Stereocomplex of poly(propylene carbonate): Synthesis of stereogradient poly(propylene carbonate) by regio- and enantioselective copolymerization of propylene oxide with carbon dioxide

Article abstract of DOI:10.1002/anie.201007958

Cobalt(III)-salen complexes with ammonium arm(s) have been used to catalyze the title transformation. Higher thermal decomposition temperatures than those of typical poly(propylene carbonate)s (PPCs) were observed for the stereogradient and the stereobloc

Full text of DOI:10.1002/anie.201007958

Communications
DOI: 10.1002/anie.201007958
Polycarbonate Synthesis
Stereocomplex of Poly(propylene carbonate): Synthesis of
Stereogradient Poly(propylene carbonate) by Regio- and
Enantioselective Copolymerization of Propylene Oxide with Carbon
Dioxide**
Koji Nakano, Shinichi Hashimoto, Mitsuru Nakamura, Toshihiro Kamada, and Kyoko Nozaki*
Stereocontrol has been one of the major challenges in
polymer synthesis for half a century, ever since the synthesis
of isotactic polypropylene by Natta et al.[1a] Not only does
tacticity arise from the relative stereochemistry of the
neighboring asymmetric carbon centers, but the absolute
configuration of the asymmetric centers can be controlled by
modern techniques.[1b,c] Thus, asymmetric polymerization of
chiral polymers has been accomplished by either asymmetric
synthesis polymerization starting from achiral monomers or
enantiomer selective polymerization from a racemic mixture
of chiral monomers.
Gradient copolymers are a new class of polymers, in which
the decrement of one component and the increment of the
other component occur sequentially from one chain end to
the other end, unlike traditional block or random copolymers.
The gradient can be realized by using two kinds of monomers
as well as by the stereoregularity of the same monomer unit.
Examples of such a stereogradient polymer are atactic–
syndiotactic poly(methacrylic acid)[2] and d-l poly(lactic
acid).[3] Herein, we report the first synthesis of a iso-enriched
stereogradient poly(propylene carbonate) (PPC) which starts
from an S-rich PPC block and ends with an R-rich PPC block.
Higher thermal decomposition temperatures were observed
relative to those of typical poly(propylene carbonate)s
(PPCs) for the stereogradient and the stereoblock PPCs
obtained.
utilization.[4–7] When using propylene oxide (PO), three diad
structures (head-to-tail, head-to-head, and tail-to-tail) would
form depending on whether the ring-opening reaction occurs
at the methylene or methine carbon atom.[8] Because PO
possesses a chiral center, stereoregular PPCs such as isotactic
and syndiotactic PPCs are possibly produced from rac-PO by
stereocontrolled copolymerization (Schemes 1a and c).
Indeed, the regio- and stereocontrolled rac-PO/CO2 copoly-
merizations have been reported, and gave iso-enriched PPC
(Scheme 1b) and syndio-enriched PPC (Scheme 1, d).[5g,6a–f,9]
On the other hand, the synthesis of an isotactic stereo-
block PPC (Scheme 1e) or an iso-enriched stereogradient
PPC (Scheme 1 f) has never been reported. Synthesis of
stereoblock or stereogradient polymers can be realized by the
regio- and enantiomer-selective copolymerization of rac-PO
and CO2 with complete conversion of rac-PO; that is, the
more reactive enantiomer of PO is preferably consumed at
Since the first report in the 1960s, copolymerization of
epoxides with carbon dioxide (CO2) has been intensively
studied as one of the most promising processes for CO2
[*] Dr. K. Nakano, S. Hashimoto, M. Nakamura, T. Kamada,
Prof. K. Nozaki
Department of Chemistry and Biotechnology
Graduate School of Engineering, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, 113-8656 Tokyo (Japan)
Fax: (+81)3-5841-7263
E-mail: nozaki@chembio.t.u-tokyo.ac.jp
[**] We are grateful to Prof. Yoshiaki Nishibayashi and Prof. Yoshihiro
Miyake for carrying out HRMS analysis. This work was supported by
a Grant-in-Aid for Scientific Research (A; 21245023), Innovative
Areas “Molecular Activation Directed toward Straightforward Syn-
thesis” from the Ministry of Education, Culture, Sports, Science and
Technology (Japan), and a Grant for Practical Application of
University R&D Results under the Matching Fund Method from the
New Energy and Industrial Technology Development Organization
(NEDO).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007958.
