhanced by adding Al
i
Bu 3,
with the incorporation of PPh 3 only slightly decreasing the cis-1,4 selectivity. In general, the use of a co-
catalyst exhibited higher dispersities and, in the cases with AlEt 3 and AlMe 3, a loss in activity.
NMR studies were conducted to better understand the loss of activity with AlMe 3 despite the better
dispersity control it displayed. These were carried out by first reacting Y(CH 2SiMe 3) 3(THF) 2 with 2
equivalents of [Ph 3C][B(C 6F 5) 4] for 10 minutes. 5 equivalents of AlMe 3 were then added and
1
H and
27
Al
NMR spectra were taken at room temperature in toluene-d 8 (Figures 3.24 and 3.25, respectively). The
1
H
Table 3.4. Aluminum alkyl additives in the homopolymerization of IP with Y(CH2SiMe3)3(THF)2.
a
Entry
PPh3
(P)
Alkylating Agent
(AlR3)
AlR3
(equiv.)
Conv.
(%)
b
Mn
(KDa)
c
Đ
c
Microstructure
d
Cis-1,4/ Trans-
1,4/3,4
1 –– AlMe3 5 68 68 1.49 54/42/4
2 –– AlMe3 10 78 43 1.49 56/37/7
3 –– AlMe3 15 49 39 1.81 58/37/5
4 –– AlEt3 5 90 25 3.31 64/19/17
5 –– AlEt3 10 84 26 4.78 63/21/16
6 –– AlEt3 15 80 22 6.41 65/21/14
7 –– Al
i
Bu3 5 >99 59 3.69 78/2/20
8 –– Al
i
Bu3 10 >99 46 4.69 84/3/13
9 –– Al
i
Bu3 15 >99 37 6.38 85/2/13
10 1 Al
i
Bu3 5 >99 42 3.07 73/4/23
11 1 Al
i
Bu3 10 >99 40 2.29 71/5/24
12 1 Al
i
Bu3 15 86 28 4.15 71/7/22
a
Conditions: Y(CH2SiMe3)3(THF)2, 10 μmol; [Ph3C][B(C6F5)4] (B), 20 μmol; toluene, 10 mL; 0-10 μmol, PPh3; [IP]/Y=500.
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures, comparing monomer peaks to polymer.
c
Determined by gel
permeation chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
d
1,4 and 3,4 selectivity
determined by
1
H NMR. Cis-1,4 and trans-1,4 selectivity determined by
13
C NMR.
85
NMR showed multiple peaks between -0.66 ppm to -1.13 ppm indicating multiple CH 3 species.
Interestingly, despite activation with two equivalents of [Ph 3C][B(C 6F 5) 4] a doublet around -0.67 ppm is
observed consistent with the Y-CH 2 monocationic species while the peak associated with the dicationic
species at -1.08 ppm is present but at a lower intensity. So, the polymerization is hypothesized to proceed
through a monocationic species thus accounting for the reduced activity. The abundance of the
monocationic species could be attributed to ligand rearrangement. Also, previous reports have shown that
AlMe 3 species do react with [Ph 3C][B(C 6F 5) 4] forming [AlR 2]
+
which will subsequently degrade the
[B(C 6F 5) 4]
-
into AlMe 3-x(C 6F 5) x species.
40
This is unlikely to be occurring with AlMe 3 as elevated
temperatures are required; however, this could be taking place with the other AlR 3 species with β-
hydrogens, which can decompose [Ph 3C][B(C 6F 5) 4] under ambient conditions.
27
Al NMR shows two broad
signals, one at 157 ppm which is consistent with a monocationic [AlR 4]
-
previously described by Okuda
and co-workers.
41
The second broad signal appears between 37-78 ppm which falls in between the 5-
coordinate Al region (30-50 ppm) and the 4-coordinate Al region (60-70 ppm), which could indicate a high
amount of dynamic binding behavior.
42
These fast interactions are supported by what we see in the
polymerization experiments were adding additional equivalents of AlMe 3 (10 or 15 equivalents) lead to the
reduction in molecular weight giving indication of facile chain transfer. NMR studies were not conducted
with either AlEt 3 or Al
i
Bu 3 as polymerizations with either showed exceptionally poor dispersities which
increased with further additions of AlR 3 (3.07-6.41). This could be due to the added steric bulk by the longer
alkyl chains which block access to coordination sites. Indeed, the light scattering GPC trace with the
addition of AlEt 3 shows multimodal distribution indicating multiple different active sites. This is coupled
with the fact that the experimental molecular weight does not decrease with higher AlEt 3 indicating that
chain transfer is not facile. In the case with Al
i
Bu 3, these ligands have an even larger steric profile, but the
light scattering GPC traces show a mostly unimodal distribution especially with higher AlR 3 equivalents.
This is accounted for by the enhanced chain transfer ability of this Lewis acid. Upon inclusion of PPh 3 into
the reaction with Al
i
Bu 3, chain transfer is seen but to a lesser extent. However, the light scattering GPC
trace is multimodal indicating that the PPh 3 could be preventing some chain transfer. The increased
86
dispersity matches results found for the yttrium methyl dicationic species, which showed higher dispersity
in the presence of Al
i
Bu 3.
28
This contrasts with the studies done by Boisson and co-workers where
polymerization activity required the addition of Al co-catalysts.
3.2.5 Extension to Other Rare Earth Metal Pre-catalysts
Traditionally, the faster catalysts for olefin and IP polymerization favor the smallest RE metals, Sc
and Lu, which unfortunately are the most expensive and least abundant. In contrast, larger metals such as
La are often inactive or show much slower diene polymerization compared to analogous complexes with
small metals.
43
With the proposed simple system, using accessible RE(CH 2SiMe 3) 3(THF) n (RE = Sc, Lu,
Tm, Yb, Y, Er, Ho, Dy, Tb, Gd, n=2; RE= Sm, La, n=3), we sought to investigate whether neutral donors
could support active and controlled catalysis for other RE metals, particularly the larger, less expensive
ones. Therefore, polymerizations of IP were pursued for RE(CH 2SiMe 3) 3(THF) n with Sm, Gd, and Tm,
representing large, medium, and small RE ions, respectively, and compared to those of Y (Table 3.5).
44,29
Previous studies of RE(CH 2SiMe 3) 3(THF) n using 5 equivalents of [NMe 2HPh][B(C 6F 5) 4] and 200
equivalents of Al
i
Bu 3 showed active catalysts for ethylene polymerization for a range of metals from Tb to
Tm, while the smallest metals Sc, Lu and Yb showed minimal polymerization activity.
24
In this case, the
larger metals showed faster polymerization activity, while the smaller active metals showed slower
catalysis. Nevertheless, the trends swapped for molecular weights, with the smaller metals allowing for
larger polymer molecular weights to be synthesized.
The synthesized RE trialkyl pre-catalysts were activated with 1 or 2 equivalents of
[Ph 3C][B(C 6F 5) 4] 10 mins prior to IP addition. IP polymerization was also carried out with the addition of
the monodentate neutral donor, PPh 3. In these reactions, 1 equivalent of PPh 3 was added to the monocationic
or dicationic RE alkyl complex after 10 mins activation time, and subsequent IP addition occurred 10 mins
after PPh 3 was introduced to the reaction. These reactions were run for 30 mins prior to characterization.
Analogous reactions were carried out at longer reaction times and the relevant data can be found in Table
3.12. The overall rate of polymerization was faster with 2 equivalents of [Ph 3C][B(C 6F 5) 4] than with only
87
1 equivalent for all pre-catalysts screened for the 30 min reactions. The yttrium catalyst with 2 equivalents
of [Ph 3C][B(C 6F 5) 4] had the fastest rate, with complete IP conversion reached within 30 mins.
Table 3.5. Polymerization of IP with RE(CH 2SiMe 3) 3(THF) n pre-catalysts with and without PPh 3.
a
Entry RE
Borate (B)
(equiv.)
PPh3 (P)
(equiv.)
Conv. (%)
b
Theor
Mn
(kDa)
c
Exp
Mn
(kDa)
d
Đ
d
Microstructure
e
Cis-1,4/Trans-
1,4/3,4
1 Sm 1 –– 13(1) 4 8(2) 1.81(3) 23/67/10
2 Sm 2 –– 30(5) 10 68(1) 1.58(2) 65/12/23
3 Gd 1 –– 43(1) 15 32(2) 1.24(1) 58/17/25
4 Gd 2 –– 63(5) 21 48(6) 2.23(28) 86/0/14
5 Y 1 –– 20(1) 7 5(1) 1.44(5) 43/42/15
6 Y 2 –– >99 34 40(2) 2.04(2) 83/0/17
7 Tm 1 –– 40(4) 14 20(1) 1.55(3) 39/46/15
8 Tm 2 –– 47(2) 16 36(4) 2.67(10) 63/9/28
9 Sm 1 1 20(4) 7 10(2) 1.19(1) 23/67/10
10 Sm 2 1 45(1) 15 55(4) 1.33(2) 64/12/24
11 Gd 1 1 31(4) 11 15(1) 1.23(7) 28/56/16
12 Gd 2 1 88(1) 30 85(1) 1.10(4) 68/8/24
13 Y 1 1 15(1) 5 5(1) 1.27(4) 30/55/15
14 Y 2 1 76(3) 26 33(2) 1.16(6) 61/13/26
15 Tm 1 1 46(2) 16 22(6) 1.37(10) 39/46/15
16 Tm 2 1 67(2) 23 28(1) 1.29(1) 48/31/22
a
Conditions: RE(CH2SiMe3)3(THF)n (RE= Sm, n=3, RE= Gd, Y, Tm, n=2), 10 μmol; [Ph3C][B(C6F5)4] (B),10-20 μmol; toluene,
10 mL; 0-10 μmol, PPh3; [IP]/RE=500; all entries are done in duplicate.
b
Determined by
1
H NMR spectroscopy of crude reaction
mixtures, comparing monomer peaks to polymer.
c
Calculated based on one alkyl initiator, [IP mol/RE mol] x IP molecular weight
x Conversion.
d
Determined by gel permeation chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
e
All selectivity data is an average of duplicate runs. 1,4 and 3,4 selectivity determined by
1
H NMR. Cis-1,4 and trans-1,4 selectivity
determined by
13
C NMR.
Without the added support of a neutral donor, the dispersity remained high for all metals, regardless
of how much [Ph 3C][B(C 6F 5) 4] was added. Additionally, with 1 equivalent of [Ph 3C][B(C 6F 5) 4], the
dispersity remained moderate (1.24-1.81) for all metals. Alternatively, with 2 equivalents of
[Ph 3C][B(C 6F 5) 4], the dispersity increased as the metal size decreased. This may be due to the more
congested steric environment for the smaller metals, where IP coordination has an inhibitive effect on
propagation. In most cases, the experimental molecular weight was much higher than expected for one
88
initiator per metal. This could be attributed to the possible decomposition of the catalyst during activation,
but prior to IP addition, leaving fewer initiators than expected (vide infra).
When PPh 3 is used, the dispersities of all polymerizations drop significantly, with no observable
trends based on the metal size. Similar to the experiments without a donor, the rates of polymerization are
faster when 2 equivalents of [Ph 3C][B(C 6F 5) 4] are used instead of 1 equivalent. When 2 equivalents of
[Ph 3C][B(C 6F 5) 4] are used with PPh 3, the yttrium pre-catalyst shows consistent molecular weight control
similar to theoretical and also has a low dispersity of 1.16. The gadolinium catalyst shows the fastest rates
and lower dispersity but shows poor molecular weight control. The largest and smallest metals, Sm and Tm
respectively, show the highest dispersities, which identifies the value in the RE metal series, allowing for
subtle fine-tuning between metal size and Lewis acidity. This is contrary to what was seen previously for
ethylene polymerization, as discussed above.
24
However, the demonstrated polymerization of IP with large
metals, such as samarium, with low dispersities near 1.3, opens opportunities to decrease the cost of
selective and living diene polymerization, as the larger RE metals are known to be cheaper and more
abundant.
45
3.2.6 Activation Conditions
When examining the IP polymerization using the Y(CH 2SiMe 3) 3(THF) 2/[Ph 3C][B(C 6F 5) 4]/donor
system, there are many questions to be addressed. For instance, does the order in which the different
components are added, or the time spaced between the addition of reagents affect the rate, selectivity, and
molecular weight control for the polymerization? In the previously discussed studies, the
Y(CH 2SiMe 3) 3(THF) 2 was first activated with [Ph 3C][B(C 6F 5) 4], followed by addition of a neutral donor.
Since polymer M n was often slightly higher than theoretical, it was hypothesized that some catalyst might
decompose prior to donor addition, leaving fewer initiators than expected. It was questioned whether donor
presence prior to activation might prevent any decomposition of the proposed dicationic yttrium active
catalyst. To test this hypothesis, experiments were carried out by first combining the pre-catalyst with PPh 3
and then adjusting the time at which [Ph 3C][B(C 6F 5) 4] was added (Table 3.6). It was observed that for all
pre-catalysts tested, insoluble materials appeared upon [Ph 3C][B(C 6F 5) 4] addition after PPh 3, which was not
89
present when [Ph 3C][B(C 6F 5) 4] was added first. This was attributed to a direct reaction between
[Ph 3C][B(C 6F 5) 4] and PPh 3, as discussed previously through NMR studies of catalyst activation.
Table 3.6. Addition order in the homopolymerization of IP with RE(CH2SiMe3)3(THF)n.
a
Entry RE
Time before
Borate addition
(min)
Order of
Addition
Conv.
(%)
b
Theor
Mn
(kDa)
c
Exp
Mn
(kDa)
d
Đ
d
Microstructure
e
Cis-1,4/Trans-
1,4/3,4
1 Sm 0 PPh3 → B 23(5) 8 10(2) 1.24(8) 45/37/18
2 Sm 10 PPh3 → B 18(2) 6 13(4) 1.18(2) 53/28/19
3 Sm 30 PPh3 → B 18(1) 6 11(5) 1.21(5) 49/34/17
4 Gd 0 PPh3 → B 58(2) 20 25(6) 1.11(8) 63/16/21
5 Gd 10 PPh3 → B 64(5) 22 34(5) 1.09(3) 62/18/20
6 Gd 30 PPh3 → B 39(3) 13 23(3) 1.16(1) 55/24/21
7 Y 0 PPh3 → B 34(3) 11 7(3) 1.25(5) 38/43/19
8 Y 10 PPh3 → B 28(1) 10 8(2) 1.23(8) 41/40/19
9 Y 30 PPh3 → B 25(4) 9 7(3) 1.29(4) 38/43/19
10 Tm 0 PPh3 → B 36(2) 12 20(1) 1.29(6) 45/38/17
11 Tm 10 PPh3 → B 50(1) 17 24(3) 1.21(3) 47/35/18
12 Tm 30 PPh3 → B 43(3) 15 24(1) 1.25(7) 53/31/16
a
Conditions: RE(CH2SiMe3)3(THF)n (RE= Sm, n=3, RE= Gd, Y, Tm, n=2), 10 μmol; 10 μmol, PPh3; [Ph3C][B(C6F5)4] (B), 20
μmol; toluene, 10 mL; [IP]/RE= 500; all entries are done in duplicate.
b
Determined by
1
H NMR spectroscopy of crude reaction
mixtures, comparing monomer peaks to polymer.
c
Calculated based on one alkyl initiator, [IP mol/RE mol] x IP molecular weight
x Conversion.
d
Determined by gel permeation chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
e
All selectivity data is an average of duplicate runs. 1,4 and 3,4 selectivity determined by
1
H NMR. Cis-1,4 and trans-1,4 selectivity
determined by
13
C NMR.
This suggested that there was likely less active catalyst in solution due to the interaction between PPh 3 and
the [Ph 3C][B(C 6F 5) 4] reagent which prevented the activation of the pre-catalysts. As expected for less
catalyst, the conversion rates were severely diminished. However, the dispersity was not greatly affected
by the change in addition order, which could suggest that the remaining active catalyst in solution is the
same. In all cases, the conversion, M n, and dispersity seemed to closely resemble the data seen where only
1 equivalent of [Ph 3C][B(C 6F 5) 4] was added prior to PPh 3 (Table 3.5, entries 9, 11, 13, and 15, respectively).
Changing the time between PPh 3 addition and [Ph 3C][B(C 6F 5) 4] did not greatly impact the polymerization.
90
Next, we adjusted the time between activation and addition of PPh 3 (Table 3.7). For these trials, RE trialkyl
pre-catalysts were activated with 2 equivalents of [Ph 3C][B(C 6F 5) 4] and 1 equivalent of PPh 3 was added in
three separate polymerizations at 0, 10, and 30 mins after activation.
