Stereospecific CO2 copolymers from 3,5-dioxaepoxides: Crystallization and functionallization

Article abstract of DOI:10.1021/ma500112c

Selective transformation of CO₂ into biodegradable polycarbonates (CO₂-based copolymers) by the alternating copolymerization with epoxides represents a most promising green polymerization process. Despite the tremendous progress this field has made, most of the CO₂-based polycarbonates are known to be amorphous, and their low thermal resistance makes them difficult to use as structural materials. Herein, we report the selective synthesis of highly isotactic CO₂ copolymers from meso-3,5-dioxaepoxides in perfectly alternating nature by the enantiopure dinuclear Co(III)-complex-mediated desymmetrization copolymerization under mild conditions. These isotactic CO₂-based polycarbonates are typical semicrystalline polymers, possessing melting points (T_m) of 179-257 C, dependent on the substitute groups at 4-position of the meso-epoxides. As a model monomer of 3,5-dioxa-epoxides, 4,4-dimethyl-3,5,8-trioxabicyclo[5.1.0] octane (CXO) was studied in detail in the asymmetric copolymerization with CO₂. The isotactic CO₂/CXO copolymer (PCXC) with >99% enantioselectivity possesses a high T_m of 242 C and a decomposition temperature of 320 C, while its atactic copolymer has a high T_g of up to 140 C. Moreover, the acid hydrolysis of highly isotactic PCXC was performed to provide stereoregular poly(1,2-bis(hydroxymethyl)ethylene carbonate)s (PCFC) with two hydroxyl groups in a carbonate unit, which showed a remarkable decrease of ～80 C in thermal decomposition temperature. This hydroxyl-functionalized CO₂ copolymer accords with an unmet need for a readily degradable biocompatible polycarbonate and was further explored to prepare bush copolymers for biomedical and pharmaceutical applications. This approach was initially demonstrated by the hydroxyl groups appended in polycarbonate backbone of a hydroxyl-functionalized terpolymer serving as macroinitiators for direct graft polymerization via organocatalytic lactide ring-opening polymerization to give fully degradable brush polymers with polycarbonate backbones and polylactide side chains. Furthermore, enantiopure dinuclear Co(III)-complex-mediated asymmetric terpolymerization of CO₂ with CXO and cyclohexene oxide (CHO) at various feed ratios was carried out in toluene solution, affording optically active terpolymers poly(CHC-co-CXC) with highly enantioselective ring-opening of the both meso-epoxides. These stereospecific terpolymers were found to be crystallizable and their crystallization capacity could be tuned by changing the feed ratio of the epoxides.

Full text of DOI:10.1021/ma500112c

Article
pubs.acs.org/Macromolecules
Stereospecific CO2 Copolymers from 3,5-Dioxaepoxides:
Crystallization and Functionallization
Ye Liu, Meng Wang, Wei-Min Ren, Ke-Ke He, Yue-Chao Xu, Jie Liu, and Xiao-Bing Lu*
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China
S
* Supporting Information
ABSTRACT: Selective transformation of CO2 into biode-
gradable polycarbonates (CO2-based copolymers) by the
alternating copolymerization with epoxides represents a most
promising green polymerization process. Despite the tremen-
dous progress this field has made, most of the CO2-based
polycarbonates are known to be amorphous, and their low
thermal resistance makes them difficult to use as structural
materials. Herein, we report the selective synthesis of highly
isotactic CO2 copolymers from meso-3,5-dioxaepoxides in
perfectly alternating nature by the enantiopure dinuclear
Co(III)-complex-mediated desymmetrization copolymeriza-
tion under mild conditions. These isotactic CO2-based
polycarbonates are typical semicrystalline polymers, possessing
melting points (Tm) of 179−257 °C, dependent on the substitute groups at 4-position of the meso-epoxides. As a model
monomer of 3,5-dioxa-epoxides, 4,4-dimethyl-3,5,8-trioxabicyclo[5.1.0]octane (CXO) was studied in detail in the asymmetric
copolymerization with CO2. The isotactic CO2/CXO copolymer (PCXC) with >99% enantioselectivity possesses a high Tm of
242 °C and a decomposition temperature of 320 °C, while its atactic copolymer has a high Tg of up to 140 °C. Moreover, the
acid hydrolysis of highly isotactic PCXC was performed to provide stereoregular poly(1,2-bis(hydroxymethyl)ethylene
carbonate)s (PCFC) with two hydroxyl groups in a carbonate unit, which showed a remarkable decrease of ∼80 °C in thermal
decomposition temperature. This hydroxyl-functionalized CO2 copolymer accords with an unmet need for a readily degradable
biocompatible polycarbonate and was further explored to prepare bush copolymers for biomedical and pharmaceutical
applications. This approach was initially demonstrated by the hydroxyl groups appended in polycarbonate backbone of a
hydroxyl-functionalized terpolymer serving as macroinitiators for direct graft polymerization via organocatalytic lactide ring-
opening polymerization to give fully degradable brush polymers with polycarbonate backbones and polylactide side chains.
Furthermore, enantiopure dinuclear Co(III)-complex-mediated asymmetric terpolymerization of CO2 with CXO and
cyclohexene oxide (CHO) at various feed ratios was carried out in toluene solution, affording optically active terpolymers
poly(CHC-co-CXC) with highly enantioselective ring-opening of the both meso-epoxides. These stereospecific terpolymers were
found to be crystallizable and their crystallization capacity could be tuned by changing the feed ratio of the epoxides.
chromium,8 cobalt,9−11 and bimetallic complexes,12,13 have
been developed for this transformation.
