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Using Lewis acid catalysts to control the ring-opening copolymerization for polyester synthesis and the deconstruction of polyethylene

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Using Lewis acid catalysts to control the ring-opening copolymerization for polyester synthesis and the deconstruction of polyethylene

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Content Copyright 2023 Yvonne Manjarrez

Using Lewis Acid Catalysts to Control the Ring-Opening Copolymerization for Polyester
Synthesis and the Deconstruction of Polyethylene
by

Yvonne Manjarrez

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

ii

Portions of Chapter 1 © 2022 Inorganic Chemistry
Portions of Chapter 2 © 2022 ChemCatChem
All other materials © 2023 Yvonne Manjarrez
iii

Dedication

This dissertation is dedicated to my loving parents and siblings for their endless love and
support.

“No le tengas miedo al día que no has visto”

iv

Acknowledgements
First, I’d like to express my deepest appreciation for my advisor, Professor Megan E. Fieser, for
her immense support and guidance throughout my graduate career. No matter how busy her
schedule was, Professor Fieser always made time to meet with me whenever I sought help whether
it be for chemistry or my personal life. Her drive for mentoring and passion for chemistry have
made me feel lucky to have had her as my advisor. I’m also greatly appreciative to my defense
committee for imparting their knowledge on my graduate journey.
Additionally, I’d also like extend my gratitude to my lab mates in the Fieser Group. I’m thankful
to have worked with such kind, bright people and to have been able to brainstorm with them about
science. Our many conversations in the student office made even the days when the chemistry
didn’t seem to work fun.
Finally, I would like to express my sincere thanks and immense gratitude to my family for their
constant support throughout my entire academic career. On the days when I felt lost and wanted
to give up, they were always there to cheer me on. Words cannot express how grateful and lucky
I am to have such an amazing family that I can always count on. I love you all so very much.

v

TABLE OF CONTENTS
Dedication ..................................................................................................................................... iii
Acknowledgements ...................................................................................................................... iv
List of Tables ............................................................................................................................... vii
List of Figures and Schemes...................................................................................................... viii
Abstract ...........................................................................................................................................x
Introduction ....................................................................................................................................1
Chapter 1: Perfectly Alternating Copolymerization of Cyclic Anhydrides and Epoxides
with Yttrium β-Diketiminate Complexes.........................................................................................7
Introduction ...................................................................................................................7
Results/Discussion ........................................................................................................9
Conclusion ..................................................................................................................18
Experimental Section ..................................................................................................19
Supplementary Information ........................................................................................22
References ...................................................................................................................24
Chapter 2: Synthesis of Rare Earth Metal Containing Ionic Liquids for the Preparation of
Aliphatic Polyesters .......................................................................................................................26
Introduction .................................................................................................................26
Results/Discussion ......................................................................................................29
Conclusion ..................................................................................................................41
Experimental Section ..................................................................................................41
Supplementary Information ........................................................................................48
References ...................................................................................................................50

vi

Chapter 3: Cationic Polymerization of Tetrahydrofuran and Cyclic Anhydrides Using a Trityl
Borate Catalyst ...............................................................................................................................51
Introduction .................................................................................................................51
Results/Discussion ......................................................................................................52
Conclusion ..................................................................................................................55
Experimental Section ..................................................................................................56
References ...................................................................................................................64
References .....................................................................................................................................65
Appendix .......................................................................................................................................70
Appendix A: The Degradation/Upcycling of Polyethylene with Lewis
Acid Catalysts through Aryl Alkylation ......................................................................70

vii

List of Tables
Table 1.1 Polymerization of BO/PA with complexes 1-3 and 1-Cl 12
Table 1.2 Polymerization of other monomers with 1-Cl, 1, 2, and 3 18
Table 1.3 Controlled Polymerizations 23
Table 1.4 Other Monomer Polymerizations 24
Table 2.1 Large scale reactions with BO and CPMA to observe molecular weight
control
39
Table 2.2 Polymer Molecular Weight Data. Representative reactions done in triplicate 49
Table 2.3 Additional BO/CPMA Polymerizations 49
Table 3.1 Preliminary ROCOP of targeted anhydrides with THF 52
Table 3.2 Polymerization reactions with shorter time frames and observed GPC data 53
Table A1 PE degradation reactions for 48 hours 73
Table A2 Large scale PE degradation reactions 74
Table A3 PE degradation reactions using TMSCl 74
Table A4 Large scale PE degradation reactions using TMSCl 75

viii

List of Figures and Schemes
Figure 0.1 Proposed ROCOP 1
Figure 0.2 Proposed ROCOP mechanism 2
Figure 0.3 Hypothesized impact of metal center Lewis acidity on RDS intermediate 3
Figure 0.4 Rare earth metals and ionic radii 3
Figure 1.1 Representative perfectly alternating copolymerization of epoxides and
cyclic anhydrides with monomers studied
7
Figure 1.2 Known rare earth metal catalysts for the ROCOP of epoxides and cyclic
anhydrides
9
Figure 1.3 Yttrium BDI complexes studied for the ROCOP of epoxides and cyclic
anhydrides
10
Figure 1.4
7
Li NMR comparison of 1-Cl to what would be 100 % LiCl impurity 22
Figure 1.5 MALDI-TOF spectrum of BO/PA oligomers 23
Figure 2.1 Proposed ROCOP 26
Figure 2.2 Catalyst systems for targeted epoxide and cyclic anhydride ROCOP 27
Figure 2.3 Monomer scope for ROCOP 29
Figure 2.4 General synthesis of MILs where Ln= Y, Gd, Ho, or Nd 30
Figure 2.5 Polymerization with Ln-WET MILs 30
Figure 2.6 Synthesis of yttrium MILs 32
Figure 2.7 Polymerizations for Y-WET, Y-DRY and the DCM analogues 34
Figure 2.8 Control reactions run with BO/PA under varying monomer conditions with
Y-DCMWET, Y-DCMDRY analogues and Y-WET, Y-DRY analogues
36
Figure 2.9 TGA spectrum of Y-WET MIL heated from 25°C–500°C. 45
Figure 2.10 TGA spectrum of Y-DRY MIL heated from 25°C–500°C. 46
Figure 2.11 TGA spectrum of Y-DCMWET MIL heated from 25°C–500°C. 46
Figure 2.12 TGA spectrum of Y-DCMDRY MIL heated from 25°C–500°C. 47
Figure 2.13 TOFs for polymerizations from Figure 2.7 including Y-AIR, Y-
DCMAIR and [H3DP]Cl
48
Scheme 3.1 Proposed ROCOP of THF and cyclic anhydrides with targeted anhydrides 51
Figure 3.1
1
H NMR spectrum of crude reaction for THF/IA, for Table 3.2, entry 3 in
CDCl3
58
Figure 3.2
1
H NMR spectrum of crude reaction for THF/GA, for Table 3.2, entry 6 in
CDCl3
59
Figure 3.3
1
H NMR spectrum of crude reaction for THF/DGA, for Table 3.2, entry 9
in CDCl3
60
Figure 3.4
1
H NMR spectrum of crude reaction for THF/THPA, for Table 3.2, entry
10 in CDCl3
61
Figure 3.5
1
H NMR spectrum of isolated polymer for THF/IA, for Table 3.2, entry 3
in CDCl3.
62
Figure 3.6
1
H NMR spectrum of isolated polymer for THF/GA, for Table 3.2, entry 6
in CDCl3
63
Figure 3.7
1
H NMR spectrum of isolated polymer for THF/DGA, for Table 3.2, entry
9 in CDCl3
64
Figure A1 Previous work with PE degradation and this work 71
Figure A2 PE and iC5 upcycling into liquid alkanes 75
Figure A3
1
H NMR spectrum of product obtained from Table A4, entry 4 in CDCl3 78
ix

Figure A4
1
H NMR spectrum of product obtained from Table A4, entry 5 in CDCl3 79
Figure A5 Representative ATR-FTIR of solid precipitate 80

x

Abstract
Although plastics (polymers) have become necessary for a variety of technologies and functions,
their large consumption has led to negative impacts on the environment as well as resulted in a
plastic pollution crisis. This urges a need to seek more environmentally friendly alternatives
(aliphatic polyesters) to mass-produced plastics in order to combat this issue. An attractive route
toward these polyesters is through the ring-opening copolymerization (ROCOP) of cyclic ethers
(epoxides or THF) and cyclic anhydrides. This route allows access to a wide scope of monomers,
thus potentially leading to a library of plastics with varying properties. Herein, Lewis acid catalysts
that are metal-free or contain rare earth metal centers were employed for the targeted ROCOP. To
further address the plastics issue, there is also a need to develop better methods to chemically
recycled/upcycle currently mass-produced plastics. As polyethylene (PE) is one of the most widely
consumed plastics, efforts described herein were directed towards using a Lewis acid catalyst
(AlCl3 or rare earth metal salts) for the degradation of PE into the value-added products, diaryl
alkanes.

1

Introduction
Plastics, made from synthetic polymers, have become an essential part of people’s lives as
we have grown to rely on them more and more heavily. From the toothbrush we grab in the
morning to the cars we drive to work, they are almost all made of synthetic polymers. However,
we’ve grown to consume so much of these plastic items that they’ve accumulated in landfills and
have leaked into our environment. If we continue to consume these materials at the current rate
with no changes, there will be more plastic than fish in the ocean, by weight, in 2050.
1,2
In an effort
to effectively combat this pressing issue, there is a need for a two-prong solution. One prong would
involve the synthesis of easily degradable plastics, while the other prong would focus on
developing methods to chemically recycle/upcycle current mass-produced plastics (e.g.
polyethylene, polystyrene).
Synthesis of Easily Degradable Plastics
While there are current plastics (polymers) on the market that are biodegradable, such as
polylactide (PLA), these plastics suffer from low melting points, brittle texture, and can be
expensive to produce.
3
The monomer to make PLA is also difficult to modify, leading to challenges
in improving physical properties of this polymer. This further emphasizes that this single plastic
cannot replace all the plastics that are produced at the industrial scale and urges the need for more
environmentally friendly plastics. A route to towards more diverse environmentally friendly
plastics that has garnered attention is the
perfectly alternating ring-opening
copolymerization (ROCOP) of epoxides
and cyclic anhydrides for the synthesis of aliphatic polyesters (Figure 0.1). The C-O bonds in
these polymers make them potentially easier to degrade than mass-produced plastics that are

Figure 0.1 Proposed ROCOP.
O
+
O
O O
Catalyst
Cocatalyst
O
n
O
O
O
2

primarily composed of strong C-C bonds. Additionally, these polymers also offer a wide monomer
scope that can lead to a large library of plastics, thus lending this method to provide alternatives
for plastics used in a multitude of every-day applications. Various catalysts have been previously
reported in the literature for this ROCOP where they’ve consisted of bimetallic and monometallic
complexes with various metal centers (e.g. Ni, Cr, Co, Al, Zn).
4
Many of the reported catalyst
systems consist of cocatalysts, such as bis(triphenylphosphine)iminium chloride ([PPN]Cl) or 4-
dimethylaminopyridine (DMAP), whose presence was found to assist with increasing rates and
polymerization control. Catalyst design has also consisted of tethering the cocatalyst to the catalyst
as this could provide better molecular weight control as well as prevent unwanted side reactions
during the copolymerization at low catalyst loadings.
5

In order to optimize the targeted copolymerization, it is important to take into consideration
the choice in metal center of the catalyst. The Lewis acidity of the metal center is hypothesized to
play an important role in overcoming the rate-determining step (RDS) for the proposed

Figure 0.2 Proposed ROCOP mechanism.
3

mechanism, which commonly consists of the ring-opening of the epoxide (Figure 0.2). In
particular, we hypothesized the intermediate formed prior to the RDS can be optimized by tuning
the metal center to exhibit an ideal Lewis acidity (Figure 0.3). The Lewis acidity of the metal
center is expected to play a crucial role
as having a metal center that is too
Lewis acidic can lead to the
carboxylate anion chelating to the
metal; however, if the metal center is
not Lewis acidic enough, this could
prevent the epoxide from coordinating
to the metal center and being activated
for the ring-opening step. Therefore,
the metal center needs to be just Lewis acidic enough for the epoxide coordination event, but not
too Lewis acidic that it would cause unwanted reactions like the chelating of the carboxylate anion.
To cater the targeted ideal scenario of the catalyst, the rare earth metals are considered to be
suitable as they display a subtle change in Lewis acidity across the series as well as a change in
their ionic radii (Figure 0.4). This enables the ability
to finely tune the Lewis acidity of the metal center and
optimize the intermediate prior to the RDS as the
monomers change. Furthermore, contrary to their
names the rare earth metals are also as abundant as some industrially used metals like Cu and Ni,
thus making them industrially relevant.
6
The use of rare earth metals would provide insight on

Figure 0.3 Hypothesized impact of metal center Lewis
acidity on RDS intermediate.
Figure 0.4 Rare earth metals and ionic
radii.
4

their reactivity towards the targeted polymerization while also working towards making the
targeted environmentally friendly plastics.
While this ROCOP strategy can change the functional groups on epoxides and cyclic
anhydrides as well as the ring size on cyclic anhydrides, one limitation of this method is the ring
size of the epoxide.
7
A way to add more tunability is to use cationic polymerization, which can
allow varying the epoxide side, while maintaining alternating polymers with cyclic anhydrides.
However, it is often difficult to achieve this type of polymerization with low dispersity control. If
dispersity control can be realized, this would also lead to a wider variety of polyesters to be
synthesized.
Chemical Recycling/Up-Cycling of Plastics
Along with making new sustainable plastics, it is also important to investigate ways to
degrade and upcycle currently mass-produced plastics (e.g. polyethylene, polystyrene). Many of
the current recycling methods for these plastics are mechanical that consist of heating and
shredding the plastics which can negatively impact the integrity of the polymers. This ultimately
leads to down-cycled products like clothes or carpet fibers that will inevitably end up in the landfill.
One of the most widely consumed plastics globally is polyethylene owing to its large use in
packaging.
8
While there are some methods that have been reported to depolymerize/upcycle
polyethylene such as pyrolysis and hydrogenolysis, these methods and the catalysts used do have
drawbacks in that they aren’t selective and are costly.
9
Therefore, it is valuable to find ways to
effectively depolymerize these plastics down into value-added products or chemically recycle
these polymers back into their respective monomers.
5

Dissertation Outline. Chapter 1 of this thesis will explore the use of simple yttrium β-diketiminate
complexes as catalysts for the targeted ROCOP. These complexes feature ligands with 1-2 neutral
donor groups that vary in field strength. The donor group field strength and the number of donor
groups was found to impact the catalyst reactivity during polymerization. Additional
polymerization conditions such as the presence of a cocatalyst and benzyl alcohol were found to
impact the polymerization rates, as well.
Following the pursuit of even simpler catalysts for the targeted polymerization, Chapter 2
will focus on the synthesis of metal-containing ionic liquids (MILs) by heating simple rare earth
metal salts and a phosphonium ionic liquid and using these catalysts for polymerization. As these
MILs were implemented on the targeted ROCOP, the impact of the dry-ness and the synthetic
route of the MIL towards the polymerization rates and control was investigated.
In an effort to further expand the scope of aliphatic polyesters synthesized, Chapter 3
focuses on the cationic copolymerization of THF and cyclic anhydrides through the use of a trityl
borate salt. As there are limited examples of this targeted polymerization with the aforementioned
monomer pairs, this would allow to gain further insight into the polymer properties for the resulting
polymers. Furthermore, the use of a trityl borate salt could provide a readily available, metal-free
catalyst toward the desired polyesters.
While exploring routes towards potentially sustainable plastics, there was also a need to
investigate routes to depolymerize currently existing plastics. Therefore, in Appendix A, Lewis
acidic metals (rare earth metals, AlCl3) were used in an attempt to depolymerize polyethylene and
chemically recycle/upcycle it into value-added chemicals through arene alkylation. The pursuit of
these projects has been able to address both prongs of the proposed solution for the growing
plastics crisis where Chapters 1-3 are focused on the synthesis of potentially easily degradable
6

plastics and Appendix A is directed towards the upcycling of current plastics on the market through
an exploratory project.