Scheme 1. Stereoregularities of PPCs.
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Angew. Chem. Int. Ed. 2011, 50, 4868 –4871

< 50% conversion, thus giving an isotactic (or iso-enriched)
PPC with the preferable absolute configuration at the chiral
centers and then the less reactive enantiomer of PO becomes
incorporated into PPC chain at > 50% conversion. However,
the reported catalysts for regio- and enantiomer-selective PO/
CO2 copolymerization cannot be applied to such a strategy
because degradation occurs at > 50% conversion of PO
(Scheme 1g), thus giving cyclic propylene carbonate (CPC) as
a by-product.[6d]
Recently, our research group has reported the first
example of selective PPC production at almost complete
PO conversion by using cobalt complex 1a.[10] The key feature
of the catalyst design is an ammonium arm in the salen-type
ligand. Taking advantage of the complete conversion ach-
ieved by our catalyst, we started to investigate the synthesis of
a stereogradient iso-enriched PPC using 1a and its analogues.
The cobalt complexes employed here and the polymeri-
zation results are summarized in Scheme 2, and Table 1,
respectively. First, we evaluated the regio- and enantioselec-
tive outcomes with complex 1a. The PO/CO2 copolymeriza-
tion with complex 1a (PO/1a = 2,000) at 258C for 2 hours
selectively produced PPC [PPC/CPC = 98/2, yield of (PPC +
PC) = 23%] (Table 1, entry 1). However, the regioselectivity
was moderate [head-to-tail linkage (HT) = 72%] and almost
no enantioselectivity (krel = 1.1) was observed in spite of its
enantiomerically pure cyclohexanediamine unit (krel is
defined as the relative rate constant of (S)-PO vs. (R)-PO
or (R)-PO vs. (S)-PO. The value is estimated by means of the
Scheme 2. Structures of cobalt complexes.
and gave PPC selectively (> 99%; Table 1, entry 3). In
addition, when using 1,2-dimethoxyethane (DME) as a
solvent, complete PO consumption was accomplished without
a concomitant production of CPC (> 99% conversion, > 99%
PPC selectivity; Table 1, entry 4). After complete PO con-
version, the obtained PPC consisted of 83% HT linkage,
which is slightly lower than that at 22% PO conversion. The
decrease in regioselectivity at higher PO conversion corre-
sponds to the fact that the more reactive (S)-PO underwent
ring-opening with higher regioselectively than the less
reactive (R)-PO.[6d]
general equation,
krel = ln[(1Àc)(1Àee)]/ln[(1Àc)(1+ee)],
where c is a conversion of PO and the ee value is of unreacted
PO). The ee value are much lower than the reference values
(Table 1, entry 10) obtained with the typical Co–salen com-
plex 4, with which the PO conversion cannot reach > 50%.
To improve the selectivities, we next investigated the
effects of amino/ammonium groups on the salen ligand.