Despite the significant progress this field has made, the scope
of epoxides used in the production of CO2 copolymers is very
narrow. Most studies have been associated with the formation
of polycarbonates from propylene oxide or cyclohexene oxide
(Figure 1). Recently, CO2 copolymers from epoxides with an
electron-withdrawing group such as epichlorohydrin or styrene
INTRODUCTION
■
The catalytic transformation of carbon dioxide into value-added
chemicals has received intense attention in recent decades,1 due
to the economic and environmental benefits arising from the
utilization of renewable source and the growing concern on the
greenhouse effect. One of the most promising reactions is the
alternating copolymerization of CO2 with epoxides (oxiranes)
to make degradable polycarbonates,2 which was first reported
oxide and its derivatives were prepared in perfectly alternating
by Inoue and co-workers in 1969.3 In contrast to the alternative
route of condensation polymerization involving the use of toxic
phosgene, this process represents an environmentally benign
approach for the synthesis of polycarbonates. These CO2-based
copolymers have potential applications as ceramic binders,
adhesives, coatings, and packaging materials, as well as in the
synthesis of engineering thermoplastics and resins.4,5 A variety
of catalyst systems, typically based on zinc,6 aluminum,7
nature using cobalt-based catalyst systems.14−16 Also, the co-
and terpolymerization of CO2 with several cyclohexene oxides
that had been functionalized at the monomer’s 4-position was
realized, affording various multiblock copolymers with
Received: January 15, 2014
Revised: January 23, 2014
Published: February 4, 2014
© 2014 American Chemical Society
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Figure 1. Epoxides employed in the copolymerization with CO2.
Scheme 1
functionalities.17 Importantly, Darensbourg and co-worker
reported the synthesis of poly(indene carbonate)s with a rigid
backbone from the coupling of CO2 with indene oxide.18
Notably, the copolymer possesses a glass transition temperature
(Tg) of up to 134 °C, the highest reported for CO2-based
polycarbonates.
provided herein also involves a study on crystallization and
functionallization of these novel CO2 copolymers.
RESULTS AND DISCUSSION
■
The desymmetrization of meso-epoxides in the alternating
copolymerization with CO2 using chiral catalysts is regard as a
valuable strategy for the synthesis of optically active
polycarbonates with main-chain chirality. Pioneer studies by
Nozaki and Coates using enantiopure zinc complexes provided
moderate enantioselectivity for CO2/cyclohexene oxide cou-
pling.25,26 Similar result was also observed in the copolymeriza-
tion of CO2 with meso-epoxides using ternary catalyst systems
consisted of optically active dinuclear aluminum complexes of
β-ketoiminate or aminoalkoxide, in conjunction with a bulky
Lewis base as catalyst activator.27 Recently, we achieved the
highly efficient synthesis of isotactic CO2 copolymers from
both cyclohexene oxide and the less reactive cyclopentene
oxide using chiral dinuclear Co(III)-based catalyst systems.28
The resultant copolymers possess an unprecedented enantio-
selectivity of 99% ee for poly(cyclopentene carbonate)s and
98% ee for poly(cyclohexene carbonate)s. However, these CO2
copolymers from cyclohexene oxide and cyclopentene oxide
have no possibility for further functionallization.
Indeed, the synthesis of degradable CO2-based polycarbon-
ates with Tgs similar to that of bisphenol A polycarbonates is
highly desired. Except for the high Tg of 150 °C, the
crystallizability associated with bisphenol A polycarbonates
made them suitable for structural materials with wide
applications, though arguments and disputes regarding the
safety of this material for foodstuff containers have never
stopped since it realized industrialization as early as 1960s.19
Unfortunately, the previously reported CO2 copolymers with
atactic structure were proved to be difficultly crystallizable. In a
recent study, we found that highly isotactic poly(cyclohexene
carbonate)s were crystallizable and possessed a high melting
point (Tm) of 215−230 °C and one decomposition temper-
ature of ∼310 °C.20 Further research confirmed that isotactic
CO2 copolymers from epichlorohydrin and phenyl glycidyl
ether were also crystallizable, but their Tms are only 108 and 75
°C, respectively.21,22 More recently, Nozaki and co-workers
reported iron−corrole-complex-catalyzed phenyl glycidyl
ether/CO2 copolymerization to provide a crystalline material
owing to the isotactic polyether moiety.23 Pursuant to our own
efforts toward the development of highly efficient stereoregular
catalysts for CO2/epoxide coupling,10,21,22,24 we became
interested in designing crystalline and functional CO2
copolymers from meso-epoxides. In the present contribution,
we report the selective synthesis of highly isotactic CO2
copolymers from 3,5-dioxaepoxides (functional meso-epoxides,)
via a stereospecific copolymerization utilizing chiral dinuclear
Co(III)-based catalyst systems (Scheme 1). The investigation
3,5-Dioxaepoxides (Figure 1), consisted of one 3-membered
epoxy-ring and one 7-membered ring with two oxygen atoms,
