Tunable and Recyclable Polyesters from CO2 and Butadiene

Abstract

Carbon dioxide is inexpensive and abundant, and its prevalence as waste makes it attractive as a sustainable chemical feedstock. Although there are examples of copolymerizations of CO2 with high-energy monomers, the direct copolymerization of CO2 with olefins has not been reported. Herein, an alternate route to tunable, recyclable polyesters derived from CO2 and butadiene via an intermediary lactone, 3-ethyl-6-vinyltetrahydro-2H-pyran-2-one, is described. Catalytic ring-opening polymerization of the lactone by 1,5,7-triazabicyclo[4.4.0]dec-5-ene yields polyesters with molar masses up to 13.6 kg/mol and pendent vinyl sidechains that can undergo post-polymerization functionalization. The polymer has a low ceiling temperature of 138 ºC, allowing for facile chemical recycling. These results mark the first example of a well-defined polyester derived solely from CO2 and olefins, expanding access to new feedstocks that were once considered unfeasible.

Full text

Tunable and Recyclable Polyesters from CO2 and Butadiene
Rachel M. Rapagnani, Rachel J. Dunscomb, Alexandra A. Fresh, and Ian A. Tonks*
University of Minnesota – Twin Cities
207 Pleasant St SE, Minneapolis, MN 55455
Abstract
Carbon dioxide is inexpensive and abundant, and its prevalence as waste makes it
attractive as a sustainable chemical feedstock. Although there are examples of
copolymerizations of CO2 with high-energy monomers, the direct copolymerization of
CO2 with olefins has not been reported. Herein, an alternate route to tunable, recyclable
polyesters derived from CO2 and butadiene via an intermediary lactone, 3-ethyl-6-
vinyltetrahydro-2H-pyran-2-one, is described. Catalytic ring-opening polymerization of
the lactone by 1,5,7-triazabicyclo[4.4.0]dec-5-ene yields polyesters with molar masses
up to 13.6 kg/mol and pendent vinyl sidechains that can undergo post-polymerization
functionalization. The polymer has a low ceiling temperature of 138 ºC, allowing for facile
chemical recycling. These results mark the first example of a well-defined polyester
derived solely from CO2 and olefins, expanding access to new feedstocks that were once
considered unfeasible.
Introduction
Carbon dioxide is often considered an ideal sustainable polymer feedstock:
inexpensive and abundant, and a significant industrial and energy sector waste product
in need of sequestration.1–7 However, the thermodynamic stability of CO2 renders
polymerizations that directly incorporate CO2 extremely challenging, requiring reaction
with high-energy comonomers such as epoxides (resulting in polycarbonates).3 The
direct copolymerization of CO2 with inexpensive commodity alkenes to form aliphatic
degradable polyesters is an attractive and potentially cost-competitive alternative to
nondegradable petroleum-based plastics. However, aliphatic alkenes simply do not
have enough energy to efficiently fix CO2 on their own, represented by the fact that direct
CO2 and ethylene copolymerization is an as-of-yet unrealized reaction owing to the
significant endothermicity (and kinetic challenges) of forming the 1:1 copolymer.8
Given the challenges of direct conversion of CO2 into polyesters, alternate strategies
involving CO2 conversion to inexpensive polymerizable intermediates are critically
O
O
n
+ CO22 steps O
O
organocatalyzed ROP
Δ
facile catalytic depolymerization
inexpensive commodity
and waste feedstocks
recyclable polyesters from CO2
• 29% by weight CO2
• first ROP of a disubstituted valerolactone
• versatile post-polymerization modification (alkene sidechain)
• recyclable back to monomer
• chemically degradables