Table 3.7. PPh 3 addition time variation in the homopolymerization of IP with RE(CH2SiMe3)3(THF)n.
a
Entry RE
Time before
PPh3 addition
(min)
Total time
before IP
addition (min)
Conv.
(%)
b
Theor
Mn
(kDa)
c
Exp
Mn
(kDa)
d
Đ
d
Microstructure
e
Cis-1,4/Trans-
1,4/3,4
1 Sm 0 10 38(1) 13 22(4) 1.17(3) 53/27/20
2 Sm 10 20 45(1) 15 55(4) 1.33(2) 64/12/24
3 Sm 30 40 43(1) 15 64(3) 1.61(6) 31/7/24
4 Gd 0 10 74(1) 25 44(2) 1.18(5) 71/3/26
5 Gd 10 20 88(1) 30 85(1) 1.09(4) 71/7/23
6 Gd 30 40 90(1) 31 120(3) 1.09(1) 75/6/20
7 Y 0 10 31(1) 11 10(3) 1.20(3) 45/34/21
8 Y 10 20 76(3) 26 33(2) 1.16(6) 61/13/26
9 Y 30 40 >99 34 42(5) 1.18(1) 68/8/24
10 Tm 0 10 60(1) 20 21(2) 1.28(4) 46/33/21
11 Tm 10 20 67(2) 23 27(1) 1.2(6) 48/31/22
12 Tm 30 40 89(2) 30 32(3) 1.34(6) 54/21/25
13 Y –– 10 >99 34 40(2) 2.04(2) 83/0/17
14 Y –– 20 >99 34 55(5) 2.59(7) 74/1/25
15 Y –– 40 >99 34 100(7) 3.92(10) 77/2/21
a
Conditions: RE(CH2SiMe3)3(THF)n (RE= Sm, n=3, RE= Gd, Y, Tm, n=2), 10 μmol; [Ph3C][B(C6F5)4] (B), 20 μmol; 0-10 μmol
PPh3; toluene, 10 mL; [IP]/RE= 500; all entries are done in duplicate.
b
Determined by
1
H NMR spectroscopy of crude reaction
mixtures, comparing monomer peaks to polymer.
c
Calculated based on one alkyl initiator, [IP mol/RE mol] x IP molecular weight
x Conversion.
d
Determined by gel permeation chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
e
All selectivity data is an average of duplicate runs. 1,4 and 3,4 selectivity determined by
1
H NMR. Cis-1,4 and trans-1,4 selectivity
determined by
13
C NMR.
In all cases, IP addition was kept constant by adding it to the reaction 10 mins after PPh 3 addition. For the
smaller metals (Y and Tm), waiting a longer time before adding PPh 3 increased the rate, while the
corresponding conversions for Sm and Gd remained similarly stagnant. For the smaller metals, a longer
91
activation time could allow the [Ph 3C][B(C 6F 5) 4] to fully react quantitatively to form the dicationic catalyst,
leading to the enhanced rate. In contrast, the more coordinatively unstable dicationic congeners of Sm and
Gd could decompose to a greater extent with a longer activation timeframe in the absence of a neutral donor.
Similarly, the experimental M n increased relative to the theoretical M n with increasing activation time, again
more so for Sm and Gd than Y and Tm. Since this discrepancy in molecular weight is characteristic of less
active catalyst species than expected being present in solution, this trend is self-consistent with the
decomposition of the dicationic complex without support from other ancillary ligands. However, since we
have established living behavior, we hypothesize this decomposition occurs before IP addition. The
discrepancy between the theoretical and the experimental M n becomes more apparent as the ionic radius of
the metal catalyst increases, which would be consistent with the fact that the larger dicationic species would
be more unstable. To further demonstrate the role that PPh 3 had in preventing the decomposition of the
catalyst, analogous studies without the PPh 3 were conducted (Table 3.7, entries 13–15). As the time before
isoprene addition increased, the dispersity and the experimental M n increased dramatically. This shows that
the catalyst is decomposing as the time before IP addition is extended.
NMR experiments analogous to the conditions in Table 3.7 with the addition of PPh 3 were
conducted to better understand how the donor impacts the activation of the catalysts. Three experiments
were carried out using Y(CH 2SiMe 3) 3(THF) 2 and 2 equivalents of [Ph 3C][B(C 6F 5) 4]. PPh 3 was then added
at 0, 10, and 30 mins after the addition of the [Ph 3C][B(C 6F 5) 4] with 0 mins meaning that the donor, catalyst,
and activator were added in at the same time. The different times of PPh 3 addition showed an impact on the
1
H NMR spectra. When adding the catalyst and the PPh 3 together at the same time, the NMR shows a
mixture of the monocationic and dicationic species (Figure 3.26). At the 10 min addition time, the spectra
shows one distinct doublet at -1.10 ppm which was assigned to the Y-CH 2 species which integrated to two
relative to the THF signal (Figure 3.27). However, when waiting for 30 mins before adding PPh 3 the
intensity of the Y-CH 2 decreased relative to the THF peak and they appear as 2 singlets (Figure 3.28)
indicating that by waiting longer before addition of PPh 3 lead to less dicationic complex present in solution
92
thus demonstrating the stabilizing effect of PPh 3. We also found experimentally that shorter activation times
led to slower polymerization rates. Also, an observation was made that at longer activation times, the
molecular weight increased dramatically for larger metals which might be due to the deactivation of the
catalyst that were observed in the NMR spectra. One important thing to note is that the concentration of the
catalyst is much higher in the NMR studies than in the polymerization conditions so any comparison may
not be wholly representative. This is exemplified by the fact that in the NMR studies solids often appeared
in the NMR tubes. Under standard catalytic conditions if full activation of the catalyst is achieved then no
solids are seen.
3.2.7 Stability of the Pre-catalyst
The stability of the Y(CH 2SiMe 3) 3(THF) 2/[Ph 3C][B(C 6F 5) 4] system under catalysis conditions, both
with and without addition of an in situ donor, was tested by sequential addition of IP (Table 3.8). Initially,
10 μmol Y(CH 2SiMe 3) 3(THF) 2 was stirred with 10 μmol [Ph 3C][B(C 6F 5) 4] for 10 mins before adding 500
equivalents of IP. In the case with the donor , 10 μmol of PPh 3 was first added, and the reaction was stirred
for 10 mins before adding IP. Reactions were stirred for 60 mins and half the reaction was quenched and
Table 3.8. Sequential polymerization of IP using Y(CH2SiMe3)3(THF)2/[Ph3C][B(C6F5)4] both with and without
PPh 3.
a
Entry
PPh3 (P)
(equiv.)
Time (min)
IP
Addition
(equiv.)
Conv. (%)
b
Theor
Mn
(kDa)
c
Exp
Mn
(KDa)
d
Đ
d
Microstructure
e
Cis-1,4/ Trans-
1,4/3,4
1
1
Step 1: 60 500 >99 34 30 1.18 58/16/26
2 Step 2: 60 250 >99 68 61 1.19 61/13/26
3 Step 3: 60 125 >99 102 113 1.16 63/11/26
4
––
Step 1: 60 500 >99 34 69 2.08 73/3/24
5 Step 2: 60 250 >99 68 139 2.39 77/3/20
6 Step 3: 60 125 >99 102 224 3.25 82/5/13
a
Conditions: Y(CH2SiMe3)3(THF)2, 10 μmol; [Ph3C][B(C6F5)4] (B), 10 μmol; toluene, 10 mL; 0-10 μmol PPh3; [IP]/Y=500
for each step.
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures, comparing monomer peaks to polymer.
c
Calculated based on one alkyl initiator, [IP mol/Y mol] x IP molecular weight x Conversion.
d
Determined by gel permeation
chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
e
1,4 and 3,4 selectivity determined by
1
H
NMR. Cis-1,4 and trans-1,4 selectivity determined by
13
C NMR.
93
fully characterized with
1
H,
13
C NMR spectroscopy, and GPC analysis. A second portion of IP (250
equivalents) was added and after 60 mins of stirring, half of the reaction was again removed and
characterized. The last step of the sequential polymerization was achieved by adding 125 equivalents of IP,
and then the entire reaction mixture was quenched and characterized after 60 mins of stirring. Throughout
the reaction with PPh 3, the dispersity remains around 1.19 indicating no termination of chains during the
sequence of polymerizations. The polymer molecular weight also increased correspondingly, with excellent
agreement between experimental and theoretical M n values, and preservation of PIP microstructure.
However, without a donor, both the dispersity and M n increased significantly during the three separate IP
additions, indicating a lack of catalyst stability between additions without the added support of the donor.
While this by no means indicates that the catalyst could be isolated after polymerization, it does denote the
stability of the catalyst under catalysis conditions, even when monomer is fully converted.
3.3 Conclusions
Herein, we have demonstrated that
simple rare earth metal trialkyl complexes
can be activated for rapid, controlled, and
living polymerization of isoprene when given
the right activation conditions (Figure 3.6).
Abstraction of two alkyls from the pre-
catalysts, using [Ph 3C][B(C 6F 5) 4], followed
by the addition of a weak field donor, PPh 3,
led to the best rate of polymerization and
molecular weight control (low dispersities
and experimental M n close to theoretical M n).
While yttrium pre-catalysts exhibited the best
overall reactivity for polymerization,
evidence suggests that controlled polymerizations with larger metals (such as Gd and Sm), could lead to
Figure 3.6. Summary of major findings of this chapter.
✓ Activation needs to occur before donor addition for optimal rates
OMe
EW
F Me H
ED
X =
✓ Electron-withdrawing groups promoted rate and cis-1,4 selectivity
✓ Activation with 2 equiv. of [Ph
3
C][B(C
6
F
5
)
4
] led to faster polymerization but
decreased M
n
and dispersity control
✓ Best M
n
control seen with mid-strength donors (PPh
3
)
✓ For smaller metals (Y, Tm) longer activation time prior to donor addition
enhances rate, but also increases weight due to catalyst deactivation
✓ In situ donor enhances stability and control of cationic complex
94
reduced catalyst costs catalyst cost and a more efficient transition to polar monomers, where large metals
often excel. The rate and order of addition of all substrates greatly impacted the catalyst reactivity. Finally,
in the presence of the donor, the catalyst demonstrates robust stability after polymerization is complete, as
the addition of more IP showed continued polymerization with maintained low dispersity. These results
suggest that proper reaction conditions can lead to controlled polymerization of isoprene, without the
lengthy synthesis of designer catalysts. Application of these methods to other monomers is currently
underway. While soft, phosphine donors are used readily in late transition metal chemistry and catalysis,
they are not commonly considered for use with hard early transition and rare earth metal ions. We hope
these findings will add these soft donors as potential resources in early transition metal and rare earth metal
catalysis.
3.4 Experimental Details and Additional Figures
3.4.1 General Considerations
All reactions involving air and moisture sensitive compounds were carried out using Schlenk line
techniques or in a Vacuum Atmospheres OMNI-LAB glovebox under an oxygen free, N 2 atmosphere.
Solvents used in air free reactions (toluene, hexane, pentane, diethyl ether, and tetrahydrofuran) were
purchased from Fisher, sparged under ultrahigh purity (UHP) grade argon and passed through two columns
of drying agent in a JC Meyer solvent purification system and dispensed directly into the glovebox. All
other solvents were used without further purification. Deuterated NMR solvents, C 6D 6 and CDCl 3, were
purchased from Cambridge Isotope Laboratories and were used as received. CDCl 3 and C 6D 6 suitable for
air sensitive compounds were dried using the following methods. C 6D 6 was dried by stirring over
Na/benzophenone for two days, followed by three freeze-pump-thaw cycles and vacuum transferred into a
flame-dried Straus flask and stored in a glovebox under a N 2 atmosphere. CDCl 3 was dried by stirring over
4Å molecular sieves for 3 days, followed by three freeze-pump-thaw cycles and vacuum transferred into a
flame-dried Straus flask and stored in a glovebox under a N 2 atmosphere. The four rare earth metal pre-
95
catalysts, RE(CH 2SiMe 3) 3(THF) n (RE= Tm, Y, Gd, n=2; RE= Sm, n=3), were synthesized following
literature procedure.
46,47
Isoprene, purchased from Sigma-Aldrich, was dried over 4Å molecular sieves for
7 days, followed by three freeze-pump-thaw cycles and a vacuum transfer into a flame-dried Straus flask
and stored in a glovebox at –35 ºC under a N 2 atmosphere. Bipyridine was purchased from Sigma-Aldrich
and sublimed 3 times before being transferred and stored in a glovebox under a N 2 atmosphere. All other
reagents and chemicals were obtained from commercial vendors (Sigma-Aldrich, TCI, Alfa Aesar, and
VWR) and were used without further purification.
3.4.2 Polymerization Methods
Preparation of stock solutions for polymerizations
In a glovebox, RE trialkyl pre-catalysts was crystallized immediately following synthesis and stored at -35
ºC and used within two weeks. Stock solutions of Trityl (tetrakis(pentafluorophenyl)borate
[Ph 3C][B(C 6F 5) 4] (20 μmol, 10 mL of a 2 M stock solution), RE trialkyl pre-catalyst (10 μmol, 200 μL of a
0.5 M stock solution) and donor (10 μmol, 600 μL of a 16 μM stock solution) were prepared using
volumetric flasks and used within 12 h and stored at -35 ºC. Stock solutions were warmed to rt prior to
catalysis.
General procedure for homopolymerization of isoprene
In a glovebox, Trityl (tetrakis(pentafluorophenyl)borate [Ph 3C][B(C 6F 5) 4] (20 μmol, 10 mL of a 2 M stock
solution) was placed in a stir bar charged 20 mL vial. RE trialkyl pre-catalyst (10 μmol, 200 μL of a 0.5 M
stock solution) was added to the vial and the mixture was stirred for 10 min. In the cases where a donor was
used, donor (10 μmol, 600 μL of a 16 μM stock solution) was then added by micro syringe and the reaction
was stirred for an additional 10 min. Isoprene (500 equiv.) was added by micro syringe in one portion, and
the polymerization was carried out for the designated time with constant stirring. The reaction mixture was
removed from a glovebox and poured into a large quantity of ethanol (100 mL) to give colorless polymer
96
as a precipitant. Collected polymer was redissolved in minimum chloroform and was washed with acetone
to remove impurities and subsequently dried in a vacuum oven at 40 ºC for 12 h to a constant weight.
3.4.3 Characterization Methods
Nuclear Magnetic Resonance Spectroscopy (NMR).
1
H and
13
C NMR spectra were recorded using a Varian
Mercury 400 MHz, Varian 500 MHz, or Varian 600 MHz spectrometers. Chemical shifts are referenced to
residual protons in the deuterated solvent or the deuterated solvent itself for
1
H (7.26 ppm for CDCl 3) or
13
C (77.16 ppm for CDCl 3) NMR spectra. All NMR spectra were recorded at room temperature in specified
deuterated solvents.
Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR spectra were recorded using an Agilent Cary 630
FT-IR equipped with a Diamond ATR sampling accessory. Accompanying MicroLab FT-IR software was
used to acquire 72 scans at 4 cm
−1
resolution with a spectral range of 400-4000 cm
−1
.
Gel Permeation Chromatography (GPC). GPC analyses were conducted using an Agilent 1260 Infinity II
GPC System equipped with a Wyatt DAWN HELEOS-II and a Wyatt Optilab T-rEX as well as an Agilent
1260 Infinity autosampler and UV-detector. The GPC system was equipped with two Agilent PolyPore
columns (5 micron, 4.6 mmID) which were calibrated using monodisperse polystyrene standards, eluted
with THF at 30 ºC at 0.3 mL/min. The number average molar mass and dispersity values were determined
from multi-angle light scattering (MALS) using dn/dc values calculated by 100% mass recovery method
from the refractive index (RI) signal.
97
3.4.4 In situ synthesis of [Y(CH 2SiMe 3) 2(THF) 2]
+
[B(C 6F 5) 4]
-
and [Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
Figure 3.7. In situ
1
H NMR spectrum of the monocationic active species
[Y(CH 2SiMe 3) 2(THF) 2]
+
[B(C 6F 5) 4]
-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 1 equiv.
[Ph 3C][B(C 6F 5) 4] in toluene-d 8 at room temperature.
98
Figure 3.8. In situ
1
H NMR spectrum of the monocationic active species
[Y(CH 2SiMe 3) 2(THF) 2]
+
[B(C 6F 5) 4]
-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 1 equiv.