has a relatively high reactivity with CO2 by its epoxy-ring in the
presence of a metal catalyst, while its 7-membered ring easily
forms two hydroxyl groups by deprotection, affording the
opportunity for further functional modification for biomedical
and pharmaceutical applications. These meso-epoxides were
easily prepared from epoxidation of 2-substituents-1,3-dioxep-
5-enes, an intermediate, formed through the nucleophilic
addition of cis-2-butene-1,4-diol to aldehyde or ketone
catalyzed by p-toluenesulfonic acid.29
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a
Table 1. Enantiopure Co(III)-Complex-Mediated Asymmetric Copolymerization CO2 with CXO
c
d
f
CO2 pressure
(MPa)
temp
(°C)
time
(h)
TOF
Mn
specific rotation
b
d
e
entry
catalyst
CXO/catalyst/PPNY
(h−1
)
(kg/mol)
PDI
ee (%)
(deg)
1
(S,S,S,S)-1a
(S,S,S,S)-1b
(S,S,S,S)-1a
(S,S,S,S)-1b
(S,S,S,S)-1c
(R,R,R,R)-1b
(S,S,S,S)-1b
(S,S,S,S)-1b
(S,S,S,S)-1b
(S,S,S,S)-1b
(S,S,S,S)-1b
(S,S,S,S)-1b
(S,S,S,S)-1b
(S,S,S,S)-1b
2a
1000/1/−
1000/1/−
1000/1/2
1000/1/2
1000/1/2
1000/1/2
1000/1/2
1000/1/2
1000/1/2
1000/1/2
1000/1/2
2000/1/2
5000/1/2
1000/1/2
1000/1/2
2
25
25
25
25
25
25
0
6
6
52
59
11.3
16.6
12.2
17.5
18.4
18.1
16.2
20.3
6.7
1.10
1.14
1.09
1.18
1.12
1.15
1.13
1.21
1.16
1.12
1.17
1.20
1.19
1.22
1.15
62 (S,S)
73 (S,S)
90 (S,S)
>99 (S,S)
>99 (S,S)
>99 (R,R)
>99 (S,S)
98 (S,S)
>99 (S,S)
>99 (S,S)
>99 (S,S)
>99 (S,S)
>99 (S,S)
>99 (S,S)
0
63 (+)
75 (+)
92 (+)
102 (+)
102 (+)
102 (−)
103 (+)
101 (+)
103 (+)
102 (+)
102 (+)
102 (+)
102 (+)
102 (+)
0
2
2
3
2
2
150
180
191
185
35
4
2
2
5
2
2
6
2
2
7
2
8
8
2
50
25
25
25
25
25
25
25
0.5
6
876
22
9
0.1
0.6
4
10
11
12
13
2
98
11.5
19.7
18.4
10.3
50.2
12.1
2
198
137
68
2
4
2
10
12
12
g
14
2
83
15
2
20
a
The reaction was performed in neat 4,4-dimethyl-3,5,8-trioxa-bicyclo[5.1.0]octane (CXO) (2.6 mL, 30 mmol) in a 20 mL autoclave. The selectivity
b
1
for polycarbonates over cyclic carbonate and carbonate linkages of all resultant polymers are >99% based on H NMR spectroscopy. Molar ratio,
PPN = bis(triphenylphosphine)iminium, Y = 2,4-dinitrophenoxide. Turnover frequency (TOF) = mol of product (polycarbonates)/mol of cat per
hour. Determined by gel permeation chromatography in THF, calibrated with polystyrene. Measured by hydrolyzing the copolymer and analyzing
the resulting diol by chiral GC. Specific rotation of the copolymers was determined by polarimeter in chloroform at 20 °C (c = 1). The reaction was
carried out in dichloromethane solution with [CXO]/[dichloromethane] = 1/2 (volume ratio).
c
d
e
f
g
1
Figure 2. H NMR spectrum of a representative sample of CO2 copolymers from 4,4-dimethyl-3,5,8-trioxa-bicyclo[5.1.0]octane in CDCl3.
Since 4,4-dimethyl-3,5,8-trioxa-bicyclo[5.1.0]octane (CXO)
is easily obtained in a great quantity, it was chosen as model
meso-epoxide for copolymerizing with CO2 using dinuclear
Co(III)-based catalyst systems. We discovered that dinuclear
complexes (S,S,S,S)-1a and 1b alone could operate at high
efficiency in catalyzing CO2/CXO copolymerization, with
turnover frequencies (TOF) of about 70 h−1 at ambient
temperature; however, the enantioselectivity was moderate
(Table 1, entries 1 and 2). We delightedly discovered that in
the presence of PPNY (PPN = bis(triphenylphosphine)-
iminium, Y = 2,4-dinitrophenoxide) as cocatalyst, enantiopure
dinuclear Co(III) complexes (S,S,S,S)-1a could effectively
catalyze this coupling reaction to selectively produce the
corresponding polycarbonate with a TOF of 150 h−1 at ambient
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Figure 3. DSC thermograms (left) in second heating and WAXD profiles (right) of (A) atactic copolymer (ee = 0%, Table 1, entry 15), and (B)
highly isotactic CO2 copolymer (ee > 99%, entry 4) from meso-CXO.
Table 2. Enantiopure Dinuclear Co(III) Complex 1b-Mediated Asymmetric Copolymerization CO2 with Other 3,5-
a
Dioxaepoxides
a
The reaction was performed in toluene (4 mL) in 20 mL autoclave at 25 °C. [(S,S,S,S)-1b]/[PPNY]/[epoxide] = 1/2/500 (molar ratio),
[epoxide]/[toluene] = 1/2 (molar ratio), Y = 2,4-dinitrophenoxide. The selectivity for polycarbonates over cyclic carbonates and carbonate linkages
b
c
1
1
of all the resulted polymer are >99% based on H NMR spectroscopy. The conversion was determined by H NMR spectroscopy. Turnover
d
frequency (TOF) = mole of product (polycarbonates)/mol of cat per hour. Determined by gel permeation chromatography in THF, calibrated with
e
polystyrene. Measured by hydrolyzing the polymer or derivatizing the resultant diol and analyzing by chiral GC or HPLC (for detailed procedures,
see Supporting Information). Specific rotation of the polymers was determined by polarimeter in chloroform at 20 °C (c = 1). The polycarbonates
f
g
in entries 1 and 4 undergo the end-capping reaction using acetic anhydride in CH2Cl2 (for detailed synthesis procedures, see Supporting
h
i
j
Information), as determined by DSC. Not detected. Not applicable. The polycarbonates were decomposed easily under the hydrolysis conditions.
temperature (Figure S6 in the Supporting Information). The
resultant copolymers possess perfectly alternating nature with
>99% carbonate unit content (Figure 2) and show a specific
rotation value of +92° (c = 1, in CHCl3) at 20 °C (entry 3).