Complex 1b, having amino/ammonium groups derived from
(S)-prolinol moiety, demonstrated higher regio- (HT= 85%)
and enantioselectivities (krel = 1.8) than complex 1a (Table 1,
entry 2). Complex 1b effectively suppressed CPC formation
even at a high PO conversion of 84% under neat condition
Steric bulk near the cobalt center was found to be crucial
for both regio- and enantioselectivities. Cobalt complex 2
with a tert-butyl group at the 3-position of one of the benzene
Table 1: Copolymerization of propylene oxide with CO2 using cobalt complexes[a]
PPC/CPC[b]
HT [%][c]
krel
Mn [gmolÀ1 [d]
]
Mw/Mn
Tg [8C][e]
Td [8C][f]
Td5 [8C][f]
[d]
Entry
Complex
t [h]
Yield of
PPC+CPC [%][b]
1
2
3
4
5
6
7
8
1a
1b
1b
1b
2
2
2
3
3
2
5
78
48
7
120
98
9
96
1
23
22
84
>99
26
83
98
26
96
23
98:2
>99:1
>99:1
99:1
99:1
98:2
99:1
98:2
98:2
98:2
72
85
86
83
86
–
81
92
89
95
1.1 (R)
1.8 (S)
–
–
2.1 (S)
–
–
3.5 (S)
–
4.3 (S)
10800
10600
39800
21400
13600
40500
22900
8200
1.13
1.13
1.16
1.13
1.12
1.15
1.16
1.13
1.15
1.13
34
–
–
35
–
–
33
–
35
35
239
–
–
228
–
–
243
–
273
224
229
–
–
224
–
–
237
–
236
222
9
13800
21300
10[g]
4
[a] Reaction conditions: PO (2.0 mL, 28.6 mmol), cobalt complex (0.014 mmol), [PO]/[Co]=2,000, CO2 (1.4 mPa) at 258C (entries 1–3, 5, 6, 8, and
10); PO (1.0 mL, 14.3 mmol), cobalt complex (0.014 mmol), [PO]/[Co]=1,000, CO2 (1.4 mPa), DME (1.0 mL) at 258C (entries 4, 7, and 9).
1
[b] Determined on the basis of H NMR spectroscopy of the crude product by using phenanthrene as an internal standard. [c] Head-to-tail linkage
determined on the basis of 13C NMR spectroscopy. [d] Determined by size-exclusion chromatography using a polystyrene standard. The molecular
weight is strongly affected by an amount of concomitant water since it works as a chain-transfer reagent.[10,11] Accordingly, the Mn values were not
consistent with those estimated from the yields. [e] Tg values were determined from the second heating scan in DSC. [f] Td =onset temperature of TG
= =
curve. Td5 =temperature of 5% weight loss in TG. [g] [Ph3P N PPh3]Cl (0.014 mmol) was added.
Angew. Chem. Int. Ed. 2011, 50, 4868 –4871
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Communications
rings exclusively gave PPC with 86% HT linkage at 26% PO
conversion. In addition, (S)-PO was consumed preferentially
over (R)-PO with krel = 2.1 (Table 1, entry 5). Accordingly,
complex 2 demonstrated higher regio- and enantioselectiv-
ities than complexes 1. Complex 2 can also produce PPC
selectively even at high PO conversion under neat and dilute
conditions (Table 1, entries 6 and 7), thus showing that only
one ammonium group is effective for suppressing CPC
formation.
Further improvement in regio- and enantioselectivities
was achieved by introducing tert-butyl group at each of the 3-
positions. Because the results with complex 2 indicated that
introduction of sterically hindered tert-butyl group(s) at the 3-
position(s) should be promising for higher selectivities, we
next designed complex 5 (Scheme 2). Attempt to synthesize
complex 5 by the same procedure as for complexes 1 and 2,
however, did not give the desired complex but gave complex 6
without a piperidinium acetate moiety.[11,12] Such easy disso-
ciation of acetic acid from complex 5 may be attributed to the
larger separation between the cobalt center and the piper-
idinyl group. By changing the acid from acetic acid to a
nonvolatile pentafluorobenzoic acid, we obtained complex 3
(Scheme 2). With complex 3, the highest regio- and enantio-
selectivities were achieved among the complexes we inves-
tigated in this study (Table 1, entry 8). Furthermore, high
selectivity for PPC was also accomplished even at complete
conversion of PO (Table 1, entry 9). Thus, we finally obtained
the iso-enriched stereogradient PPC (Scheme 1, f).
Figure 1. Thermogravimetric curves of PPCs in a) Table 1 and
b) Table 2. Broken lines: concentrated from AcOEt; solid lines: repreci-
pitated from AcOEt/MeOH (see the Supporting Information for the
solvent ratios).
Stereogradient and stereoblock PPCs were found to
possess high thermal decomposition temperature. Thermal
properties of the obtained PPCs (reprecipitated from CH2Cl2/
MeOH) in Table 1 were analyzed by differential scanning
calorimetry (DSC) and thermogravimetry (TG). Glass-tran-
sition temperatures (Tg) determined by DSC were almost
independent on regio- and stereoregularities. On the other
hand, decomposition (Td) and 5% weight loss (Td5) temper-
atures of stereogradient PPC (Table 1, entry 9) were higher
than those of PPCs with lower regio- and stereoselectivities
(Table 1, entries 1, 4, and 7) and even iso-enriched PPC
(Table 1, entry 10 and Figure 1a).