[Ph 3C][B(C 6F 5) 4] after IP addition in toluene-d 8 at room temperature.
Figure 3.9. In situ
19
F NMR spectrum of the monocationic active species
[Y(CH 2SiMe 3) 2(THF) 2]
+
[B(C 6F 5) 4]
-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 1 equiv.
[Ph 3C][B(C 6F 5) 4] in toluene-d 8 at room temperature.
99
Figure 3.10. In situ
1
H NMR spectrum of the dicationic active species [Y(CH 2SiMe 3) 2(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2 equiv. [Ph 3C][B(C 6F 5) 4] in toluene-d 8 at room
temperature.
Figure 3.11. In situ
19
F NMR spectrum of the dicationic active species [Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 1 equiv. [Ph 3C][B(C 6F 5) 4] in toluene-d 8 at room
temperature.
100
3.4.5 GPC analysis of select polymerizations
Figure 3.12. GPC spectrum of PIP 500 equivalents generated by Y(CH 2SiMe 3) 3(THF) 2 and 1 equivalent
[Ph 3C][B(C 6F 5) 4] from Table 3.1, entry 1 (30 min): (left) LS; (right) RI.
Figure 3.13. GPC spectrum of PIP 500 equivalents generated by Y(CH 2SiMe 3) 3(THF) 2 and 1 equivalent
[Ph 3C][B(C 6F 5) 4] from Table 3.3, entry 1 (7 h): (left) LS; (right) RI.
101
Figure 3.14. GPC spectrum of PIP 500 equivalents generated by Y(CH 2SiMe 3) 3(THF) 2, 2 equivalents
[Ph 3C][B(C 6F 5) 4], and 1 equivalent PPh 3 from Table 3.2, entry 8 (30 min): (left) LS; (right) RI.
3.4.6 In situ NMR Studies with PPh 3
Figure 3.15. In situ
31
P NMR spectrum of the dicationic active species [Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2 equiv. [Ph 3C][B(C 6F 5) 4] and 1 equiv. PPh 3 added
10 min after activation in toluene-d 8 at room temperature. Reaction monitored overtime,
31
P NMR taken in
5 minute intervals.
102
Figure 3.16. In situ
31
P NMR spectrum of 1 equiv. PPh 3 with 2 equiv. [Ph 3C][B(C 6F 5) 4] in toluene-d 8 at
room temperature. Reaction monitored overtime,
31
P NMR taken in 5 minute intervals.
103
Figure 3.17. In situ
31
P NMR spectrum of the dicationic active species [Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2 equiv. [Ph 3C][B(C 6F 5) 4] and 1 equiv. PPh 3 in
toluene-d 8 at -80 ºC.
Figure 3.18. DOSY NMR spectrum of the dicationic active species [Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2 equiv. [Ph 3C][B(C 6F 5) 4] in toluene-d 8 at room
temperature.
104
Figure 3.19. DOSY NMR spectrum of the dicationic active species [Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2 equiv. [Ph 3C][B(C 6F 5) 4] and 1 equiv. PPh 3 in
toluene-d 8 at room temperature.
3.4.7 GPC spectra of polymerizations with different equivalents of [Ph 3C][B(C 6F 5) 4]
Figure 3.20. GPC spectrum of PIP 500 equivalents generated by Y(CH 2SiMe 3) 3(THF) 2 and 1.5 equivalents
[Ph 3C][B(C 6F 5) 4] from Table 3.3, entry 3 (7 h): (left) LS; (right) RI.
105
Figure 3.21. GPC spectrum of PIP 500 equivalents generated by Y(CH 2SiMe 3) 3(THF) 2 and 2 equivalents
[Ph 3C][B(C 6F 5) 4] from Table 3.3, entry 5 (7 h): (left) LS; (right) RI.
Figure 3.22. GPC spectrum of PIP 500 equivalents generated by Y(CH 2SiMe 3) 3(THF) 2 and 2.5 equivalents
[Ph 3C][B(C 6F 5) 4] from Table 3.3, entry 7 (7 h): (left) LS; (right) RI.
106
3.4.8 In situ reaction of Y(CH 2SiMe 3) 3(THF) 2 with 3 equiv. [Ph 3C][B(C 6F 5) 4]
Figure 3.23. In situ
1
H NMR spectrum of the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 3 equiv.
[Ph 3C][B(C 6F 5) 4] in toluene-d 8 at room temperature. NMR taken 10 minutes after catalyst addition.
107
3.4.9 In situ reaction of Y(CH 2SiMe 3) 3(THF) 2 with 2 equiv. [Ph 3C][B(C 6F 5) 4] and 5 equiv. AlMe 3
Figure 3.24. In situ
1
H NMR spectrum of the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2 equiv.
[Ph 3C][B(C 6F 5) 4] and 5 equiv. AlMe 3 in toluene-d 8 at room temperature.
Figure 3.25. In situ
27
Al NMR spectrum of the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2 equiv.
[Ph 3C][B(C 6F 5) 4] and 5 equiv. AlMe 3 in toluene-d 8 at room temperature.
108
Figure 3.26. In situ
1
H NMR spectrum of the dicationic active species [Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2 equiv. [Ph 3C][B(C 6F 5) 4] and PPh 3 added at time
0 min in toluene-d 8 at room temperature.
109
Figure 3.27. In situ
1
H NMR spectrum of the dicationic active species [Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2 equiv. [Ph 3C][B(C 6F 5) 4] and PPh 3 added at time
10 min in toluene-d 8 at room temperature.
110
Figure 3.28. In situ
1
H NMR spectrum of the dicationic active species [Y(CH 2SiMe 3)(THF) 2]
2+
[B(C 6F 5) 4]
2-
from the reaction of complex Y(CH 2SiMe 3) 3(THF) 2 with 2 equiv. [Ph 3C][B(C 6F 5) 4] and PPh 3 added at time
30 min in toluene-d 8 at room temperature.
111
3.4.10 Hammett Plot
3.4.11 Living Polymerization
General procedure for time point studies
In a glovebox, [Ph 3C][B(C 6F 5) 4] (47.2 μmol, 10 mL of a 2 M stock solution) was placed in a stir bar charged
20 mL Teflon capped Schlenk flask. RE trialkyl pre-catalyst (23.6 μmol, 200 μL of a 0.5 M stock solution)
was add to the Schlenk flask and the mixture was stirred for 10 min. In the cases where a donor was used,
donor (23.6 μmol, 600 μL of a 16 μM stock solution) was then added by micro syringe and the reaction was
stirred for an additional 10 min. Isoprene (500 equiv.) was added by micro syringe in one portion, and the
polymerization was stirred at 800 RPM. Aliquots were removed from the reaction at intervals throughout
the polymerization and quenched with isopropanol. All quenched aliquots were removed from a glovebox
and poured into a large quantity of ethanol (100 mL) to give colorless polymer precipitants. Collected
polymer was redissolved in minimum chloroform and was washed with acetone to remove impurities and
subsequently dried in a vacuum oven at 40 ºC for 12 h to a constant weight.
Table 3.9. IP polymerization with Y(CH2SiMe3)3(THF)2, 2 equiv. [Ph3C][B(C6F5)4], and different para substituted
donors.
a
Entry Donor
Time
(min)
Conv. (%)
b
Theor
Mn
(kDa)
c
Exp
Mn
(kDa)
d
Đ
d
Microstructure
e
Cis-1,4/ Trans-
1,4/3,4
1 P(Ph-p-OMe)3 30 20(1) 7 12(2) 1.85(8) 48/31/21
2 P(p-tolyl)3 30 36(2) 12 11(1) 1.78(15) 58/17/25
3 PPh3 30 76(3) 26 33(2) 1.16(4) 61/13/26
4 P(Ph-p-F)3 10 65(2) 22 67(5) 1.69 75/0/25
a
Conditions: Y(CH2SiMe3)3(THF)2, 10 μmol; [Ph3C][B(C6F5)4](B), 20 μmol; toluene, 10 mL; 10 μmol Donor; [IP]/Y= 500.
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures, comparing monomer peaks to polymer.
c
Calculated for one
alkyl initiator, [IP mol/Y mol] x IP molecular weight x Conversion.
d
Determined by gel permeation chromatography (GPC) in
THF using a Wyatt DAWN HELEOS II MALS detector.
e
All selectivity data is an average of duplicate runs. 1,4 and 3,4 selectivity
determined by
1
H NMR. Cis-1,4 and trans-1,4 selectivity determined by
13
C NMR.
112
Table 3.10. Living plot homopolymerization of IP with Y(CH2SiMe3)3(THF)2.
a
Entry RE Time (min) Conv. (%)
b
Mn (KDa)
c
Đ
c
Microstructure
d
Cis-1,4/ Trans-
1,4/3,4
1 Y 5 30 27 1.58 79/0/21
2 Y 12 56 38 1.79 79/0/21
3 Y 18 76 44 1.72 82/0/18
4 Y 24 89 53 1.68 81/0/19
5 Y 30 91 54 1.74 81/0/19
a
Conditions: Y(CH2SiMe3)3(THF)2, 24 μmol; [Ph3C][B(C6F5)4], 47 μmol; toluene, 26 mL; [IP]/Y=500.
b
Determined by
1
H NMR
spectroscopy of crude reaction mixtures, comparing monomer peaks to polymer.
c
Determined by gel permeation chromatography
(GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
d
1,4 and 3,4 selectivity determined by
1
H NMR. Cis-1,4 and
trans-1,4 selectivity determined by
13
C NMR.
Table 3.11. Living plot homopolymerization of IP with Y(CH2SiMe3)3(THF)2 and PPh 3.
a
Entry RE Time (min) Conv. (%)
b
Mn (KDa)
c
Đ
c
Microstructure
d
Cis-1,4/ Trans-
1,4/3,4
1 Y 10 49 17 1.1 71/5/24
2 Y 21 76 31 1.1 71/5/24
3 Y 31 87 38 1.15 64/7/29
4 Y 41 97 44 1.11 63/8/29
5 Y 51 99 47 1.12 63/9/28
a
Conditions: Y(CH2SiMe3)3(THF)2, 24 μmol; [Ph3C][B(C6F5)4], 47 μmol; toluene, 26 mL; [IP]/Y= 500; 24 μmol PPh3.
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures, comparing monomer peaks to polymer.
c
Determined by gel
permeation chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
d
1,4 and 3,4 selectivity determined
by
1
H NMR. Cis-1,4 and trans-1,4 selectivity determined by
13
C NMR.
113
3.4.12 Extended reaction times for IP polymerization with RE trialkyl complexes
Table 3.12. Homopolymerization of IP with RE trialkyl pre-catalysts both with and without PPh 3.
a
Entry RE
Borate (B)
(equiv.)
PPh3 (P)
(equiv.)
Conv. (%)
b
Theor
Mn
(kDa)
c
Exp
Mn
(kDa)
d
Đ
d
Microstructure
e
Cis-1,4/Trans-
1,4/3,4
1 Sm 1 –– 51(3) 17 48(2) 1.72(10) 58/30/20
2 Gd 1 –– 85(8) 29 44(7) 1.22(11) 16/73/11
3 Gd 2 –– >99 34 89(7) 1.61(9) 69/10/21
4 Tm 1 –– 74(1) 25 25(3) 1.36(5) 24/61/15
5 Tm 2 –– >99 34 54(1) 2.03(5) 61/10/29
6 Sm 1 1 54(4) 18 50(3) 1.42(6) 28/60/12
7 Gd 1 1 73(3) 25 51(6) 1.21(2) 17/70/13
8 Gd 2 1 >99 34 102(11) 1.21(7) 69/9/22
9 Tm 1 1 74(1) 25 21(2) 1.36(1) 28/57/15
10 Tm 2 1 >99 34 31(4) 1.39(14) 51/25/24
a
Conditions: RE(CH2SiMe3)3(THF)n (RE= Sm, n=3, RE= Gd, Y, Tm, n=2), 10 μmol; [Ph3C][B(C6F5)4](B), 10-20 μmol; toluene,
10 mL; [IP]/RE=500; all entries are done in duplicate.
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures,
comparing monomer peaks to polymer.
c
Calculated for one alkyl initiator, [IP mol/RE mol] x IP molecular weight x Conversion.
d
Determined by gel permeation chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
e
1,4 and 3,4
selectivity determined by
1
H NMR. Cis-1,4 and trans-1,4 selectivity determined by
13
C NMR.
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116
CHAPTER 4
Homopolymerizations and Block Polymerizations of Polar and Non-Polar
Monomers with Trialkyl Rare Earth Metal Complexes
117
4.1 Introduction
A majority of synthetic rubbers are composed of block copolymers or homopolymers of styrene
(S), isoprene (IP), and/or butadiene (BD) and are used in a wide array of applications, from adhesives to
footwear.
1
As the demand for synthetic rubber has risen, a need has been identified to create a more
sustainable future for this class of material.
2
First, to reduce our reliance on petroleum resources and to
lower the environmental impact such as greenhouse gases (GHG) emissions that come from extracting and
refining them, switching to biomass derived monomers has quickly become an attractive strategy.
3
One
class of biomass derived monomers that could be particularly useful in creating synthetic rubber material
are terpenes such as β-myrcene (Myr) and farnesene which can serve as a mimic for monomers such as IP.
4
For example, the Hillmyer and Hoye groups developed a ABA type block copolymer of methyl S and Myr
using living ionic polymerization which exhibited excellent thermoplastic elastomer behavior comparable
to petroleum derived products.
5
Myr is quite a desirable target because it is very abundant, cheap, and easy
to purify.
6
Similar to IP it has multiple double bonds leading to four different possible isomers cis-1,4,
trans-1,4, 3,4 and 1,2 which has a large effect on the properties of the polymer such as the glass transition
temperature (T g).
7
However, this monomer is somewhat challenging to polymerize as it has an additional
diene in the structure which can participate in side reactions that are detrimental to the polymerization such
as crosslinking or chelating to the metal during polymerization.
8
Several methods have been used to
polymerize Myr. Chief among them is coordination polymerization with Myr because it can produce
stereoregulated polymer, producing either high content of cis-1,4, trans-1,4, or 3,4 with a range of different
metals such as neodymium, lanthanum, lutetium, titanium, and chromium.
9–14
However, despite these
advances in stereoselectivity, other types of polymerization control such as molecular weight and
polydispersity still remain challenging.
Copolymerization of chemically similar and dissimilar monomers is an emerging approach to high
value polymers. Combining polar monomers with useful synthetic rubbers such as IP and BD gives the
material enhanced properties.
15
For example, it can enhance the materials resistance to chemical agents,
118
tearing, and aging. To accomplish this, many different strategies have been developed to overcome the
difficult challenge of copolymerizing these different types of monomers in a single reaction. These methods
have mostly relied on modifications to the end groups of each block in order to couple them together. The
first method relies on modifying both end groups and linking them together post polymerization. The
second is performed by modifying the end group of the first block to make it amenable to the polymerization
of the second monomer type. Many living polymerization methods to copolymerize two different nonpolar
monomers have been proposed but very few address the transition from nonpolar to polar monomers.
16
Recently the Métafiot group demonstrated the block copolymerization of Myr and glycidyl methacrylate
using nitroxide-mediated controlled radical polymerization.
17
The second way in which we could create a more sustainable future for synthetic rubbers is by
including different functionalities in the blocks that could be selectively targeted for degradation in order
to recover and reuse the nondegradable portion. In this case, the source of the monomer would not matter
if it could be reused multiple times. Recently, as mentioned in chapter 2, we identified that poly(1,3-dienes)
could be chemically recycled and, upon recovery, remade into block copolymers with similar properties.
This gives new options to create a circular economy for materials that previously did not have any current
chemical or mechanical recycling strategies.
As described in Chapter 3, we determined that simple rare earth (RE) metal alkyl complexes
RE(CH 2SiMe 3) 3(THF) n (RE= Tm Y, Gd, n=2; RE= Sm, n=3) could be activated for high activity and good
control of IP polymerization with a neutral donor. This has led us to experiment to see whether this catalyst
system could extend to other non-polar and polar monomers with the same control as was seen with IP and
whether these catalysts would be able to perform the difficult switch from one type to another in a single
reaction. Herein, we tested simple RE metal complexes RE(CH 2SiMe 3) 3(THF) n (RE= Tm Y, Gd, n=2)
supported with an in situ donor for the homopolymerization or block copolymerization of a diverse set of
monomers.
119
4.2 Results and Discussion
4.2.1 Homopolymerization of 1,3-Diene/Olefins
The RE metal pre-catalysts, RE(CH 2SiMe 3) 3(THF) 2 (RE = Tm, Y, Gd), were first screened for
their effectiveness at polymerizing a range of different 1,3-
diene/olefin monomers with the metals ranging from mid-sized to
small in the RE metal series.