After hydrolysis of the resulting polycarbonate with aqueous
NaOH, the enantiomeric excess (ee) of the resulting diol was
determined by chiral GC to be 90% with an S,S-configuration.
It was found that the decrease in the steric hindrance of the
phenolate ortho substituents significantly improved product
enantioselectivity and also increased the reaction rate to a
certain extent (entries 4 and 5). Substituted for (S,S,S,S)-1a
with complex (S,S,S,S)-1b or (S,S,S,S)-1c resulted in an
enantioselectivity from 90% increasing to >99% for S,S-
configuration, corresponding to a specific rotation value of
+102° (c = 1, in CHCl3). Similar activity and enantioselectivity
were observed in the (R,R,R,R)-1b mediated CO2/CXO
copolymerization, and the obtained copolymer has a specific
rotation value of −102° (c = 1, in CHCl3) (entry 6). Notably,
the (S,S,S,S)-1b-based catalyst system could be performed at an
elevated temperature of 50 °C with a TOF of 876 h−1, affording
the copolymer with 98% enantioselectivity (entry 8). Addi-
tionally, changes in CO2 pressure or/and catalyst loading had
no effect on product enantioselectivity and the selectivity for
copolymer, but caused a significant influence on reaction rate
(entries 9−13). With the use of dichloromethane as an organic
solvent, we achieved the complete conversion in the
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copolymerization performed at 25 °C within prolonged time,
affording the polycarbonates with a high molecular weight of
50200 without any loss in enantioselectivity (entry 14, Figure
S8 in the Supporting Information). It is worthwhile noting here
parenthetically that binary catalyst system consisted of
enantiopure mononuclear salenCo(III) complex and PPNY
exhibited significantly lower activity (TOF < 2 h−1) and
enantioselectivity (ee = 12%) for CXO/CO2 copolymerization
at 25 °C and 2.0 MPa CO2 pressure.
bicyclo[5.1.0]octane. The exo-isomer showed a neglected
reactivity, but the endo-isomer could be copolymerized with
CO2 smoothly to produce the corresponding polycarbonate
(Table 2, entries 3 and 4). It might be attributable to the steric
hindrance of the phenyl, significantly retarding the nucleophilic
attack of the propagating chain at the coordinated epoxide from
its back. The crystallization behaviors of these isotactic
copolymers were investigated by DSC method. It was found
that CO2 copolymers from 3,5,8-trioxaspiro[bicyclo[5.1.0]-
octane-4,1′-cyclohexane] and endo-4-phenyl-3,5,8-trioxa-
bicyclo[5.1.0]octane were crystallizable and their crystallization
endothermic peaks appeared in 179 and 257 °C, respectively.
Perhaps owning to relatively low enantioselectivity (92% ee) of
CO2 copolymer from simple 3,5,8-trioxa-bicyclo[5.1.0]octane
or overlapping of the melting point and decomposition
temperature, only a Tg of ∼118 °C was found in the DSC
trace (Figure S18 in the Supporting Information).
More importantly, the ketal-protecting group of the primary
hydroxyl in the CO2 polymers from 3,5-dioxaepoxides can be
easily removed through dilute acid treatment, affording two
hydroxyl groups in a carbonate unit, which benefits for further
functional modification. In a recent communication, Grinstaff
and co-worker reported the synthesis of atactic and isotactic
linear poly(benzyl 1,2-glycerol carbonate)s via the Co(III)-
complex-mediated ring-opening copolymerization of rac-/(R)-
benzyl glycidyl ether with CO2. Deprotection of the resultant
polymers afforded a functionalizable primary hydroxyl group
appended polycarbonates, poly(1,2-glycerol carbonate)s, which
showed a remarkable increase in degradation rate compared to
poly(1,3-glycerol carbonate)s.30
For the functional modification of CO2 copolymers from 3,5-
dioxa-epoxides, in a typical procedure, the polymer (e.g., Mn =
17.5 kg/mol; PDI = 1.18; Table 1, entry 4) was dissolved in
DMF, and then hydrogen chloride/methanol solution was
added at 0 °C. The deprotection is rapid and complete. No
methyl signal located at 1.35 ppm was observed in the NMR
spectrum of the resultant polymers (Figures 5 and S16 in the
Supporting Information). Removal of the ketal-protecting
group resulted in a reduced molecular weight (Mn = 13.0 kg/
mol; PDI = 1.11). Owning to a great amount of hydroxyl
groups in the resultant polymers, poly(1,2-bis(hydroxymethyl)-
ethylene carbonate)s are not soluble in a non-polar or low polar
organic solvent such as dichloromethane but are easily
dissolved in DMF or DMSO. Similar to poly(1,2-glycerol
carbonate)s, poly(1,2-bis(hydroxymethyl)ethylene carbonate)s
were also found to be sensitive to moisture and easily
decomposed at mild temperatures. The thermolysis curve
indicates a broad range 210−270 °C of 10%−90% thermal
decomposition (Figure 6), compared to a narrow range, 315−
330 °C, before deprotection.