PPC was achieved through reprecipitation from AcOEt/
MeOH; Table 2, entries 2 and 4).[14] The Td values approached
2808C and were remarkably high compared to those of the
typical PPCs. No increase in thermal decomposition temper-
ature was observed for enantiopure isotactic PPCs ((S)-PPC
and (R)-PPC; Table 2, entries 6 and 8) and their equimolar
mixture (Table 2, entry 10). One possible explanation for such
high thermal decomposition temperature of the stereogra-
dient PPC and the stereoblock PPC is the stereocomplex
formation between a (S)-PPC block and a (R)-PPC block in
the same chain.[3] Reprecipitation from MeOH (poor solvent)
may accelerate the stereocomplex formation, thus resulting in
higher thermal decomposition temperatures. No increase in
Td values of an equimolar mixture of (S)-PPC and (R)-PPC
indicate that the stereocomplex formation was facilitated by
proximity between a (S)-PPC block and a (R)-PPC block.
In conclusion, we have demonstrated the first synthesis of
stereogradient PPC that consists of two enantiomeric struc-
tures on each end by using optically active cobalt–salen
complexes with ammonium arm(s). Substituents at the 3-
positions of the salicylidene units had great influence on
regio- and enantioselectivities. The obtained stereogradient
PPC as well as stereoblock PPC were found to possess higher
thermal decomposition temperature than the typical PPCs.
The present report has demonstrated the possibility of
stereocomplex formation of PPC and indicated a promising
method for increasing thermal properties of PPC. Further
investigations on the stereocomplex formation of PPCs are
underway.
To investigate the relationship between the stereose-
quence and thermal decomposition property, we synthesized
isotactic (R)-PPC and (S)-PPC (Scheme 1, a) with (R)-PO
= =
and (S)-PO as a monomer, respectively, using 4/[Ph3P N
PPh3]Cl as the catalyst system.[6d,13] In addition, isotactic
stereoblock PPC (Scheme 1, e) was synthesized by stepwise
(S)-PO/CO2 and (R)-PO/CO2 copolymerization using com-
plex 3 according to our previously reported procedure.[10]
These PPCs and stereogradient polymers (Table 2, entry 9)
were purified by reprecipitation from CH2Cl2/MeOH and
subsequent column chromatography on silica gel (AcOEt as
an eluent). Among the PPC samples obtained after concen-
tration from AcOEt (Table 2), iso-enriched stereogradient
PPC (Table 2, entry 1) and isotactic stereoblock PPC
(Table 2, entry 3) demonstrated higher Td and Td5 values
than enantiopure isotactic PPCs ((S)-PPC and (R)-PPC;
Table 2, entries 5 and 7) and their equimolar mixture (Table 2,
entry 9). Interestingly, further increase of the Td and
Td5 values for the stereogradient PPC and the stereoblock
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Angew. Chem. Int. Ed. 2011, 50, 4868 –4871

[5] Chromium-based catalysts: a) S.
Mang, A. I. Cooper, M. E. Col-
Table 2: Relationship between the stereosequence and thermal properties of PPCs.
Entry PPC[a]
Preparation
method
Mn [gmolÀ1 [b]
]
Mw/
Tg [8C][c] Td [8C][d] Td5 [8C][d]
clough,
N.
Chauhan,
A. B.
[b]
Mn
Holmes, Macromolecules 2000, 33,
303; b) D. J. Darensbourg, J. C.
Yarbrough, J. Am. Chem. Soc.
2002, 124, 6335; c) R. Eberhardt,
M. Allmendinger, B. Rieger, Mac-
romol. Rapid Commun. 2003, 24,
194; d) D. J. Darensbourg, R. M.
Mackiewicz, J. L. Rodgers, C. C.
