Electronic Effects of Aluminum Complexes in the Copolymerization of Propylene Oxide with Tricyclic Anhydrides: Access to Well-Defined, Functionalizable Aliphatic Polyesters

Article abstract of DOI:10.1021/jacs.5b12888

The synthesis of well-defined and functionalizable aliphatic polyesters remains a key challenge in the advancement of emerging drug delivery and self-assembly technologies. Herein, we investigate the factors that influence the rates of undesirable transesterification and epimerization side reactions at high conversion in the copolymerization of tricyclic anhydrides with excess propylene oxide using aluminum salen catalysts. The structure of the tricyclic anhydride, the molar ratio of the aluminum catalyst to the nucleophilic cocatalyst, and the Lewis acidity of the aluminum catalyst all influence the rates of these side reactions. Optimal catalytic activity and selectivity against these side reactions requires a careful balance of all these factors. Effective suppression of undesirable transesterification and epimerization was achieved even with sterically unhindered monomers using a fluorinated aluminum salph complex with a substoichiometric amount of a nucleophilic cocatalyst. This process can be used to synthesize well-defined block copolymers via a sequential addition strategy.

Full text of DOI:10.1021/jacs.5b12888

Article
pubs.acs.org/JACS
Electronic Effects of Aluminum Complexes in the Copolymerization
of Propylene Oxide with Tricyclic Anhydrides: Access to Well-
Defined, Functionalizable Aliphatic Polyesters
†
Nathan J. Van Zee, Maria J. Sanford, and Geoffrey W. Coates*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States
*
S Supporting Information
ABSTRACT: The synthesis of well-defined and functionaliz-
able aliphatic polyesters remains a key challenge in the
advancement of emerging drug delivery and self-assembly
technologies. Herein, we investigate the factors that influence
the rates of undesirable transesterification and epimerization
side reactions at high conversion in the copolymerization of
tricyclic anhydrides with excess propylene oxide using
aluminum salen catalysts. The structure of the tricyclic
anhydride, the molar ratio of the aluminum catalyst to the
nucleophilic cocatalyst, and the Lewis acidity of the aluminum
catalyst all influence the rates of these side reactions. Optimal catalytic activity and selectivity against these side reactions requires
a careful balance of all these factors. Effective suppression of undesirable transesterification and epimerization was achieved even
with sterically unhindered monomers using a fluorinated aluminum salph complex with a substoichiometric amount of a
nucleophilic cocatalyst. This process can be used to synthesize well-defined block copolymers via a sequential addition strategy.
INTRODUCTION
(triphenylphoshine)iminium chloride (PPNCl) is required to
achieve high catalytic activity. The key advantage of this process
over the ROP of lactones for synthesizing functionalizable
aliphatic polyesters is the large number and diversity of
functional monomers that are either commercially available or
easily synthesized.
■
Aliphatic polyesters are important alternatives to petroleum-
1
based materials because they can be produced using renewable
2
feedstocks and disposed of with minimal environmental impact.
Current commercial aliphatic polyesters, such as polylactide and
polycaprolactone, are used in applications ranging from
3
Unfortunately, current catalysts undergo undesirable side
reactions at high monomer conversion especially in the presence
biomedical devices to consumer products, such as polyurethane
4
5
foams and packaging materials. However, the applicability of
current aliphatic polyesters in emerging drug delivery and self-
assembly technologies is limited because of their hydrophobicity,
semicrystallinity, and lack of functionality. A promising approach
to address these deficiencies is the development of functionaliz-
able aliphatic polyester copolymers that would allow for facile
tuning of physical properties and functionality via postpolyme-
1
0a,b
10h,11c
of excess epoxide (Scheme 1b). Inoue,
Chisholm,
and
13a
Nozaki noted that metal porphyrin and corrole complexes
homopolymerize epoxides when the copolymerization is run to
full conversion with excess epoxide. Duchateau and co-
1
1a
workers
observed that chromium salen and porphyrin
catalysts form cyclic polymers via intramolecular transester-
10i
2g
ification based on MALDI-TOF-MS analyses. Our group also
found that chromium and cobalt salen catalysts undergo
transesterification and epimerization side reactions when
copolymerizing an excess of propylene oxide (PO) with the
Diels−Alder adduct of maleic anhydride and α-terpinene (1a).