References
1. World Economic Forum, Ellen MacArthur Foundation and McKinsey & Company. The
New Plastics Economy—Rethinking the future of plastics
(2016, http://www.ellenmacarthurfoundation.org/publications).
2. The PEW Charitable Trusts. Breaking the Plastic Wave: A Comprehensive Assessment of
Pathways Towards Stopping Ocean Plastic Pollution (2020,
https://www.pewtrusts.org/en/projects/preventing-ocean-plastics).
3. Anisko, J.; Barczewski, M. Adv. Sci. Technol. Res. J., 2021, 15, 9-29.
4. Longo, J. M.; Sanford, M. J.; Coates, G. W. Chem. Rev., 2016, 116, 15167–15197.
5. Abel, B. A.; Lidston, C. A. L.; Coates, G. W. J. Am. Chem. Soc. 2019, 141, 12760-12769.
6. Haxel, G.B.; Hedrick, J.B.; Orris, G.J. Rare Earth Elements-Critical Resources for High
Technology; USGC Fact Sheet 087-02; U.S. Geological Survey, 2002.
7. Tang, T.; Oshimura, M.; Yamada, S.; Takasu, A.; Yang, X.; Cai, Q. J. Polym. Sci., 2012,
50, 3171-3183.
8. Polyethylene demand and capacity worldwide from 2015 to 2022.
https://www.statista.com/statistics/1246675/polyethylene-demand-capacity-forecast-
worldwide/ (accessed 2023-07-02).
9. Kosloski-Oh, S. C.; Wood, Z. A.; Manjarrez, Y.; de los Rios, J. P.; Fieser, M. E. Mater.
Horiz. 2021, 8, 1084–1129.

*A portion of the publication were taken from the publication: Manjarrez, Y.; Cheng-Tan, M. D.
C. L.; Fieser, M. E. Inorg. Chem. 2022, 61, 7088-7094.
7
Chapter 1: Perfectly Alternating Copolymerization of Cyclic Anhydrides and Epoxides with
Yttrium β-Diketiminate Complexes
Introduction*
As rates of accumulation of plastics in landfills and the environment grow, there is an
urgent need for alternatives to petroleum-based, nondegradable plastics.
1-3
Aliphatic polyesters
have shown to be promising as sustainable plastics due to their biodegradability and their
obtainability from renewable monomers.
4-7
The perfectly alternating ring-opening
copolymerization (ROCOP) of cyclic anhydrides and epoxides is an attractive route to aliphatic
polyesters with various thermal properties (Figure 1.1).
8,9
This method allows for more precise
control of molecular weight in comparison to polycondensation of diacids and diols. Additionally,
a wide monomer breadth can
provide polymers with more
versatile properties in comparison
to the ring-opening
polymerization (ROP) of cyclic
esters.
Current efforts to increase catalyst activity have been through elaborate ligand design, with
modified Schiff base ligands being the most common.
10-13
The modulations of the ligands required
to reach competitive rates can be difficult and expensive to synthesize, making it desirable to
identify metals that can be controlled and efficient for the target ROCOP with easy-to-synthesize
ligands. For example, the route to cyclopropenium-substituted ligands
14,15
and ammonium salt
pendant ligands
10,16
require six or more steps to reach just the ligands themselves. The rare earth
metals are ideal to address these challenges, as catalysts with these metals have been modified
with many simple ligands to be efficient for polymerization of a wide range of monomers,

Figure 1.1 (Top) Representative perfectly alternating
copolymerization of epoxides and cyclic anhydrides.
8,9

(Bottom) Particular monomers studied herein.
8

including nonpolar olefins and polar cyclic esters.
17-21
Additionally, the rare earth metal series can
support a wide range of ligand environments and exhibits a subtle decrease in ionic radius, thus
lending enormous tunability to the series.
22
It is worth noting that contrary to their nickname, rare
earth metals are reasonably abundant, with some of the metals in the series being as abundant as
Cu or Ni.
23
Only two reports have identified rare earth metal catalysts as active for the target ROCOP
of epoxides and cyclic anhydrides. Ko and coworkers reported bimetallic yttrium complexes
consisting of two benzotriazole iminophenolate ligands and two ancillary ligands (Figure 1.2a).
24
Aside from achieving the ROP of racemic lactide, they were able to use these complexes for the
copolymerization of cyclohexene oxide (CHO) and phthalic anhydride (PA) without the use of a
cocatalyst, thus demonstrating the feasibility of this type of copolymerization with rare earth
metals. However, this catalyst requires high temperatures (110 °C) and long reaction times (24 h)
to achieve high conversion of CHO and PA, affording polymers with only moderate dispersities.
Additionally, high conversions could not be achieved for 1-hexene oxide under the conditions
used. Arnold and coworkers recently reported bimetallic Ce(IV) complexes with bridging aryl-
oxide ligands that are also active for the target polymerization (Figure 1.2b).
25
These ligands could
be altered to support catalysts that are much faster than the previous yttrium dimer, in which high
conversions could be achieved within 0.5-2.5 hours at 100 °C for several monomers using an
excess of epoxide equivalents (100:800 ratio of anhydride to epoxide). However, polymer
molecular weights were often much lower than expected, bimodal distributions were common, and
homopolymerization of epoxides was observed for all epoxides studied, indicating the presence of
undesirable side reactions. It would be ideal to identify simple, monometallic rare earth metal
9

complexes to directly observe how ligand structure and Lewis acidity impact polymerization rates
and the presence of side reactions.
Herein, we report the use of discrete monometallic yttrium complexes with simple β-
diketiminate (BDI) ligands as active and controlled catalysts for the ROCOP of epoxides and
cyclic anhydrides. These catalysts can polymerize both mono- and di-substituted epoxides, as well
as bi- and tricyclic anhydrides, under mild temperatures. Additionally, the identity and number of
pendant neutral donors on the BDI ligands greatly impacted the rate of polymerization.

Results/Discussion
Catalyst Synthesis and Design. The first goals were to identify monometallic complexes
with inexpensive, easy-to-synthesize ligands, that could catalyze the ROCOP of epoxides and
anhydrides, and to determine which conditions led to the best polymerization rates without the
appearance of side reactions. BDI ligands have been used to support numerous monometallic rare
earth metal catalysts for the ring-opening polymerization of cyclic esters and were selected as the
first supporting ligands of interest for the target copolymerization.
17
Additionally, BDI ligands

Figure 1.2 Known rare earth metal catalysts for the ROCOP of epoxides and cyclic anhydrides.
24,25

10

have been used to support Zn complexes for the perfectly alternating copolymerization of epoxides
and cyclic anhydrides, in which changes to the ligand greatly impacted the rate of
polymerization.
26
As polymerization trends could prefer large or small ionic radii, yttrium was
used first to confirm that monometallic rare earth metal complexes could catalyze the target
ROCOP, as it is a mid-sized rare earth metal that is diamagnetic in the +3 oxidation state.
BDI ligands containing one or
two pendant neutral donors were targeted
as the spectator ligands, and two bulky
alkyl (CH2SiMe3) ligands were targeted
as initiating groups that are known to
stabilize monometallic, coordinatively
unsaturated rare earth metals of a wide range in ionic radii (Figure 1.3). Keeping catalysts
monometallic allows for discrete modifications to address factors that impact reactivity of the
catalyst for the target ROCOP. Moreover, a major goal of this study was to identify how the neutral
donor strength and the number of neutral donors impact the rate of catalysis and the presence of
side reactions.
Complexes 1-3, which have been previously synthesized and used for the polymerization
of cyclic esters, were synthesized by modified procedures from the literature.
26,27,30
Complex 1,
containing a pendant amine group, has been shown to rapidly polymerize ε-caprolactone at room
temperature.
30
With a range of rare earth metals explored, smaller metals were identified to have
slightly faster rates of polymerization. Additionally, the alkyl groups on the pendant amine
impacted the rate of polymerization, indicating how changes to the neutral donor can affect ROP
reactions. Complexes 2 and 3 were identified as highly active catalysts for the ROP of racemic

Figure 1.3 Yttrium BDI complexes studied for the
ROCOP of epoxides and cyclic anhydrides.
11

lactide, with indistinguishable rates within the scope of the reported conditions.
26,27
These
particular ligands can be used to directly analyze the donor strength impacts (comparing 1 and 2)
and the number of donors (comparing 2 and 3).
While the alkyl ligands were used to stabilize the metal complex, it was unclear how readily
they might initiate polymerization. Attempts to exchange the alkyls with alkoxides, which are well
known to initiate the target polymerization, in the presence of stoichiometric BnOH led to
dissociation and protonation of the BDI ligand as an undesirable side reaction.
8, 29
Chlorides have
also been reported as one of the most common initiating ligands for the target ROCOP.
8
Therefore,
1-Cl was synthesized according to literature procedures to directly compare initiating anions.
28

Attempts to synthesize 2-Cl and 3-Cl were conducted, however the solubility of the resulting
complexes made it difficult to remove LiCl, making them unsuitable candidates for catalysis.
Purification of 1-Cl from LiCl remnants also proved to be difficult in some cases. It was anticipated
that the halophilic nature of the rare earth metals could make chlorides weaker initiators than alkyls
for yttrium.
Polymerization of BO/PA. One of the most common pairs of monomers studied in the
literature is propylene oxide (PO) with PA. To stay below the boiling point of the epoxide, while
still heating to reasonable temperatures, 1-butene oxide (BO) was used as a substitute for PO.
Reaction conditions were screened to identify if 1-3 are efficient for the copolymerization of BO
and PA, with the base conditions heated at 60 °C for 16 hours with an epoxide:cyclic
anhydride:catalyst ratio of 500:100:1 (Table 1.1). The alkyl initiators were found to be
nucleophilic enough to initiate the copolymerization of BO and PA without the presence of a
cocatalyst (Table 1.1, entries 1-3). While conversions are low for 2 and 3, 73 % conversion was
observed for 1. This indicated that a stronger neutral donor was more important for the
12

polymerization rate than number of neutral donors in the absence of a cocatalyst. 1-Cl was also
able to initiate the copolymerization of BO and PA with a similar rate to that of 1, achieving a
conversion of 69 % (Table 1.1, entry 4).
Table 1.1 Polymerization of BO/PA with complexes 1-3 and 1-Cl.
a

Entry Catalyst Time
(h)
BnOH [PPN]Cl Conv.
b
% ester
c
M n,theor
(kDa)
d

M n,GPC
(kDa)
e

Đ
e

1 1 16 – – 73 88 8.5 17.7 1.13
2 2 16 – – 13 83 – – –
3 3 16 – – 29 86 – – –
4 1-Cl 16 – – 69 97 7.6 11.7 1.32
5 1 16 2 eq. – 67 94 7.5 22.9 1.13
6 2 16 2 eq. – 19 81 – – –
7 3 16 2 eq. – 24 83 – – –
8 1 16 – 1 eq. >99 98 7.5 11.7 1.21
9 2 16 – 1 eq. 59 98 4.4 7.7 1.08
10 3 16 – 1 eq. >99 99 7.4 8.7 1.05
11 1-Cl 16 – 1 eq. >99 99 7.4 9.6 1.28
12 1 16 2 eq. 1 eq. >99 96 7.5 11.5 1.24
13 2 16 2 eq. 1 eq. 78 98 5.8 5.5 1.07
14 3 16 2 eq. 1 eq. >99 98 7.4 8.0 1.43
15 1-Cl 8 – 1 eq. >99 93 7.7 6.7 1.17
16 1 8 – 1 eq. 96 97 7.1 8.5 1.12
17 3 8 – 1 eq. 61 99 4.4 10.1 1.10
18 1 8 2 eq. 1 eq. >99 97 7.2 7.5 1.28
19 3 8 2 eq. 1 eq. 74 97 5.3 5.2 1.10
20 1 5 2 eq. 1 eq. 85 95 6.5 5.2 1.11
21 1-Cl
f
5 – 1 eq. 72 >99 5.4 6.8 1.08
a
[BO]:[PA]:[catalyst] was 500:100:1.
b
Determined using
1
H NMR spectra of crude reaction mixtures, comparing the
conversion of PA monomer to polymer.
c
Determined using
1
H NMR spectra of purified polymers, comparing the
polyether signal to a polyester signal.
d
Calculated for 2 initiating ligands for conditions without cocatalyst, for 3
initiating ligands if cocatalyst was present.
e
Identified by GPC in THF using a Wyatt DAWN HELEOS II MALS
detector.
f
The catalyst showed remnants of LiCl that were unable to be removed.
Substitution of the alkyl groups with alkoxides in situ, through addition of two equivalents
of BnOH to the polymerization mixture directly for catalysts 1-3, did not lead to observed
protonation of the ligand differing from the exchange reactions done previously (as protonated
ligand was not observed in the
1
H NMR spectra.
29
However, the presumed alkoxide initiators did
13

not improve the rate of polymerization (Table 1.1, entries 5-7). In the case of 1 and 3, conversions
were lower than the cases without BnOH. It should be noted that all conditions studied without the
presence of a cocatalyst (Table 1.1, entries 1-7) showed significant quantities of polyether,
suggesting an undesirable side reaction of epoxide homopolymerization. Additionally, the
polymers isolated from conditions of 1 with or without BnOH (Table 1.1, entries 1 and 5) showed
a substantially higher molecular weight than anticipated for the two initiators. We hypothesize that
this is due to incomplete initiation or catalyst decomposition without the presence of a cocatalyst.
Presence of [PPN]Cl as a cocatalyst, has been shown to increase the rate of polymerization
and molecular weight control for many catalysts in the literature.
8
Indeed, one equivalent of
[PPN]Cl led to a significant increase in polymerization conversion for 1-3, with both 1 and 3
reaching full conversion within 16 hours (Table 1.1, entries 8-10). The conversion for 2 was rather
sluggish at 59 %, indicating the need for a stronger neutral donor or a larger number of neutral
donors for achieving faster rates of polymerization. With [PPN]Cl, 1-Cl also showed full
conversion within 16 hours (Table 1.1, entry 11). Presence of BnOH and [PPN]Cl led to an
increase in conversion for 2, while no difference in rate could be identified for 1 and 3 at the 16
hour reaction time (Table 1.1, entries 12-14).
Shortening the reaction time to 8 hours distinguished 1 and 1-Cl to be the fastest catalysts
in the presence of cocatalyst, reaching 96 % and >99% conversion, respectively, while 3 achieved
just 61 % conversion (Table 1.1, entries 15-17). Addition of BnOH led to an increase in
conversion, in which 1 reached full conversion within the 8 hours, while 3 only reached 74 %
conversion (Table 1.1, entries 18 and 19). Shortening the reaction to 5 hours identified 1 with
BnOH to be the fastest condition of the catalysts studied, in which 85 % conversion was achieved
(Table 1.1, entry 20).
14