For a comparison purpose, dinuclear Co(III) complex 2a
with nonchiral ethylenediamine backbone was synthesized and
used as catalyst for preparing atactic CO2 copolymer from CXO
(Table 1, entry 15). Although the accurate assignation of the
microstructure of this polycarbonate proved to be very difficult,
13C NMR study could clearly reveal significant differences in
both the carbonyl and methine region between the atactic and
highly isotactic copolymers (Figure S9 in the Supporting
Information). It is deserved to be emphasized that the atactic
CO2 copolymer possesses a high Tg of up to 140 °C. Regarding
the highly isotactic copolymer with >99% ee, one should first
notice from the DSC (differential scanning calorimetry) trace
that the Tg peak has disappeared, while a quite sharp and high
crystallization endothermic peak is now found at 242 °C with
ΔHm = 29.89 J/g (Figure 3, left plot), indicating a very high
degree of crystallinity. Figure 3 (right plots) shows the wide-
angle X-ray diffraction (WAXD) profiles of atactic and highly
isotactic (S,S)-copolymers with >99% ee. No diffraction was
observed for the corresponding atactic copolymer, confirming
its amorphous feature. On the contrary, for highly isotactic
copolymers, sharp diffraction peaks were observed at 2θ values
of 11.8°, 14.3°, 16.9°, and 20.4°, demonstrating that the
isotactic copolymer is a typical semicrystalline polymer.
Furthermore, a variety of 3,5-dioxa-epoxides derivatives were
investigated in the asymmetric copolymerization with CO2
using enantiopure dinuclear Co(III) complex (S,S,S,S)-1b/
PPNY catalyst system (for synthetic procedures of epoxides,
see Supporting Information) (Table 2). In consideration with
the solid or slimy state of these employed epoxides at ambient
temperature, the copolymerization was carried out in toluene.
High TOFs and excellent enantioselectivities were observed in
the systems regarding 3,5-dioxaepoxides with H or cyclohexyl
substituent at 4-positions (Table 2, entries 1 and 2). As
expected, the asymmetric copolymerization provided the
complete alternating copolymers with an S,S-configuration.
After hydrolysis of the resulting polycarbonates with aqueous
NaOH, we succeeded in isolating the corresponding diols in
crystal state (Figure 4). X-ray single crystal analysis revealed its
S,S-configuration. Interestingly, there was a huge difference in
activity between the exo- and endo-4-phenyl-3,5,8-trioxa-
Moreover, the asymmetric terpolymerization of CO2 with
meso-epoxide mixtures of CXO and cyclohexene oxide (CHO)
at various feed ratios were performed in toluene solution with
use of (S,S,S,S)-1b-based/PPNY catalyst system (for detailed
procedures, see Supporting Information). The resulting
terpolymers {designated as poly(CHC-co-CXC)} with different
carbonate units from CXO monomer (CXC carbonates) have
been synthesized by changing the feed ratios of CHO and CXO
(Figures S25−S30 in the Supporting Information). The acid
hydrolysis of CXC carbonate in the resultant terpolymers
provided the crystalline and hydroxyl-functionalized terpol-
ymers, poly(cyclohexene carbonate-co-1,2-bis(hydroxymethyl)-
ethylene carbonate)s {designated as poly(CHC-co-CFC)}. It
Figure 4. Molecular structure of (S,S)-diol produced from the
hydrolysis of isotactic CO2 copolymers from 3,5,8-trioxaspiro-
[bicyclo[5.1.0]octane-4,1′-cyclohexane] (hydrogen atoms and uncoor-
dinated solvent omitted for clarity). Thermal ellipsoids are at the 30%
probability level.
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1
Figure 5. H NMR spectra of PCXC and poly(1,2-bis(hydroxymethyl)ethylene carbonate)s in DMSO-d6.
Figure 6. Thermolysis curves of (A) isotactic PCXC (Mn = 17.5 kg/
mol; PDI = 1.18; Table 1, entry 4), and (B) its deprotection product,
isotactic poly(1,2-bis(hydroxymethyl)ethylene carbonate)s (Mn = 13.0
kg/mol; PDI = 1.11).
Figure 7. DSC thermograms of various poly(CHC-co-CFC) in second
heating: (A) CHC/CFC = 3/2; (B) CHC/CFC = 4/1; (C) CHC/
CFC = 20/1.
was found that the content of 1,2-bis(hydroxymethyl)ethylene
carbonate (CFC) units significantly affected the crystallization
capacity (Figure 7)). The presence of hydroxyl groups allows
for further graft modification to afford degradable brush
polymers (Scheme 2).
To demonstrate the feasibility of the pathways proposed in
Scheme 2, a poly(CHC-co-CFC) terpolymer with ∼10
hydroxyl groups each polycarbonate chain in a unimodal
distribution of molecular weights with a Mn value of 13000 g
mol−1 and PDI of 1.21 was prepared. These hydroxyl groups
appended in polycarbonate backbone could serve as macro-
initiators that allow direct graft polymerization via organo-
catalytic lactide ring-opening polymerization to obtain fully
degradable brush polymers with polycarbonate backbones and
polylactide side chains. In the subsequent experiment, certain
quantities of lactide and DBU (1,8-diazabicyclo[5.4.0]undec-7-
ene) were added in situ into the reaction solution to prepare
predesigned poly(CHC-co-CFC)-g-PLA brush copolymers (for
detailed procedures, see Supporting Information). The GPC
trace of the resultant brush copolymers is shown in Figure 8,
where it was apparent that the side polylactide chains leads to
an increase in the polymer molecular weight. More importantly,
the copolymer displays a monomodal weight distribution with a
narrow PDI value of 1.18, demonstrating the successful chain
extension from the hydroxyl groups of poly(CHC-co-CFC) to
afford the predesigned poly(CHC-co-CFC)-g-PLA brush
copolymers. The structure of the brush copolymer was also
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with 98% ee in an enhanced TOF of 876 h−1. In addition, the
isotactic CO2/CXO copolymer (PCXC) is a typical semi-
crystalline thermoplastic, and possesses a high Tm of 242 °C,
while its atactic copolymer has a high Tg of up to 140 °C, the
highest record for the reported CO2 copolymers from epoxides.
Moreover, the isotactic CO2 copolymers from 3,5,8-trioxaspiro-
[bicyclo[5.1.0]octane-4,1′-cyclohexane] and endo-4-phenyl-
3,5,8-trioxa-bicyclo[5.1.0]octane were found to be crystallizable
and their crystallization endothermic peaks appear in 179 and
257 °C, respectively.