Fang, D. R. Billodeaux, J. H. Rei-
benspies, Inorg. Chem. 2004, 43,
6024; e) D. J. Darensbourg, R. M.
Mackiewicz, J. Am. Chem. Soc.
2005, 127, 14026; f) D. J. Dare-
nsbourg, R. M. Mackiewicz, D. R.
Billodeaux, Organometallics 2005,
24, 144; g) B. Li, G. P. Wu, W. M.
Ren, Y. M. Wang, D. Y. Rao, X. B.
Lu, J. Polym. Sci. Part A 2008, 46,
6102.
1
2
stereogradient
PPC
(iso-enriched)
AcOEt
AcOEt/MeOH
18300
17900
1.14
1.16
23
33
254
281
242
264
3
4
stereoblock PPC
(isotactic)
AcOEt
AcOEt/MeOH
11320
10900
1.29
1.34
25
24
260
277
251
253
5
6
AcOEt
AcOEt/MeOH
13900
13900
1.15
1.16
33
25
245
233
237
219
(S)-PPC (isotactic)
(R)-PPC (isotactic)
7
8
AcOEt
AcOEt/MeOH
13500
13200
1.14
1.15
28
28
248
245
236
227
9
10
(S)-PPC+(R)-PPC AcOEt
(isotactic) AcOEt/MeOH
12800
13100
1.20
1.19
20
22
235
238
228
226
[a] All PPCs were purified before use by reprecipitation from CH2Cl2/MeOH (see the Supporting
Information for the solvent ratios) and the subsequent column chromatography on silica gel with AcOEt
as the eluent. Stereogradient PPC: entry 9 in Table 1. (S)-PPC + (R)-PPC: a mixture of equal amounts of
(S)-PPC and (R)-PPC. [b] Determined by size-exclusion chromatography analysis using a polystyrene
standard. [c] Tg values were determined from the second heating scan in DSC. [d] Td =onset
temperature of TG curve. Td5 =temperature of 5% weight loss in TG.
[6] Cobalt-based catalysts: a) Z. Q.
Qin, C. M. Thomas, S. Lee, G. W.
Coates, Angew. Chem. 2003, 115,
5642; Angew. Chem. Int. Ed. 2003,
42, 5484; b) X. B. Lu, Y. Wang,
Angew. Chem. 2004, 116, 3658; Angew. Chem. Int. Ed. 2004,
43, 3574; c) R. L. Paddock, S. T. Nguyen, Macromolecules 2005,
38, 6251; d) C. T. Cohen, T. Chu, G. W. Coates, J. Am. Chem. Soc.
2005, 127, 10869; e) X. B. Lu, L. Shi, Y. M. Wang, R. Zhang, Y. J.
Zhang, X. J. Peng, Z. C. Zhang, B. Li, J. Am. Chem. Soc. 2006,
128, 1664; f) C. T. Cohen, G. W. Coates, J. Polym. Sci. Part A
2006, 44, 5182; g) E. K. Noh, S. J. Na, S. Sujith, S. W. Kim, B. Y.
Lee, J. Am. Chem. Soc. 2007, 129, 8082; h) H. Sugimoto, K.
Kuroda, Macromolecules 2008, 41, 312; i) S. Sujith, J. K. Min,
J. E. Seong, S. J. Na, B. Y. Lee, Angew. Chem. 2008, 120, 7416;
Angew. Chem. Int. Ed. 2008, 47, 7306; 3574.
Experimental Section
Propylene oxide (2.0 mL, 29 mmol) and cobalt complex (1.4ꢀ
10À2 mmol) were added to a 50-mL autoclave under argon that
contained a magnetic stirring bar. After CO2 (1.4 mPa) was introduced,
the reaction mixture was stirred at 258C for the required time. The CO2
pressure was released, and the polymerization mixture was transferred
into a round-bottom Schlenk flask. The flask was connected to a trap,
which was cooled with liquid nitrogen, and the unchanged propylene
oxide monomer was collected in the trap under reduced pressure. The
remaining polymerization mixture was diluted with dichloromethane,
and phenanthrene was added as an internal standard. A small aliquot
of the mixture was removed and concentrated. Analysis of the residue
[7] Zinc-based catalysts: a) M. Cheng, E. B. Lobkovsky, G. W.