For most conditions with the [PPN]Cl cocatalyst, dispersities were quite low (<1.3) and
there was no indication of bimodal distributions. Dispersities were only identified to being above
1.2 in cases where the reaction went to full conversion. To identify if this was an effect of
transesterification after polymerization was complete, the conditions were instead conducted for 5
hours (Table 1.1, entry 20). Within this timeframe, an 85 % conversion and a turnover frequency
of 17 h
−1
were achieved, with a dispersity of 1.11. This indicates that polymerization is very well
controlled until polymerization is complete, in which transesterification reactions start to occur.
This further suggests that the disagreement between the theoretical and experimental molecular
weights in entries 8, 11 and 12 may also be the result of transesterification. The same conditions
with 1-Cl identified that 1 displays faster polymerization activity than 1-Cl, and the fastest activity
of all the complexes in this study towards BO/PA copolymerization (Table 1.1, entry 21). As
mentioned earlier, difficulties were found in 1-Cl purification from LiCl, thus emphasizing 1 as a
far more suitable catalyst than 1-Cl. Additionally, the presence of the [PPN]Cl cocatalyst appears
to slow the presence of epoxide homopolymerization, as almost all conditions tested have > 95 %
polyester content in the resulting polymer. The turnover frequency is not yet competitive with
aluminum and cobalt complexes with designer ligands in the literature, which can achieve turnover
frequencies well over 100 h
−1
.
10-13, 27
However, these studies indicate the promise of rare earth
metal catalysts with simple ligands to be tunable and controlled for the ROCOP of epoxides and
cyclic anhydrides. Adjustments to the complexes, through further changes to the ligand and metal
center, are ongoing.
With two alkyl (or alkoxide) groups and one chloride, three initiators would be expected.
In many cases with [PPN]Cl present, the experimental molecular weights did match well for the
expected three initiators, while others showed a higher molecular weight that fit best with two
15

active initiators. To identify which initiators were active, oligomers of the optimized conditions
with 1, [PPN]Cl, BnOH, BO and PA (Table 1.1, entry 20) were synthesized using just 20
equivalents of anhydride instead of 100 equivalents. MALDI-TOF analysis of the oligomers
identified two main species, with the major initiating group observed to be benzyl alkoxide and
the minor initiating group observed to be chloride (Figure 1.5). As expected, the chloride initiates
through ring-opening of epoxide, while the generated alkoxide appears to initiate through ring-
opening of the cyclic anhydride, as shown previously for an aluminum complex with an
isopropoxide initiator and expected for an alkoxide propagating anion.
28
No evidence of the alkyl-
initiated oligomers was observed under these conditions. This indicates that benzyl alcohol is
indeed exchanging with the alkyls in 1 to form alkoxide initiating groups, in lieu of functioning as
a chain transfer agent.
For control reactions, we aimed to identify how the metal complexes were impacting the
catalysis by testing the activity of the cocatalyst and any byproducts that could be present in
solution (Table 1.3). We first tried [PPN]Cl on its own, which turned out to be faster than with the
fastest catalyst studied, 1, in which 97 % conversion was observed in 5 hours under the same
conditions. However, once polymerization was complete, prevalence for side reactions was high,
in which the dispersity of the polymer increased over time. This suggests that the metal catalysts
can help prevent or slow rapid side reactions. Additionally, the polymer molecular weights were
much lower than expected for the single chloride initiator at short reaction times, which could be
sensitivity to trace moisture in the epoxide. A prior study identified that protonated ligands were
more active than the corresponding metal complexes with the ligands coordinated.
30
A
combination of the protonated ligand of 1 and [PPN]Cl showed similar side reactions to [PPN]Cl
alone. However, the molecular weight of the polymer suggested the ligand as an initiating species
16

for polymerization, since the molecular weight was less than half that for [PPN]Cl alone. Finally,
we tested combinations of the Y(CH2SiMe3)3(THF)2 and [PPN]Cl (with and without benzyl
alcohol) to compare a complex without a supporting ligand (Table 1.3, entries 3-6). This complex
was found to be slower than that of 1, and molecular weights were substantially higher than
expected, indicating incomplete initiation or catalyst decomposition. These controls indicate how
the right ancillary ligand bound to a catalyst can encourage improvements in polymerization rates
and can also help prevent undesirable control reactions.
Other Monomers. As mentioned previously, catalysts for the perfectly alternating
ROCOP often do not work for a wide range of monomers. In particular, disubstituted epoxides and
tricyclic anhydrides often require catalysts with different metal ions and/or different ligand
structures. Therefore, 1, 1-Cl, 2, and 3, were reacted with three new monomer pairs (CHO/PA,
BO/CPMA and CHO/CPMA) to identify the versatility of these complexes in polymerizing a range
of monomers all within the presence of [PPN]Cl as a cocatalyst (Table 1.2, Table 1.4). All four
complexes showed the highest activity towards the polymerization of disubstituted CHO with
bicyclic PA. Complex 1 showed the highest efficiency towards the monomer pair reaching full
conversion within 8 hours when BnOH was present and 73 % conversion without BnOH (Table
1.2, entries 2 and 3). Similar to results found with the BO/PA monomer combination, 1-Cl was a
less efficient catalyst than 1 for CHO/PA achieving only 62 % conversion after 8 hours, although
it showed excellent molecular weight control (Table 1.2, entry 1). Complexes 2 and 3 displayed a
similar trend as that found for BO/PA in the absence of BnOH (51% and 69%, respectively, Table
1.4, entries 1 and 2) and were comparable to one another when BnOH was present (Table 1.2,
entries 4 and 5). Unfortunately, homopolymerization of CHO was more prevalent than with BO
for complex 1, in which all CHO/PA polymers had 69-93 % polyester. While complexes 2 and 3
17

showed slower rates of polymerization for CHO/PA, lack of epoxide homopolymerization
demonstrates better polymerization control. This further emphasizes the need for a library of
complexes for different monomer pairs as there is no one-size-fits-all catalyst.
Both 1 and 1-Cl were much slower catalysts when a tricyclic anhydride was used,
regardless of the epoxide identity. For the combination of BO and CPMA, 1 with BnOH and
[PPN]Cl was the most efficient condition, reaching 34 % conversion in 8 hours (Table 1.2, entry
6). Interestingly, 2 and 3 were the fastest catalysts for the monomer combination of CHO and
CPMA, reaching 34 % and 29 % conversion, respectively, in 8 hours (Table 1.2 entry 7, Table
1.4 entry14). All polymerizations with the CPMA monomer had significant quantities of polyether,
further suggesting these catalysts are not a good fit to open this tricyclic anhydride. These results
identify that the ligand and initiators also impact how well an yttrium catalyst can polymerize
various monomers, as shown previously for aluminum, chromium and cobalt.
31

18

Table 1.2 Polymerization of other monomers with 1-Cl, 1, 2, and 3.
a

Entry Catalyst Epoxide Anhydride BnOH Conv.
b
%ester
c

M n,Theo
(kDa)
d
M n,GPC
(kDa)
e

Đ
e

1 1-Cl CHO PA – 62 71
f
6.0 8.6 1.07
2 1 CHO PA – 73 68
f
7.2 14.2 1.10
3 1 CHO PA 2 eq. >99 84
f
8.9 8.2 1.09
4 2 CHO PA 2 eq. 86 >99 7.2 5.6 1.05
5 3 CHO PA 2 eq. 82 >99 6.7 5.5 1.04
6 1 BO CPMA 2 eq. 34 91
g
– – –
7 2 CHO CPMA 2 eq. 34 – – – –
a
[epoxide]:[anhydride]:[catalyst]:[[PPN]Cl] was 500:100:1:1. Reactions were heated at 60 °C for 8 hours.
b
Determined using
1
H NMR spectra of crude reaction mixtures, comparing the conversion of anhydride monomer to
polymer.
c
Determined using
1
H NMR spectra of purified polymers, comparing the polyether signal to a polyester
signal.
d
Calculated for 3 initiating ligands.
e
Identified by GPC in THF using a Wyatt DAWN HELEOS II MALS
detector.
f
Isolated polymer showed diethyl ether impurities still present in
1
H NMR spectrum, therefore, the %ester
may be higher than those stated.
g
Due to low conversions crude
1
H NMR was used and may show impurities in the
baseline thus making the value lower than what is shown.

Conclusion
We have demonstrated the first example of a monometallic rare earth metal complex to be active
and controlled for the perfectly alternating ring opening copolymerization of epoxides and cyclic
anhydrides. BDI ligands were found to support yttrium alkyl complexes for the polymerization of
BO/PA monomer combinations with limited side reactions, such as homopolymerization of
epoxides and transesterification. Use of cocatalyst and generating alkoxide initiating groups in situ
led to the faster rates of polymerization. Identity and quantity of pendant neutral donors in BDI
ligands were found to have a large impact on the polymerization rate for the polymerization of BO
and PA, with donor strength being more important than the number of donors. High conversions
O
+
O
O O
Catalyst
[PPN]Cl
O
n
O
O
O
60
o
C
PA CPMA
O
O O
O
O O
O
O
BO CHO
19

could also be obtained with the CHO and PA monomers, although homopolymerization of CHO
was more prevalent than with BO. Reactions with a tricyclic anhydride (CPMA) with either
epoxide were sluggish and contained significant amounts of polyether from epoxide
homopolymerization.
The impact of subtle changes to well-known BDI ligands on the ROCOP rates for yttrium
catalysts suggests that rare earth metal catalysts could be tunable to reach faster rates and address
a wide range of monomers. However, more complexes need to be studied to identify the promise
of these metal ions in this polymerization. Efforts to identify how metal size, ligand geometries
and Lewis acidity impact polymerization rate, are currently under way.
Experimental Section
Materials and Methods. All manipulation of air and water sensitive compounds was carried out
under nitrogen in a Vacuum Atmospheres OMNI glovebox or by using standard Schlenk line
technique. Solvents for air sensitive reactions (tetrahydrofuran (THF), toluene, hexanes, pentane)
were purchased from Fisher, sparged under ultrahigh purity (UHP) grade argon and passed through
two columns of drying agent in a JCMeyer solvent purification system and dispensed directly in
the glovebox. Otherwise, solvents (chloroform, THF, toluene, benzene, ethyl acetate,
dichloromethane, diethyl ether (Et2O), hexanes, pentane) and other reagents were used as received.
NMR solvents were purchased from Cambridge Isotope Laboratories and used as received unless
otherwise noted. Deuterated benzene was dried with Na/benzophenone, degassed by three freeze-
pump-thaw cycles, vacuum transferred to an oven and flame-dried Straus flask, and then stored in
a glovebox under a nitrogen atmosphere. Epoxides (1-butene oxide (BO) and CHO) were stirred
over calcium hydride for at least 3 days, degassed by three freeze-pump-thaw cycles, vacuum
transferred to an oven and flame-dried Straus flask, and then stored in a glovebox under a nitrogen
20

atmosphere. Carbic anhydride (CPMA) was crystallized from a 70:30 hexanes:ethyl acetate
mixture and dried under vacuum before being stored in a glovebox under a nitrogen atmosphere.
PA was sublimed under vacuum and stored in the glovebox under a nitrogen atmosphere.
Bis(triphenylphosphoranylidene)ammonium chloride ([PPN]Cl) was recrystallized by layering
Et2O onto a saturated dichloromethane solution. The crystals were ground into a powder and dried
at 100 °C under vacuum and stored in the glovebox under a nitrogen atmosphere. All other
chemicals and reagents were purchased from commercial sources (Aldrich, TCI, Alfa Aesar,
Acros, Fisher, and VWR) and used without further purification.
Instrumentation and measurements.
1
H NMR spectra were obtained using a Varian 400 MHz NMR instrument. Chemical shifts (δ) for
1
H NMR spectra were referenced to residual protons in the deuterated solvent. Gel permeation
chromatography (GPC) analyses were carried out using an Agilent 1260 Infinity II GPC System
equipped with an Agilent 1260 Infinity autosampler and UV-detector, as well as a Wyatt DAWN
HELEOS-II MALS detector and a Wyatt Optilab T-rEX. The Agilent GPC was equipped with an
Agilent PolyPore column (5 micron, 4.6 mmID) which was eluted with THF at 30 °C at 0.3
mL/min and calibrated using monodisperse polystyrene standards. The number average molar
masses and dispersity values were determined from MALS using dn/dc values calculated by 100
% mass recovery method from the refractive index (RI) signal. High resolution mass spectra were
recorded using a Bruker Autoflex Speed MALDI-TOF spectrometer. Samples were prepped as 2
mL stock solutions in THF where 0.2 mL of it was mixed with 0.2 mL of sodium trifluoroacetate
followed by the addition of 25 microliters of a dihydroxybenzoic acid matrix that had been
previously dissolved in THF at 40 mg/mL.