Scheme 2
Importantly, the acid hydrolysis of these isotactic CO2
copolymers from meso 3,5,8-trioxa-epoxides further provided
stereoregular poly(1,2-bis(hydroxymethyl)ethylene carbonate)s
with two hydroxyl groups in a carbonate unit, which showed a
remarkable decrease of ∼80 °C in thermal decomposition
temperature. This hydroxyl-functionalized CO2 copolymer
accords with an unmet need for a readily degradable
biocompatible polycarbonate and also affords chances of graft
modification to prepare bush copolymers for biomedical and
pharmaceutical applications. This approach was initially
demonstrated by organocatalytic ring-opening polymerization
of lactide onto a hydroxyl-functionalized terpolymer, in which
the hydroxyl groups appended in polycarbonate backbone serve
as macroinitiators and DBU (1,8-diazabicyclo[5.4.0]undec-7-
ene) as catalyst, affording fully degradable brush polymers with
polycarbonate backbones and polylactide side chains.
Furthermore, enantiopure dinuclear Co(III)-complex-based
catalyst systems were demonstrated to be efficient in catalyzing
asymmetric terpolymerization of CO2 with CXO and cyclo-
hexene oxide at various feed ratios, with excellent chiral
induction for desymmetric ring-opening of meso-epoxides
incorporated into the polycarbonates. The crystallization
capacity of the resultant terpolymers could be tuned by altering
the feed ratio of the both epoxides. This result provides us
possibility for designing various crystalline CO2 copolymers
with desired properties, which will become our next goal. In
addition, further studies on detailed crystallization behavior and
kinetics are currently underway in our laboratory.
Figure 8. GPC traces of (A) poly(CHC-co-CFC) and (B) poly(CHC-
co-CFC)-g-PLA.
1
revealed by H NMR spectroscopy. That is, as illustrated in
Figure S31 in the Supporting Information, the resonance at 4.7
ppm from the polycarbonate backbone and resonance at 5.2
and 1.5 ppm from polylactide side chains were clearly seen.
It is worthwhile noting here parenthetically that the excellent
chiral induction for desymmetric ring-opening of both CXO
and CHO was also observed in the (S,S,S,S)-1b-mediated
terpolymerization with CO2, though the random distribution of
CXC and CHC units in the terpolymers resulted in a decrease
in crystallization capacity. Importantly, the acid hydrolysis of
CXC units in the resultant terpolymers and further graft
modification via organocatalytic lactide ring-opening polymer-
ization did not have any influence on the main-chain chirality.
ASSOCIATED CONTENT
■
S
* Supporting Information
General experimental procedures and characterizations of
catalyst, organic bases, and copolymers and a .cif crystallo-
graphic file. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
CONCLUDING REMARKS
■
*E-mail: (X.-B.L.) lxb-1999@163.com.
In summary, we have realized the highly selective synthesis of
isotactic CO2 copolymers from meso-3,5-dioxa-epoxides in
perfect alternating nature through stereospecific polymerization
using enantiopure dinuclear Co(III)-complex catalyst systems.
In the presence of a nuclephilic cocatalyst PPNY (Y = 2,4-
dinitrophenoxide), (S,S,S,S)-1b could efficiently catalyze the
asymmetric copolymerization of CO2 with 4,4-dimethyl-3,5,8-
trioxa-bicyclo[5.1.0]octane (CXO) at ambient temperature,
affording the corresponding polycarbonates with more than
99% enantioselectivity for S,S-configuration. Moreover, the
catalyst system proved to be very effective at an elevated
temperature of 50 °C to provide highly isotactic copolymers
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the National Natural Science
Foundation of China (NSFC, Grant 21134002, 21104007), and
Program for Changjiang Scholars and Innovative Research
Team in University (IRT13008). X.-B.L. gratefully acknowl-
edges the Chang Jiang Scholars Program (T2011056) from the
Ministry of Education of the People’s Republic of China.
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7306−7309. (d) Nakano, K.; Hashimotoa, S.; Nozaki, K. Chem. Sci.
2010, 1, 369−373.
REFERENCES
■
(1) (a) Darensbourg, D. J.; Holtcamp, M. W. Coord. Chem. Rev.
1996, 153, 155−174. (b) Dell’Amico, D. B.; Calderazzo, F.; Labella,
L.; Marchetti, F.; Pampaloni, G. Chem. Rev. 2003, 103, 3857−3898.
(c) Sakakura, T.; Choi, J. C.; Yasuda, H. Chem. Rev. 2007, 107, 2365−
2386. (d) Carbon Dioxide as Chemical Feedstock; Aresta, M., Ed.;
Wiley−VCH: Weinheim, Germany, 2010. (e) Federsel, C.; Jackstell,
R.; Beller, M. Angew. Chem., Int. Ed. 2010, 49, 6254−6257. (f) Cokoja,
(12) (a) Kember, M. R.; Knight, P. D.; Reung, P. T. R.; Williams, C.
K. Angew. Chem., Int. Ed. 2009, 48, 931−933. (b) Kember, M. R.;
White, A. J. P.; Williams, C. K. Inorg. Chem. 2009, 48, 9535−9542.
(c) Jutz, F.; Buchard, A.; Kember, M. R.; Fredriksen, S. B.; Williams, C.
K. J. Am. Chem. Soc. 2011, 133, 17395−17405. (d) Lehenmeier, M.
W.; Kissling, S.; Altenbuchner, P. T.; Bruckmeier, C.; Deglmann, P.;
Brym, A. K.; Rieger, B. Angew. Chem., Int. Ed. 2013, 52, 9821−9826.