Chapter 3 describes optimized conditions for these trisalkyl pre-
catalysts towards IP polymerization were developed. Initially,
these same conditions will be used to test the different 1,3-
diene/olefin monomers (Table 4.1). The RE pre-catalysts were
mixed with two equivalents of [Ph 3C][B(C 6F 5) 4] for ten minutes, then the donor, triphenylphosphine
(PPh 3), was added followed by monomer addition. As BD is structurally similar to IP, it was expected that
the catalyst would have a similar behavior. The Y catalyst demonstrated the fastest polymerization of BD
reaching 90% conversion within 30 mins which is similar to the analogous case with IP (78% conversion)
but with a higher dispersity than what was exhibited with IP (1.37 vs 1.16). Contrary to IP
polymerization, all three metals showed almost exclusive 1,4 selectivity with only minor 3,4
polymerization. More cis-1,4 selectivity could be realized as the ionic radii of the metal center increased.
All catalysts showed good molecular weight control but had similar broadened dispersities (1.34-1.36) for
BD polymerization suggesting that PPh 3 may not be the correct donor for this monomer.
Polymerization with Myr was slower (57%) and with a very high dispersity (2.56). Polymerization
rates of Myr with the three different metal catalysts increases as the ionic radii increases. While the
dispersity with Tm and Y were very high ( Đ= 2.56 and 2.87, respectively) the Gd catalyst showed a
phenomenal dispersity of 1.09. The discrepancy of the experimental molecular weight vs the theoretical
molecular weight could be due to the deactivation of the metal center prior to monomer addition which was
previously seen for these systems. Additionally, it could be caused by crosslinking of the dienes which is
Figure 4.1. Trialkyl rare earth
metal pre-catalysts.
120
commonly seen for Myr polymerization.
8
The polymers made from the different metal centers showed
approximately the same 1,2 and 3,4 selectivity but the cis-1,4 selectivity increased as the ionic radii
increased. This same trend was seen for these metals for IP polymerization. All catalysts showed very low
polymerization activity for styrene (Sty) polymerization with very high dispersities (2.23-2.93).
Table 4.1. Homopolymerization of 1,3-dienes and olefins with RE(CH2SiMe3)3(THF)2.
a
Entry Cat.
Time
(min)
Monomer
Conv.
(%)
b
Theor
Mn
(kDa)
c
Exp
Mn
(kDa)
d
Đ
d
Microstructure
e
Cis-1,4/
Trans-1,4/ 1,2/ 3,4
1 Tm 30 BD 61(1) 17 21 1.37 30/66/––/4
2 Y 30 BD 90(1) 25 30(5) 1.37(4) 45/50/––/5
3 Gd 30 BD 70(1) 20 17 1.34 53/42/––/5
4 Tm 30 Myr 54(3) 37 84 2.87 40/27/3/30
5 Y 30 Myr 57(2) 39 50(18) 2.56(21) 43/22/2/33
6 Gd 30 Myr >99 68 118(11) 1.09(1) 56/16/2/26
7 Tm 30 Sty trace –– 3(1) 2.93(1) ––
8 Y 30 Sty trace –– 6(1) 2.23(28) ––
9 Gd 30 Sty
trace
–– 4 2.49 ––
10 Tm 6 BD >99 27 21 1.49 ––
11 Y 6 BD >99 27 31(2) 1.69(4) ––
12 Gd 6 BD >99 27 16(3) 2.05(13) ––
13 Tm 6 Myr >99 67 61(11) 4.68(28) ––
14 Y 6 Myr >99 67 75(6) 2.56(43) ––
15 Gd 6 Myr >99 67 100(16) 1.36(17) ––
a
Conditions: RE(CH2SiMe3)3(THF)2 (RE = Tm, Y, Gd), 10 μmol; [Ph3C][B(C6F5)4] (B), 20 μmol; toluene, 10 mL; 10
μmol, Donor; [Monomer]/RE= 500; entries preformed in duplicate are indicated by inclusion of error.
b
Determined by
1
H
NMR spectroscopy of crude reaction mixtures, comparing monomer peaks to polymer.
c
Calculated based on one alkyl
initiator, [monomer mol/Y mol] x monomer molecular weight x conversion.
d
Determined by gel permeation chromatography
(GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
e
1,4 and 3,4 and 1,2 selectivity determined by
1
H NMR.
Cis-1,4 and trans-1,4 selectivity determined by
13
C NMR.
121
The reaction time for all 1,3-diene monomers were extended to six hours but with the same
conditions in order to see if monomers reached full conversion and to see if there were any side reactions
once full conversion was reached. All catalysts achieved full conversion for the BD monomer during the
six hour reactions. As the ionic radii increased, these reactions exhibited higher dispersity for the smaller
RE metal centers (Tm and Y) only a mild increase in dispersity was seen with fairly good agreement
between the theoretical and experimental molecular weights. However, for the much larger Gd a much
larger dispersity of 2.05 was seen indicating loss of control at higher conversions. Increasing the time to six
hours with Myr showed mild increase in dispersity in comparison to the shorter reactions but similar
molecular weight control was observed.
4.2.2 Homopolymerization of Cyclic Ester Monomers
Polymerizations with cyclic ester monomers ɛ-caprolactone (CL) and 𝛿 -valerolactone (VL) were
carried in the same manner as the 1,3-diene/olefin polymerization but with shortened reaction times (5
mins). Polymerization of CL was extremely facile reaching full conversion for all catalyst within 5 mins
(Table 4.2, entries 1- 3). However, there was a large difference between the theoretical molecular weight
and the experimental molecular weight suggesting there was either a large amount of transesterification or
cyclic oligomer formation. The dispersity for Gd and Tm were 1.49 and 1.58, respectively, while Y showed
a narrower dispersity of 1.25. The polymerization with Gd did not show as low of an experimental
molecular weight as the other two catalysts. A possible explanation could be that the size increase might
allow for the incoming monomer to be bound faster than the side reaction can occur. Another possibility is
that the decreased Lewis acidity of the metal could make the oxygen on the growing polymer chain less
nucleophilic. To test whether the low molecular weight of this reaction was caused by transesterification or
by cyclic polymer formation, the reaction time was extended to three hours using the Y and Tm catalysts
(Table 4.2, entries 5 and 6). In these cases, the molecular weight doubled in size, but the dispersity only
broadened slightly. This suggests the possibility of post-polymerization reactions such as coupling of
polyester chains similar to a step-growth mechanism.
122
The homopolymerization of VL was tested for all 3 pre-catalysts. VL has historically been more
difficult to polymerize due to the stability of its six-membered ring. The midsized metal precatalyst Y
demonstrated the fastest rate of VL polymerization (76%) within 5 mins. The larger Y metal center provides
more space for the monomer to bind, which would improve the rate.
18
Even though the Gd precatalyst is
larger, its center is less Lewis acidic, which provides less activation for the carbonyl of the monomer, and
it makes the initiator less nucleophilic. Tm showed no side reactions in the shorter experiment with good
molecular weight control matching the theoretical molecular weight. In comparison, the molecular weight
for Y is lower than its corresponding theoretical molecular weight (Table 4.2, entry 5).
Table 4.2. Homopolymerization of cyclic esters with RE(CH
2
SiMe
3
)
3
(THF)
2 pre-catalysts.
a
Entry Cat. Time Monomer Conv. (%)
b
Theor
Mn (kDa)
c
Exp
Mn (kDa)
d
Đ
d
1 Tm 5 min CL >99 56 9(3) 1.58(2)
2 Y 5 min CL >99 56 7 1.25
3 Gd 5 min CL >99 56 20(1) 1.49(10)
4 Tm 5 min VL 32(1) 16 15(1) 1.22(13)
5 Y 5 min VL 76(5) 38 23(5) 1.16(6)
6 Gd 5 min VL 50 25 –– ––
7 Tm 3 h CL >99 56 19(4) 1.61(9)
8 Y 3 h CL >99 56 16(5) 1.59(4)
9 Tm 3 h VL 79(1) 39 16(1) 1.48(6)
10 Y 3 h VL 87(1) 44 26(1) 1.31(14)
a
Conditions: RE(CH2SiMe3)3(THF)2 (Tm, Y, Gd), 10 μmol; [Ph3C][B(C6F5)4] (B), 20 μmol; toluene, 10 mL; 10 μmol,
Donor; [IP]/Y= 500; all entries are done in duplicate.
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures,
comparing monomer peaks to polymer.
c
Calculated based on one alkyl initiator, [monomer mol/Y mol] x monomer
molecular weight x conversion.
d
Determined by gel permeation chromatography (GPC) in THF using a Wyatt DAWN
HELEOS II MALS detector. e1,4 and 3,4 and 1,2 selectivity determined by
1
H NMR. Cis-1,4 and trans-1,4 selectivity
determined by
13
C NMR.
123
To test if there were side reactions that occurred after polymerization was complete, the reaction
time was extended. When reaction times for VL were extended to three hours, the conversion did not reach
full conversion (Table 4.2, entries 9 and 10). One reason for this could the increased viscosity of the
reaction mixture at high conversion. The dispersity also broadened slightly, indicating presence of
transesterification. Indeed, even though the monomer reached a higher conversion, the molecular weight
did not change significantly. This is similar to the case with CL where there was evidence of possible cyclic
polymer formation or other side reactions that lead to small oligomer fragments.
4.2.3 Block Copolymerization of 1,3-Diene/Olefins and Cyclic Esters
Block copolymerization reactions were carried out in a stepwise manner using the Y and the Gd
pre-catalysts both with and without the presence of PPh 3 (Table 4.3). These reactions were doubled in scale
as the other polymerizations (20 μmol). Catalysts were combined with two equivalents of [Ph 3C][B(C 6F 5) 4]
and stirred for ten minutes. In the cases where donor was used, it was stirred with the activated catalyst for
ten minutes prior to monomer addition. When donor was not used, ten minutes after catalyst activation,
monomer was added to solution. After polymerization of the first block, half of the solution was removed
in order to be characterized. The second monomer was added to the remaining solution and then
polymerization was carried out for ten minutes. In all cases, full conversion of monomer was achieved
except in the case of Myr and CL block copolymerization with the addition of PPh 3.
Block copolymerization of IP and CL with Y with the addition of a donor decreased the dispersity
from 2.14 to 1.18 (Table 4.3, entries 2 and 4) showing that in this case the donor added to dispersity control.
This was probably due to the instability of the Y catalyst without the PPh 3 to stabilize it before monomer
addition. However, in the case of the block copolymerization of Myr and CL with Y, the donor contributed
significantly to the broadening of the dispersity (Table 4.3, entries 6 and 8). When examining the molecular
weight, we see that for the homopolymerization of Myr with Y and without donor, the molecular weight
was significantly higher than with the donor (Table 4.3, entries 7 and 5). This could be due to the
crosslinking of the Myr monomer due to the increased steric hindrance of the metal center preventing
124
monomer enchainment and favoring crosslinking reactions. Notable, the molecular weight of the Myr and
CL block copolymers with and without donor (Table 4.3, entries 6 and 8) were significantly lower than the
experimental, and in fact, they were lower than their respective homopolymer as well (Table 4.3, entries 5
and 7). This can be attributed to the presence of short CL oligomer contributing to the lower molecular
Table 4.3. Step-wise block copolymerization with RE(CH2SiMe3)3(THF)2 pre-catalysts.
a
Entry Cat. Donor (PPh3) Monomer(M1:M2) Conv. (%)
b
Theor
Mn (kDa)
c
Exp
Mn (kDa)
d
Đ
d
1 Y –– IP >99 38 51 2.04
2 Y –– IP:CL >99 90 72 2.14
3 Y 1 IP >99 38 45 1.18
4 Y 1 IP:CL >99 90 68 1.23
5 Y –– Myr >99 67 165 1.22
6 Y –– Myr:CL >99 124 62 2.29
7 Y 1 Myr 92 63 67 2.18
8 Y 1 Myr:CL 92 119 39 4.18
9 Gd –– IP >99 38 60 3.41
10 Gd –– IP:CL >99 90 157 1.24
11 Gd 1 IP >99 38 71 1.08
12 Gd 1 IP:CL >99 90 92 1.27
13 Gd –– Myr >99 67 137 1.89
14 Gd –– Myr:CL >99 124 92 2.55
15 Gd 1 Myr >99 67 126 1.28
16 Gd 1 Myr:CL >99 124 114 1.22
a
Conditions: RE(CH2SiMe3)3(THF)2 RE= Y, Gd, 20 μmol; [Ph3C][B(C6F5)4] (B), 40 μmol; toluene, 20 mL; 20
μmol, Donor; [IP]/RE= 500;
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures, comparing monomer
peaks to polymer. cCalculated based on one alkyl initiator, [IP mol/Y mol] x IP molecular weight x Conversion.
d
Determined by gel permeation chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
e
1,4 and 3,4 selectivity determined by
1
H NMR. Cis-1,4 and trans-1,4 selectivity determined by
13
C NMR.
125
weight average.
Considering the reactions with the Gd catalyst, the donor showed clear indication of adding
control to the homopolymerization of IP (Table 4.3, entries 9 and 11) and the block copolymerization of
Myr and CL (Table 4.3, entries 14 and 16) by lowering the dispersity. As noted with the aforementioned
homopolymerization of CL, the Gd catalyst showed the highest molecular weight which indicated that
there were fewer side reactions. This can be seen in the block copolymerizations with the Gd catalyst
where the molecular weight did not decrease relative to the theoretical molecular weight as much as with the
Y catalyst. Evidence of CL oligomer was seen in the DOSY NMR where two diffusion coefficients were
seen (Figure 4.2). One corresponded with the homopolymerization of CL and the other corresponded
with both the 1,3- diene and the cyclic ester monomers indicating that there was both homopolymer and
block copolymer in solution. Homopolymer can be separated from the block copolymer through an
acetone wash (Figure 4.3).
4.3 Conclusions
In this chapter it was demonstrated that rare earth metal trialkyl precatalyst were active for the
homopolymerization of a range of 1,3-dienes/olefins and cyclic esters monomers. We were able to achieve
high conversion and low dispersity with the homopolymerization of Myr, which has been shown in the
literature as difficult to achieve such control. While there was evidence of homopolymerization in the block
copolymerization of 1,3-diene and cyclic ester monomers, the stabilizing effect of PPh 3 can be clearly seen
with larger rare earth metals and good molecular weight control and dispersity for the block
copolymerization of Myr and CL was achieved. Further optimization of the different monomers needs to
be explored but the reactivity that we have shown in this chapter shows we can manipulate the conditions
for these simple catalyst to obtain good control.
4.4 Experimental Details and Additional Figures
4.4.1 General Considerations
All reactions involving air and moisture sensitive compounds were carried out using Schlenk line
126
techniques or in a Vacuum Atmospheres OMNI-LAB glovebox under an oxygen free, N2 atmosphere.
Solvents used in air free reactions (toluene, hexane, pentane, diethyl ether, and tetrahydrofuran) were
purchased from Fisher, sparged under ultrahigh purity (UHP) grade argon and passed through two columns
of drying agent in a JC Meyer solvent purification system and dispensed directly into the glovebox. All
other solvents were used without further purification. Deuterated NMR solvents, C 6D 6 and CDCl 3, were
purchased from Cambridge Isotope Laboratories and were used as received. CDCl 3 and C 6D 6 suitable for
air sensitive compounds were dried using the following methods. C 6D 6 was dried by stirring over
Na/benzophenone for two days, followed by three freeze-pump-thaw cycles and vacuum transferred into a
flame-dried Straus flask and stored in a glovebox under a N 2 atmosphere. CDCl 3 was dried by stirring over
4Å molecular sieves for 3 days, followed by three freeze-pump-thaw cycles and vacuum transferred into a
flame-dried Straus flask and stored in a glovebox under a N 2 atmosphere. The four rare earth metal pre-
catalysts, RE(CH 2SiMe 3) 3(THF) n (RE= Tm, Y, Gd, n=2), were synthesized following literature procedure.