21

Complex Synthesis
Complexes {MeC(NDIPP)CHC(Me)[N(2-OMeC6H4)]}Y(CH2SiMe3)2 (DIPP = 2,6-
i
Pr2C6H3) (2),
32
CH{C(Me)[N(2-OMeC6H4)]}2Y(CH2SiMe3)2 (3),
26,33
and
[MeC(NDIPP)CHC(Me)(NCH2CH2NMe2)]YCl2 (1-Cl)
34
were synthesized and purified
according to literature procedures. In some cases, isolated 1-Cl contained residual LiCl byproduct
from the salt metathesis reaction. Based on
7
Li NMR spectra, a solution of LiCl in THF that
represents a 100 % impurity in 1-Cl shows a significantly more intense Li signal than that of 1-Cl
in a C6D6/THF mixture, suggesting an extremely minor impurity (Figure 1.4).
[MeC(NDIPP)CHC(Me)(NCH2CH2NMe2)]Y(CH2SiMe3)2 (1). Alternative synthesis to
that previously reported.
35
1-Cl (141 mg, 0.288 mmol) was stirred in toluene to which
LiCH2SiMe3 in 1.0 M pentane (0.58 mL, 0.58 mmol) was added dropwise. After stirring for 4
hours, the solution was separated through centrifugation followed by evaporation of the volatiles.
The resulting oil residue was charged with 10 mL hexanes followed by separation through
centrifugation. The solution was concentrated to approximately 3 mL and cooled to −35 °C in the
glovebox freezer to afford the product as a pale yellow-orange microcrystalline solid (81 mg, 0.14
mmol, 48% yield.
29

General polymerization procedure
Appropriate quantities of catalyst, cocatalyst and cyclic anhydride (described in Tables 1.1
and 1.2) were combined in an oven-dried 8 dram vial with a Teflon-lined cap and a stir bar.
Epoxide or a stock solution of benzyl alcohol (BnOH) in epoxide were added to the vial. The vial
was capped, taped closed with electrical tape, and brought out of the glovebox to be heated in a
22

heating block, pre-heated to 60 °C, for 5-16 hours. Once cooled to room temperature, the samples
were dissolved in chloroform. An aliquot was removed for conversion analysis by
1
H NMR
spectroscopy. The remaining chloroform solution was evaporated. The remaining residue was
dissolved in a minimal amount of dichloromethane and was added dropwise to a stirring solution
of pentane. Lower conversion polymer samples were further purified with a Et2O wash. The
resulting polymer was collected via filtration and dried in a vacuum oven at 55 °C for 14 hours.
Final polymers were characterized by
1
H NMR spectroscopy and SEC-MALS.

Supplementary Information

Figure 1.4
7
Li NMR comparison of 1-Cl (bottom) to what would be 100 % LiCl impurity (top).

23

Figure 1.5 MALDI-TOF spectrum of BO/PA oligomers. Reaction conditions:
[1]:[[PPN]Cl]:[BnOH]:[PA]:[BO] was 1:1:2:20:500, 60 °C, 75 minutes.

Table 1.3. Controlled Polymerizations
Entry Catalyst Time
(h)
BnOH [PPN]Cl Conv.
b
% ester
b
M n,theor
(kDa)
c

M n,GPC
(kDa)
f

Đ
f

1 – 5 – 1 eq. 97 >99 21.2
d
16.7 1.04
2 – 16 – 1 eq. >99 >99 22.3
d
24.5 1.6
3 YR3(THF)2 16 – 1 eq. >99 >99 5.6 11.5 1.18
4 YR3(THF)2 5 – 1 eq. 30 >99 – – –
5 YR3(THF)2 16 2 eq. 1 eq. >99 >99 5.6 8.9 1.46
6 YR3(THF)2 5 2 eq. 1 eq. 42 >99 – – –
7 H-1 16 – 1 eq. >99 >99 10.8
e
8.6 1.56
b
Determined using
1
H NMR spectra of crude reaction mixtures, comparing the conversion of anhydride monomer to
polymer.
c
Calculated for 4 initiating ligands.
d
Calculated for 1 initiating ligand.
e
Calculated for 2 initiating ligands
f
Identified by GPC in THF using a Wyatt DAWN HELEOS II MALS detector
24

Table 1.4 Other Monomer Polymerizations.
a

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Entry Catalyst Epoxide Anhydride BnOH Conv.
b

% ester
c
M n,Theo
(kDa)
d
M n,GPC
(kDa)
e

Đ
e

1 2 CHO PA – 51 92 4.4 7.0 1.03
2 3 CHO PA – 69 93 5.8 6.5 1.04
3 1-Cl BO CPMA – 15 94
f
– – –
4 1 BO CPMA – 20 93
f
– – –
5 2 BO CPMA – 21 96
f
– – –
6 3 BO CPMA – 25 96
f
– – –
7 2 BO CPMA 2 eq. 27 97
f
– – –
8 3 BO CPMA 2 eq. 29 99
f
– – –
9 1-Cl CHO CPMA – 21 – – – –
10 1 CHO CPMA – 6 84
f
– – –
11 2 CHO CPMA – 17 – – – –
12 3 CHO CPMA – 25 – – – –
13 1 CHO CPMA 2 eq. 4 73
f
– – –
14 3 CHO CPMA 2 eq. 29 – – – –
a
[epoxide]:[anhydride]:[catalyst]:[[PPN]Cl] was 500:100:1:1. Reactions were heated at 60 °C for 8 hours.
b
Determined using
1
H NMR spectra of crude reaction mixtures, comparing the conversion of anhydride monomer
to polymer.
c
Determined using
1
H NMR spectra of purified polymers, comparing the polyether signal to a polyester
signal.
d
Calculated for 3 initiating ligands.
e
Identified by GPC in THF using a Wyatt DAWN HELEOS II MALS
detector.
f
Determined using
1
H NMR spectra of crude reaction mixtures comparing the conversion of anhydride
monomer to polymer.
25

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M. E. ChemCatChem 2023, e202300319.
26
Chapter 2: Synthesis of Rare Earth Metal Containing Ionic Liquids for Synthesis of
Epoxide/Cyclic Anhydride Copolymers
Introduction*
Development of inexpensive and efficient methods for the synthesis of diverse degradable
polymers is a critical challenge to commercialize degradable polymer alternatives to combat
growing plastic pollution. The perfectly
alternating ring-opening copolymerization
(ROCOP) of epoxides and cyclic anhydrides
has been a promising method for the production of a wide array of aliphatic polyesters with a range
of thermal properties (Figure 2.1).
1
However, future directions need to consider cost and synthesis
of catalysts while working to enhance polymerization activity and control (i.e. dispersity and
selectivity).
1, 2
Recently, there has been an acceleration of studies with simple, inexpensive metal
salts as active and controlled catalysts for the target polymerization.
In particular, metal carboxylate salts have been identified as active catalysts for the targeted
polymerization without the need of a cocatalyst. Of these salts, cesium pivalate has been shown to
be an active catalyst and has been used to make a wide range of polyesters through the target
ROCOP (Figure 2.2A).
3-8
While Cs-pivalate has been shown to exhibit the highest activity of a
series of alkali metal carboxylates tested for copolymerization of various monomer pairs, these
polymerizations were met with some challenges such as long reaction times, the need for an
initiator along with the catalyst, and air-free conditions. Additionally, extremely high monomer
loadings are needed to acquire high molecular weight polymers (>30 kDa). We have previously
identified yttrium based catalysts are active for ROCOP of epoxides and cyclic anhydrides, in
which the environment around the metal center can be tuned to impact the rate and control of
polymerization.
9,10
Simple yttrium-based chloride salts, such as YCl3(THF)3.5 and YCl3•6H2O,

Figure 2.1 Proposed ROCOP.
O
+
O
O O
Catalyst
Cocatalyst
O
n
O
O
O
27

along with a cocatalyst, were highly active for the perfectly alternating copolymerization of ten
epoxide/cyclic anhydride monomer pairs, while maintaining good molecular weight control and
suppressed side reactions (Figure 2.2B).
10
While both yttrium catalysts displayed record turnover
frequencies (TOFs) for various monomer pairs, there were some challenges for each catalyst. The
YCl3•6H2O catalyst displayed high conversions,
however, the molecular weight of the samples fell
below the expected theoretical molecular weight,
likely from the water acting as a chain transfer agent.
While YCl3(THF)3.5 was synthesized in an inert
atmosphere, preventing water content in the reaction,
the rates of polymerization for most monomer pairs
suffered in comparison to the hydrate catalyst.
Additionally, the synthesis of the air-free
YCl3(THF)3.5 adduct is low-yielding, as the starting
YCl3 salt is only slightly soluble in THF. Alongside
these challenges, both catalysts required the use of an
expensive cocatalyst, bis(triphenylphosphoranylidene)ammonium chloride ([PPN]Cl), for the
optimized conditions for the targeted polymerization. Despite these challenges, these results show
promise for design of future catalysts with rare earth metals (which includes scandium, yttrium
and the lanthanide series).
To improve the use of these simple yttrium salts (and other lanthanide salts) as catalysts
for the target ROCOP, it is important to identify easy synthetic methods for highly soluble metal
salt alternatives that could maintain fast and controlled rates of polymerization, while avoiding
Figure 2.2 Catalyst systems for targeted
epoxide and cyclic anhydride ROCOP.
28

coordination of water to prevent lowering the polymer molecular weights. The introduction of an
ionic liquid (IL) into the catalyst system could be expected to minimize the presence of water in
the catalyst owing to the fact that the coordination sphere of the metal would be saturated with the
counter anions from the ionic liquid leading to a visual loss of water, in some cases.
12
Sesto and
coworkers demonstrated that mixing an ionic liquid and gadolinium hexahydrate salt through a
solvent-assisted method led to a metal-containing ionic liquid (MIL) with minimal water present.
When used with rare earth metals, ionic liquids have been employed in various chemical
transformations (e.g. Friedel-Crafts alkylation, ring-opening of cyclic esters
13, 14
). However, the
ionic liquid in these reports served as a method for catalyst recovery and as a solvent. Some studies
did find that the use of ionic liquid also improved some of the chemical transformations in
comparison to when no ionic liquid was present.
14,15
These prior studies demonstrate the catalytic
value of ionic liquids for rare earth metal catalysis, and support the hypothesis that MILs can limit
water access around the metal ion.
If designed carefully, the IL that is paired with the rare earth metal salt could serve three
roles; a solubilizing agent for the metal salt, a coordinating anion to prevent water coordination
and a cocatalyst. Herein, we report the use of a phosphonium-based ionic liquid, mixed with
YCl3•6H2O, to synthesize MIL catalysts that can maintain or increase high polymerization
activities and improved ability to achieve high molecular weight polymers than the previous
YCl3•6H2O/[PPN]Cl cocatalyst system (Figure 2.2C). MIL synthetic conditions, drying and
storage all made a difference on the rate and molecular weight control of ROCOP. To our
knowledge, this is the first example of a rare earth MIL being used as a cooperative catalyst, in
which both ionic liquid and metal salt are important for the reactivity, for any polymerization.

29

Results/Discussion
First, it was important to identify whether ionic liquids could form MILs with YCl3•6H2O,
and if these MILs were then active for ROCOP of epoxides and cyclic anhydrides. As previously
identified, YCl3(THF)3.5 shows very slow activity and poor control for the target ROCOP without
the presence of a cocatalyst, so it is important to identify if the ionic liquid can serve as a cocatalyst.
At a catalytic scale, YCl3•6H2O was mixed with varying ILs at different ratios (1-6 equivalent IL
to metal salt) to identify what IL could convert the metal salt to a low melting point MIL. If the
mixture of YCl3•6H2O and IL showed successful formation of MIL at 60 °C, it was then used to
copolymerize six different monomer pairs.
ROCOP of epoxides and cyclic anhydrides offers the advantage of having a wide monomer
scope than the ring-opening polymerization of cyclic esters. However, catalysts in the literature
are not often universally active or controlled for the ROCOP of all the monomers commercially
available. Therefore, initial screening needs a variety of monomers to identify the versatility of the
catalyst system. For epoxides, 1-butene oxide (BO) was used as a common monosubstituted
epoxide with a higher boiling point than the
more frequently used propylene oxide, while
cyclohexene oxide (CHO) is used as the most
common disubstituted epoxide (Figure 2.3). Glutaric anhydride (GA), phthalic anhydride (PA),
and carbic anhydride (CPMA) were selected as representative monocyclic, bicyclic and tricyclic
anhydrides, respectively.

Figure 2.3 Monomer scope for ROCOP
30

The initial screening identified phosphonium-based ILs (trihexyltetradecylphosphonium
chloride ([H3DP]Cl), tributyltetradecylphosphonium chloride ([B3DP]Cl)) to be the most
promising candidates for the targeted copolymerization (see SI for further details). YCl 3•6H2O
was mixed with one equivalent at a time of each phosphonium-based IL at 95 °C until the mixture
reached a homogeneous liquid. In the case of [B3DP]Cl, the IL is a solid at room temperature and
the mixture at 95 °C proved to be extremely viscous and the reaction required five equivalents of
IL to make a homogeneous mixture. With [H3DP]Cl, which is a viscous liquid at room
temperature, the reaction only required four equivalents to reach a homogeneous liquid. Since
[H3DP]Cl offers the advantage of already
being a liquid and added atom economy
through needing less equivalents to reach
a homogenous liquid, MILs with
[H3DP]Cl were further pursued for this study.
Since the screening reactions required very small weights of metal salt and IL, it was
important to scale up the synthesis of the MIL catalyst for more consistent comparisons of
reactions. First, four rare earth metal salts
(LnCl3•6H2O, Ln= Gd, Nd, Ho, and Y)
were mixed with four equivalents of
[H3DP]Cl neat at 95°C and stirred until
the formation of a homogeneous melt that
was liquid at room temperature (Figure
2.4). The different rare earth metals allow
for the tunability in the size and Lewis

Figure 2.4 General synthesis of MILs where Ln= Y,
Gd, Ho, or Nd.
Figure 2.5 Polymerization with Ln-WET MILs.
[MIL]:[anhydride]:[epoxide] (1:200:1000) at 110 °C.
For full description of the reactions, see experimental.
31

acidity of the metal in the MILs, potentially impacting the catalyst reactivity during
polymerization. Additionally, three paramagnetic lanthanides were selected due to the possible
catalyst recovery through use of a magnet, as has been seen for iron-containing MILs.
12
Since no
efforts were made to drive off water, these MILs were defined as Ln-WET for clarity.
The polymerization reactions were done at a scale of [1]:[200]:[1000]:
[MIL]:[anhydride]:[epoxide] over the time frames designated for each monomer pair (Figure 2.5).
The reactivity of each MIL for all six monomer pairs was compared using single point studies,
which were converted to turnover frequency with respect to the rare earth metal ion (h
–1
, TOF).
While single point studies are not a perfect comparison of reaction rates, the conditions for each
monomer pair were kept constant to allow for the best comparison between metal ions.
Interestingly, the MILs with different metals showed similar TOFs for the same monomer pair, in
most cases all four metals studied were within error of each other. We hypothesize that the anions
from the IL saturate the coordination sphere for all the rare earth metals used, limiting the impact
of Lewis acidity or metal ion size on polymerization activity. For all MILs, the rates of
polymerization were fastest for monomer pairs with the PA anhydride, with polymerization rates
for monomer pairs with GA and CPMA being much slower. These trends were similar to those
observed in the previously reported YCl3•6H2O/[PPN]Cl and YCl3(THF)3.5/[PPN]Cl systems
showing these MILs as promising catalysts for the targeted polymerization.
4
Owing to yttrium’s
similar reactivity to the other metals, its lower cost, and diamagnetic properties, yttrium MILs were
pursued for the remaining studies presented herein.
Initial characterization of the molecular weight of the polymers produced for Figure 2.5
revealed smaller molecular weights than would be expected for seven chloride initiators
(Supplementary Info, Table 2.2). Through Attenuated Total Reflectance-Fourier Transform
32