These side reactions result in mixtures of polymer architectures
and microstructures that can be detrimental to the bulk
properties of the resulting material.
rization modification. Most recent examples are based on the
ring-opening polymerization (ROP) of functionalizable lac-
6
7
tones and glycolide derivatives. This strategy unfortunately has
several disadvantages. Notably, functional lactones require
multiple synthetic steps to make, and the production of well-
defined polymers at high monomer conversion is precluded by
transesterification side reactions.
An alternative route to aliphatic polyesters is the alternating
copolymerization of epoxides with cyclic anhydrides (Scheme
There are many practical advantages to running polymer-
izations to full conversion with an excess of a volatile epoxide
such as PO if side reactions can be eliminated. First, because the
8
9
10
1
a). Our group and others have reported zinc, aluminum,
10c−i,11 10d,f,g,13 13a
10c−i,12
chromium,
cobalt,
manganese,
iron, and
11b
9
g
polymerization rate depends on the concentration of epoxide,
magnesium complexes that produce highly alternating
copolymers. The metal complex serves as a Lewis acid that
activates the epoxide for ring-opening, and, in the case of metal
salen- and porphyrin-based catalysts, the addition of an onium
salt such as tetrabutylammonium chloride or bis-
the polymerization proceeds under pseudo-zero-order kinetics,
Received: December 9, 2015
©
XXXX American Chemical Society
A
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Journal of the American Chemical Society
Article
Scheme 1. Synthesis of Aliphatic Polyesters via Alternating
Ring-Opening Copolymerization and Common Side
Reactions
amorphous, have higher glass transitions than polylactide (T >
g
10i
60 °C), and can be readily functionalized through thiol−ene
a
11f,15
click chemistry.
Herein, we investigate the factors that
influence the rates of undesirable side reactions at high
conversion in the copolymerization of tricyclic anhydrides with
excess PO. We report an optimized process that utilizes a
fluorinated salph aluminum complex to produce a variety of well-
defined, functionalizable aliphatic polyesters.
RESULTS AND DISCUSSION
■
The cyclic anhydrides 1a, 1b, 1c, and 1e were synthesized via
Diels−Alder reactions as previously described, and monomer 1d
was prepared by catalytic hydrogenation of 1c (Table 1). Cyclic
anhydrides 1b−e were copolymerized with an excess of PO using
2
a and PPNCl under conditions that were previously found to be
10i
optimal for the copolymerization of 1a with PO. When the
copolymerizations were quenched before full conversion, all
resulting copolymers are perfectly alternating and have narrow
molecular weight distributions (M /M < 1.20). The cis
w
n
stereochemistry of each monomer is also retained in the
16
corresponding polymers (Table 1, trxn ≤ 60 min). The turnover
−1
frequencies (TOFs) observed for 1b−e (TOF = 95−185 h ,
entries 2−5, t ≤ 40 min) are higher than the one observed with
rxn
−1
a
+
the bulkier monomer 1a (TOF = 63 h , entry 1, t = 60 min).
rxn
P = polymer, M = metal center, C = countercation.
Although minimal transesterification and epimerization are
observed in polymerizations with 1a at high conversion (entry 1,
trxn = 150 min), polymerizations with 1b−d at high conversion
resulting in no appreciable decrease in the rate of polymerization
as a function of conversion. In addition, excess epoxide may be
easily recovered and reused via distillation of the crude
polymerization mixture, and well-defined block copolymers
can be synthesized.
We recently discovered that (salph)AlCl [2a, salph = N,N′-
bis(3,5-di-tert-butylsalicylidene)-1,2-diaminobenzene] and
PPNCl copolymerize 1a and PO with perfect alternation and
minimal side reactions even when the copolymerization is run to
full conversion with excess PO. This aliphatic polyester and
related copolymers derived from other tricyclic anhydrides are
highly interesting because they exhibit unique properties
compared to commercial aliphatic polyesters. They are
yield polymers with broad molecular weight distributions (M /
w
Mn = 1.64−1.69) and significant amounts of trans-diester
1
7
14
linkages (Table 1, entries 2−4, t = 150 min). These
rxn
copolymers are likely more susceptible to transesterification and
epimerization compared to poly(1a-alt-PO) because of the
decreased steric bulk along the polymer chain. However, no
1
13
polyether linkages were observed by H or C NMR
spectroscopy. The polymerization with 1e surprisingly yields a
well-defined copolymer even at high monomer conversion. The
resulting poly(1e-alt-PO) has an M of 8.8 kDa, an M /M of
1
0i
n
w
n
1.15, and a % cis-diester content of 97%. The α-methyl groups
along poly(1e-alt-PO) sterically hinder the exo-face of the diester
Table 1. Effect of Cyclic Anhydride Structure on Turnover Frequency in Copolymerizations with PO Using 2a and PPNCl (PPNCl
a
=
[Ph P−N=PPh ]Cl)
3
3
short reaction time (trxn ≤ 60 min)
long reaction time (trxn = 150 min)
b
TOF (h−1
c
d
d
e
b
d
d
n
e
entry
anhyd.