(13) (a) Chen, X.-H.; Shen, Z.-Q.; Zhang, Y.-F. Macromolecules 1991,
24, 5305−5308. (b) Liu, B.-Y.; Zhao, X.-J.; Wang, X.-H.; Wang, F.-S. J.
Polym. Sci., Part A: Polym. Chem. 2001, 39, 2751−2754. (c) Quan, Z.;
Wang, X.-H.; Zhao, X.-J.; Wang, F.-S. Polymer 2003, 44, 5605−5610.
(d) Sun, X.-K.; Zhang, X.-H.; Liu, F.; Chen, S.; Du, B.-Y.; Wang, Q.;
Fan, Z.-Q.; Qi, G.-R. J. Polym. Sci., Part A: Polym. Chem. 2008, 46,
3128−3139.
M.; Bruckmeier, C.; Rieger, B.; Herrmann, W. A.; Kuhn, F. E. Angew.
̈
Chem., Int. Ed. 2011, 50, 8510−8689. (g) Omae, I. Coord. Chem. Rev.
2012, 256, 1384−1405. (h) Lu, X.-B.; Darensbourg, D. J. Chem. Soc.
Rev. 2012, 41, 1462−1484.
(2) For recent reviews on CO2/epoxides copolymerization, see:
(a) Coates, G. W.; Moore, D. R. Angew. Chem., Int. Ed. 2004, 43,
6618−6639. (b) Darensbourg, D. J.; Mackiewicz, R. M.; Phelps, A. L.;
Billodeaux, D. R. Acc. Chem. Res. 2004, 37, 836−844. (c) Sugimoto,
H.; Inoue, S. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 5561−5573.
(d) Chisholm, M. H.; Zhou, Z. J. Mater. Chem. 2004, 14, 3081−3092.
(e) Darensbourg, D. J. Chem. Rev. 2007, 107, 2388−2410. (f) Luinstra,
G. A. Polym. Rev. 2008, 48, 192−219. (g) Kember, M. R.; Buchard, A.;
Williams, C. K. Chem. Commun. 2011, 47, 141−163. (h) Klaus, S.;
Lehenmeier, M. W.; Anderson, C. E.; Rieger, B. Coord. Chem. Rev.
2011, 255, 1460−1479. (i) Lu, X.-B.; Ren, W.-M.; Wu, G.-P. Acc.
Chem. Res. 2012, 45, 1721−1735.
(14) (a) Wu, G.-P.; Wei, S.-H.; Lu, X.-B.; Ren, W.-M.; Darensbourg,
D. J. Macromolecules 2010, 43, 9202−9204. (b) Wu, G.-P.; Wei, S.-H.;
Ren, W.-M.; Lu, X.-B.; Li, B.; Zu, Y.-P.; Darensbourg, D. J. Energy
Environ. Sci. 2011, 4, 5084−5092. (c) Wu, G.-P.; Darensbourg, D. J.;
Lu, X.-B. J. Am. Chem. Soc. 2012, 134, 17739−17745.
(15) Wei, R.-J.; Zhang, X.-H.; Du, B.-Y.; Sun, X.-K.; Fan, Z.-Q.; Qi,
G.-R. Macromolecules 2013, 46, 3693−3697.
(16) Wu, G.-P.; Wei, S.-H.; Ren, W.-M.; Lu, X.-B.; Xu, T.-Q.;
Darensbourg, D. J. J. Am. Chem. Soc. 2011, 133, 15191−15199.
(17) Kim, J. G.; Cowman, C. D.; LaPointe, A. M.; Wiesner, U.;
Coates, G. W. Macromolecules 2011, 44, 1110−1113.
(18) (a) Darensbourg, D. J.; Wilson, S. J. J. Am. Chem. Soc. 2011, 133,
18610−18613. (b) Darensbourg, D. J.; Wilson, S. J. Macromolecules
2013, 46, 5929−5934.
(19) Schnell, H. Chemistry and Physics of Polycarbonates; Wiley: New
York, 1964.
(20) (a) Wu, G.-P.; Ren, W.-M.; Luo, Y.; Li, B.; Zhang, W.-Z.; Lu, X.-
B. J. Am. Chem. Soc. 2012, 134, 5682−5688. (b) Wu, G.-P.; Jiang, S.-
D.; Lu, X.-B.; Ren, W.-M.; Yan, S.-K. Chin. J. Polym. Sci. 2012, 30,
487−492.
(21) Wu, G.-P.; Xu, P.-X.; Lu, X.-B.; Zu, Y.-P.; Wei, S.-H.; Ren, W.-
M.; Darensbourg, D. J. Macromolecules 2013, 46, 2128−2133.
(22) Ren, W.-M.; Liang, M.-W.; Xu, Y.-C.; Lu, X.-B. Polym. Chem.
2013, 4, 4425−4433.
(23) Nakano, K.; Kobayashi, K.; Ohkawara, T.; Imoto, H.; Nozaki, K.
J. Am. Chem. Soc. 2013, 135, 8456−8459.
(24) (a) Ren, W.-M.; Zhang, W.-Z.; Lu, X.-B. Sci. China: Chem. 2010,
53, 1646−1652. (b) Ren, W.-M.; Liu, Y.; Wu, G.-P.; Liu, J.; Lu, X.-B. J.
Polym. Sci., Part A: Polym. Chem. 2011, 49, 4894−4901.
(25) (a) Nozaki, K.; Nakano, K.; Hiyama, T. J. Am. Chem. Soc. 1999,
121, 11008−11009. (b) Nakano, K.; Nozaki, K.; Hiyama, T. J. Am.
Chem. Soc. 2003, 125, 5501−5510. (c) Nakano, K.; Hiyama, T.;
Nozaki, K. Chem. Commun. 2005, 1871−1873.
(26) (a) Cheng, M.; Darling, N. A.; Lobkovsky, E. B.; Coates, G. W.