Infrared Spectroscopy (ATR-FTIR) studies of the yttrium MIL, water was revealed to be present
in Y-WET. Thermal gravimetric analysis (TGA) was then used to characterize a rough estimate
of how much water was remaining in the MIL (Figures 2.9-2.12). By ramping up the temperature
to 100°C and maintaining this temperature for 5 hours revealed a drop in mass in the sample that
correlates to approximately six equivalents of water for every yttrium center. This suggested that
this water was likely acting as a chain transfer agent, leading to more initiators than expected and
therefore, lower molecular weight than theoretically calculated. The resultant small polymer
molecular weights suggested water content must be minimized in the catalytic system to achieve
better molecular weight control (Figure 2.6).
The first aim to minimize water in the MILs was to dry Y-WET by heating the MIL at 50
°C for 36 hours under dynamic vacuum (10
–3
Torr). Upon drying, this MIL, labelled as Y-DRY,
was transferred to and stored in a glovebox. When quickly removed from the glovebox, Y-DRY
showed no observable water present by ATR-FTIR. Using the TGA method described above, only
2.5 equivalents of water were identified
for Y-DRY, suggesting success in
removing water from Y-WET.
Alternatively, a prior report combined
GdCl3•6H2O with three equivalents of
[H3DP]Cl through a DCM/water
solvent assisted synthesis method,
which led to a MIL with no observable
water by FTIR.
12

Figure 2.6 Synthesis of yttrium MILs.
33

It was rationalized that while the neat method is more atom economical, the solvent-
assisted method may ensure a more homogeneous mixture, while making it easier to remove water.
This solvent-assisted method was used for YCl3•6H2O, which was mixed with four equivalents of
[H3DP]Cl at room temperature in DCM and water, with the resulting MIL being labelled Y-
DCMWET (as it was only dried for a couple hours on the Schlenk line to evaporate any remaining
solvent). While water was also not observed in the ATR-FTIR spectrum of Y-DCMWET, the
TGA method described above identified 2.5 equivalents of water per yttrium center. This
suggested that the solvent-assisted method removed as much water as the neat synthesis after days
of drying Y-WET on the Schlenk line. Drying Y-DCMWET on the Schlenk line, analogous to
that for Y-WET, only led to minor water removal for what is labelled as Y-DCMDRY, with TGA
studies finding just 2 equivalents of water for each yttrium ion. Synthesis of a second batch of Y-
WET and Y-DCMWET revealed lower water content than the first synthesis (2.8 and 1.4
equivalents of water, respectively), which indicates variability batch to batch. Therefore, single
batches were used to compare reactivity, discussed below. The first question to address was how
the synthesis (neat or solvent-assisted), as well as the dryness (synthesized as is or dried further),
impact the rate of ROCOP for the targeted monomer pairs.
The “wet” and “dry” MILs were used for the targeted polymerization with the same six
monomer pairs used above. All polymerizations were run at 110°C under neat conditions for 10-
30 minutes, where the epoxide served as the solvent. The polymerizations carried out using the
“wet” catalyst used monomers that were dried and kept under atmospheric conditions, while those
that used “dry” catalysts used monomers that were dried, purified, and stored under a nitrogen
glovebox atmosphere. The monomer conversion for the polymerizations was quantified by taking
34

an aliquot from the reaction and analyzed via
1
H NMR spectroscopy. The polymerizations were
done in triplicate unless otherwise stated
with the turnover frequencies seen in
Figure 2.7.
When comparing the TOFs, the
“wet” catalysts resulted in higher
reactivity than their representative “dry”
catalysts across all monomer pairs where
the highest TOFs were observed for monomer pairs using PA. This could be attributed to the
residual water driving reaction rates, as indicated in prior work comparing YCl 3•6H2O and
YCl3(THF)3.5. These rapid turnover frequencies for both Y-WET and Y-DCMWET are all faster
than the YCl3•6H2O/[PPN]Cl system, now representing the fastest reported TOFs for BO/PA,
BO/GA and CHO/GA monomer pairs. Interestingly, the catalysts made through the solvent
assisted methods (Y-DCMWET and Y-DCMDRY) showed similar reactivity to the comparative
neat MILs when considering reactions conducted in butylene oxide (BO). However, reactions
conducted in cyclohexene oxide (CHO) showed a dramatic difference in reactivity between the
neat catalysts and the solvent assisted catalysts, with the solvent assisted MILs showing slower
rates of polymerization. These trends do not track with water content found from TGA studies,
suggesting other reasons for this reactivity difference. A plausible hypothesis is that the
coordination environment of Y is not the same between the neat and solvent-assisted method,
which could alter how well the epoxide can be activated for ring-opening. As shown in previous
literature, the speciation of rare earth metal-containing ILs can be dependent on stoichiometry or
reagents and reaction conditions.
16,17
All MILs studied showed suppression of epoxide
Figure 2.7 Polymerizations for Y-WET, Y-DRY and
the DCM analogues. [MIL]:[anhydride]:[epoxide]
(1:200:1000) at 110 °C.
35

homopolymerization (<6 % for all conditions, see SI for details). Contrary to the
YCl3•6H2O/[PPN]Cl system, epimerization of CPMA-based polymers was observed prior to
reaction completion.
To further understand the impact of the rare earth metal in the MILs, control experiments
were conducted where the YCl3•6H2O and 4 equivalents of [H3DP]Cl were weighed and added
separately into the reaction vial as cocatalysts rather than together as an MIL (Figure 2.7). The
series of these controls was found to have TOFs comparable to the “wet” catalysts where in the
case of CHO/PA, the control displayed even faster rates than Y-WET. These results suggest that
the MIL does show similar reactivity to cases where the metal salt and phosphonium salt are used
as cocatalysts. An additional control study was performed where only 4 equivalents of [H3DP]Cl
was used as the catalyst for a set of polymerizations (Figure 2.13). The TOFs for those reactions
only using [H3DP]Cl as the catalyst showed slower rates than the control of adding the metal salt
and IL separately, thus demonstrating that the presence of the rare earth metal does aid the rate of
polymerization.
Since reactions with the wet catalysts used dried monomers stored outside the glovebox,
while reactions with dry catalysts used dried monomers stored inside the glovebox, it was
important to identify if the catalyst, reaction conditions or monomer stock were responsible for the
dramatic differences in rate between the wet and dry catalysts. Initial studies were performed with
Y-DRY and Y-DCMDRY for all six monomer pairs, using the monomers that were dried and
stored outside the glovebox (Figure 2.13). In all cases, these reactions showed faster TOFs than
the analogous dry conditions, some of which reached comparable rates to those of the “wet”
catalysts with the same monomers. These results identified that monomer and reaction conditions
did impact the rate of polymerization.
36

Since the BO/PA monomer
pair showed the biggest difference in
reactivity between catalysts, this pair
was used for more in-depth studies
(Figure 2.8). When using monomers
dried and stored outside the glovebox,
both Y-DRY and Y-DCMDRY
showed comparable rates to reactions
previously conducted with Y-WET
and Y-DCMWET, respectively. In the
case where polymerizations were
prepared in the glovebox with “dry”
catalysts and monomers stored in the
glovebox, a decrease in reactivity for the “dry” catalysts is observed in comparison to the reactions
being done with wet monomers with the “dry” catalysts. The hypothesis is that presence of water
in the monomers could be driving the rate to compare more with the wet catalysts than the dry
catalysts. Therefore, polymerizations were run with “dry” catalysts using monomers dried and
stored in the glovebox with the reaction mixture exposed to air for 5 minutes before the
polymerizations were run. In these conditions, a slightly faster rate was observed for Y-
DCMDRY, while Y-DRY showed comparable results to the reaction with no air exposure,
indicating the atmospheric conditions were not greatly impacting the catalyst activity. An
additional set of reactions performed was the use of monomers stored and dried in the glovebox
with the “wet” catalysts as well. Results obtained from these control experiments showed that the
Figure 2.8 Control reactions run with BO/PA under
varying monomer conditions with a) Y-DCMWET, Y-
DCMDRY analogues and b) Y-WET, Y-DRY
analogues. [MIL]:[PA]:[BO] (1:200:1000) at 110 °C.
37

dry-ness of the monomers led to slower rates for the polymerizations. For instance, both Y-WET
and Y-DCMWET showed a decrease in rate when reacted with the dry monomers from the
glovebox. This demonstrates that the water found in the monomers is contributing to the increase
in rates for these reactions. The aforementioned observations further support that the presence of
water contributes to the higher conversions observed, which is hypothesized to occur through
hydrogen bonding throughout the rate determining step of epoxide ring-opening.
Since water has been shown to act as a chain transfer agent, it is important to improve the
catalyst systems that have limited water exposure. Although the “dry” MIL catalysts showed
slower polymerization than their “wet” counterparts, these MILs still showed significant
improvements in the turnover frequency for yttrium simple salts. For example, the turnover
frequencies for all 6 monomer pairs for Y-DRY and Y-DCMDRY were faster or comparable to
those of the YCl3•6H2O/[PPN]Cl system. The difference in rate is likely due to the added
equivalents of [H3DP]Cl (4 equiv) vs. 1 equiv [PPN]Cl or due to the charge attraction between the
cations and anion of the MIL, which keeps the catalyst more closely associated than with the parent
system. These much-improved rates with less presence of water could lead to rapid synthesis of
high molecular weight polymers, as originally hypothesized.
To gain further insight into the impact of the dry-ness of the catalysts on the resulting
polymer, the polymer samples were analyzed through gel permeation chromatography (GPC). The
BO/CPMA monomer pair was selected as the ideal candidate to study any trends observed in the
data as high molecular weight polymers for the BO/CPMA monomer pair were synthesized with
the previously reported yttrium salts.
4
In most cases, reactions run with monomers stored outside
the glovebox showed low molecular weights with the standard conditions of 1:200:1000 ratio of
[MIL]:[anhydride]:[epoxide]. In many cases, the molecular weight was between what would be
38

expected for seven chloride initiators with and without the added water determined by TGA studies
acting as initiators as well (Table 2.3). This suggested that not all initiators present in solution are
initiating polymer chains. Interestingly, several reactions with “dry” catalysts showed much higher
molecular weights than expected. Since these MILs had the least amount of water, it could be
possible that aggregation of the metal ions creates bridging chloride interactions that are inactive
for polymerization.
To lower the impact of water in the monomers, reactions were scaled up and used dried
monomers stored in the glovebox (Table 2.1). The polymer obtained with Y-WET, the catalyst
with the most water present, was close to what is expected for 7 chloride initiators, achieving a
13.4 kDa polymer in just 75 minutes (Table 2.1, entry 1). In comparison, when the monomers
stored outside the glovebox are used, Y-WET only formed polymers with a molecular weight of
4.2 kDa (Table 2.1, entry 2). Alternatively, Y-DCMWET, which showed much less water in the
MIL than Y-WET, was able to achieve a high molecular weight polymer of 27.5 kDa with a
reasonably controlled dispersity of 1.31 within the same 75 minutes (Table 2.1, entry 3a). A
duplicate reaction produced a polymer with even higher molecular weight polymer of 34.3 kDa
with slightly higher dispersity (1.55) (Table 2.1, entry 3b). Analogous reactions with Y-DRY and
Y-DCMDRY, conducted under a nitrogen atmosphere, showed similar high molecular weight
polymers >22 kDa, albeit after longer reaction times of 110 minutes (Table 2.1, entries 4 and 5).
These results suggest that not all chlorides are initiating polymerization for MILs that contain less
water. As described above, we hypothesize that lack of water could lead to bridging chloride
interactions, which could alter how easily the anions can initiate polymerization. Since the MIL is
a dynamic liquid catalyst, the metal anion could aggregate differently depending on conditions,
explaining the variability in molecular weight and dispersity between duplicate reactions. Since
39

the dispersity remains reasonably low for most cases, it is expected that these chlorides are not
being initiated slowly over time.
When reactions with Y-DRY and Y-DCMDRY are exposed to air at this larger scale, even
higher molecular weight polymers are obtained in 72 or 60 minutes, respectively (Table 2.1,
entries 6 and 7). In the case of Y-DRY, duplicate reactions lead to polymers of approximately 80
kDa, with moderate dispersities near 1.5. At just over 50 % conversion, these polymers are more
than ten times larger than expected for the initiators present in Y-DRY. Shockingly, the Y-
DCMDRY MIL with the least amount of water showed an even higher molecular weight polymer
of 185 kDa in just one hour with a dispersity of 1.43. It is clear that the exposure to air has a
significant impact on the initiation of the polymer chains, even if it doesn’t impact the rate of
polymerization.
Table 2.1 Large scale reactions with BO and CPMA to observe molecular weight control.
Entry Catalyst Time
(mins)
%
Conv.
b

%
Ester
c

Epim %
d
Polymer
Growth
Rate
(kDa/h)
Mn,theor
(kDa)

(7 init)
Mn,theo
r
(kDa)
e

Mn,exp
(kDa)
f

Đ
f

1 Y-WET 75 73 >99 14 11 9.98 5.40 13.4 1.44
2
g
Y-WET 70 92 99 12 4 12.46 6.71 4.2 1.22
3a Y-DCMWET 74 68 >99 21 22 9.25 6.83 27.5 1.31
3b Y-DCMWET 74 69 >99 18 28 9.32 6.89 34.3 1.55
4
h
Y-DRY 110 72 >99 16 23 9.80 7.24 42.9 1.16
4b
h
Y-DRY 110 68 >99 15 13 9.28 6.86 24.2 1.25
5a
h
Y-DCMDRY 110 76 >99 18 17 10.27 8.00 31.7 1.36
5b
h
Y-DCMDRY 110 71 >99 17 12 9.69 7.55 22.6 1.51
6a Y-DRY 72 53 99 15 68 7.18 - 81.9 1.56
6b Y-DRY 72 54 >99 14 69 7.39 - 83.4 1.43
7 Y-DCMDRY 60 44 >99 16 185 6.02 - 185.4 1.43
8
g,i
[H3DP]Cl 70 81 99 15 4 19.43
j
- 5.0 1.33
9
i
[H3DP]Cl 70 53 >99 17 66 12.84
j
- 76.4 1.74
a
Reaction conditions were a 1:400:2000 ratio for [catalyst]:[CPMA]:[BO] with a scale of 2 mL BO, reacted at 110
C with exposure to air. Monomers that were dried, purified and stored in the glovebox were used.
b
Determined
using
1
H NMR spectra of crude reaction mixtures, comparing the conversion of anhydride monomer to polymer.
c
Determined using
1
H NMR spectra of purified polymers.
d
%Epim = {2 x A 2.7ppm/ (A 6.0-6.5ppm)}x100.
e
Initiators
calculated from TGA for: Y-WET:13, Y-DCMWET & Y-DRY: 9.5, Y-DCMDRY: 9.
f
Identified by GPC in THF
using a Wyatt DAWN HELEOS II MALS detector.
g
Reactions used monomers dried and stored outside the
glovebox.
h
Reaction conducted in N 2.
i
Catalyst loading was 4 mol% instead of 1mol%.
j
Theoretical M n was
calculated for 4 initiators instead of 7.
40