Isoprene, β-myrcene, ɛ-caprolactone, styrene, and 𝛿 -valerolactone purchased from Sigma-Aldrich, was
dried over 4Å molecular sieves for 7 days, followed by three freeze-pump-thaw cycles and a vacuum
transfer into a flame-dried Straus flask and stored in a glovebox at –35 °C under a N 2 atmosphere. Butadiene
(20 wt. %) purchased from Sigma-Aldrich was decanted into a Schlenk flask with activated 4Å molecular
sieves and kept in an ice bath for two days. Butadiene solution was then syringe transferred into another
Straus flask containing 4Å of molecular sieves. Straus was then transferred into a glovebox and decanted
into a new flask with fresh molecular sieves and stored at -35 °C. Bipyridine was purchased from Sigma-
Aldrich and sublimed 3 times before being transferred and stored in a glovebox under a N 2 atmosphere. All
other reagents and chemicals were obtained from commercial vendors (Sigma-Aldrich, TCI, Alfa Aesar,
and VWR) and were used without further purification.
4.4.2 Polymerization Methods
Preparation of Stock Solutions for polymerizations
In a glovebox, RE trialkyl pre-catalysts was crystallized immediately following synthesis and stored at -35
127
°C and used within two weeks. Stock solutions of Trityl (tetrakis(pentafluorophenyl)borate
[Ph3C][B(C6F5)4] (20 μmol, 10 mL of a 2 M stock solution), RE trialkyl pre-catalyst (10 μmol, 200
μL of a 0.5 M stock solution) and donor (10 μmol, 600 μL of a 16 μM stock solution) were prepared
using volumetric flasks and used within 12 h and stored at -35 °C. Stock solutions were warmed to rt
prior to catalysis.
General procedure for homopolymerization of 1,3-denes/ olefin or cyclic esters
In a glovebox, Trityl (tetrakis(pentafluorophenyl)borate [Ph 3C][B(C 6F 5) 4] (20 μmol, 10 mL of a 2 M stock
solution) was placed in a stir bar charged 20 mL vial. RE trialkyl pre-catalyst (10 μmol, 400 μL of a 0.25
M stock solution) was added to the vial and the mixture was stirred for 10 min. In the cases where a donor
was used, donor (10 μmol, 600 μL of a 16 μM stock solution) was then added by micro syringe and the
reaction was stirred for an additional 10 min. Monomer (500 equiv.) was added by micro syringe in one
portion, and the polymerization was carried out for the designated time with constant stirring. The reaction
mixture was removed from a glovebox and poured into a large quantity of ethanol (100 mL) to give colorless
polymer as a precipitant. Collected polymer was redissolved in minimum chloroform and was washed with
acetone to remove impurities and subsequently dried in a vacuum oven at 40 ºC for 12 h to a constant
weight.
General procedure for the block copolymerization of 1,3-denes and cyclic esters
In a glovebox, Trityl (tetrakis(pentafluorophenyl)borate [Ph 3C][B(C 6F 5) 4] (40 μmol, 20 mL of a 2 M stock
solution) was placed in a stir bar charged 20 mL vial. Rare earth metal pre-catalysts (20 μmol, 800 μL of a
0.25 M stock solution) was added to the vial and reaction was stirred for 10 minutes. In the cases where a
donor was used, donor (20 μmol, 1.20 mL of a 16 μM stock solution) was then added by micro syringe and
the reaction was stirred for an additional 10 min. 1,3-diene (1000 equiv.) was added by micro syringe in
one portion, and the polymerization was carried out for the designated time with constant stirring during
which the reaction turned from orange to gold. Exactly half of the reaction was removed by syringe,
quenched with EtOH and subsequently characterized. Cyclic ester (500 equiv.) was added by micro syringe
128
to the second half of the reaction mixture and polymerization was continued for the designated time. The
gel like reaction mixture was poured into a large quantity of ethanol (100 mL) to give colorless copolymer
that was dried in a vacuum oven at 40 ºC for 12 h to a constant weight (0.90 g, 100%).
4.4.3 Characterization Methods
Nuclear Magnetic Resonance Spectroscopy (NMR).
1
H and
13
C NMR spectra were recorded using a Varian Mercury 400 MHz, Varian 500 MHz, or Varian 600
MHz spectrometers. Chemical shifts are referenced to residual protons in the deuterated solvent or the
deuterated solvent itself for 1H (7.26 ppm for CDCl 3) or S27
13
C (77.16 ppm for CDCl 3) NMR spectra. All
NMR spectra were recorded at room temperature in specified deuterated solvents. 1,4 and 3,4 and 1,2
selectivities were determined using
1
H NMR. Distinction between cis-1,4 and trans-1,4 was determined by
13
C NMR.
19,20
Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR spectra were recorded using an Agilent Cary 630
FT-IR equipped with a Diamond ATR sampling accessory. Accompanying MicroLab FT-IR software was
used to acquire 72 scans at 4 cm
−1
resolution with a spectral range of 400-4000 cm
−1
.
Gel Permeation Chromatography (GPC). GPC analyses were conducted using an Agilent 1260 Infinity II
GPC System equipped with a Wyatt DAWN HELEOS-II and a Wyatt Optilab T-rEX as well as an Agilent
1260 Infinity autosampler and UV-detector. The GPC system was equipped with two Agilent PolyPore
columns (5 micron, 4.6 mmID) which were calibrated using monodisperse polystyrene standards, eluted
with THF at 30 °C at 0.3 mL/min. The number average molar mass and dispersity values were determined
from multi-angle light scattering (MALS) using dn/dc values calculated by 100% mass recovery method
from the refractive index (RI) signal.
129
Figure 4.2. DOSY NMR spectrum of Myr-b-CL block copolymer generated by Gd(CH2SiMe3)3(THF)2 with 2
equiv. [Ph3C][B(C6F5)4] in CDCl 3 at room temperature.
130
Figure 4.3. DOSY NMR spectrum of Myr-b-CL block copolymer generated by Gd(CH2SiMe3)3(THF)2 with 2
equiv. [Ph3C][B(C6F5)4] in CDCl 3 at room temperature after acetone wash.
4.5 References
(1) Bates, C. M.; Bates, F. S. 50th Anniversary Perspective: Block Polymers—Pure Potential. Macromolecules
2017, 50(1), 3 - 22.
(2) Bubshait, A. K. Butadiene Rubber in the Petrochemical Industry. Int. Ann. Sci. 2021, 11, 22-26.
(3) Wilbon, P. A.; Chu, F.; Tang, C. Progress in Renewable Polymers from Natural Terpenes, Terpenoids, and Rosin
Macromol. Rapid Commun. 2013, 34, 8-37.
(4) Métafiot, A.; Kanawati, Y.; Gérard, J.; Defoort, B.; Marić, M. Synthesis of β-Myrcene-Based Polymers and
Styrene Block and Statistical Copolymers by SG1 Nitroxide-Mediated Controlled Radical Polymerization.
Macromolecules 2017, 50 (8), 3101-3120.
(5) Bolton, J.; Hillmyer, M.; Hoye, T. Sustainable Thermoplastic Elastomers from Terpene-Derived. ACS Macro
Letters 2014, 3 (8), 717-720.
(6) A. Behr, A.; Johnen, L.; Vorholt, A. J. Telomerization of Myrcene and Catalyst Separation by Thermomorphic
Solvent Systems. ChemCatChem 2010, 2, 1271.
(7) Makhiyanov, N.; Temnikova, E.V. Glass-transition temperature and microstructure of polybutadienes. Polym.
Sci. Ser. A 2010, 52, 1292–1300.
(8) Loughmari, S.; Hafid, A.; Bouazza, A.; El Bouadili, A.; Zinck, P.; Visseaux, M. Highly stereoselective
coordination polymerization of β-myrcene from a lanthanide-based catalyst: Access to bio-sourced elastomers. J.
Polym. Sci. A Polym. Chem. 2012, 50, 2898-2905.
131
(9) Naddeo, M.; Buonerba, A.; Luciano, E.; Grassi, A.; Proto, A.; Capacchione, C. Stereoselective polymerization
of biosourced terpenes beta-myrcene and beta-ocimene and their copolymerization with styrene promoted by
titanium catalysts. Polymer 2017, 131, 151-159.
(10) Liu, B.; Li, L.; Sun, G.; Liu, D.; Li, S.; Cui, D. Isoselective 3,4-(co)Polymerization of Biorenewable Myrcene
Using NSN-Ligated Rare-earth Metal Precursors: Approach to New Elastomer. Chem. Commun. 2015, 51, 1039.
(11) Georges, S.; Touré, A. O.; Visseaux, M.; Zinck, P. Coordinative Chain Transfer Copolymerization and
Terpolymerization of Conjugated Dienes. Macromolecules 2014, 47, 4538-4547.
(12) Liu, B.; Liu, D.; Li, S.; Sun, G.; Cui, D. High trans-1,4 (co)polymerization of β-myrcene and isoprene with an
iminophosphonamide lanthanum catalyst. Chinese J. Poly. Sci. 2016, 34, 104-110.
(13) Huang, Y.; He, J.; Cai, G.; Liu, Z.; Li, T.; Du, T.; Zhang, S.; Yao, B.; Li, X. cis-1,4-specific carbocationic
polymerization and copolymerization of 1,3-dienes initiated by (S,S)-bis(oxazolinylphenyl)amine chromium
complexes. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 1250.
(14) Lamparelli, D. H.; Winnacker, M.; Capacchione, C. Stereoregular Polymerization of Acyclic Terpenes.
ChemPlusChem 2022, 87, e20210036.
(15) Ricci, G.; Pampaloni, G.; Sommazzi, A.; Masi, F. Dienes Polymerization: Where We Are and What Lies
Ahead. Macromolecules 2021 54 (13), 5879-5914.
(16) Satoh, K. Controlled/living polymerization of renewable vinyl monomers into bio-based polymers. Polym. J.
2015, 47, 527–536.
(17) Métafiot, A.; Gérard, J.-F.; Defoort, B; Marić, M. Synthesis of β-myrcene/glycidyl methacrylate statistical and
amphiphilic diblock copolymers by SG1 nitroxide-mediated controlled radical polymerization. J. Polym. Sci. Part
A: Polym. Chem. 2018, 56, 860-878.
(18) Wu, L.; Lee, W.; Ganta, P. K.; Chang, Y.; Chang, Y.; Chen, H. Multinuclear metal catalysts in ring-opening
polymerization of ε‑caprolactone and lactide: Cooperative and electronic effects between metal centers. Coord.
Chem. Rev., 2023, 475, 214847.
(19) Dai, Q.; Jia, X.; Yang, F.; Bai, C.; Hu, Y.; Zhang, X. Iminopyridine-Based Cobalt(II) and Nickel(II)
Complexes: Synthesis, Characterization, and Their Catalytic Behaviors for 1,3-Butadiene Polymerization. Polymers
2016, 8, 12.
(20) Zhang, J.; Aydogan, C.; Patias, G.; Smith, T.; Al-Shok, L.; Liu, H.; Eissa A.M.; Haddleton DM.
Polymerization of Myrcene in Both Conventional and Renewable Solvents: Postpolymerization Modification via
Regioselective Photoinduced Thiol-Ene Chemistry for Use as Carbon Renewable Dispersants. ACS Sustain Chem
Eng. 2022, 10(29), 9654-9664.
132
CHAPTER 5
Investigation into the Mechanism of the Block Copolymerization of Isoprene
and Caprolactone Using Pincer Complexes
133
5.1 Introduction
The development of the Ziegler-Natta catalysts in the 1950s has given the world easy access to
versatile polyolefin material such as high-density polyethylene (HDPE) and isotactic polypropylene (PP).
1
Polyolefins have become the largest volume class of polymers, making up over 50% of plastic produced
worldwide, with an annual increase of 7%.
2
In addition, millions of tonnes of polyolefin and poly(1,3-diene)
based block copolymers are produced every year for a diverse range of applications, including adhesives,
footwear, and construction.
3
Catalysts that produce highly stereoselective and regioselective 1,3-diene and
olefin block copolymers are desirable targets for synthesizing valuable materials. While polyolefins and
poly(1,3-diene) block copolymers are useful, they also exhibit low stress, have poor ability against tear
and abrasion, and are easily aged by oxygen.
4
They also have strong C-C bonds in the backbone and are
often synthesized as a mixture of two immiscible polymers makes them difficult to recycle with current
mechanical or chemical recycling practices. Overcoming these limitations by incorporation of
heteroatoms in the backbone of the polymer using monomers such as cyclic esters or carbonates have
rarely been investigated but would be desirable as it adds a degree of degradability to the block
copolymer.
5
Another approach to making more sustainable polymers is synthesizing copolymers of
polyesters such as copolymers of polylactic acid (PLA) and polycaprolactone (PCL), which has shown to
exhibit properties similar to thermoplastic elastomers.
6
However, in the case of polar-polar block
copolymers all value would be lost when both blocks degrade. Since both of these polar polymers are
polyesters, they will degrade under similar conditions which would leave nothing behind to be remade
into a new product. In comparison, non-polar and polar block copolymers have added value in comparison
because a portion can be recovered for future use. However, block copolymerization of non-polar and polar
monomers is challenging as current industrial polymerization methods of non-polar monomers with either
anionic or Ziegler-Natta catalysts are not suited for expansion to polar functionalities.
7
134
Bis(alkyl) rare earth (RE) metal complexes boasting many different ligand motifs have emerged as
amenable to the block copolymerization of 1,3-dienes/olefins and cyclic esters. Four bis(alkyl) RE metal
complexes have previously been reported that are able to perform the block copolymerization of isoprene
(IP) and ε-caprolactone (CL) in a one-pot manner (Table 5.1).
8–11
All of these catalysts demonstrated fast
rates. However, these catalysts have very different ligand frameworks and different metals, which makes
comparisons between them difficult. Furthermore, they use different solvents, cocatalysts, temperatures,
and catalyst concentrations which further complicates comparisons. For example, Cui and coworkers
synthesized a Lu amidinate complex with a N-heterocycle carbene (NHC) chelate arm which demonstrated
high 3,4 selectivity (Table 5.1, entry 1).
10
In comparison, Shi and coworkers later synthesized a similar
amidinate ligand but replaced the NHC with a pyridine chelate, which made the catalyst trans-1,4 selective
(Table 5.1, entry 2).
11
Despite the subtle ligand change, different metals were used so it is difficult to
determine if the selectivity change was due to the ligand or the metal.
Table 5.1. Comparison of literature pre-catalyst polymerization conditions for the block copolymerization of IP
and CL.
Entry Cat. Feed Ratio
(IP:CL)
Time Conv.
(%)
Mn
(KDa)
Đ Cis-
1,4
Trans-
1,4
3,4 Eff
1
a
A
10
500:500 30/120 >99 102 1.16 - - 99 89
2
b
B
11
500:500 150/180 100 166 1.18 0.2 97 3 55
3
c
C
8
800:300 45/90 100 154 1.15 99 - - 58
4
d
D
9
500:500 45/5 93 108 1.70 96 0 3.8 84
Conditions:
a
Chlorobenzene, 10 mL; cat. 10 μmol; 1:1 [cat.]/[Ph3C][B(C6F5)4]; 25 ºC.
b
Toluene, 6 mL; cat. 20 μmol; 1:1
[cat.]/[PhMe2NH][B(C6F5)4]; 25 ºC.
c
Toluene, 10 mL; cat. 10 μmol; 1:1 [cat.]/[Ph3C][B(C6F5)4]; 25 ºC.
d
Toluene, 10 mL;
cat. 10 μmol; 1:1 [cat.]/[Ph3C][B(C6F5)4]; 30 ºC.
135
RE metal catalysts supported by ancillary pincer ligands have gained attention in the literature due
to the ease at which both steric and electronic properties can be tuned.
12
They can also add considerable
steric bulk without occupying many coordination sites. Pincer RE metal catalysts are efficient at the
controlled homopolymerization of both cyclic esters and olefins.
13–16
Li et al. have developed a PNP
carbazolide bis(alkyl) RE metal catalyst capable of producing diblock copolymers of cis-1,4 selective IP
and CL in a living manner (Table 5.1, entry 3).
8
This catalyst was able to produce IP at a fast rate with a
low dispersity (1.06–1.24) depending on conditions. Notably, the cis- 1,4 selectivity was extremely high
(>99%). Pan et al. recently synthesized an anilido-oxazoline ligand for the block copolymerization of IP
and CL supporting a Y metal center (Table 5.1, entry 4).
9
They were able to achieve very high cis-1,4
selectivity with very fast conversions. Hou et al. have developed a PNP amido RE metal complex able to
perform the living copolymerization of 1,4 selective IP and butadiene (Figure 5.1).
17
This catalyst
exhibited high cis-1,4 selectivity (>99%) with low
dispersities of 1.11; however, this catalyst was slower
than the Li catalyst, showing only a 10% efficiency
compared to the 58% efficiency of the Li catalyst.