important. In this regard, 3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one (EVL) is an
ideal candidate for polyester synthesis. EVL can be synthesized via the high-yielding and
efficient telomerization of butadiene with CO2 (Figure 1, Top).9–15 Butadiene is also an
economical platform chemical that can also be derived from biomass. Polyesters derived
from the ring-opening of EVL-derived molecules are thus extremely attractive targets:
they would have high CO2 content, be constructed from commodity chemicals, and also
have pendent alkene sidechains for rapid post-polymerization functionalization.
Figure 1. Overview of EVL synthesis and examples of its incorporation into polymers to
date, plus the unmet challenge of polymerization of EVL derivatives.
Although significant work has been carried out by Behr and others into the use of
EVL as a feedstock chemical, its direct use in polymerization reactions is quite limited.16
All prior attempts to directly ring-open polymerize (ROP) EVL, semi-hydrogenated 6-
ethyl-3-ethylidenetetrahydro-2H-pyran-2-one (EEtL), and fully-hydrogenated 3,6-
+ CO2
0.06% Pd(dba)2
0.18% P(o-OMePh)3
MeCN, 80 ºC
20 h
2O
O
73% yield
29 wt % CO2
EVL: promising platform chemical from butadiene + CO2 telomerization
O
O
Me
Me
O
O
O
O
Me
nO
O
O
O
n
48% yield
Mn = 85,000 g/mol
Đ = 1.5
Tg = 192 ºC
64% yield
Mn = 1,100 g/mol
Đ = 3.7
Tg = -28.6 ºC
38 : 62 EVP : BBL
O
O
BBL
O
O
O
O
O
O
57% yield
Mn = 2,200 g/mol
Đ = 2.0
Tg = 42 ºC
1% Sc(OTf)3
60 oC, 1.5 h
1% V-40
50% ZnCl2
(CH2O)2CO
100 oC, 24 h
5% TBD
25 oC, 48 h
O
O
attempted ROP
various
catalysts
XO
O
n
O
O
EtVL or DEL
O
O
n
• high weight content CO2
• first successful ROP of EVL derivative
• post-polymerization modification
• recyclable back to monomer
• chemically degradables
• biodegradable (OECD-301)
unmet challenge: direct conversion to uniform polyesters via ROP
underdeveloped in polymer synthesis
Nozaki 2014
radical polymerization
Ni 2021
random copolymerization
Eagan 2021
conjugate addition/
polymerization
x
n
this work: successful ROP of semi-hydrogenated EVL derivatives

diethyltetrahydro-2H-pyran-2-one (DEL) to their respective polyesters have been
unsuccessful.17 In fact, an outstanding question in these prior studies is whether ROP of
disubstituted d-valerolactone derivatives is thermodynamically viable given the lack of
ring strain and the deleterious impact of substitution from the Thorpe-Ingold effect.18–20
Nonetheless, several landmark recent reports motivate continued study into this
class of lactones in CO2-derived polymer synthesis (Figure 1, Middle). For example,
Nozaki demonstrated that EVL could undergo radical polymerization to form a random
copolymer comprised of 29% CO2 by mass.21 Lin later found that radical polymerization
of EVL could be thermally initiated by O2 in air, reaching molar masses up to 239
kg/mol.22 Ni has also demonstrated the random copolymerization of EVL with b-
butyrolactone (BBL) via cationic ROP, albeit with low molar masses and broad
dispersities.23 Most recently, Eagan has reported that EVL can undergo a combination
of vinylogous conjugate addition and polymerization to degradable macromolecules.24
In the interest of further understanding the potential of CO2-derived lactones to
undergo ROP, we sought to re-investigate EVL polymerization with a specific focus on
reduced EVL derivatives lacking a,b-unsaturation, envisioning that a limiting factor in
successful ROP may be unwanted side reactions occurring as a result of the ester
conjugation. Herein, we report the organocatalyzed ROP of the semi-hydrogenated 3-
ethyl-6-vinyltetrahydro-2H-pyran-2-one (EtVL) to poly(EtVL) and fully hydrogenated 3,6-
diethyl-tetrahydro-2H-pyran-2-one (DEL) to poly(DEL), resulting in low-Tg and low-Tc
polyesters that can be chemically recycled to monomer or degraded in dilute basic
conditions. These polymers are the first examples of uniform polyester polymers derived
from EVL and have a CO2 weight content of 29%. Importantly, the 6-vinyl group of EtVL
remains intact in the polymer, providing a facile handle for post-polymerization via simple
alkene reactions.
Results and Discussion
EVL13 was selectively hydrogenated to EtVL (86% yield) via conjugate reduction of
the a,b-unsaturated ester (Figure 1A).25 Exhaustive hydrogenation of EtVL to DEL was
then accomplished via reduction with p-TsNHNH2 (77% yield). Although these
reductions are convenient for academic lab-scale production of EtVL and DEL; larger
scale reductions with more economically viable conditions (Mg0 and Pd/C + H2,
respectively) are also possible.26,27