In order to compare the overall impact of the MIL, in comparison to the [H3DP]Cl IL itself,
control reactions were done at the same scale with four equivalents of [H3DP]Cl. With monomers
stored outside the glovebox, the resulting polymer is much lower in molecular weight than that
expected for 4 initiators, as would be expected with water present (Table 2.1, entry 8). However,
when monomers are used from inside the glovebox, a 76 kDa polymer is obtained within 70
minutes (Table 2.1, entry 9). These results are similar to that of Y-DRY when exposed to air
(Table 2.1, entry 6), however the higher dispersity of 1.74 reveals the value of the metal ion in the
MIL. It is surprising that the IL by itself also produced a polymer with a higher molecular weight
than expected. It is unclear if this is due to anions being inaccessible for initiation due to the IL
network, similar to that hypothesized for the MILs.
Although these results suggest that it is difficult to predict the polymer molecular weight,
based on the expected initiators in the MIL catalyst, they suggest that higher molecular weight
polymers can be achieved with less monomer excess and in shorter times than prior literature.
Described as polymer growth rate in Table 2.1, optimized catalyst systems show a range of 11-
186 kDa/h. The MILs demonstrate a drastic improvement in obtaining high molecular weight
polymers, in comparison to the (YCl3•6H2O or YCl3 (THF)3.5) /[PPN]Cl catalyst systems, where
30+ kDa polymers could only be achieved after 8 hours under the same reaction conditions
(temperature, monomer ratio and catalyst loading), which is approximately 5 kDa/h. With this
significant increase in reaction rate and polymer growth rate, these catalysts maintain polyester
selectivity with no evidence of epoxide homopolymerization. Even for other monomer pairs,
catalysts in the literature are not able to achieve high molecular weight polymers without extreme
monomer excess and their polymer growth rates are commonly <10 kDa/h.
18-22

41

Conclusion
Combination of phosphonium ionic liquids and yttrium trichloride salts are reported to
form metal-ionic liquids (MIL) that are active and controlled for the ring-opening polymerization
of epoxides and cyclic anhydrides. Synthetic methods were found to impact the amount of water
in the resulting MIL, with solvent-assisted methods more efficiently eliminating water than mixing
the two components via neat conditions. Analogous to prior studies, dryness of both the catalyst
and monomers proved to impact the rate and growth of polymer molecular weight of these
polymerizations in conflicting ways. Presence of water increases the rate of propagation, while
also acting as a chain transfer agent, lowering the polymer molecular weight from expected.
Nevertheless, this study represents the first example of a rare earth metal MIL conducting ring-
opening polymerization. Additionally, this system can achieve high molecular weight polymers
much faster than the original (YCl3•6H2O or YCl3 (THF)3.5/[PPN]Cl catalyst systems and other
catalysts in the literature. Efforts are currently underway to better control the environment around
the metal center in MILs to tailor the number of initiating and spectator anions for enhanced
molecular weight control.

Experimental Section
Instrumentation and measurements
1
H NMR spectra were obtained using a Varian 400-MR 2-Channel NMR. Chemical shifts (δ) for
1
H NMR spectra were referenced to residual protons in the deuterated solvent. Gel permeation
chromatography (GPC) analyses were carried out using an Agilent 1260 Infinity II HPLC System
equipped with an Agilent 1260 Infinity autosampler and UV-detector, as well as a Wyatt DAWN
HELEOS-II MALS detector and a Wyatt Optilab T-rEX refractice index detector. The Agilent
42

GPC was equipped with two Agilent PolyPore columns (5 micron, 4.6 mmID) which was eluted
with THF at 30 °C at 0.3 mL/min and calibrated using monodisperse polystyrene standards. The
number average molar masses and dispersity values were determined from MALS (multi-angle
light scattering) using dn/dc values calculated by 100 % mass recovery method from the refractive
index (dRI) signal. Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-
FTIR) analyses were carried out using an Agilent Cary 630 FTIR instrument. Thermal gravimetric
analysis (TGA) data were collected using a Mettler-Toledo STARe System TGA/DSC3+ equipped
with STARe software, a TA SDTA Sensor LF, XP1 Balance, and a sample robot. Sample weight
of MIL was between 10-22 mg and sealed in a 40 µL aluminum crucible fitted with a pierceable
lid. General method involved heating from 25°C to 100°C, holding it at 100°C for 5 hours, then
heating to 500°C at a scan rate of 10°C/min under a constant flow of N2 (15 mL/min).
Materials and Methods. All manipulation of air and water sensitive compounds was carried out
under nitrogen in a Vacuum Atmospheres OMNI glovebox or by using standard Schlenk line
technique. Solvents (chloroform, THF, toluene, benzene, ethyl acetate, dichloromethane, diethyl
ether (Et2O), hexanes, pentane) and other reagents were used as received. NMR solvents were
purchased from Cambridge Isotope Laboratories and used as received unless otherwise noted.
Epoxides (1-butene oxide (BO) and CHO) for air sensitive reactions were stirred over calcium
hydride (CaH2) for at least 3 days, degassed by three freeze-pump-thaw cycles, vacuum transferred
to an oven and flame-dried Straus flask, and then stored in a glovebox under a nitrogen atmosphere.
Epoxides for polymerizations under ambient conditions were stirred over CaH2 for 24 hours,
filtered through a fine-fritted filter and stored in a Teflon-sealed flask under air. Carbic anhydride
(CPMA) was crystallized from a 70:30 hexanes:ethyl acetate mixture followed by solvent
evaporation which was then used for polymerizations under ambient conditions. For air sensitive
43

polymerizations, the recrystallized CPMA was dried under vacuum before being stored in a
glovebox under a nitrogen atmosphere. Phthalic anhydride (PA) was sublimed under vacuum and
stored in the glovebox under a nitrogen atmosphere or under ambient conditions depending on the
conditions necessary for polymerizations. All other chemicals and reagents were purchased from
commercial sources (io-lo-tec, Aldrich, TCI, Alfa Aesar, Acros, Fisher, and VWR) and used
without further purification.
MIL General Synthesis
Neat. In a 250 mL round bottom flask, 0.800 g (2.64 mmol) of YCl3•6H2O was stirred with 5.48
g (10.55 mmol) [H3DP]Cl at 95°C until mixture became a homogenous melt (~48 hours). The
other metal MILs (Ho, Gd, Nd) were prepared using analogous methods.
Solvent-assisted MIL. Prepared according to literature procedure.
12
In a 250 mL round bottom
flask, 3.48 g (11.48 mmol) YCl3•6H2O and 23.85 g (45.92 mmol) [H3DP]Cl was stirred with ~100
mL dichloromethane and ~50 mL DI H2O. The mixture was stirred overnight (~14 hours) at room
temperature to form two layers. The organic MIL layer was collected and dried in vacuo for
approximately 2 hours to evaporate any remaining dichloromethane.
Drying MILs. All dry analogues of the MILs were dried by heating to 50°C while having vacuum
pulled via Schlenk line for 36 hours. MILs were then transferred and stored in a glovebox under
nitrogen atmosphere.
General polymerization procedure. A ratio of 1:200:1000 for [catalyst]:[anhydride]:[epoxide]
(unless otherwise stated) were combined in an 8 dram vial with a Teflon-lined cap and a stir bar.
Epoxide was added to the vial that was then capped, taped closed with electrical tape, and heated
in a heating block, pre-heated to 110 °C, for 10-30 minutes. The samples were then quenched with
44

approximately 1 mL chloroform followed by the removal of an aliquot for conversion analysis by
1
H NMR spectroscopy. The remaining chloroform solution was charged with ~10 mL of pentane
to form a polymer precipitate. Lower conversion polymer samples were further purified with a
Et2O wash. The resulting polymer was collected via decantation and dried in a vacuum oven at 65
°C for BO polymers and 110 °C for CHO polymers for 14 hours. Final polymers were
characterized by
1
H NMR spectroscopy and SEC-MALS.

45

TGA Spectra
As described in methods, TGA studies were used to identify the amount of water remaining in the
MIL catalysts. To do this, we heated the sample to 100 °C and left it for 5 hours before continuing
to ramp up the temperature. In all cases, decomposition was not seen before 350 °C. However, a
drop in mass was identified at 100 degrees after the 5 hours. This mass loss was then used to
calculate the approximate amount of water remaining in the MIL.

Figure 2.9 TGA spectrum of Y-WET MIL heated from 25°C–500°C. Sample was heated from
25°C–100°C, held at 100°C for 5 h, then heated to the final temperature of 500°C. Dip in spectrum
accredited to loss in water from catalyst at 100 °C.

46

Figure 2.10 TGA spectrum of Y-DRY MIL heated from 25°C–500°C. Sample was heated from
25°C–100°C, held at 100°C for 5 h, then heated to the final temperature of 500°C. Dip in spectrum
accredited to loss in water from catalyst at 100 °C. Sample prepared in a glovebox with an N2
atmosphere.

Figure 2.11 TGA spectrum of Y-DCMWET MIL heated from 25°C–500°C. Sample was heated
from 25°C–100°C, held at 100°C for 5 h, then heated to the final temperature of 500°C. Dip in
spectrum accredited to loss in water from catalyst at 100 °C.

47

Figure 2.12 TGA spectrum of Y-DCMDRY MIL heated from 25°C–500°C. Sample was heated
from 25°C–100°C, held at 100°C for 5 h, then heated to the final temperature of 500°C. Dip in
spectrum accredited to loss in water from catalyst at 100 °C. Sample prepared in a glovebox with
an N2 atmosphere.

48

Supplementary Information

Figure 2.13 TOFs for polymerizations from Figure 2.7 including Y-AIR, Y-DCMAIR and
[H3DP]Cl. “AIR” indicates reactions where Y-DRY or Y-DCMDRY were weighed under an N2
atmosphere and used monomers dried/stored outside of the glovebox. Reaction conditions:
[MIL]:[anhydride]:[epoxide] (1:200:1000) and [H3DP]Cl (reaction conditions: [4 equiv.
[H3DP]Cl]:[200 anhydride]: [1000 epoxide]). All reactions performed at 110 °C.

49

Table 2.2 Polymer Molecular Weight Data. Representative reactions done in triplicate.
Entry Catalyst Mn,theor(kDa)
a
(7 init)/[TGAinit]
Mn,exp(kDa)
b

Ɖ
b

BO/CPMA
1 Y-WET 4.81/2.60[13] 3.6 1.24
2 Y-DCMAIR 5.20 5.7 1.15
BO/PA
3 Y-WET 4.01/3.08[13] 2.5 1.23
4 Y-AIR 5.4 5.1 1.12
5 Gd-WET 6.37 3.6 1.11
6 Ho-WET 5.48 3.4 1.01
7 Y-DCMWET 6.04/4.47[9.5] 3.8 1.17
CHO/PA
8 Y-WET 6.78/3.65[13] 4.1 1.04
9 Y-AIR 6.98 4.2 1.04
10 Y-AIR 7.06 2.3 1.25
11 Gd-WET 7.13 4.7 1.06
12 Gd-WET 7.26 1.7 1.48
13 Gd-WET 7.17 3.5 1.05
14 Ho-WET 7.09 2.9 1.07
15 Ho-WET 7.031 1.7 1.28
16 Nd-WET 6.78 3.4 1.09
17 Y-DCMWET 7.09/5.25[9.5] 2.9 1.28
18 Y-DCMAIR 7.12 2 1.3
CHO/GA
19 Gd-WET 5.38 2.7 1.02
a
Initiators M n, theo was calculated for 7 initiators and for initiators calculated from TGA anlyses where [TGAinit] = Y-
WET:13, Y-DCMWET & Y-DRY: 9.5, Y-DCMDRY: 9.
b
Identified by GPCin THF using a Wyatt DAWN HELEOS
II MALS detector.

Table 2.3 Additional BO/CPMA Polymerizations.
Entry Catalyst Time
(mins)
% Conv.
b
% Ester
c
% Epim.
d
Mn,theor
(kDa)

(7 init)
Mn,theor
(kDa)
e

[# initTGA]
Mn,exp
(kDa)
f

Đ
f

1 Y-WET 36 85 94 13 5.72 3.23 [13] 4.5 1.14
3 Y-DCMWET 40 82 >99 20 5.61 4.24[9.5] 11.0 1.25
6
g,h
Y-DRY 57 72 >99 15 5.04 3.74[9.5] 29.4 1.23
8
h
Y-AIR 37 96 99 15 6.52 - 3.2 1.37
a
Reaction conditions were a 1:200:1000 ratio for [catalyst]:[CPMA]:[BO] with a scale of 1 mL BO, reacted at 110 C
with exposure to air. Monomers that were dried, purified and stored in the glovebox were used.
b
Determined using
1
H
NMR spectra of crude reaction mixtures, comparing the conversion of anhydride monomer to polymer.
c
Determined
using
1
H NMR spectra of purified polymers.
d
%Epim = {2 x A 2.7ppm/ (A 6.0-6.5ppm)}x100.
e
Initiators calculated from
TGA where [#InitTGA]: Y-WET:13, Y-DCMWET & Y-DRY: 9.5, Y-DCMDRY: 9.
f
Identified by GPC in THF
using a Wyatt DAWN HELEOS II MALS detector.
g
Reaction conducted in N2.
h
Reactions used monomers dried and
stored outside the glovebox.
i
Catalyst loading was 4 mol% instead of 1mol%.