These conditions were not completely analogous since
different solvent and different catalyst loadings were
used. Despite this, the question arises whether the
disparity in rate is a result of the flexibility differences in the ligand backbone or from the different
initiators. In addition, the presence of the THF in the Hou catalyst could also impact the polymerization as
it can compete with the binding of monomer.
18
Gaining inspiration from these works, we sought to develop a series of bis(alkyl) RE metal
complexes featuring pincer ancillary ligands with only subtle structural changes but retaining their excellent
cis-1,4 selectivity and fast rates. These complexes would feature the same anionic pincer ligand framework
but differ in the type of alkyl initiator in order to test and understand their selectivity towards the
copolymerization of olefins/1,3-dienes and cyclic esters.
Figure 5.1. PNP pincer pre-catalysts for 1,3-diene
polymerization.
8,17
136
5.2 Results and Discussion
5.2.1 Pincer Ligand Design
The target ancillary ligands are monoanionic and feature a central amine, having either a
carbazole with a bond in the backbone which will be referred to as “closed” or a diarylamido without
a bond in the backbone which will be referred to as “open”. The carbazole is less basic compared to
diarylamido and would have less flexibility around the metal center. This could cause the ligand to
lie in one plane which could provide stereoselectivity of IP polymerization.
19
The two arms of the
pincer ligands have neutral phosphine donors, which is a soft labile group that could show fluctional
behavior. Another factor that can be changed is either having a proton or a withdrawing group that could
affect the electronic properties of the ligand.
PNP pincer ligands
H
open and
tbu
closed analogous to the ligands designed by the Hou and Li
groups, respectively, were synthesized following literature
procedure (Figure 5.2).
8,17
These complexes were not directly
comparable, due to the absence of the tert-butyl in the
backbone in
H
open and the presence of it in the
tbu
closed
ligand. It is challenging to synthesize the closed ligands
without a tert-butyl, as they are necessary as a directing group
to install the arms of the ligand ortho to the N atom. We instead targeted an analogous diarylamido ligand
with tert-butyl groups para to the amine group. This was accomplished using 4,4’-di-tert-
butylphenylamine as the starting material. First, ortho bromination of 4,4’- di-tert-butylphenylamine was
carried out using bromine at reduced temperatures. The second step involved the replacement of the
bromines with diphenyl phosphine through the deprotonation with n-butyl lithium and reaction with
chlorodiphenylphosphine (PPh 2Cl). Successful synthesis of this new ligand was confirmed with
31
P NMR
showing a singlet at -19 ppm.
Figure 5.2. Synthesized pincer ligands
as ancillary supports for rare earth metal
complexes.
137
5.2.2 Rare Earth Metal Complex Design
The yttrium metal pre-catalysts that were targeted are supported by the synthesized pincer ligand
and two alkyl initiators (Figure 5.3). Complexes were first
synthesized with Y as it serves as a midsized diamagnetic RE
metal in the +3 oxidation state. In addition, Y is spin ½,
which gives an excellent diagnostic handle as atoms bound to
the metal have characteristic splitting patterns. Two different
types of alkyl groups were selected as support ligands,
trimethylsilyl methyl (CH
2
SiMe
3
) and benzyl (CH
2
Bn).
CH
2
SiMe
3 was chosen because it is the most prevalent alkyl
initiator in the literature for the polymerization of 1,3
dienes/olefins using RE metal complexes. CH
2
Bn has only a few examples in the literature but was chosen
because it can bind either η
1
or η
3
which affects the steric environment around the metal center.
Herein we successfully synthesized
all five of the target complexes: 1
H
open,
1
tbu
closed, 1
tbu
open, 2
tbu
closed, 2
tbu
open.
PNP complexes were synthesized through
a protonolysis reaction between ligand and
either Y(CH
2
SiMe
3
)
3
THF
2 and
Y(CH
2
Ph)
3
THF
2 for one hour at reduced
temperatures. Confirmation of successful
synthesis was seen with a characteristic
doublet and as a downfield shift from approximately -19 ppm to in between -4 to -11 ppm depending on
the structure of the complex in the
31
P NMR (Figures 5.7 – 5.12). Additionally, single crystals of
complex 2
tbu
open were crystallized out of a THF/pentane solution grown at -35 °C. An x-ray diffraction
Figure 5.3. Synthesized rare earth metal
complexes.
Figure 5.4 Crystal structure of 2
tbu
open crystallized from
THF/pentane solution at -35 °C.
138
study revealed that 2
tbu
open showed a bound ligand to the RE metal center with two benzyl alkyls bound
η
1
to the metal center (Figure 5.4). Two THF molecules were also seen bound to the metal center but this
may not be completely representative of the pre-catalyst in polymerization conditions as they are
performed in toluene.
5.2.3 Kinetics
Activation of 1
H
open was monitored by NMR in order to better understand the active
polymerization species. The activation of the 1
H
open complex was monitored by
31
P NMR, reacting the
complex in C
6
D
6 with trityl tetrakis(pentafluorophenyl)borate [Ph 3C][B(C 6F 5)] which abstracts one alkyl,
leaving a cationic complex.
31
P NMR showed a downfield chemical shift of the doublet at -11 ppm to -6
ppm indicating complete conversion of the neutral complex into the cationic species (Figure 5.13). This
was left in a J Young tube for over 12 hours and the
31
P NMR remained unchanged indicating that the
cationic species was highly stable. In addition, the
19
F NMR spectrum did not shift, which shows that the
borate is not directly interacting with the complex and that they are a separated ion pair and it also shows
one set of C
6
F
5 resonances, indicating no interaction between the fluoride atoms and the metal center in
solution (Figure 5.14).
Complexes were tested for reactivity towards IP and CL polymerization. The cationic 1
H
open
complex, generated in situ, showed reactivity towards IP polymerization. Complete conversion was reached
after an hour with a feed ratio of [IP]:[CAT] at 800:1. Interestingly, this complex showed facile
polymerization of CL with complete conversion reached after 16 min with a feed ratio [CL]:[CAT] at 800:1.
This is surprising because, while similar complexes have demonstrated higher reactivity towards IP, the
PNP complex demonstrated higher reactivity towards CL. The cationic 1
tbu
closed complex’s reactivity
towards CL polymerization was also studied. Complete conversion to PCL was equally facile.
First, we examined the rate of IP polymerization with the 1
H
open. This was accomplished through
in situ NMR monitoring by mixing catalyst with one equivalent of [Ph
3
C][B(C
6
F
5
)]. Reaction was treated
with ten equivalents of IP and monitored by
1
H spectroscopy at room temperature. The reaction followed
139
first order kinetics (rate= k obs[IP]) and the concentration vs time data was fit using COPASI with a k obs=
0.000615(10)s
-1
(Figure 5.15 and 5.16).
20
Next, we sought to determine if the flexibility between the carbazole and the diphenylamido
impacted the rate of CL polymerization. To this end, in situ NMR studies were conducted on 1
H
open and
1
tbu
closed using one equivalent of [Ph
3
C][B(C
6
F
5
)] in toluene-d
8
. These were carried out by mixing catalyst
with one equivalent of [Ph
3
C][B(C
6
F
5
)] for ten minutes prior to injection of CL. Reaction was monitored
by NMR for one hour at -30 °C with quantitative conversion of CL after fifteen mins. The concentration
versus time data was fit to a first-order rate expression of (rate = k
obs
[CL]) using COPASI.
20
The cationic
1
H
open demonstrated the value of k
obs
= 0.00376(7) s
-1
while the analogous closed complex had a k
obs
=
0.0616(11) s
-1
, Figures 5.18 and 5.19 respectively. The enhanced rate of the closed complex might be due
to the reduced basicity of the ligand. An important note is that after 100% conversion was achieved, a
31
P
NMR spectrum was taken and it showed complete decomposition of the complexes to one phosphorus
signal that matched the
protonated ligand. Variable
temperature (VT)
31
P NMR
studies were done to further
investigate the decomposition
of 1
H
open during CL
polymerization (Figure 5.5).
The studies were conducted in
toluene-d
8 at -30 ℃ and
demonstrated a gradual
decrease of the Y bound doublet at -11 ppm and the evolution of a singlet at -19 ppm consistent with
unbound ligand. Figure 5.5. VT-
31
P NMR of polymerization of ε- caprolactone in toluene-d
8 at -30 ºC
Figure 5.5. VT-
31
P NMR of polymerization of ε-caprolactone in toluene-d 8 at -
30 ºC.
140
Next, we sought to investigate whether the rate for ROP of CL was faster for an alkyl or an alkoxide.
This is particularly relevant for the block copolymerization of 1,3-dienes and cyclic esters because at the
end of the 1,3-diene polymerization an alkyl group is bound to the metal, while during propagation of cyclic
ester polymerization, an alkoxide unit is bound to the metal. In order to investigate this, the open complex
was reacted with two equivalents of either dry isopropanol or benzyl alcohol to exchange the alkyls for
alkoxides. However,
31
P NMR of the reactions indicate only a singlet at -19 ppm indicating that the ligand
also gets protonated off. This occurred even under highly dilute conditions and was not pursued further.
5.2.4 Initial Polymerization Data
To make a direct comparison of the IP polymerization rates with respect to the flexibility of the
ligand backbone and the initiator the complexes featuring tert-butyl groups in the backbone were tested. IP
polymerization was tested with 1
tbu
closed, 2
tbu
closed, 1
tbu
open, and 2
tbu
open with one equivalent of
[Ph
3
C][B(C
6
F
5
)
4
] in toluene. After 30 min, IP polymerization with the 2
tbu
closed reached 35% conversion
while 2
tbu
open reached 59% conversion within the same time (Table 1.1, entries 1 and 2, respectively).
This indicates that the non-confined geometry might have allowed for more facile binding of the IP
monomer to enhance the rate. The same trend is observed with the CH
2
SiMe
3 initiator where the open
catalyst has a faster rate than the closed. Comparing the effects of the two different initiators (Table 1.1,
entries 1 and 2 vs 3 and 4, respectively) it is clearly seen that the CH
2
SiMe
3 initiator has an enhanced rate.
This was somewhat surprising as the CH
2
SiMe
3 groups are bulkier than the benzyl group.
21
This
polymerization behavior can either be explained by considering that the benzyl group can act as a chelate
which can block coordination sites and thus increasing the steric hindrance or because the benzyl group has
decreased nucleophilicity. GPC analysis showed a consistent low dispersity (1.09 – 1.16) for all catalysts.
Experimental number average molecular weights matched very well with the theoretical molecular
weights indicating good catalyst efficiency (~65-97 %). Additionally, all catalysts showed excellent
selectivity toward cis-1,4 polyisoprene (99%). Interestingly, both closed complexes showed a slight trans-
141
1,4 impurity (1% – 3%), but as only a single data point was collected duplicate runs should be conducted
in order to confirm the consistent presence of this impurity. Complexes were also tested towards styrene
(S) polymerization. 2
tbu
closed only showed low conversion (16%) and 2
tbu
open showed no
polymerization activity (Table 5.2, entries 5 and 6, respectively). Finally,
stepwise block copolymerization of IP and CL was conducted using catalyst 1
tbu
open. This was carried out
by reacting catalyst with one equivalent of [Ph
3
C][B(C
6
F
5
)] for ten minutes prior to IP addition. Reaction
was run for 2 hours, then CL was added to the reaction. The polymer showed high molecular weight (108
kDa) in good agreement with the theoretical molecular weight and a low dispersity of 1.11. No evidence of
homopolymerization was seen in the DOSY NMR and it showed excellent thermal properties (T d = 292 °C)
(Figure 5.21).
Table 5.2. Polymerization isoprene with targeted pre-catalysts.
a
Entry Cat. Monomer
Time
(min)
Conv.
(%)
b
Mn
(kDa)
c
Đ
c
Cis-
1,4
d
Trans-
1,4
d
3,4
d
1 2
tbu
closed IP 30 35 23 1.16 96 3 1
2 2
tbu
open IP 30 59 38 1.09 99 0 1
3 1
tbu
closed IP 30 90 54 1.12 98 1 1
4 1
tbu
open IP 30 >99 58 1.10 98 0 2
5 2
tbu
closed S 30 16 - - - - -
6 2
tbu
open S 30 0 - - - - -
a
Conditions: [cat.] 10 μmol; [Ph3C][B(C6F5)4], 10 μmol; IP 0.80 M; [IP or S]/[cat.]=800; toluene, 10 mL; room temperature.
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures, comparing monomer peaks to polymer.
c
Determined by
gel permeation chromatography (GPC) in THF using a Wyatt DAWN HELEOS II MALS detector.
d
1,4 and 3,4 selectivity
determined by
1
H NMR. Cis-1,4 and trans-1,4 selectivity determined by
13
C NMR.
142
5.3 Conclusions
Herein, we synthesized five bis(alkyl) pincer RE metal complexes, with three of them not
previously reported in the literature. All synthesized complexes demonstrated excellent dispersity and
molecular weight control towards IP polymerization. It was demonstrated that subtle changes to both the
ligand structure and the initiator impacted the rate of polymerization. In particular, the more nucleophilic,
bulkier initiator showed faster polymerization rates, suggesting that the cationic complex was not too
sterically hindered to allow for good binding of the IP even with the bulkier initiator. The benzyl ligand
could also have been undergoing some fluctuation in denticity which could make the metal more crowded,
thus slowing the rate of polymerization. Many questions remain to be answered such as how the different
initiators affect CL polymerization. More broadly, do these initiators have a similar trend as IP with
olefin/1,3-diene monomers. Can other ligands be synthesized with slightly stronger binding to prevent the
ligand from being protonated off the complex during polymerization, but still remain weak enough of a
donor to not hinder the excellent polymerization rate, selectivity, and control of these complexes?
5.4 Experimental Details and Additional Figures
5.4.1 General Considerations
All reactions involving air and moisture sensitive compounds were carried out using Schlenk line
techniques or in a Vacuum Atmospheres OMNI-LAB glovebox under an oxygen free, N
2 atmosphere.
Solvents used in air free reactions (toluene, hexanes, pentane, and tetrahydrofuran) were purchased from
Fisher, sparged under ultrahigh purity (UHP) grade argon and passed through two columns of drying agent
in a JC Meyer solvent purification system and dispensed directly into the glovebox. All other solvents
were used without further purification. Deuterated NMR solvents, C
6
D
6
, CDCl
3
, and toluene-d
8 were
purchased from Cambridge Isotope Laboratories and were used as received. Toluene-d
8 and C 6D 6
suitable for air sensitive compounds were dried by stirring over Na/benzophenone for two days, followed
by three freeze- pump-thaw cycles and vacuum transferred into a flame-dried Straus flask and stored in a
glovebox under a N
2 atmosphere. Complexes Y(CH
2
SiMe
3
)
3
(THF)
2
,
22
Y(CH
2
Ph)
3
(THF)
3
23
were
143
synthesized following literature procedure. Compounds bis(2-bromo-4-tert-butylphenyl)amine, 1,8-
Dibromo-3,6-di-tert-butyl-9H-carbazole were prepared following literature procedure.
24
Isoprene,
purchased from Sigma-Aldrich, was dried over 4Å molecular sieves for 7 days, followed by three freeze-
pump-thaw cycles and a vacuum transfer into a flame-dried Straus flask and stored in a glovebox at -35
ºC under a N
2 atmosphere. ε-Caprolactone was purchased from Sigma-Aldrich and were dried over CaH
2
for 3 days followed by three freeze-pump-thaw cycles and a vacuum transfer to a flame-dried Straus flask
and stored in a glovebox at -35 ºC under a N
2 atmosphere. Styrene was purchased from Sigma-Aldrich
and dried over 4Å molecular sieves for days followed by three freeze-pump-thaw cycles and a vacuum
transfer to a flame-dried Straus flask and stored in a glovebox at -35 ºC under a N
2 atmosphere All other
reagents and chemicals were obtained from commercial vendors (Sigma-Aldrich, TCI, Alfa Aesar, and
VWR) and were used without further purification.
5.4.2 Characterization Methods
NMR Spectroscopy.
1
H,
13
C, and
19
F,
31
P NMR spectra were recorded using a Varian Mercury 400 MHz, Varian 500 MHz, or
Varian 600 MHz spectrometers. Chemical shifts are referenced to residual protons in the deuterated
solvent or the deuterated solvent itself for
1
H (7.26 ppm for CDCl 3, 2.08 for toluene-d
8
) or
13
C (77.16 ppm
for CDCl
3
) NMR spectra. Temperature and deuterated solvents that the NMR spectra were calibrated
using an MeOH standard and recorded at are specified in the figure description.
Gel Permeation Chromatography (GPC).