Figure 2. A: Synthesis of EtVL and DEL. B: optimization of bulk EtVL polymerization.
Reaction conditions: 1% PPA initiator, 5% catalyst, 72 h. Conversion and Mn (end group
analysis) determined by 1H NMR. a10% NaOMe. bHNTf2 led to some monomer
decomposition. c0.5% PPA initiator. C: bulk DEL polymerization catalyzed by a urea
organocatalyst.
Bulk-phase ROP of EtVL to poly(EtVL) was then examined using 1 mol% 3-
phenylpropanol (PPA) initiator combined with a variety of well-established ROP catalyst
systems (Figure 2B). Brønsted acid catalysts such as diphenyl phosphate (DPP) are
efficient catalysts for well-controlled ROP of e-caprolactone and d-valerolactone.28,29
Bulk polymerization of EtVL with DPP was very slow, reaching only 40% conversion over
3 days. Kinetic analysis of the DPP-catalyzed EtVL polymerization revealed kobs = 0.029
M/h, an order of magnitude slower (despite higher catalyst loading) than the rapid DPP-
catalyzed polymerization of monosubstituted d-hexalactone (kobs = 0.32 M/h) and a-
methylvalerolactone (kobs = 0.92 M/h).18 Since both a- and d-monosubstituted
valerolactones undergo DPP-catalyzed ROP at slower rates than valerolactone (kobs =
O
O0.2 equiv. HMPA
2 equiv. Cl3SiH
CH2Cl2
0 ºC to RT, 4 h
86% yield
O
O
EVL EtVL
(2:1 trans:cis)
O
O
DEL
(2:1 trans:cis)
Mn = 9,700 g/mol
Đ = 1.27
Tg = -42 ºC
Td,95% = 250 ºC
2 equiv.
H2NNHTs
o-xylene
reflux, 4 h
77% yield
A
O
O
EtVL
(2:1 trans:cis)
O
O
n
1% PPA
catalyst
bulk, RT
a
poly(EtVL)
B
O
O
DEL
(2:1 trans:cis)
O
O
n
3% NaOMe
bulk, RT, 24 h
70% conversiona
poly(DEL)
N
HN
H
O
Cy
Ph
1%
for entry 9:
Mn = 13,600 g/mol
Đ = 1.32
Tg = -39 ºC
Td,95% = 300 ºC
C