50

References
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2021, 54, 2, 713–724.
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55, 9373-9383.
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5999-6009.
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2022, 13, 5469–5477.
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Irabien, A. Sep. Purif. Technol. 2012, 97, 26-33.
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B. L.; Wilkes, J. S.; P. Williams, P. Chem. Commun. 2008, 447-449.
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51

Chapter 3: Cationic Polymerization of Tetrahydrofuran and Cyclic Anhydrides using a Trityl
Borate Catalyst
Introduction
The pursuit of aliphatic polyesters is an avenue that has been widely pursued owing to the
fact that they represent an environmentally friendly alternative to currently mass-produced plastics
(e.g. polyethylene, polystyrene). A route to aliphatic polyesters that was discussed in previous
chapters was through the ring-opening copolymerization (ROCOP) of epoxides and cyclic
anhydrides. This route does offer a wide monomer scope towards the targeted polymers and is
partially driven through the ring strain present in the epoxide. In an effort to develop a wider array
of environmentally friendly plastics, it would be ideal to expand the monomer scope to encompass
cyclic ethers of larger ring sizes. While there have been examples of simple catalysts for the
copolymerization of epoxides and cyclic anhydrides, these catalysts may not be suitable for cyclic
ethers of large ring sizes, like tetrahydrofuran (THF).
1,2
The use of THF provides an aspect of
tunability in the length of the polymer backbone between polyester functional groups while also
being a cheap and readily available reagent. A report by Tang and coworkers demonstrated the
ability to form aliphatic polyesters from THF and cyclic anhydrides through cationic
polymerization using an organic catalyst (nonafluorobutanesulfonimide, Nf2NH).
3
While this
system does provide the advantage of having a
metal-free catalyst, the polymerizations
resulted in polymers with high dispersities
suggesting poor molecular weight control.
Therefore, this urges the need for a catalyst
system that can perform the targeted
polymerization as well as provide molecular

Scheme 3.1 Proposed ROCOP of THF and cyclic
anhydrides with targeted anhydrides.
[Ph
3
C][B(C
6
F
5
)
4
]
toluene
Monomers:
110 C n
DGA CPMA PA THPA
MA IA GA
52

weight control during the targeted ROCOP. Previous reports have shown trityl salts to
homopolymerize tetrahydrofuran, thus suggesting that they can potentially be used for the targeted
ROCOP of THF and cyclic anhydrides.
4
As trityl tetrakis(pentafluorophenyl)borate
([Ph3C][B(C6F5)4]) is a metal-free reagent and readily available, it will be pursued for the cationic
copolymerization of THF and cyclic anhydrides of ranging ring size. Throughout these reactions
the THF monomer also served as the solvent alongside a toluene co-solvent (Scheme 3.1).
Results/Discussion
Upon designing the polymerization reaction conditions, preliminary reactions were run
with the trityl borate salt ([Ph3C][B(C6F5)4]) to determine the air and moisture sensitivity of the
salt. It was found that the trityl borate catalyst did not exhibit any degradation upon exposure to
air. Therefore, the polymerizations for these studies were prepared via bench top where the
anhydrides and any solvents were used as received. The trityl borate was stored under a glovebox
atmosphere and taken out as it was needed. Furthermore, reactions were heated to 110 °C, as
previously reported reactions for the targeted polymerizations were run at similar elevated
temperatures as lower temperatures can lead
to THF homopolymerization.
3
The
polymerizations were initially run for 24
hours with conditions as trityl
borate:anhydride:toluene:THF as
1:100:200:300, respectively (Table 3.1).
The use of toluene was included in the
reaction conditions as it has previously been
reported to increase reaction conversion.
3
An initial set of reactions were performed using the
Table 3.1 Preliminary ROCOP of targeted
anhydrides with THF
Entry Monomers Time
(h)
Conv. %
b
Ester
%
b
1 PA 24 85 89
2 MA 24 86 89
3 IA 24 >99 88
4 GA 24 >99 98
5 DGA 24 >99 89
6 THPA 24 >99 86
7 CPMA 24 87 95
a
Reaction conditions were a 1:100:200:300 ratio for
[Ph 3C][B(C 6F 5) 4]:[anh]:[THF]:[toluene] reacted at 110°C
for 24 hours.
b
Determined using
1
H NMR spectra of crude
reaction mixtures, comparing the conversion of anhydride
monomer to polymer.
53

designated reaction conditions and were run for an initial time of 24 hours where seven cyclic
anhydrides were screened that ranged in functionality from number of rings as well as size of the
rings. Reactions were quenched with chloroform from which an aliquot was collected to analyze
through
1
H NMR. Afterwards, the polymer was collected as a precipitate when the reaction vial
was charged with petroleum ether. This set of reactions demonstrated the ability for the trityl borate
salt to be able to perform the targeted polymerization as all the conversions were 85% or higher.
Amongst these reactions, an anhydride that resulted in one of the lower conversions was phthalic
anhydride (PA) (Table 3.1, entry 1). This result was surprising as phthalic anhydride was one of
the best anhydrides for the ROCOP of epoxides and cyclic anhydrides as seen in chapters 1 and 2.
As the other four anhydrides (itaconic anhydride (IA), glutaric anhydride (GA), diglycolic
anhydride (DGA), and tetrahydrophthalic anhydride (THPA)) that were used in the screening
reached full conversion, further studies were conducted to obtain better insight to the reactivity of
the trityl borate catalyst. To do this, the reactions were performed again for shorter time frames
and the results can be seen in Table 3.2. While the time frames were shortened for each of the
four anhydrides to 10 hours, each respective reaction still resulted in >90% conversion.
Table 3.2 Polymerization reactions with shorter time frames and observed GPC data.
Entry Monomers Time
(h)
Conv. %
b
Ester %
c
Theo. Mn
kDa
Exp. Mn
d
kDa
Ɖ
d
1 20 >99 >99 - - -
2 IA 16 >99 >99 18.5 15.5* 2.99
3 10 >99 86 19.7 17.0 1.98
4 20 98 >99 18.4 13.0 2.75
5 GA 16 96 >99 18.3 12.3 1.83
6 10 >99 92 19.1 11.0 3.71
7 20 >99 - - - -
8 DGA 16 >99 - - - -
9 10 >99 98 19.0 10.9 1.86
10 THPA 16 92 - - - -
a
Reaction conditions were a 1:100:200:300 ratio for [Ph 3C][B(C 6F 5) 4]:[anh]:[THF]:[toluene] reacted at 110°C.
b
Determined using
1
H NMR spectra of crude reaction mixtures, comparing the conversion of anhydride monomer
to polymer.
c
Determined using
1
H NMR spectra of purified polymers.
d
Identified by GPC in THF using a Wyatt
DAWN HELEOS II MALS detector. *GPC sample did not completely dissolve in THF for GPC analysis.
54

In order to obtain a better understanding on the properties of the resultant polymers as well
as how well the trityl borate controlled the polymerization, the polymer samples were analyzed
through gel permeation chromatography (GPC). When considering the GPC data obtained from
the GA/THF reactions, the lowest dispersity achieved was for the reaction run for 16 hours with a
dispersity of 1.83 (Table 3.2, entry 5). This dispersity highlights the control of the trityl borate
catalyst for this polymerization as a prior report from Tang and coworkers obtained a dispersity of
2.47.
3
Furthermore, this report was able to achieve a 96% conversion of the GA/THF monomer
pair in 8 hours with a molecular weight of 0.8 kDa at 120°C. While the GA/THF reaction reported
herein was run for 16 hours to achieve this dispersity, the resulting polymer had a much higher
molecular weight of 12.3 kDa. Additionally, the reaction was able to display this high molecular
weight along with low dispersity while exhibiting >99% ester. GPC data was also obtained for the
IA/THF and DGA/THF where dispersities of 1.98 and 1.86 were obtained, respectively (Table
3.2, entries 3 and 9). While these are much higher than the ideal dispersity of 1.00, to our
knowledge this is one of the first reports in regard to GPC data for these monomer pairs. Moreover,
while the molecular weights (Mn) of most of the tested monomer pairs did not meet the theoretical
molecular weight, the molecular weights were all >10 kDa.
Although only preliminary data was obtained for the PA/THF and MA/THF monomer
pairs, the conversions obtained for these reactions demonstrated the trityl borate catalyst’s ability
to reach high conversions in a shorter amount of time. Tang and coworkers reported conversions
of 98% and 75% for PA/THF and MA/THF, respectively, in 30 hours. The trityl borate catalyst
reported herein was able to achieve conversions of 85% and 86% for PA/THF and MA/THF,
respectively, in 24 hours (Table 3.1, entries 1 and 2). This emphasizes the trityl borate salt as a
55

highly active catalyst for the targeted polymerization and encourages further studies in its use as a
catalyst for this ROCOP as well as optimization in reaction conditions.

Conclusion
The use of [Ph3C][B(C6F5)4] was employed as a catalyst for the copolymerization of THF
and cyclic anhydrides. As there are very few reports for this targeted polymerization, the studies
presented herein provide another potential avenue to access these polymers, thus broadening the
overall scope of aliphatic polyesters. The trityl borate salt was found to lead to complete conversion
for six of the seven monomer pairs investigated. Polymer characterization data revealed high
molecular weights with dispersities from 1.83-3.71. While the dispersities are higher than 1.00,
they provide insight on the resulting polymers as there is limited data on these types of aliphatic
polyesters and motivate the pursuit of more in-depth studies. The use of a readily available catalyst
and bench-top reaction preparation also allow for this system to easily translate to an industry
setting. Future studies for this work would include further characterization of the resultant
polymers such as decomposition temperatures and tensile testing in an effort to compare them to
the aliphatic polyesters synthesized from the ROCOP of epoxides and cyclic anhydrides mentioned
in the previous two chapters.

56

Experimental
Instrumentation and measurements
1
H NMR spectra were obtained using a Varian 400-MR 2-Channel NMR. Chemical shifts (δ) for
1
H NMR spectra were referenced to residual protons in the deuterated solvent. Gel permeation
chromatography (GPC) analyses were carried out using an Agilent 1260 Infinity II HPLC System
equipped with an Agilent 1260 Infinity autosampler and UV-detector, as well as a Wyatt DAWN
HELEOS-II MALS detector and a Wyatt Optilab T-rEX refractice index detector. The Agilent
GPC was equipped with two Agilent PolyPore columns (5 micron, 4.6 mmID) which was eluted
with THF at 30 °C at 0.3 mL/min and calibrated using monodisperse polystyrene standards. The
number average molar masses and dispersity values were determined from MALS (multi-angle
light scattering) using dn/dc values calculated by 100 % mass recovery method from the refractive
index (dRI) signal.
Materials and Methods. The [Ph3C][B(C6F5)4] salt was stored in a glovebox atmosphere and
taken out as needed. Solvents (chloroform, THF, toluene, benzene, ethyl acetate, dichloromethane,
diethyl ether (Et2O), hexanes, pentane) and other reagents were used as received. NMR solvents
were purchased from Cambridge Isotope Laboratories and used as received unless otherwise noted.
Carbic anhydride (CPMA) was crystallized from a 70:30 hexanes:ethyl acetate mixture followed
by solvent evaporation which was then used for polymerizations under ambient conditions.
Phthalic anhydride (PA) was sublimed under vacuum and stored under ambient conditions. All
other chemicals and reagents were purchased from commercial sources (Aldrich, TCI, Alfa Aesar,
Acros, Fisher, and VWR) and used without further purification.

57

General polymerization procedure. A ratio of 1:100:200:300 for
[Ph3C][B(C6H5)4]:[anh]:[THF]:[toluene] were combined in an 8 dram vial with a Teflon-lined cap
and a stir bar. The reaction vial was charged with 200µL HPLC-grade THF and 300µL toluene
then capped, taped closed with electrical tape, and heated in a heating block, pre-heated to 110°C,
for 24 to 10 hours. The samples were then quenched with approximately 1 mL chloroform followed
by the removal of an aliquot for conversion analysis by
1
H NMR spectroscopy. The remaining
chloroform solution was charged with ~10 mL of petroleum ether to form a polymer precipitate.
The resulting polymer was collected via decantation and dried in a vacuum oven at 115 °C for 14
hours.

58

1
H NMR Spectra of Polymerization Reactions
Crude Reaction Polymer
1
H NMR Spectra
Spectra presented in this section are from aliquots taken upon having quenched the polymerization
reactions with CHCl3.

Figure 3.1
1
H NMR spectrum of crude reaction for THF/IA, for Table 3.2, entry 3 in CDCl3.

59

Figure 3.2
1
H NMR spectrum of crude reaction for THF/GA, for Table 3.2, entry 6 in CDCl3.

60

Figure 3.3
1
H NMR spectrum of crude reaction for THF/DGA, for Table 3.2, entry 9 in CDCl3.

61

Figure 3.4
1
H NMR spectrum of crude reaction for THF/THPA, for Table 3.2, entry 10 in CDCl3.

62

Isolated Polymer
1
H NMR Spectra

Figure 3.5
1
H NMR spectrum of isolated polymer for THF/IA, for Table 3.2, entry 3 in CDCl3.

63

Figure 3.6
1
H NMR spectrum of isolated polymer for THF/GA, for Table 3.2, entry 6 in CDCl3.

64

Figure 3.7
1
H NMR spectrum of isolated polymer for THF/DGA, for Table 3.2, entry 9 in CDCl3.