trxn (min)
conv (%)
)
M (kDa)
n
M /Mn
w
% cis
conv (%)
M (kDa)
n
M /M
w
% cis
f
1
1a
1b
1c
1d
1e
60
35
40
30
26
63
71
63
76
80
63
7.4
6.4
6.0
6.2
7.2
1.11
1.13
1.14
1.18
1.14
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
10.2
11.5
13.7
11.7
8.8
1.16
1.64
1.65
1.69
1.15
97
48
58
25
97
2
3
4
5
122
95
152
185
a
b
1
c
[
PO]:[1]:[2a]:[PPNCl] = 500:100:1:1. Conversion of cyclic anhydride, determined by H NMR spectroscopy. TOF = Turnover frequency, mol
consumed · mol 2a− · h . Determined by GPC in THF, calibrated with polystyrene standards. Determined by C NMR spectroscopy. ref 10i.
1
−1
d
e
13
f
1
B
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Journal of the American Chemical Society
Article
+
units, which evidently decreases the rate of transesterification and
We hypothesized that [PPN] alkoxides are largely respon-
sible for the observed transesterification and epimerization side
reactions because they are the most highly basic and nucleophilic
species present at the end of the copolymerization. By contrast,
epimerization is not observed when using an excess of cyclic
epimerization.
1
0a,b
On the basis of the mechanisms proposed by Inoue
and
1
0h,11c
Chisholm,
the identity of the catalytic species at the end of
the copolymerization depends on [PO]/[1]. When the
copolymerization is performed with an excess of PO, the
resulting species after consumption of the cyclic anhydride likely
+
anhydride because the [PPN] carboxylates formed at the end of
the copolymerization are considerably less basic than [PPN]+
alkoxides. Transesterification is also not observed in this case
because it is thermodynamically unfavorable for carboxylates to
undergo transesterification with the main chain ester bonds.
We thus sought to compensate for the reduced steric bulk of
the copolymers based on 1b−d by modulating the reactivity of
the species at the end of the copolymerization without destroying
the activity of the catalyst. We first investigated the effect of
changing [PPNCl]/[2] on the degree of transesterification and
epimerization after reaching full conversion of 1c using an excess
+
consist of an aluminum salen alkoxide and a [PPN] alkoxide,
which are in equilibrium with a hexacoordinate dialkoxide
aluminate species (Scheme 2a). Conversely, with an excess of
Scheme 2. Proposed Equilibria at the End of the
Copolymerization When Using (a) an Excess of Epoxide and
a
(b) an Excess of Cyclic Anhydride
+
of PO. If [PPN] alkoxides are primarily responsible for the side
reactions, then these side reactions may be suppressed by using
an excess of 2a to drive the equilibrium toward the presumably
less reactive dialkoxide aluminum complex (Scheme 2a).
Monomer 1c was used as a model monomer because it is
inexpensive and the resulting polymers are easily functionali-
11f,15
zed.
Copolymerizations were performed under similar
conditions as described in Table 1 using 0.5, 0.9, and 1.1 equiv
of PPNCl. Turnover frequencies (TOFs) were determined by
quenching before reaching full conversion (Table 2, trxn = 25
min). TOFs increased as more PPNCl was used. For example,
use of 0.5 equiv of PPNCl resulted in a TOF of 36 h− (entry 1,
trxn = 25 min), while use of 0.9 and 1.1 equiv of PPNCl resulted in
1
−1
TOFs of 92 and 106 h , respectively (entries 2 and 3, t = 25
rxn
min). These results are in contrast to previous work by
10h,11c
11b
Chisholm
and Darensbourg, who both reported that
variation of the ratio of PPNCl to metal complex did not
significantly affect the initial rate of copolymerization for
comparable catalytic systems. We could not detect any trans-
diester linkages by 13C NMR spectroscopy for any loading of
PPNCl investigated when the copolymerization was quenched
before reaching full conversion.
a
P = polymer, PPNCl = [Ph P−N=PPh ]Cl.