Chem. Commun. 2000, 2007−2008. (b) Xiao, Y.; Wang, Z.; Ding, K.
Chem.−Eur. J. 2005, 11, 3668−3678. (c) Abbina, S.; Du, G. D.
Organometallics 2012, 31, 7394−7403.
(27) Nishioka, K.; Goto, H.; Sugimoto, H. Macromolecules 2012, 45,
8172−8192.
(28) Liu, Y.; Ren, W.-M.; Liu, J.; Lu, X.-B. Angew. Chem., Int. Ed.
2013, 52, 11594−11598.
(3) Inoue, S.; Koinuma, H.; Tsuruta, T. Makromol. Chem. 1969, 130,
210−220.
(4) (a) Pang, H.; Liao, B.; Huang, Y. H.; Cong, G. M. Chin. J. Appl.
Chem. 2001, 18, 347−350. (b) Throat, S. D.; Phillips, P. J.; Semenov,
V.; Gakh, A. J. Appl. Polym. Sci. 2003, 89, 1163−1176. (c) Czaplewski,
D. A.; Kameoka, J.; Mathers, R.; Coates, G. W.; Craighead, H. G. Appl.
Phys. Lett. 2003, 83, 4836−4838. (d) Qin, Y.; Wang, X. Biotechnol. J.
2010, 5, 1164−1180.
(5) Presently, China National Offshore Oil Company, Novomer in
the USA, and SK in Korea have undertaken the industrial production
of poly(propylene carbonate) by catalytic CO2/propylene oxide
copolymerization.
(6) (a) Darensbourg, D. J.; Holtcamp, M. W. Macromolecules 1995,
28, 7577−7579. (b) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am.
Chem. Soc. 1998, 120, 11018−11019. (c) Darensbourg, D. J.;
Wildeson, J. R.; Yarbrough, J. C.; Riebenspies, J. H. J. Am. Chem.
Soc. 2000, 122, 12487−12496. (d) Moore, D. R.; Cheng, M.;
Lobkovsky, E. B.; Coates, G. W. Angew. Chem., Int. Ed. 2002, 41,
2599−2602.
(7) (a) Aida, T.; Inoue, S. J. Am. Chem. Soc. 1983, 105, 1304−1309.
(b) Aida, T.; Inoue, S. Acc. Chem. Res. 1996, 29, 39−48. (c) Sugimoto,
H.; Kuroda, K. Macromolecules 2008, 41, 312−317.
(8) (a) Darensbourg, D. J.; Yarbrough, J. C. J. Am. Chem. Soc. 2002,
124, 6335−6342. (b) Eberhardt, R.; Allmendinger, M.; Rieger, B.
Macromol. Rapid Commun. 2003, 24, 194−196. (c) Darensbourg, D. J.;
Mackiewicz, R. M. J. Am. Chem. Soc. 2005, 127, 14026−14038. (d) Li,
B.; Wu, G.-P.; Ren, W.-M.; Wang, Y.-M.; Rao, D.-Y.; Lu, X.-B. J. Polym.
Sci., Part A: Polym. Chem. 2008, 46, 6102−6113. (e) Nakano, K.;
Nakamura, M.; Nozaki, K. Macromolecules 2009, 42, 6972−6980.
(9) (a) Qin, Z.-Q.; Thomas, C. M.; Lee, S.; Coates, G. W. Angew.
Chem., Int. Ed. 2003, 42, 5484−5487. (b) Cohen, C. T.; Chu, T.;
Coates, G. W. J. Am. Chem. Soc. 2005, 127, 10869−10878.
(c) Paddock, R. L.; Nguyen, S. T. Macromolecules 2005, 38, 6251−
6253.
(10) (a) Lu, X.-B.; Wang, Y. Angew. Chem., Int. Ed. 2004, 43, 3574−
3577. (b) Lu, X.-B.; Shi, L.; Wang, Y.-M.; Zhang, R.; Zhang, Y.-J.;
Peng, X.-J.; Zhang, Z.-C.; Li, B. J. Am. Chem. Soc. 2006, 128, 1664−
1674. (c) Shi, L.; Lu, X.-B.; Zhang, R.; Peng, X.-J.; Zhang, C.-Q.; Li, J.-
F.; Peng, X.-M. Macromolecules 2006, 39, 5679−5685. (d) Ren, W.-M.;
Liu, Z.-W.; Wen, Y.-Q.; Zhang, R.; Lu, X.-B. J. Am. Chem. Soc. 2009,
131, 11509−11518.
(29) (a) Brannock, K. C.; Lappin, G. R. J. Org. Chem. 1956, 21,
1366−1368. (b) Gianni, M. H.; Cody, R.; Asthana, M. R. J. Org. Chem.
1977, 42, 365−367. (c) Pavelyev, R. S.; Gnevashev, S. G.; Vafina, R.
M.; Gnezdilov, O. I.; Dobrynin, A. B.; Lisovskaya, S. A.; Nikitina, L. E.;
Klimovitskii, E. N. Mendeleev Commun. 2012, 22, 127−128.
(30) (a) Zhang, H.; Grinstaff, M. W. J. Am. Chem. Soc. 2013, 135,
6806−6809. (b) Geschwind, J.; Frey, H. Macromolecules 2013, 46,
3280−3287.
(11) (a) Nakano, K.; Kamada, T.; Nozaki, K. Angew. Chem., Int. Ed.
2006, 45, 7274−7277. (b) Noh, E. K.; Na, S. J.; Sujith, S.; Kim, S. W.;
Lee, B. Y. J. Am. Chem. Soc. 2007, 129, 8082−8083. (c) Sujith, S.; Min,
K. K.; Seong, J. E.; Na, S. J.; Lee, B. Y. Angew. Chem., Int. Ed. 2008, 47,
1276
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