2.88 M/h), it stands to reason that an a,d-disubstituted valerolactone like EtVL would
polymerize even more slowly.
Base catalysts such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5,7-
triazabicyclo[4.4.0]dec-5-ene (TBD) have been broadly used for both lactide and lactone
ROP. TBD, in particular, is one of the most active organocatalysts for ROP owing to its
basicity and bifunctional mode of activation.30,31 Excitingly, ROP of bulk EtVL with TBD
at room temperature led to 78% conversion and an Mn of 10,700 g/mol (measured by
end-group analysis), and kinetic analysis revealed that the initial rate of polymerization
was significantly faster (kobs = 1.4 M/h) than the corresponding DPP-catalyzed
polymerization.
A screen of additional catalysts was carried out (Figure 2B). NaOMe17 catalyzes very
rapid polymerization, but required high loadings (10%) for productive ROP, limiting the
overall molar mass. Strong acids resulted in lower conversions/molar masses (e.g.
MSA32, 66% conv., Mn = 8,600 g/mol) or unproductive side reactions instead of
polymerization (e.g. HNTf233); DBU in combination with benzoic acid34 or 1-cyclohexyl-3-
phenylurea35 did not polymerize EtVL at all. Other metal-based catalysts such as
Sn(Oct)2 require temperatures too close to the poly(EtVL) Tc and result in low conversion
(vide infra).
Based on these results, TBD was determined to be the most effective ROP catalyst
for EtVL. High molar mass poly(EtVL) (13.6 kg/mol, Đ = 1.36) can be synthesized with
0.5% PPA initiator at room temperature (Figure 2B, entry 9). The glass transition
temperature (Tg) of poly(EtVL) is -38.8 ºC, making this potentially suitable as a soft block
in thermoplastic elastomers.36,37 This value is approximately 10 ºC higher than
comparable monosubstituted d-lactones (e.g. d-hexalactone and d-heptalactone18),
most likely from impeded chain rotation from the additional substituent.
Success with semi-hydrogenated EtVL led us to re-evaluate DEL and EVL as
candidates for ROP. Conditions slightly modified from the EtVL ROP conditions (1%
benzyl alcohol/5% TBD/-15 ºC) led to only 18% conversion of DEL after two days.
However, further exploration and optimization of catalysts and conditions led to the
discovery that Waymouth’s NaOMe/1-cyclohexyl-3-phenylurea catalyst system38 is
effective for DEL ROP, leading to 70% conversion after one day at room temperature
(Mn = 9.7 kg/mol, Đ = 1.27) (Figure 2C). Polymerization attempts of EVL with NaOMe
resulted in similar conjugate addition/ROP competition as recently observed by Eagan.24
The poor ROP behavior of EVL may be a reflection of the stability of the s-cis
conformation of the a,b-unsaturated ester, which may disfavor ring-opening nucleophilic
attack and promote unwanted conjugate addition reactivity.
van’t Hoff analysis of TBD-catalyzed polymerization of EtVL revealed thermodynamic
parameters of ΔHp = -2.26 ± 0.23 kcal/mol and ΔSp = -5.48 ± 0.70 cal/mol•K, resulting in
a ceiling temperature (Tc) of 138 ºC (Table 1). Similarly, van’t Hoff analysis of 1-
cyclohexyl-3-phenylurea-catalyzed polymerization of DEL revealed ΔHp = -2.82 ± 0.23
kcal/mol and ΔSp = -7.34 ± 0.68 cal/mol•K, resulting in a ceiling temperature (Tc) of 110
ºC. These low Tc values open the possibility of facile chemical recycling (vide infra).

Comparison of these values to unsubstituted poly(d-valerolactone) (ΔHp = -2.92 kcal/mol,
ΔSp = -2.27 cal/mol•K) and poly(d-hexalactone) (ΔHp = -3.3 kcal/mol, ΔSp = -5.5
cal/mol•K) reveals that the thermodynamics of EtVL and DEL polymerizations are
surprisingly similar to poly(d-hexalactone), likely owing to the fact that a-substitution has
a limited impact on the entropy of polymerization of 6-membered lactones (Table 1).18
Importantly, the combined polymerization results of EtVL and DEL demonstrate that
ROP of disubstituted valerolactones is thermodynamically feasible, although choice of
the appropriate catalyst to engender kinetically-accessible polymerizations remains
unpredictable.
Table 1. Comparison of the thermodynamics of polymerization of various substituted
valerolactones (bulk conditions). adata from ref. 19. bdata from ref. 18. cdata calculated
at 1 M.
Conjugate reduction of EVL to EtVL results in an approximate 2:1 ratio of the trans
and cis diastereomers, consistent with the computationally predicted (B3LYP/6-31G*)
thermodynamic equilibrium ratio (ΔGo = 0.527 kcal/mol). TBD-catalyzed polymerization
of the 2:1 trans:cis diastereomeric mixture of EtVL results in the formation of a
stereorandom polymer where the relative sidechain stereochemistry is indistinguishable
by 1H NMR, while remaining unconsumed EtVL monomer is still in a 2:1 trans:cis ratio
(Figure 3, top). TBD-catalyzed polymerization of a diastereomerically-enriched 5.5:1
trans:cis sample of EtVL (Figure 3, bottom) resulted in a near-identical polymer to the
2:1 feedstock, and the remaining unconsumed EtVL was again found in a 2:1 trans:cis
ratio by 1H NMR analysis. Monitoring the polymerization over time revealed that the
monomer was epimerized rapidly compared to polymerization, with the 2:1 trans:cis ratio
being re-established within 20 minutes. Thus, EtVL undergoes rapid TBD-catalyzed a-
epimerization under the polymerization conditions, and it stands to reason that the
polymer itself would similarly undergo TBD-catalyzed epimerization. We are currently
exploring the development of stereoretentive polymerization conditions to investigate
the potential differential polymerization behavior and properties of each antipode.

Figure 3. 1H NMR spectra of TBD-catalyzed EtVL polymerizations demonstrating
concurrent TBD-catalyzed cis-trans interconversion of EtVL via a-epimerization.
Next chemical recycling of poly(EtVL) to recover EtVL was explored, taking
advantage of the low Tc of the polymer; chemical recyclability is an important feature
when considering the overall sustainability of a material.39–41 Using a vacuum distillation
apparatus, the isolated polymer was exposed to 3% Sn(Oct)2 at 165 ºC, from which 84%
pure monomer was obtained in less than 2 hours (Figure 4B). The Coates group recently
reported poly(1,3-dioxolane) can similarly be chemically recycled via distillation.42
Further, the hydrolytic degradation potential of poly(EtVL) was determined by monitoring
mass loss over time in basic (0.1 M NaOH) and acidic (0.1 M HCl) solutions at 50 ºC and
in 0.01 M phosphate-buffered saline solution (PBS) at 37 ºC (Figure 4A). The polymer
almost fully degraded in the basic solution over a period of 13 weeks, compared to only
a loss of about 4% in both the HCl and PBS solutions in the same amount of time.
Finally, biodegradation studies of this polymer in an aerobic environment using
respirometry is currently ongoing. These studies demonstrate that a CO2-derived
polyester such as poly(EtVL) has the potential for sustainable closed-loop recycling,
while also being potentially degradable in the environment in instances where recycling
is not possible.
O
O
trans-EtVL
O
O
cis-EtVL
O
O
n
poly(EtVL)
0.81.01.21.41.61.82.02.22.42.62.83.03.23.43.63.84.04.24.44.64.85.05.25.45.65.86.0
ppm
1
2
3
4
2:1 trans:cis-EtVL
(starting)
2:1 trans:cis-EtVL
(after poly merization)
remaining
EtVL monomer
5:1 trans:cis-EtVL
(starting)
2:1 trans:cis-EtVL
(after poly merization)
TBD-catalyzed
!-epimerization!
1% PPA
5% TBD
1% PPA
5% TBD

Figure 4. A. Hydrolytic degradation of poly(EtVL) under basic (red), acidic (blue), and
buffered (orange) conditions. B. poly(EtVL) can be chemically recycled to EtVL via high-
temperature catalytic depolymerization and distillation.
In addition to recyclability, another attractive feature of poly(EtVL) is the pendent
alkene sidechain, which allows for facile post-polymerization modification via high-
yielding and well-established alkene reaction chemistry. Prior examples of alkene
functionalization in polyesters include Michael-type addition of thiols to side-chain
appended acrylates in poly(γ-acryloyloxy-ε-caprolactone)43 or backbone acrylates in
poly(a-methylene-γ-butyrolactone)44, as well as photoinitiated thiol-ene click reactions
with poly(6-allyl-ε-caprolactone-co-ε-caprolactone).45 Reactions with mercaptoacetic
acid can tune polymer solubility46, while protected cysteine provides a handle for peptide
coupling.45 Amine additions have also been applied to polymers
like poly(dodecylitaconate)47, and olefin metathesis has been demonstrated on poly(β-
heptenolactone).48
Figure 5 shows several proof-of-principle examples of the myriad simple and robust
post-polymerization modifications available to poly(EtVL). Butyl-3-mercaptopropionate
was installed via radical thiol-ene reaction to poly(EtVL), resulting in poly(EtVL-BMP)
with 97.5% of vinyl sidechains converted by 1H NMR and a decreased Tg of -57 ºC
(Figure 5A). Thiol-ene reaction of poly(EtVL) with 2-diethylaminoethane thiol resulted in
96.5% conversion to the amine-appended polymer, which could then undergo
quaternization to cationic poly(EtVL-NR4) via reaction with benzyl bromide (Figure 5B).
poly(EtVL)
3% Sn(Oct)2
EtVL monomer
(84%)
165 °C
2 h
A
B

We targeted this modification since polymers containing quaternized amine sidechains
have been shown to afford antimicrobial properties.46,49 Next, crosslinked polymers of
poly(EtVL) were synthesized using various molar ratios of a multi-mercapto coupling
agent, trimethylolpropane tris(3-mercaptopropionate) (TMPT). Crosslinking with 10%,
6%, or 3% TMPT resulted in materials that were more solid than the homopolymer and
were slightly sticky, with gel fractions of 75.5%, 57.9%, and 15% and swelling ratios of
26.9%, 20%, and 4.4%, respectively (Figure 5C). Crosslinked materials are used in many
different contexts such as coatings, foams, adhesives, and hydrogels; however, the use
of aliphatic polyesters for these materials is less common, and most are instead
employed as elastomers.50–52 Finally, diblock copolymers53,54 of poly(EtVL) and L-lactide
can be synthesized via chain extension of a 9 kg/mol sample using DBU as a catalyst
(Mn = 13.9 kg/mol, Đ = 1.42) (Figure 5D). Overall, these post-polymerization modifications
demonstrate that the utility of EtVL-based materials extends beyond simply its high CO2
content.
Figure 5. Post-polymerization modification of poly(EtVL).
In conclusion, conjugate reduction of EVL to remove a,b-unsaturation unlocks
access to well-defined recyclable and biodegradable polyesters with high CO2 content
from commodity (butadiene) and waste (CO2) feedstocks. This work represents the first
example of a polyester homopolymer derived from CO2 and an olefin feedstock,
addressing a longstanding challenge in polymer synthesis. The resulting polymers also
contain pendent alkene sidechains, which broadens the potential application of CO2-
derived polyesters through facile post-polymerization functionalization.
Acknowledgements
The funding for this work was provided by the NSF Center for Sustainable Polymers
(CHE-1901635) at the University of Minnesota. Instrumentation for the University of
Minnesota Chemistry NMR facility was supported from a grant through the National
Institutes of Health (S10OD011952).
Conflicts of Interest
O
O
n
vinyl functional handle
for post-polymerization
modification
O
O
n
SO
O
nBu
O
O
n
SO
O
Et
O
O
n
SN
Et
Et
Ph
Br
O
O
n
O
O
m
HS O
O
nBu
0.3 eq. DPMA
385-400 nm, 6 h
2 eq.
HS O
O
Et
3
0.02 eq. DPMA
385-400 nm, 10 min
0.01-0.03 eq.
HS NEt2
1. 0.1 eq. DMPA
CHCl3, 385-400 nm, 4 h
2. 3 eq. BnBr
16 h
2 eq.
0.02 equiv. DBU
CH2Cl2, 1 h
O
O
OO
A. side-chain functionalization
(reduced Tg)
C. polymer crosslinking
B. side-chain functionalization
(quaternary amines)
D. diblock copolymers
Mn = 20,600 g/mol
Mw = 34,900 g/mol
Đ = 1.59
Tg = -57 oC
poly
poly
1:1.6 n:m by 1H NMR
15-76% gel fraction

I.A.T. and R.M.R. are co-inventors on a provisional US patent based on this work.
Supporting Information Available
Full experimental details and analytical data, as well as computational details for cis-
and trans-EtVL (.pdf)
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