3
3
A similar series of polymerizations were performed but
quenched approximately 2 h after reaching full conversion of 1c.
The ratio [PPNCl]/[2a] has a strong effect on the rate of
transesterification and epimerization, although no PO homo-
polymerization was observed in any case. Polymerizations using
cyclic anhydride, the resulting species after all of the epoxide is
consumed are likely an aluminum salen carboxylate and a
+
[
PPN] carboxylate, which are in equilibrium with a
hexacoordinate dicarboxylate aluminate complex (Scheme 2b).
a
Table 2. Variation of the Loading of PPNCl in the Copolymerization of 1a with PO using 2a
short reaction time (trxn = 25 min)
long reaction time (trxn ≥ 180 min)
b
TOF (h−1
c
d
d
e
b
d
d
n
e
entry
[PPNCl]/[2a]
conv (%)
)
M (kDa)
n
M /Mn
w
% cis
trxn (min)
conv (%)
M (kDa)
n
M /M
w
% cis
f
f
1
2
3
0.5
0.9
1.1
15
39
44
36
n.d.
n.d.
>99
>99
>99
300
180
180
>99
>99
>99
8.1
7.5
9.8
1.46
1.66
1.64
88
64
50
92
3.5
3.7
1.16
1.18
106
a
b
1
c
[
PO]:[1]:[2a] = 500:100:1. Conversion of cyclic anhydride, determined by H NMR spectroscopy. TOF = Turnover frequency, mol 1 consumed
−1
−1
d
e
13
f
·
mol 2a · h . Determined by GPC in THF, calibrated with polystyrene standards. Determined by C NMR spectroscopy. ref 10i.
C
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Journal of the American Chemical Society
.9 and 1.1 equiv of PPNCl (entries 2 and 3, trxn = 180 min) yield
Article
0
three copolymerizations on 7× scale (8.96 mmol 1c) using 1.0
equiv of 2a, 2b, and 2c, respectively. Since using a slight excess of
the metal complex provides better retention of cis-diester
stereochemistry and less transesterification as noted above, 0.9
equiv of PPNCl were used. A small amount of THF (1.0 mL) was
added to increase the solubility of 1c in the reaction mixture.
Each of these large-scale copolymerizations was evenly divided
into 7 separate vials, all of which were run simultaneously and
then quenched sequentially at the desired time. This strategy
allowed us to easily produce enough poly(1c-alt-PO) at each
time point to determine the proportion of cis-diester linkages by
polymers with similarly broad molecular weight distributions
1.66 and 1.64, respectively), although poly(1c-alt-PO)
(
produced using 0.9 equiv has significantly more cis-diester
linkages than the polymer produced using 1.1 equiv (64 vs 50%
cis, respectively). Improved selectivity against transesterification
and epimerization is realized when [PPNCl]/[2a] is reduced to
0
8
.5, as the resulting poly(1c-alt-PO) has an M /M of 1.46 and
8% cis-diester linkages (entry 1, trxn = 300 min).
w
n
While decreasing [PPNCl]/[2a] improves selectivity against
side reactions, significant transesterification and epimerization
are still observed even when only 0.5 equiv of PPNCl are used.
Furthermore, the rate of polymerization is significantly retarded
as [PPNCl]/[2a] is decreased. We sought to overcome these
issues by further tuning the Lewis acidity of the aluminum
1
3
C NMR spectroscopy.
We observed that more electron deficient catalysts exhibit
lower rates of polymerization (TOF = 88, 80, and 49 h− for 2a,
1
2b, and 2c, respectively, Scheme 3a), which is consistent with our
previous results. All three catalysts produce polymers with
narrow molecular weight distributions and exclusively cis-diester
stereochemistry prior to reaching full conversion of 1c (Scheme
b and 3c, respectively); 1c is considerably more reactive than
the ester bonds of the polymer chain, likely giving rise to the
observed selectivity prior to reaching full conversion.