Coates, J. Am. Chem. Soc. 1998, 120, 11018; b) D. R. Moore,
M. Cheng, E. B. Lobkovsky, G. W. Coates, Angew. Chem. 2002,
114, 2711; Angew. Chem. Int. Ed. 2002, 41, 2599; c) D. R. Moore,
M. Cheng, E. B. Lobkovsky, G. W. Coates, J. Am. Chem. Soc.
2003, 125, 11911.
[8] M. H. Chisholm, Z. P. Zhou, J. Am. Chem. Soc. 2004, 126, 11030.
[9] The stereocontrolled copolymerization was also achieved using
cyclohexene oxide as a co-monomer: a) K. Nozaki, K. Nakano,
T. Hiyama, J. Am. Chem. Soc. 1999, 121, 11008; b) M. Cheng,
N. A. Darling, E. B. Lobkovsky, G. W. Coates, Chem. Commun.
2000, 2007; c) K. Nakano, K. Nozaki, T. Hiyama, J. Am. Chem.
Soc. 2003, 125, 5501; d) C. T. Cohen, C. M. Thomas, K. L. Peretti,
E. B. Lobkovsky, G. W. Coates, Dalton Trans. 2006, 237; e) L.
Shi, X. B. Lu, R. Zhang, X. J. Peng, C. Q. Zhang, J. F. Li, X. M.
Peng, Macromolecules 2006, 39, 5679.
[10] K. Nakano, T. Kamada, K. Nozaki, Angew. Chem. 2006, 118,
7432; Angew. Chem. Int. Ed. 2006, 45, 7274.
[11] During our investigation, the cobalt–salen complex with the
same salen ligand as complex 6 was reported for PO/CO2
copolymerization, see: B. Y. Liu, Y. H. Gao, X. Zhao, W. D.
Yan, X. H. Wang, J. Polym. Sci. Part A 2010, 48, 359.
[12] The reaction scheme is described in the Supporting Information.
[13] For the synthesis of (R)-PPC, the enantiomer of 4 was used as a
catalyst.
1
by H NMR spectroscopy and gel permeation chromatography gave
the yield of the copolymer and cyclic propylene carbonate, molecular
weight, and molecular-weight distribution.
Received: December 16, 2010
Published online: March 4, 2011
Keywords: carbon dioxide · cobalt · copolymerization ·
.
homogeneous catalysis · polycarbonates
[1] a) G. Natta, P. Pino, P. Corradini, F. Danusso, E. Mantica, G.
Mazzanti, G. Moraglio, J. Am. Chem. Soc. 1955, 77, 1708 – 1710;
b) Y. Okamoto, T. Nakano, Chem. Rev. 1994, 94, 349; c) S. Ito, K.
Nozaki in Catalytic Asymmetric Synthesis, 3rd ed. (Ed.: I.
Ojima), Wiley-VCH, Weinheim, 2010.
[2] a) Y. Miura, T. Shibata, K. Satoh, M. Kamigaito, Y. Okamoto, J.
Am. Chem. Soc. 2006, 128, 16026; b) K. Ishitake, K. Satoh, M.
Kamigaito, Y. Okamoto, Angew. Chem. 2009, 121, 2025; Angew.
Chem. Int. Ed. 2009, 48, 1991.
[3] N. Spassky, M. Wisniewski, C. Pluta, A. LeBorgne, Macromol.
Chem. Phys. 1996, 197, 2627.
[4] Reviews: a) G. W. Coates, D. R. Moore, Angew. Chem. 2004,
116, 6784; Angew. Chem. Int. Ed. 2004, 43, 6618; b) H. Sugimoto,
S. Inoue, J. Polym. Sci. Part A 2004, 42, 5561; c) D. J. Dare-
nsbourg, Chem. Rev. 2007, 107, 2388; d) G. A. Luinstra, Polym.
Rev. 2008, 48, 192.
[14] The Mn values before and after reprecipitation were almost the
same. Accordingly, the increase of thermal decomposition
temperature after reprecipitation was not attributed to exclusion
of lower-molecular-weight PPCs.
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