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70

Appendix
Appendix A: The Degradation/Upcycling of Polyethylene with Lewis Acid Catalysts through
Aryl Alkylation
Goals
• Investigate different Lewis acid catalysts for degradation/upcycling reactions of
polyethylene
• Screen different xylene reagents for the synthesis of various diaryl alkanes
• Increase scale of reactions to obtain more quantitative and further characterize the resulting
product
Background
The growing plastic crisis urges the need for more sustainable and environmentally friendly
alternative plastics to those that are currently produced at an industrial scale. While this avenue is
pursued, a better form of recycling/upcycling current mass-produced plastics is also an urgent
matter as these are the plastics that are presently filling up landfills and leaking into the
environment. The most globally consumed plastic on the market is polyethylene due to its wide
use in different varieties of packaging.
1
Owing to its wide use, it would be beneficial to develop
new avenues towards polyethylene deconstruction and upcycling. Due to the strong carbon-carbon
and carbon-hydrogen bonds, polyethylene (PE) depolymerization/upcycling methods (e.g.
pyrolysis, hydrogenolysis, cross-alkane metathesis) have faced limitations such as not being
selective, high costs, and requiring the use of expensive catalysts.
2
These limitations urge the need
to develop better degradation/upcycling methods for polyethylene that can produce value-added
products using a cheap and effective catalyst system. Conk and coworkers reported the use of an
Ir, Ru and Pd catalyst system in a dehydrogenation isomerization ethenolysis (DIE) process for
the formation of propylene from PE (Figure A1a).
3
This shows promise in taking PE waste and
converting it into desirable monomers, like propylene, that is typically a by-product of steam
cracking. Although this work led to the formation of monomers, it required the use of precious
metals to carry out the polymerization. Furthermore, a recent report by Zhang and coworkers,
71

demonstrated the use of a heterogenous Pt/γ-
Al2O3 catalyst system for the upcycling of
polyethylene into linear alkylbenzenes
(Figure A1b).
4
Linear alkylbenzenes are
considered higher value products than the low
value alkanes that result from energy
intensive pyrolysis processes and are
desirable for their use in detergents. While the
system by Zhang and coworkers did not
require the use of a solvent and did show
capabilities for catalyst recovery, the system
did lead to a various products thus demonstrating its lack of selectivity. To overcome the
limitations and challenges met in aforementioned reports, this work is geared towards developing
a cheap catalyst system with high selectivity towards high-value alkyl aromatic products. The
catalyst system herein will use a Lewis acid catalyst with chlorotrimethylsilane (TMSCl) and a
series of xylenes for the degradation/upcycling of PE into alkyl aromatic products (Figure A1c).
Based on previous studies, polyolefins have been introduced into polystyrene polymers to
synthesize polystyrene/polyolefin blends.
5
Previous work showed that the use of AlCl3 allowed
for the alkylation of the polyolefin chain (i.e. PE). This process was inspired through the use of
the Friedel-Crafts technique. As this allowed for the incorporation of the PE to the PS chain, this
inspires the ability to develop a method for the degradation of PE. Taking inspiration from this
method, the commonly used AlCl3 catalyst for Friedel Crafts reactions will be employed for the
upcycling of PE into highly desired diaryl products.
6
While previous reports used the polystyrene
Figure A1. Previous work with PE degradation
and this work.
3,4
72

as the source for the aryl group, the use of xylenes will act as the source of the aryl group in the
targeted product in this case. The use of the xylene will not only act as the aromatic group for the
desired aromatic product, but it will also serve as the solvent for the proposed reactions. The
introduction of TMSCl will also be studied as this can potentially serve as a silylium cation that is
activated by the AlCl3 catalyst employed for the targeted reaction. Throughout this work a series
of three different xylenes were also screened (e.g. o-xylene, m-xylene, p-xylene) to determine
any difference in reactivity.
Work Done
Owing to its literature precedence for PE degradation/upcycling, AlCl3 was initially
targeted as the catalyst for the proposed work. Additionally, the use of rare earth metals were also
considered for the catalyst system as they also display a range in Lewis acidity and have
precedence as catalysts for Friedel-Crafts alkylation of aromatics.
6
The reactions were carried out
by charging a pressure flask with metal salt, xylene, TMSCl, and PE pellets. The flask was then
heated at 130°C for varying amounts of time. Preliminary reactions were done using anhydrous o-
xylene to test the need of TMSCl where one reaction flask had TMSCl and one did not where these
reactions were run side-by-side for approximately 7 days. Upon analysis through
1
H NMR
spectroscopy, it was observed that the reaction without the use of TMSCl led to observable product
peaks in comparison to the reaction in which it was added. Taking this observation into
consideration, a series of reactions were then conducted to test m-xylene with varying metal
catalysts over approximately 48 hours as listed in Table A1. Due to the small scale of the reactions,
the products were obtained in minimal quantities that was all used for NMR characterization,
therefore, no yields were obtained, However, the suggested presence of the desired product was
recorded and is listed in Table A1. As the reactions with YCl3•6H2O and AlCl3 as the metal
73

catalyst (entries 2-4) resulted in the formation of the desired diaryl product, this further encouraged
the pursuit of these metals in larger scale reactions in effort to obtain higher yields.
A series of larger scale reactions (~100 mg PE) were further conducted with p- and o-xylene for
approximately 6 days. Throughout these reactions, anhydrous and wet xylenes were both used in
an effort to determine if the presence of water would affect the reaction rates as water has been
reported to deactivate AlCl3 as the catalyst.
7
However, these reactions did not result in any product
observed upon stopping the reaction after the alotted time.
In an effort to drive the reaction forward, a higher mol% of metal catalyst was used for set
of reactions (Table A2, entries 1 and 2), however, this did not result in the product being observed
for these reactions. Furthermore, p-xylene, both wet and anhydrous, were investigated, but the
presence of the product was not observed in the NMR spectrum (Table A2, entries 3 and 4). Based
on all the xylenes that were screened throughout these studies, the one that resulted in the least
successful was p-xylene. The use of p-xylene lead to unknown impurities observed in the NMR
alongside what were suspected to be the targeted product peaks. The poor results from p-xylene
could be attributed to the sterics of this reagent only allowing for one possible orientation of the
Table A1. PE degradation reactions for 48 hours.
Entry Metal Mol% Product (Y/N)
a
1 AlCl3 5 N
2 AlCl3 5 Y
3 AlCl3 10 Y
4 YCl3•6H2O 10 Y
Conditions: A pressure flask charged with 4 mL of m-xylene, metal, and ~30 mg PE stirred and heated
at 130°C.
a
Product presence determined from
1
H NMR spectrum in CDCl 3.

74

xylene to couple with the other xylene ring. Since the reactions using p-xylene resulted in data
that was difficult to distinguish, the use of p-xylene was no longer pursued.
With this in mind, the use of TMSCl (6 equivalents) was reconsidered for these larger scale
reactions in an effort to obtain higher yields of the targeted diaryl product (Table A3). The data
obtained from this series of reactions demonstrated that the use of TMSCl led to higher yields of
the desired product according to the
1
H NMR obtained for the reactions as well is potentially aiding
the PE degradation throughout the reaction (entries 1-3). Furthermore, the success of using both
wet and dry xylenes demonstrates that while water is present, the reaction is still able to form
product.
While the product was observed with the introduction of TMSCl to the system, minimal
amounts of the suggested product were still obtained. Therefore, the reactions were performed at
a larger scale (Table A4). Overall, product yields of up to 1.83% were collected, however, these
values were not considered quantitative as some of the product could have been evaporated or
sublimed through during product purification. It was also found that the best Lewis acid metal
Table A3. PE degradation reactions using TMSCl.
Entry Metal Time (days) n-xylene
(wet/dry)
Product (Y/N)
a
1 AlCl3 5 m (dry) Y
2 YCl3•6H2O 5 m (dry) Y
3 AlCl3 6 o (wet) Y
Conditions: A pressure flask charged with 25 mL xylene, metal (10 mol%), TMSCl(6 equiv.), and ~150
mg PE stirred and heated at 130°C.
a
Product presence determined from
1
H NMR spectrum in CDCl 3.

Table A2. Large scale PE degradation reactions.
Entry Metal Time (days) n-xylene
(wet/dry)
Product (Y/N)
c
1 YCl3•6H2O
a
4 m (dry) N
2 AlCl3
a
4 m (dry) N
3 AlCl3
b
5 p (dry) N
4 AlCl3
b
5 p (wet) N
Conditions: A pressure flask charged with 25 mL xylene, metal (20 mol%), and PE (
a
= 135 mg,
b
= 150
mg) stirred and heated at 130°C.
c
Product presence determined from
1
H NMR spectrum in CDCl 3.

75

catalysts for the targeted polyethylene degradation and upcycling were YCl3•6H2O and AlCl3
when used with TMSCl. This preliminary work demonstrated the potential for these Lewis acid
catalyst systems to be able to perform this PE degradation and form value-added products. Further
optimization of these reactions still needs to be done to effectively carry-out this polymer
degradation and properly characterize the resulting product and obtain quantitative yields.

Future Work
The work presented in this appendix demonstrate the ability for simple catalysts to perform
the PE degradation, however, further efforts are needed to optimize these reactions. Owing to
previous literature precedence where the ionic liquid n-butyl pyridinium was mixed with AlCl3 to
form a molten-salt or metal-containing ionic liquid, this
further pushes the pursuit of using MILs as those discussed
in Chapter 2 for the upcycling of PE (Figure A2).
8
These
could potentially serve as catalysts for this reaction and
could also demonstrate catalyst recovery as well especially when paramagnetic metals are
incorporated into the MIL catalyst. Additionally, other rare earth metal salts could also be pursued
for this targeted degradation to identify if other rare earth metals are able to perform the PE

Figure A2. PE and iC 5 upcycling
into liquid alkanes.
Table A4. Large scale PE degradation reactions using TMSCl.
Entry Metal Time (days) n-xylene
(wet/dry)
Product (Y/N)
d
1 LaCl3•7H2O
a
6 m (dry) N
2 AlCl3
b
8 o (wet) Y
3 YCl3•6H2O
b
8 m (dry) Y
4 AlCl3
c
14 o (wet) Y
5 YCl3•6H2O
c
9 m (dry) Y
Conditions: A pressure flask charged with 25 mL xylene, metal (10 mol%), TMSCl (6 equiv.), and PE
(
a
= 160 mg PE,
b
= 250 mg,
c
= 400 mg) stirred and heated at 130°C.
d
Product presence determined from
1
H NMR spectrum in CDCl 3.

76

degradation/upcycling. Further work in this project would also include performing more thorough
studies on the use of TMSCl in the reaction to determine the role it plays in this system as well as
whether the quantity of it directly impacts the successful product formation. The results found in
these studies show promise in the design of a simple system for PE degradation and further prompts
optimization of the reaction conditions to obtain product for in-depth characterization and
confirmation of the diaryl product synthesis.

Experimental Section
Instrumentation and measurements
1
H NMR spectra were obtained using a Varian 400-MR 2-Channel NMR. Chemical shifts (δ) for
1
H NMR spectra were referenced to residual protons in the deuterated solvent. Attenuated Total
Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) analyses were carried out
using an Agilent Cary 630 FTIR instrument.
Materials and Methods. All manipulation of air and water sensitive compounds was carried out
under nitrogen in a Vacuum Atmospheres OMNI glovebox or by using standard Schlenk line
technique. Solvents (chloroform, THF, toluene, benzene, ethyl acetate, dichloromethane, diethyl
ether (Et2O), hexanes, pentane) and other reagents were used as received. NMR solvents were
purchased from Cambridge Isotope Laboratories and used as received unless otherwise noted.
Anhydrous xylenes were stored in a glovebox atmosphere and the AlCl3 was stored in a dessicator.
All other chemicals and reagents were purchased from commercial sources (Aldrich, TCI, and
VWR) and used without further purification.

77

General PE Degradation Procedure
Sample Reaction Procedure
A 100 mL pressure flask was charged with the 0.252 g (8.98 mmol), 6 equivalents of TMSCl (53.9
mmol), 10 mol% AlCl3 (0.897 mmol), 25 mL o-xylene and a stir bar. The reaction flask was then
stirred and heated at 130°C in a silicone oil bath for approximately 8 days.

General Reaction Work-Up
The reaction was removed from heat and the reaction solution was separated from the unreacted
PE precipitate through gravity filtration. The filtrate was collected and dried via Schlenk line or
heat evaporation after which it was analyzed through
1
H NMR. The precipitate was collected and
washed with isopropanol (3 x 10 mL) after which the precipitate was dried in a vacuum oven at
110°C for ~14 hours.

78

Sample
1
H NMR Spectra of Obtained Products
In this section, reaction product chemical shifts were evaluated in comparison to reported
1
H NMR
values for the expected diaryl products.
9
Figure A3.
1
H NMR spectrum of product obtained from Table A4, entry 4 in CDCl3.
79

Figure A4.
1
H NMR spectrum of product obtained from Table A4, entry 5 in CDCl3.

80

ATR-FTIR of Reaction Precipitate
Figure A5. Representative ATR-FTIR of solid precipitate that was collected from a reaction using
AlCl3 (50 mol%) and o-xylene overlayed on an ATR-FTIR spectrum of a PE pellet demonstrating
the unreacted PE after a reaction.

81

References
1. Polyethylene demand and capacity worldwide from 2015 to 2022. (accessed 2023-07-02)
https://www.statista.com/statistics/1246675/polyethylene-demand-capacity-forecast-
worldwide/
2. Kosloski-Oh, S. C.; Wood, Z. A.; Manjarrez, Y.; de los Rios, J. P.; Fieser, M. E. Mater.
Horiz. 2021, 8, 1084–1129.
3. Conk, R. J.; Hanna, S.; Shi, J. X.; Yang, J.; Ciccia, N. R.; Qi, L.; Bloomer, B. J.; Heuvel,
S.; Wills, T.; Su, J.; Bell, A. T.; Hartwig, J. F. Science 2022, 377, 1561-1566.
4. Zhang, F.; Zeng, M.; Yappert, R. Y.; Sun, J.; Lee, Y-H.; LaPointe, A. M.; Peters, B.; Abu-
Omar, M. M.; Scott, S. L. Science 2020, 370, 437-441.
5. Guo, Z.; Tong, L.; Xu, Z.; Fang, Z. Polym. Eng. Sci., 2007, 47, 951-959.
6. Mantri, K.; Komura, K.; Kubota, Y.; Sugi, Y. J. Mol. Catal. A: Chem. 2005, 236, 168-175.
7. Prakash, G.K.S.; Yan, P.; Torok, B.; Bucsi, I.; Tanaka, M.; Olah, G. A. Catal. Lett., 2003,
85, 1-6.
8. Zhang, W.; Kim, S.; Wahl, L.; Khare, R.; Hale, L.; Hu, J.; Camaioni, D. M.; Gutierrez, O.
Y.; Liu, Y.; Lercher, J. A. Science 2023, 379, 807-811.
9. Yonezawa, N.; Hino, T.; Tokita, Y.; Matsuda, K.; Ikeda, T. Tetrahedron 1997, 53, 14287-
14296.

Abstract (if available)

Abstract Although plastics (polymers) have become necessary for a variety of technologies and functions, their large consumption has led to negative impacts on the environment as well as resulted in a plastic pollution crisis. This urges a need to seek more environmentally friendly alternatives (aliphatic polyesters) to mass-produced plastics in order to combat this issue. An attractive route toward these polyesters is through the ring-opening copolymerization (ROCOP) of cyclic ethers (epoxides or THF) and cyclic anhydrides. This route allows access to a wide scope of monomers, thus potentially leading to a library of plastics with varying properties. Herein, Lewis acid catalysts that are metal-free or contain rare earth metal centers were employed for the targeted ROCOP. To further address the plastics issue, there is also a need to develop better methods to chemically recycled/upcycle currently mass-produced plastics. As polyethylene (PE) is one of the most widely consumed plastics, efforts described herein were directed towards using a Lewis acid catalyst (AlCl3 or rare earth metal salts) for the degradation of PE into the value-added products, diaryl alkanes.

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Creator Manjarrez, Yvonne (author)

Core Title Using Lewis acid catalysts to control the ring-opening copolymerization for polyester synthesis and the deconstruction of polyethylene

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

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Degree Conferral Date 2023-08

Publication Date 08/09/2023

Defense Date 07/18/2023

Publisher University of Southern California. Libraries (digital)

Tag Lewis acid,OAI-PMH Harvest,plastics,polymers,rare earth metals,ring opening copolymerization

Language English

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