The electronic properties of the ligands have a dramatic impact
on the rates of transesterification and epimerization at high
conversion. As expected, polymer formed using 2a after reaching
full conversion exhibits broad molecular weight distributions and
low cis-diester contents (Scheme 3b and 3c, red traces). Complex
10i
10i
catalyst. As suggested in our previous work, increasing the
Lewis acidity of the catalyst causes the equilibrium shown in
Scheme 2a to lie more toward the less reactive hexacoordinate
aluminate species. We thus synthesized two analogues of 2a in
which we varied the 5-position of the two salicylidene moieties.
Complex 2b is unsubstituted in these positions, and complex 2c
has fluoro substituents (Scheme 3a). We expected that the
3
Scheme 3. Copolymerization of PO and 1c Using 2a−c and
a
PPNCl
2
b exhibits improved selectivity, as transesterification and
epimerization of the resulting polymer proceed at a slower rate
than observed when using 2a (Scheme 3b and 3c, blue traces).
No evidence of transesterification or epimerization is observed
by GPC and 13C NMR spectroscopy, respectively, when using
the fluorinated complex 2c even several hours after the
copolymerization reaches full conversion (Scheme 3b and 3c,
black traces). We attribute the exceptional selectivity of 2c to its
enhanced Lewis acidity and postulate that it shifts the
equilibrium depicted in Scheme 2a more toward the less reactive
19
hexacoordinate aluminate complex compared to 2a and 2b.
We further compared the selectivity of 2c to 2a by performing
copolymerizations with high catalyst loading relative to 1c ([1c]/
[
2] = 25) to produce low molecular weight polymers suitable for
analysis by MALDI-TOF-MS (Scheme 4). Both polymerizations
were run for much longer than it takes to reach full conversion of
1c. We did not observe any evidence of polyether linkages in
either experiment. The MALDI-TOF-MS spectrum of the
polymer produced using 2a (Scheme 4a) revealed the expected
α-Cl,ω-OH-terminated copolymer (structure A) as the major
distribution. Several other minor distributions were also
observed, including the α-OH,ω-OH-terminated copolymer
a
(
a) Effect of catalyst structure on turnover frequency (TOF =
−1
−1
turnover frequency, mol 1c consumed · mol 2 · h ), see ref 18. (b)
Plot of M /M versus time. (c) Plot of diester stereochemistry (% cis)
versus time. (Open circle = copolymerization quenched prior to
reaching full conversion of 1c, based on H NMR spectroscopy).
w
n
1
(
structure B), the α-Cl,ω-Cl-terminated (structure C), and the
α-Cl,ω-COOH-terminated copolymer (structure D), as well as
the cyclic copolymer (structure E). The presence of these
structures is consistent with inter- and intramolecular trans-
esterification. By contrast, the MALDI-TOF-MS spectrum of the
polymer produced using 2c (Scheme 4b) exhibits a major
distribution of α-Cl,ω-OH-terminated copolymer (structure A)
and a minor distribution of α-Cl,ω-COOH-terminated copoly-
mer (structure D), suggesting that no significant transester-
ification takes place under these conditions.
While tuning the Lewis acidity of the catalyst affords excellent
suppression of side reactions, this selectivity remains dependent
on the ratio [PPNCl]/[2c]. A copolymerization was performed
using a slight excess of PPNCl (1.1 equiv) relative to 2c (1.0
equiv) under the conditions described in entry 3 of Table 2. After
difference in steric bulk between these substituents would have a
negligible effect on catalytic activity because they are distal from
the metal center. The ligands were prepared by condensation of
the appropriate salicylaldehydes with 1,2-diaminobenzene, and
the corresponding aluminum complexes were synthesized in the
1
13
same way as 2a to give monometallic complexes. The H and C
NMR spectra of each complex were consistent with C symmetry.
s
Complex 2c exhibits a single resonance at −128.03 ppm in
19
DMSO-d by F NMR spectroscopy. All complexes in this study
6
are surprisingly stable to moisture, as NMR spectral analyses
were carried out in wet DMSO without degradation.