GPC analyses were conducted using an Agilent 1260 Infinity II GPC System equipped with a Wyatt
DAWN HELEOS-II and a Wyatt Optilab T-rEX as well as an Agilent 1260 Infinity autosampler and UV-
detector. The GPC system was equipped with an Agilent PolyPore column (5 micron, 4.6 mmID) which
was calibrated using monodisperse polystyrene standards, eluted with THF at 30 ºC at 0.3 mL/min. The
number average molar mass and dispersity values were determined from multi-angle light scattering
(MALS) using dn/dc values calculated by 100% mass recovery method from the refractive index (RI)
144
signal.
Thermal Gravimetric Analysis (TGA).
All TGA traces were recorded using Mettler-Toledo STARe System TGA/DSC 3+ equipped with STARe
software, a TA SDTA Sensor LF, XP1 Balance, and a sample robot. Data from these four samples were
collected on a TGA Q50 instrument. Sample weight of purified polymer between 5-15 mg was sealed in a
40 μL aluminum crucible fitted with a pierceable lid. General method involves heating from 25 ºC to 500
ºC at a scan rate of 10 ºC/min under a constant flow of N
2 (15 mL/min).
5.4.3 Synthetic Procedures
Bis(2-bromo-4-tert-butylphenyl)amine
Neat Br
2 (3.6 mL, 0.071 mol) was syringe transferred dropwise into a stirring slurry of 4,4
’
-di-tert-
butylphenylamine (10 g, 0.036 mol) in acetic acid at -5 ºC. Following addition, the solution was stirred
warmed to room temperature for 16 h. A dilute solution of aqueous Na
2
S
2
O
4 (500 mL) was added, and the
resulting solution was stirred for 15 min. The solids were collected on a frit and washed with H
2
O (3 ×
200 mL). The crude product was purified by crystallization at -20 °C from MeOH/CHCl 3 as a white solid
(10 g, 80%).
145
3,6-(
t
Bu) 2-1,8-(PPh 2) 2-amine) [
tbu
open]
On a Schlenk line 11.2 ml (97.44 mmol) of
n
BuLi (2.5 M in hexane) was added dropwise to 3.39 g (30.09
mmol) of 2,2-dibromodiphenylamine in Et
2
O (20 mL) at 0 ºC. The reaction mixture was stirred for 90 min.
5 ml (97.48 mmol) diphenylchlorophosphine in Et
2
O (20 ml) was added dropwise at 0 ºC and the solution
was stirred at room temperature overnight. The reaction mixture was then hydrolyzed with concentrated
HCl (5 mL), the solution washed with water, and the product precipitated with EtOH. The colorless 2,2
bis(diphenylphosphino)diphenylamine was purified by recrystallization from CH
2
Cl
2 and EtOH. Yield:
1.34 g (48%).
Figure 5.6.
31
P NMR spectrum of
tbu
open in CDCl
3 at 25 ºC.
General procedure for the synthesis of 2
tbu
closed and 2
tbu
open
146
3,6-(
t
Bu)
2
-1,8-(PPh
2
)
2
-carbazole) (0.24g, 0.50mmol) or 3,6-(
t
Bu)
2
-1,8-(PPh
2
)
2
-amine) (0.098 g, 0.15 mmol)
were dissolved in tetrahydrofuran (5mL) and cooled to -35 ºC. Y(CH
2
Ph)
3
THF
3 (1 equivalent) was
dissolved in tetrahydrofuran (2 mL) and cooled to -35 ºC. Ligand solution was added dropwise to
YBn
3
THF
3
. The solution turned yellow right away and was left to stir at room temperature for one hour.
The reaction was filtered and concentrated to a dark yellow residue. Oil residue was triturated with pentane
(3 mL) twice and pumped down to a light-yellow solid. Solids were washed with pentane and crystalized
from a concentrated toluene solution at -35 ºC (78% yield 2
tbu
closed or 90% 2
tbu
open).
147
Figure 5.7.
1
H NMR spectrum of 2
tbu
closed in C
6
D
6 at 25 ºC. relative integrations consistent with two benzyl
groups and one THF bound to the metal center.
Figure 5.8.
31
P NMR spectrum of 2
tbu
closed in C
6
D
6 at 25 ºC. Doublet at -4.8 ppm indicates ligand bound to an Y
metal center.
148
Figure 5.9.
1
H NMR spectrum of 2
tbu
open in C
6
D
6 at 25 ºC. Relative integration of the
t
Bu on the ligand to the CH
2
of the benzyl group indicates two alkyls bound to the metal center.
Figure 5.10
31
P NMR spectrum of 2
tbu
open in C
6
D
6 at 25 ºC. Doublet at -5.5 suggests ligand is a cleanly bound to
the yttrium metal center.
149
General procedure for the synthesis of 1
tbu
closed and 1
tbu
open
3,6-(
t
Bu)
2
-1,8-(PPh
2
)
2
-carbazole) (0.098 g, 0.15 mmol) or 3,6-(
t
Bu)
2
-1,8-(PPh
2
)
2
-amine) (55 mg, 0.08
mmol) was dissolved in tetrahydrofuran (5 mL) and added to a solution of Y(CH
2
SiMe
3
)
3
(THF)
2 (1
equivalent) in tetrahydrofuran (2 mL) at room temperature in a nitrogen atmosphere. The reaction was
stirred at room temperature for 1 h, then concentrated to a yellow powder. The residue was dissolved in
pentane, filtered and the solvent removed under reduced pressure. Crystals were grown by dissolving the
powder in THF (1 mL), layering the top with pentane (2 mL), and storing the solution at -35 ºC (90%
yield 1
tbu
closed or 89% yield 1
tbu
open).
150
Figure 5.11.
31
P NMR spectrum of 1
tbu
closed in C
6
D
6 at 25 ºC.
Figure 5.12.
31
P NMR spectrum of 1
tbu
open in C
6
D
6 at 25 ºC.
151
5.4.4. NMR Monitoring and Polymerization Kinetics Data
Figure 5.13. In situ
31
P NMR spectrum cationic active species synthesized by reacting 1 equivalent 1
H
open
with 1 equiv. [Ph
3
C][B(C
6
F
5
)
4
] in Toluene-d
8 at room temperature.
Figure 5.14. In situ
19
F NMR spectrum of cationic active species synthesized by reacting 1 equivalent
1
H
open with 1 equiv. [Ph
3
C][B(C
6
F
5
)
4
] in toluene-d
8 at room temperature.
152
Figure 5.15. Representative
1
H NMR of the polymerization of IP with 1
H
open activated by 1 equiv.
[Ph
3
C][B(C
6
F
5
)
4
] in Toluene-d
8 at -30 ºC.
153
Figure 5.16. COPASI fit for IP polymerization (concentration = 0.408 M) with 1
H
open. Filled in colored circles
represent experimental data. (Blue = PIP, Red = IP). Lines represent the fit data for their respective species. The
open circles represent how good the fit was to the experimental data.
154
Figure 5.17. Representative
1
H NMR of the polymerization of CL with 1
H
open activated by 1 equiv.
[Ph
3
C][B(C
6
F
5
)
4
] in Toluene-d
8 at -30 ºC.
155
Figure 5.18. COPASI fit for CL polymerization (concentration = 0.059 M) with 1
H
open. Filled in colored circles
represent experimental data. (Blue = PCL, Red = CL). Lines represent the fit data for their respective species. The
open circles represent how good the fit was to the experimental data.
156
Figure 5.19. COPASI fit for CL polymerization (concentration = 0.059 M) with 1
tBu
closed. Filled in colored circles
represent experimental data. (Blue = PCL, Red = CL). Lines represent the fit data for their respective species. The
open circles represent how good the fit was to the experimental data.
5.4.5 Bulk Polymerization Data
General Procedure for the Homopolymerization of 1,3-dienes
In a glovebox, catalyst (10 μmol, 200 μL of a 0.5 M stock solution) was placed in a stir bar charged 20 mL
vial and diluted in toluene (7 mL). Trityl (tetrakis(pentafluorophenyl)borate [Ph
3
C][B(C
6
F
5
)
4
] (10 μmol, 1
mL of a 0.01 M stock solution) was added to the vial and the mixture was stirred for 10 minutes. 1,3-diene
(300-800 equiv.) was added by micro syringe in one portion, and the polymerization was carried out for the
designated time with constant stirring during which the reaction turned from orange to gold. The reaction
157
mixture was poured into a large quantity of ethanol (100 mL) to give colorless copolymer that was dried in
a vacuum oven at 40 ºC for 12 h to a constant weight (0.90 g, 100%).
General Procedure for the Block Copolymerization of 1,3-dienes and Cyclic Esters
In a glovebox, catalyst (10 μmol, 200 μL of a 0.5 M stock solution) was placed in a stir bar charged 20 mL
vial and diluted in toluene (7 mL). Trityl (tetrakis(pentafluorophenyl)borate [Ph
3
C][B(C
6
F
5
)
4
] (10 μmol, 1
mL of a 0.01 M stock solution) was add to the vial and reaction was stirred for 10 minutes. 1,3-diene (800
equiv.) was added by micro syringe in one portion, and the polymerization was carried out for the
designated time with constant stirring during which the reaction turned from orange to gold. Then cyclic
ester (300 equiv.) was added by micro syringe to the above system and polymerization was continued for
the designated time. The gel like reaction mixture was poured into a large quantity of ethanol (100 mL) to
give colorless copolymer that was dried in a vacuum oven at 40 ºC for 12 h to a constant weight (0.90 g,
100%).
Figure 5.20.
13
C NMR spectrum of polyisoprene-b-caprolactone diblock copolymer generated by 1
tbu
open (500
MHz, CDCl
3
).
158
Figure 5.21. Thermal gravimetric analysis of polyisoprene-b-caprolactone diblock copolymer generated by
1
tbu
open.
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173
APPENDIX A
Triblock Polymerization of 1,3-Diene/Olefin and Polar Monomers with Divalent Samarium Complexes
Goal: Synthesize poly(1,3-dienes) centered “ABA” type block copolymers using divalent samarium
complexes. This will be accomplished first through the reductive polymerization of olefins/1,3-dienes to
form a telechelic growing poly(1,3-dienes) then with the addition of the polar monomer the samarium metal
center will switch to the ring-opening polymerization (ROP) of the polar cyclic ester monomer and thus
propagating two polar blocks on either side of the poly(1,3-dienes).
Background: A major limitation of the one-pot block copolymerization of 1,3-diene and cyclic
ester monomer using rare earth metal complexes is that only block copolymer morphologies can be made.
This is because once the transition from the 1,3-diene to the cyclic ester monomer has occurred an alkoxide
is left on the rare earth metal center which cannot reinitiate the 1,3-diene monomer. An alternative strategy
is using a telechelic initiator to first synthesize the soft (low T g) center block, then use this as a
macroinitiator to install the two harder blocks on either side. The Cheng group synthesized a binuclear
scandium half-sandwich precatalyst
which was able to make
caprolactone-b-styrene-b-
caprolactone triblock copolymers
using the bridging
bis(aminobenzyl) group as an
initiator.
1
While this was an
interesting strategy, these triblock
Figure A.1. General polymerization scheme for the formation of ABA block copolymers using
divalent samarium complexes.
Figure A.2. Initial targeted divalent samarium complexes tested for
isoprene polymerization.
174
copolymers had the softer blocks (caprolactone) on the outside of the harder block (styrene) which was the
opposite arrangement to what thermoplastic elastomer materials required. An alternative strategy is using
a divalent rare earth catalyst to first reduce an olefin or a 1,3-diene monomer, forming the softer inner block.
Then the polar blocks could be installed on either side after the addition of the polar monomer. There have
only been a few reports that have identified Sm
II
catalysts as being able to transition from ethylene
polymerization to CL polymerization.
2 –6
Work Done: In 2003, Evans and coworkers used divalent rare earth metal iodide salts (SmI 2, TmI 2,
DyI 2, and NdI 2) as well as their corresponding solvated THF adducts to catalyze the polymerization of
isoprene.
7
Motivated by this, similar conditions were modeled to reproduce their conditions by using SmI 2
(0.1 M) THF solution purchased from Sigma to polymerize isoprene in either hexane (this is what the Evans
group did) or in toluene. However, reactions stirred for 24 h at room temperature formed no polymer. The
conditions were adjusted by heating the solution to 60 ºC to increase the reduction potential, but still no
polymer was produced.
We then wondered if presence of THF was interfering with polymerization activity, so it was
removed. This was accomplished by heating the SmI 2 (0.1 M) THF solution on the Schlenk line at 130 ºC
for 18 h. Removal of THF was confirmed using Fourier transform infrared spectroscopy (FT-IR) and
resulted in a green powder. Polymerization of isoprene was carried out with the now solvent free SmI 2
which did not lead to any polymer. In order to make the Sm metal center more reducing a phosphoramide
was used as a coordinative ligand.
Hexamethylphosphoramide (HMPA) was the
phosphoramide chosen as it has been commonly used
in conjunction with SmI 2 to boost the reduction
potential. Most notably, Endo and coworkers used the
Sm
II
I 2/HMPA catalyst system to perform the living
polymerization of tert-butyl-4-vinylbenzoate.
8
Figure A.3. Two proposed synthetic pathways to
synthesize (BPh 4) 2Sm
II
(THF) x (c) and
[N(SiMe 3) 2] 2Sm
II
(THF) 2 (d).
175
However, modeling these conditions using 4-6 equivalents of HMPA relative to SmI 2 did not produce
polymer.
With no success with SmI 2, focus was turned to making samarium divalent complexes that would
be more reducing. The next targeted complex (Figure A.2c) has two tetraphenylborate groups supporting
the metal center by either acting as more of an outersphere cation or by binding via the phenyl rings to help
support the metal. This has the potential to leave a lot of space around the metal center during
polymerization to allow for monomer binding. This is an unknown complex so two methods to synthesize
it was envisioned (Figure A.3). The synthesis of (BPh 4) 2Sm
II
(THF) x was attempted using path 1. NaBPh 4,
which had been dried on the Schlenk line for four days and brought into the glovebox, was combined with
the SmI 2 THF solution and left to stir at room temperature for 18 h. No color change was seen indicating
that no reaction had taken place. In addition, after the reaction was pumped down, the residue was not
soluble in solvents such as hexane or toluene, so it was difficult to determine if any NaI had formed. Next,
a new divalent samarium complex was targeted (Figure A.2d) through path 2. SmI 2 THF solution was
reacted with K[N(SiMe 3) 2] to form [N(SiMe 3) 2] 2Sm
II
(THF) 2. A definitive color change from green to dark
purple was observed. The reaction was pumped down to dryness, and the purple residue was then soluble
in hexane and a colorless solid was removed by filtration. It was presumed that this solid removed was KI
as it was insoluble in hexanes. Polymerization of isoprene using [N(SiMe 3) 2] 2Sm
II
(THF) 2 was attempted
both at room temperature and at -35 ºC, but no polyisoprene was observed. Additionally, no color change
occurred during the polymerization, indicating that no reduction reaction had taken place.
The third targeted complex (Cp
*
) 2Sm
II
(THF) 2 (Figure A.2e) was endeavored to be synthesized
following the method developed by Evans and coworkers.
9
The first step in the synthesis was the
deprotonation of HCp
*
using K[N(SiMe 3) 2] which produced a colorless solid that was assumed to be KCp
*
.
In the second step of the synthesis, Sm
II
I 2 THF solution was combined with KCp
*
and a distinct color
change from green to purple was seen, which is what the literature describes. Polymerization of isoprene
using (Cp
*
) 2Sm
II
(THF) 2 was attempted at both room temperature and at -35 ºC using 2-4 equivalents of
176
isoprene. In both cases the solution changes color from purple to dark yellow. This is a possible indication
of the oxidation of Sm to the 3+ oxidation state. However, no change to the isoprene is observed in the
1
H
NMR spectrum.
Future: Despite the lack of initial success, this project is still an exciting possibility. However,
there are several different pathways that this project could take. One such pathway would include targeting
more complex ligand frameworks to support the divalent Sm metal center. These ligand frameworks should
be tuned to make the Sm metal more reducing. Alternatively, introduction of a sacrificial olefinic species
that could facilitate the oxidation of the metal to form a binuclear bridging alkyl species between two metal
centers could also be pursued. Finally, a switching to other divalent rare earth metals such as Tm, which
has a higher oxidation potential could be necessary in order to facilitate this polymerization.
Perfectly Alternating Copolymerization of Epoxides and Cyclic Anhydrides with Pincer Complexes
Goal: Design and synthesize rare earth pincer catalysts capable of initiating the perfectly alternating
copolymerization of epoxides and cyclic anhydrides.
Background: The world has become inundated with commodity, synthetic polymer (plastic) waste,
as only 2% of them undergo a closed-loop recycling scheme.
10
One of the largest contributors to this
problem are single use plastics such as utensils, masks, and packaging which accounts for nearly 30% of
the total amount of plastics produced.
11
Recently, the PEW released a report showing the projection of
plastics that will enter our oceans if we do not curtail our over production of nondegradable plastics.
12
The
report also indicated that an 80% reduction of plastic entering the ocean could be reached by 2040 if a
system change could be implemented. This system change would need to involve synthesis of degradable
Figure A.4. General scheme for the perfectly alternating
copolymerization of epoxides and cyclic anhydrides.
177
or recyclable replacements for non-degradable polymers. Once such promising material that is currently on
the market is polylactic acid (PLA), which is often used in place of polystyrene utensil. However, PLA is
only an industrial compostable plastic meaning that it requires specialized conditions for it to degrade. In
addition, it is derived from a single monomer so it lacks structural diversity which limits the range of
applications it can be used in. that to expand the range of applications a multitude of additives will need to
be included in the material which raises toxicity and other environmental concerns.
The perfectly alternating ring-opening copolymerization (ROCOP) of epoxides and cyclic
anhydrides is a promising alternative because there exists a large library of many different monomers both
from petroleum and biomass
resources which could lead to
an extensive range of
different materials. Many of
these monomers are still
unexplored but are expected
to lead to materials with a
broad range of glass
transition temperatures (T g= -
44 °C – 184 °C). A range of
different catalysts have been
explored featuring different
metals such as Al, Cr, Zn, and
Mg which are mostly
supported with porphyrin or
salen ligands. The
mechanism is proposed to proceed through alternating ring opening of the epoxide and the cyclic anhydride
Figure A.5. Proposed mechanism for the perfectly alternating ring-opening
copolymerization of epoxides and cyclic anhydrides.
178
(Figure A.5).
13
The rate-determining step (RDS) is hypothesized to be the ring-opening of the epoxide. The
Lewis acidity and the steric environment around the rare earth metal center as well as the co-catalyst are
important for the activation of the epoxide as well as the ring-opening of the monomer itself. Of critical
importance is making sure that the carboxylate anion in step I is sufficiently activated to allow for the ring
opening of the epoxide. To accomplish this, it is important to identify the right steric environment around
the metal center to ensure that the carboxylate anion chain end is not bound too tightly nor too far from the
metal center.
Work Done:
Pincer rare earth metal complexes are
excellent at providing steric control for
polymerizations. Two alkyl catalysts were
envisioned for this polymerization (Figure A.6).
Complex A was a bis-alkyl pincer ligand
featuring an anionic ligand support with labile
phosphine arms which was previously discussed
in chapter 5. The second complex B features a
central pyridine center moiety with two anionic amine arms. The dianionic ligand was synthesized in two
steps following a modified literature procedure (details can be found in the experimental section). First, the
diisopropylaniline was deprotonated with n-
butyllithium, then two equivalents of this species
were reacted with 2,6-bis(bromomethyl)pyridine
to generate the dianionic ligand. Purification by
crystallization generated a pure product with a
yield of 52%. Additional synthetic details for the ligand and complex synthesis are found below.
Figure A.6. Synthesized pincer rare earth metal
complexes for the ROCOP of epoxides and cyclic
anhydrides.
Figure A.7. Initial monomers tested for the rare earth
metal catalyzed ROCOP of epoxides and cyclic
anhydrides.
179
These two rare earth metal complexes were tested for their reactivity towards the ROCOP of
epoxides and cyclic anhydrides. These reactions were performed under neat conditions with the addition of
one equivalent [PPN]Cl as a cocatalyst and five hundred equivalents of epoxide and one hundred
equivalents of cyclic anhydride. The chosen epoxides were a monosubstituted 1-butene oxide (BO) and a
disubstituted cyclohexene oxide (CHO) (Figure A.7). The selected cyclic anhydrides were bicyclic phthalic
anhydride (PA) and the tricyclic carbic anhydride (CPMA) as they were the most commonly used in the
literature (Figure A.7).
First, complex A with the addition of 1 equivalent of [PPN]Cl as the cocatalyst, showed full
conversion of BO and PA within 16 hours (Table A.1, entry 1). Polymerization with the tricyclic CPMA
instead of PA resulted in a lower conversion at 78% (Table A.1, entry 2). Polymerization with CHO and
PA exhibited a substantial decrease in activity with conversions reaching 40% (Table A.1, entry 3).
Reactions with the CHO and CPMA monomer combinations showed no conversion after 16 hours. These
results demonstrate that this catalyst shows potential for the copolymerization with less sterically crowded
monomer pairs.
The second complex B was polymerized with the same cocatalyst as A. Overall, complex B
exhibited slower polymerizations than complex A which may not be surprising as it has one less initiator.
Of note, the monomer combination of BO and PA reached a conversion of 95% after 16 hours (Table A.1,
entry 5). The polymerization of BO and CPMA resulted in a conversion of 68% (Table A.1, entry 6).
180
Table A.1. Preliminary screening reactions for ROCOP of BO or CHO with PA and CPMA.
a
Entry Catalyst Epoxide Anhydride Cocatalyst Conv. (%)
b
1 A BO PA [PPN]Cl >99
2 A BO CPMA [PPN]Cl 78
3 A CHO PA [PPN]Cl 40
4 A CHO CPMA [PPN]Cl 0
5 B BO PA [PPN]Cl 95
6 B BO CPMA [PPN]Cl 68
a
Conditions: Catalyst, 1 equiv.; [epoxide]/Y= 500; anhydride, 100 equiv.; [PPN]Cl, 1 equiv.; neat; 60 ºC; 16 h.
b
Determined by
1
H NMR spectroscopy of crude reaction mixtures, comparing remaining anhydride to polymer peaks.
Future Work: This polymerization reactivity exemplifies how the ligand structure can greatly impact this
polymerization type. More work needs to be done to understand the unusual reactivity of complex A with
respect to the bicyclic anhydride vs its fast polymerization rate with the mono cyclic anhydride.
Additionally, quantification of the polymers molecular weight and dispersity with these catalysts should be
investigated to identify presence of side reactions such as the homopolymerization of epoxides and
transesterification reactions.
Experimental Details:
2,6-
i
Pr 2C 6H 3NHLi(THF)
2,6-diisopropylaniline (2.5 mL, 0.0133 mol) was added into a Schlenk tube charged with 4Å molecular
sieves and stirred for 2 days. Three freeze-pump-thaw cycles were performed after the 2 days and it was
transferred into a glovebox. 2,6-diisopropylaniline was decanted from the molecular sieves and dissolved
181
in dry THF (15 mL). At 1000 RPM stirring speed,
n
BuLi (2.5 M in hexane, 8.3 mL,0.0133 mol) was added
dropwise. Reaction was left to stir at room temperature for 30 min. All solvents were removed under
reduced pressure. The colorless solids were washed with dry hexane two times leaving pure product (yield
59%).
2,6-[(2,6-Diisopropylphenyl)HNCH 2] 2NC 5H 3
A THF (25 mL) solution of LiNHR (2.57 g, 66.77 mmol) was added dropwise to a stirring THF (20 mL)
solution of 2,6-bis(bromomethyl)pyridine (1.24 g, 33.37 mmol) at -78 °C. The mixture was stirred at room
temperature and stirred for 12 h. Reaction was quenched with a saturated NaHCO 3 solution (30 mL) and
extracted with diethyl ether. The solvent was removed in vacuo, and the resulting solid was dissolved in a
minimum quantity of hexanes and cooled to -30 °C for 12 h. A white crystalline solid was isolated by
filtration and dried under vacuum (1.54 g, 17.02 mmol, 51%).
General procedure for the polymerization of epoxides and cyclic anhydrides
Typical polymerization reactions were performed under a nitrogen atmosphere. The catalyst (1 equiv.),
[PPN]Cl (1 equiv.), and anhydride (100 equiv.) were loaded into an 8 mL vial charged with a stir bar.
Epoxide (500 equiv., 0.5 mL) was then added to the vial. Vial was capped with a green teflon lined cap and
wrapped in electrical tape and removed from the glovebox. Vial was placed in a Chemglass high throughput
heating block. Polymerization reactions were heated to 60 ℃ and left to stir for 16 h. The polymerization
was quenched with the addition of prodio chloroform. Conversion was determined by NMR analysis of an
aliquot of this solution. Volatiles were removed from the solution on a rotovap. Residues were dissolved in
a minimal amount of dichloromethane which was pipetted into a stirring solution of pentanes to precipitate
out the polymer. The isolated polymer was dried in a vacuum oven for twelve hours at 50 ℃.
182
Synthesis of Novel NNN Pincer Complexes
Goal: To synthesize novel NNN pincer complexes for the block copolymerization for chemically dissimilar
monomers.
Background:
In chapter 5, pincer rare earth metal complexes were used to synthesize block copolymers of 1,3-
dienes and cyclic ester monomers. One challenge that emerged was that the ligand was unstable during
cyclic ester polymerization and was subsequently protonated off. This could have a negative impact over
the control of future polymerizations, so a pincer complex analogous to those in chapter 5 but with stronger
chelate arms was designed. In particular, amine donors were selected because RE metals could coordinate
stronger to them as they favor these harder ligands. While NNN-type pincer complexes are ubiquitous in
the literature, none have been synthesized that are perfectly analogous to the phosphine pincer complexes
in chapter 5.
Work Done:
To synthesize an analogous complex, a synthetic strategy was developed using a double Buchwald-
Hartwig cross coupling reaction as shown in the experimental details. This was accomplished using a Pd
catalyst to install diphenylamine onto both arms of either 1,8-dibromo-3,6-di-tert-butyl-9H-carbazole or
bis(2-bromo-4-tert-butylphenyl)amine. Purification by column chromatography resulted in
Figure A.8. Targeted
NNN pincer complexes.
183
spectroscopically pure ligands (Figure A.10 and A.11, respectively). Confirmation of successful synthesis
was confirmed using crystallography of a crystal grown from a deprotonation reaction with NaH.
Once the ligands were dried, efforts to synthesize the yttrium trimethylsilylmethyl alkyl complex
was attempted similar to the methods that were used for the PNP complexes in chapter 5. This was carried
out through the protonolysis reaction with the NNN ligand and Y(CH 2SiMe 3) 3(THF) 2. However,
1
H NMR
spectrum still revealed the NH peak indicating that the ligand was still protonated. Different reaction
conditions were adjusted such as changing the solvent, heating the reactions, longer reaction times, ball
milling, etc. in order to try to produce the desired complex, but to no avail.
A different synthetic approach was envisioned by first deprotonating the ligand with n-butyllithium
then adding in YCl 3 to form the NNNCl 2 complex (Figure 5). But because the chloride in this complex
(NNNclosedYCl 2) is not NMR active, we could not tell for sure if this complex made. However, we
suspected that we did
make it because the
reaction was very clean
(no excess NMR peaks),
and the ligand was still
deprotonated. In order to
add the -CH 2SiMe 3 groups to replace the chlorides, the NNNclosedYCl 2 complex was reacted with 2
equivalents of CH 2SiMe 3. The
1
H NMR spectrum of this reaction was messy, but it was promising because
the ligand was still deprotonated. The reaction was repeated under reduced temperatures and monitored it
via
1
H NMR. The reaction completed in 2 hours, and it was purified leaving a clean spectrum. Unfortunately,
the complex is insoluble in pentane so LiCl could not be separated from the product. LiCl is insoluble in
DCM but the complex decomposed upon addition of this solvent as clearly seen with the generation of
SiMe 4 in the
1
H NMR spectrum.
Figure A.9. Proposed synthetic route to synthesize Y bisalkyl NNN pincer
complexes (NNNclosedY(CH 2SiMe 3) 2(THF) x).
‘’ ‘’
184
Reactions with Y(CH 2SiMe 3) 3THF 2 and the NNN pincer ligand were unsuccessful due to the
basicity of the NNN ligand. However, protonolysis reactions with the Y(CH 2Ph) 3THF 2 at reduced
temperatures demonstrated appropriate integration in the NMR matching one ligand with two bound benzyl
groups. The Y-CH 2 peaks shifted relative to the trisalkyl species and a doublet could be seen. The complex
is unstable as seen by the fast loss of the CH 2 peak.
Future Work: Herein we were able to successfully synthesize two new NNN pincer type ligands;
however, formation of bisalkyl rare earth metal complexes seem to be unstable and warrant further synthetic
investigation.
Experimental Details:
3,6-(
t
Bu) 2-1,8-(NPh 2) 2-carbazole) [
ph
NNNClosed]
In a nitrogen fill glovebox, Pd 2(dba) 3 (0.116 g, 0.127 mmol) and
t
Bu 3P (0.05 g, 0.247 mmol) was dissolved
in toluene (10 mL) and loaded in a 200 mL Schlenk flask. The Schlenk flask was removed from the
glovebox and placed on a Schlenk line. The side arm of the Schlenk flash was evacuated and backfilled
with dry nitrogen three times. 1,8-dibromo-3,6-di-tert-butyl-9H-carbazole (0.548 g, 2 mmol.),
diphenylamine (0.423 g, 2.5 mmol.), and
t
BuONa (0.72 g, 7.5 mmol) was added to the reaction mixture
before heating under N 2 atmosphere at reflux temperature for 24 h. The reaction mixture was then filtered
through a Celite plug, the filtrate evaporated under vacuum and subjected to column chromatography on
silica gel (n-hexane/DCM, 10/3) to get a yellow solid (0.85 g, 87%).
185
Figure A.10.
1
H NMR spectrum of
ph
NNNClosed in CDCl 3 at 25 ºC.
3,6-(
t
Bu) 2-1,8-(NPh 2) 2-amine) [
ph
NNNOpen]
Bis(2-bromo-4-tert-butylphenyl) amine (0.548 g, 2 mmol.), diphenylamine (0.423 g, 2.5 mmol.), Pd 2(dba) 3
(0.116 g, 0.127 mmol),
t
Bu 3P (0.05 g, 0.247 mmol), and
t
BuONa (0.72 g, 7.5 mmol) were combined in
toluene (10 mL). The reaction mixture was evacuated and backfilled with dry nitrogen three times before
heating under N 2 atmosphere at reflux temperature for 24 h. The reaction mixture was then filtered through
a Celite plug, the filtrate evaporated under vacuum, and subjected to column chromatography on silica gel
(n-hexane/DCM, 10/1) to get a white solid (0.55 g, 76%).
186
Figure A.11.
1
H NMR spectrum of
ph
NNNopen in CDCl 3 at 25 ºC.
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copolymerization of ethylene with polar monomers by the unique catalytic function of organolanthanide complexes.
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Abstract (if available)
Abstract
Development of catalysts for the stereospecific living polymerization of 1,3-dienes and olefins is an area of interest for both academic and industrial laboratories. This is attributed to the exceptional physical and mechanical properties exhibited by these polymers, as well as the versatility of poly(1,3-diene) materials in numerous applications. Although Ziegler-Natta catalysts have been traditionally associated with this polymerization, their effectiveness is hindered by their limited tolerance towards polar functional groups. Alkyl rare earth metal complexes supported by a diverse range of ligand frameworks have demonstrated promising selectivity for the polymerization of 1,3-dienes, while maintaining stable in the presence of polar monomers. Rare earth metals present an intriguing avenue for studying 1,3-diene polymerization and copolymerizations, as they possess a gradual change in ionic radii and Lewis acidity unmatched by other elements on the periodic table. Furthermore, steric and electronic properties of supporting ligand frameworks can be tailored to influence the rate and selectivity of these polymerizations. However, while ligand frameworks often play a crucial role in providing stability, their synthesis can be costly and time-consuming. Additionally, few studies have delved into understanding the trends that lead to the development of fast and selective catalysts, especially for copolymerizing multiple monomers. This work aims to identify optimal rare earth metal centers, catalyst ligand environments, activation conditions, and reaction parameters for achieving rapid, controlled, and selective polymerization of 1,3-dienes, as well as copolymerization of 1,3-dienes with cyclic esters. Insights gained from these investigations will hopefully pave the way for the optimization of similar systems.
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Kosloski-Oh, Sophia
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Rational design of simple and complex rare earth metal catalyst systems to enable reactive, controlled and selective block copolymerization of chemically dissimilar monomers
School
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Chemistry
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2023-08
Publication Date
08/21/2024
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alkyl complexes
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living isoprene polymerization
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