To compare the degree of transesterification and epimeriza-
tion observed over time when using each catalyst, we prepared
D
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Article
20
Scheme 4. MALDI-TOF-MS Spectra of Poly(1c-alt-PO) Produced Using (a) 2a and (b) 2c
running the copolymerization for 3 h at 60 °C, the resulting
copolymer had an M of 10.1 kDa, an M /M of 1.24, and a cis-
Scheme 5. Synthesis of Poly[(1c-alt-PO)-b-(1d-alt-PO)] via a
Sequential Addition Strategy Using 2c and PPNCl
a
n
w
n
diester content of 87%. Although more selective than 2a under
these conditions, the use of 2c does not afford perfect selectivity.
To achieve both high selectivity against side reactions and a high
rate of copolymerization, the ratio of [PPNCl] to [2c] and the
Lewis acidity of the catalyst must be carefully balanced.
Finally, to demonstrate the exceptional control of this process,
we synthesized a diblock copolymer using 2c (1.0 equiv) and a
substoichiometric amount of PPNCl (0.9 equiv., Scheme 5a).
The first block was formed by reacting 25 equiv of 1c to full
conversion with an excess of PO (125 equiv). A small amount of
THF was used for facile transfer of the reaction mixture into the
reaction flask used for the copolymerization. The M /M of the
w
n
first block was narrow (1.08), and the M (3.2 kDa) closely
n
corresponded with the theoretical M of 2.9 kDa (Scheme 5b).
n
The next block was produced by adding 1d (25 equiv), PO (125
equiv), and THF to the reaction mixture in one portion. After
running to full conversion of 1d, the molecular weight
distribution did not significantly broaden (M /M = 1.13), and
w
n
the M (6.4 kDa) was also close to the theoretical M of 5.9 kDa
n
n
13
(
Scheme 5b). The C NMR spectrum of the diblock further
confirmed its well-defined structure. The carbonyl region was
diagnostic, as the diblock exhibits a set of signals that are
consistent with a combination of nonepimerized poly(1c-alt-
PO) and poly(1d-alt-PO) (Scheme 5c).
CONCLUSION
■
We report the copolymerization of tricyclic anhydrides with an
excess of PO using aluminum-based salen complexes and PPNCl.
Copolymers derived from tricyclic anhydrides such as 1b−d are
more susceptible to transesterification and epimerization than
copolymers derived from bulkier monomers such as the terpene-
based cyclic anhydride 1a. Using 1c as a model monomer, we
found that these side reactions can be suppressed by decreasing
the ratio of PPNCl to salph aluminum complex. Elimination of
side reactions can be achieved by tuning the Lewis acidity of the
aluminum complex; we were unable to detect evidence of
a
a) Synthesis of poly[(1c-alt-PO)-b-(1d-alt-PO)]. (C+ = counter-
(
cation). (b) GPC traces of the first block (in red) and the final diblock
copolymer (in blue). (c) Comparison of the carbonyl regions in the
13
C NMR spectra of the diblock copolymer (top trace, in blue),
poly(1c-alt-PO) (middle trace, from Table 1, entry 3, trxn = 40 min),
and poly(1d-alt-PO) (bottom trace, Table 1, entry 4, trxn = 30 min).
transesterification or epimerization side reactions when using the
fluoro-substituted complex 2c with a substoichiometric amount
of PPNCl. We propose that these strategies reduce the
E
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Article
concentration of [PPN]+ alkoxides at the end of the
copolymerization, which we suspect are highly reactive and the
cause for the observed side reactions. The exceptional selectivity
of 2c can be used to synthesize well-defined block copolymers.
We plan to utilize this process to synthesize functional aliphatic
polyester block copolymers and telechelic copolymers.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b12888.
■
*
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Synthetic procedures, raw polymerization data, supple-
mentary experiments, and NMR characterization data.
(
(
PDF)
(
2
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AUTHOR INFORMATION
Corresponding Author
coates@cornell.edu
Present Address
Institute for Complex Molecular Systems and Laboratory of
Macromolecular and Organic Chemistry, Eindhoven University
of Technology, P.O. Box 513, 5600 MB Eindhoven, The
Netherlands.
■
(
d) Zhu, L.; Liu, D.; Wu, L.; Feng, W.; Zhang, X.; Wu, J.; Fan, D.; Lu
Lu, R.; Shi, Q. Inorg. Chem. Commun. 2013, 37, 182. (e) Wu, L.-y.; Fan,
D.-d.; Lu, X.-q.; Lu, R. Chin. J. Polym. Sci. 2014, 32, 768. (f) Liu, D.-F.;
Wu, L.-Y.; Feng, W.-X.; Zhang, X.-M.; Wu, J.; Zhu, L.-Q.; Fan, D.-D.; Lu
̈
, X.;
*
̈
†
̈
,
X.-Q.; Shi, Q. J. Mol. Catal. A: Chem. 2014, 382, 136. (g) Saini, P. K.;
Romain, C.; Zhu, Y.; Williams, C. K. Polym. Chem. 2014, 5, 6068.
(h) Liu, Y.; Xiao, M.; Wang, S.; Xia, L.; Hang, D.; Cui, G.; Meng, Y. RSC
Adv. 2014, 4, 9503. (i) Winkler, M.; Romain, C.; Meier, M. A. R.;
Williams, C. K. Green Chem. 2015, 17, 300. (j) Zhu, Y.; Romain, C.;
Williams, C. K. J. Am. Chem. Soc. 2015, 137, 12179.
Notes
The authors declare no competing financial interest.
(
10) (a) Aida, T.; Inoue, S. J. Am. Chem. Soc. 1985, 107, 1358. (b) Aida,
ACKNOWLEDGMENTS
Funding for this project was provided by the Center for
Sustainable Polymers, an NSF Center for Chemical Innovation
T.; Sanuki, K.; Inoue, S. Macromolecules 1985, 18, 1049. (c) DiCiccio, A.
M.; Coates, G. W. J. Am. Chem. Soc. 2011, 133, 10724. (d) Robert, C.; de
Montigny, F.; Thomas, C. M. Nat. Commun. 2011, 2, 586. (e) Hosseini
Nejad, E.; van Melis, C. G. W.; Vermeer, T. J.; Koning, C. E.; Duchateau,
R. Macromolecules 2012, 45, 1770. (f) Hosseini Nejad, E.; Paoniasari, A.;
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H.; Paoniasari, A.; van Melis, C. G. W.; Koning, C. E.; Duchateau, R.
Macromolecules 2013, 46, 631. (h) Bernard, A.; Chatterjee, C.;
Chisholm, M. H. Polymer 2013, 54, 2639. (i) Van Zee, N. J.; Coates,
G. W. Angew. Chem., Int. Ed. 2015, 54, 2665.
■
(
CHE-1413862). N.J.V.Z. acknowledges the NSF for financial
support through the IGERT program (DGE-0903653). This
work made use of the CCMR Shared Experimental Facilities,
which are supported by the NSF (DMR-1120296).
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(
(
8
2
́
3
(
8
̈
(
(
1
(16) A detailed discussion regarding the stereochemistry of these
(
polymers can be found in the Supporting Information.
+
(17) We observed no degradation of [PPN] under these conditions.
5
Experimental details are reported in the Supporting Information.
(18) Copolymerizations were performed according to Procedure E in
the Supporting Information with a molar ratio of [PO]:[1c]:[2]:
[PPNCl] = 500:100:1:0.9.
(19) A comparable set of copolymerizations was performed in the
absence of THF to rule out significant solvent effects (Table S4,
Supporting Information). Compared to copolymerizations performed
with THF as presented in Scheme 3 and Tables S1−S3 (Supporting
(
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E.; Harrell, J.; Sathiyakumar, V.; Meschievitz, M.; Katz, J.; Adcock, K.;
Harth, E. Macromolecules 2010, 43, 5665. (c) Blanquer, S.; Tailhades, J.;
Darcos, V.; Amblard, M.; Martinez, J.; Nottelet, B.; Coudane, J. J. Polym.
Sci., Part A: Polym. Chem. 2010, 48, 5891. (d) Darcos, V.; El Habnouni,
F
DOI: 10.1021/jacs.5b12888
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Information), the copolymerization rates with 2a, 2b, and 2c were
higher in the absence of THF. The resulting copolymers exhibited
similar M /M values and % cis-diester contents compared to the
w
n
corresponding copolymers prepared in the presence of THF.
20) Copolymerizations were performed according to Procedure D in
the Supporting Information with a molar ratio of [PO]:[1c]:[2]:
PPNCl] = 500:25:1:0.9.
(
[
G
DOI: 10.1021/jacs.5b12888
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX