A One-Step Route to CO2-Based Block Copolymers by Simultaneous ROCOP of CO2/Epoxides and RAFT Polymerization of Vinyl Monomers

Article abstract of DOI:10.1002/anie.201710734

The one-step synthesis of well-defined CO₂-based diblock copolymers was achieved by simultaneous ring-opening copolymerization (ROCOP) of CO₂/epoxides and RAFT polymerization of vinyl monomers using a trithiocarbonate compound bearin

Full text of DOI:10.1002/anie.201710734

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Accepted Article
Title: One-Step Route to CO2-based Block Copolymers via
Simultaneous ROCOP of CO2/Epoxides and RAFT
Polymerization of Vinyl Monomers
Authors: Yong Wang, Yajun Zhao, Yunsheng Ye, Haiyan Peng,
Xingping Zhou, Xiaolin Xie, Xianhong Wang, and Fosong
Wang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201710734
Angew. Chem. 10.1002/ange.201710734
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10.1002/anie.201710734
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COMMUNICATION
One-Step Route to CO2-based Block Copolymers via
Simultaneous ROCOP of CO2/Epoxides and RAFT Polymerization
of Vinyl Monomers
Yong Wang,[a][b] Yajun Zhao,[a] Yunsheng Ye,[a] Haiyan Peng,[a] Xingping Zhou,[a] Xiaolin Xie,*[a]
Xianhong Wang,*[b] and Fosong Wang[b]
ROCOP of CO2/epoxides is extremely sensitive to catalyst and
reaction temperature because of the quite small energy barrier
between polycarbonates and corresponding cyclic by-
products,[2c,2d] making it difficult to meet the harsh requirements
mentioned above, thus, one-step synthesis of CO2-based block
copolymers remains a great challenge that have been rarely
achieved. Coates and coworkers have donated the first example
of one-step synthesis of CO2-based block copolymer via pre-rate-
determining polymerization of cyclohexene oxide (CHO), CO2 and
diglycolic anhydride,[7] while the seminal work by Williams[8] and
Rieger[9] is based on switchable polymerization between ring
opening polymerization (ROP) of lactones and ROCOP of
CHO/CO2. These achievements are encouraging, but they rely on
the distinctive reactivity of CHO and are not applicable for other
epoxides such as propylene oxide (PO)-the most valuable
epoxide in industry. More importantly, current synthetic strategies
are not able to meet the stringent demand for functionality
control.[10]
Abstract: Herein, we report the unprecedented one-step synthesis of
well-defined CO2-based diblock copolymers via simultaneous ring
opening copolymerization (ROCOP) of CO2/epoxides and RAFT
polymerization of vinyl monomers using a trithiocarbonate compound
bearing carboxylic group (TTC-COOH) as bifunctional chain transfer
agent (CTA). The double chain transfer effect allows for independent
and precise control over the molecular weight of the two blocks and
ensures narrow polydispersities of the resultant block copolymers
(1.09-1.14). It’s notable that unusual axial group exchange reaction
between aluminium porphyrin catalyst and TTC-COOH impedes the
formation of homopolycarbonates. By taking the advantage of RAFT
technique, it is able to meet the stringent demand for functionality
control to well expand the application scopes of CO2-based
polycarbonates.
Since the seminal work by Inoue in 1969,[1] ring opening
copolymerization (ROCOP) between CO2 and epoxides has
gained intensive attentions both from economic and
environmental considerations, as it offers a sustainable and
environmental
benign
approach
to
biodegradable
polycarbonates.[2] However, CO2-based polycarbonates suffer
from poor mechanical and thermal properties as well as limited
functionalities, which greatly hinder their application scopes.[3]
Inspired by nature’s sophisticated sequence control over
biomacromoleculars (e.g., DNA, protein), block copolymers are
gaining momentum in polymer chemistry, as they offer the
opportunity to tailor the properties, morphologies as well as
functionalities of polymers at molecular scale.[4]
Last decades have witnessed significant achievements for
fine control over the properties of CO2-based polycarbonates
through block copolymer synthesis,[5] predominant among them
relying on sequential monomer addition and macroinitiation.
Considering the high CO2 pressure and water sensitive reaction
conditions required for ROCOP, one-step strategies are much
more desirable for synthesis of CO2-based block copolymers,
albeit the requirement for compatibility of polymerization
mechanisms, match of reaction conditions and polymerization
rates as well as tolerance among the reagents.[6] Nonetheless,
Scheme 1. Synthesis of CO2-based block copolymers via one-step
terpolymerization of CO2, epoxide and vinyl monomer.
Here we report the unprecedented terpolymerization of CO2,
epoxides and vinyl monomers for one-step synthesis of various
CO2-based block copolymers with completely alternating
[a]
Dr. Y. Wang, Y. J. Zhao, Prof. Dr. Y.S. Ye, Prof. Dr. H. Y. Peng,
Prof. Dr. X. P. Zhou, Prof. Dr. X. L. Xie.
School of Chemistry and Chemical Engineering,
Huazhong University of Science and Technology
Wuhan 430074, P. R. China
polycarbonate segments using
a
carboxy/trithiocarbonate
bifunctional chain transfer agent (TTC-COOH) (Scheme 1). This
protocol is of high interest as RAFT polymerization provides facile
functionality control.[11] The core issue lies in the judicious choice
of 5,10,15,20-tetrakis(2-chlorophenyl)porphyrin aluminum(III)
chloride [(TPP2-Cl)Al(III)Cl] as catalyst for ROCOP. On the one
hand, optimal reaction temperature for (TPP2-Cl)Al(III)Cl is
E-mail: xlxie@hust.edu.cn
[b]
Prof. Dr. X. H. Wang, Prof. Dr. F. S. Wang.
Key Laboratory of Polymer Ecomaterials,
Changchun Institute of Applied Chemistry, CAS,
Changchun 130022, P. R. China
o
between 65-70 C, meeting the requirement for RAFT initiation;
on the other hand, (TPP2-Cl)Al(III)Cl shows high tolerance with
thiocarbonates and radicals.[12] Although catalysts with cobalt as
the main component have shown distinguished catalytic
E-mail: xhwang@ciac.ac.cn
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201710734
Angewandte Chemie International Edition
COMMUNICATION
Figure 1. MALDI-TOF mass spectrum of PCHC end-capped with -OH and -
O2C-TTC (Table 1, entry 6).
performances and monomer universality for ROCOP of
CO2/epoxides, they suffer from loss of activity at elevated
temperature and are highly sensitive to radicals,[13] while other
catalysts effective for immortal ROCOP are generally not
applicable for terminal epoxides.[14]
Typically, the axial groups of (Salen)M(III)X or
(Porphyrin)M(III)X have been generally regarded to serve as
Table 1. ROCOP of CO2/epoxides in the presence of TTC-COOH and (TPP2-
Cl)Al(III)Cl. [a]
initiators
for
copolymerization
between
CO2
and
epoxides,[13b,16] and therefore some of resultant polymers are
expected to be terminated with X despite the presence of chain
transfer agent.[17] Williams and Darensbourg succeded in
obtaining α,ω-dihydroxyl CO2-based polycarbonates using
catalysts with trifloroacetate (TFA) as axial group, given that
TFA group could quickly undergo hydrolysis in the presence of
water.[18] However, there has been none reports about
syntheis of homogeneous α-X-ω-hydroxyl polycarbonates.
Therefore, both α-TTC-ω-hydroxyl and α-Cl-ω-hydroxyl
polycarbonates were anticipated to form by (TPP2-Cl)Al(III)Cl in
the presence of TTC-COOH. However, MALDI-TOF-MS
spectra of poly(cyclohexene carbonate) (PCHC) exhibited only
one series of peaks in accordance with [(TTC-
COOH)(CHO)1(CHO-CO2)n]Na+ (Figure 1), suggesting that all
the propagation chains were initiated by –O2C-TTC instead of
the axial Cl of (TPP2-Cl)Al(III)Cl. Thus, it could be tentatively
assumed that fast axial group exchange between (TPP2-
Cl)Al(III)Cl and TTC-COOH occurs before initiation of ROCOP,
[Cat.]/
[CTA]
t
Mn(GPC) [b]
Mn(NMR) [c]
Entry
epoxide
PDI[b]
(h)
(g mol-1)
(g mol-1)
1
2
3
4
5
6
7
8
PO
PO
PO
PO
PO
CHO
HO
HO
1/5
1/8
8
8
8
5
20
8
12
8
8200
6900
5600
4200
-
4500
7200
4100
8600
6600
5400
4300
-
4300
7600
4200
1.13
1.09
1.05
1.05
-
1.06
1.06
1.05
1/10
1/10
1/15
1/10
1/10
1/10
[a] The reactions were carried out in autoclave pressured at 65 oC with 3.0 MPa
CO2, and the mole ratio of [PO]/[Cat.]/[PPNCl] was set as 5000/1/0.8. [b]
Determined by GPC, calibrated by polystyrene standards. [c] Calculated from
1H NMR spectra.
At first, ROCOP of CO2 and PO were conducted in order to
investigate the compatibility between TTC-COOH and aluminum
porphyrin complexes (Table S1). All of the catalysts displayed
well-controlled manner between 65-75 oC, yielding polymeric
products with uniform and narrow molecular weight distributions.
Especially, polycarbonates obtained from (TPP2-Cl)Al(III)Cl
exhibited completely alternating structure as indicated by 1H NMR
spectra (Figure S7), although polyether unites have been
generally regarded as inevitable for catalysts with Al as the main
component (Table S1, entries 5-7)[15]. Furthermore, monomer
generality and effect of TTC-COOH loading have been explored
1
to address this, H NMR spectral investigation was made for
equal molar mixture of (TPP2-Cl)Al(III)Cl and TTC-COOH in d6-
DMSO at room temperature (Figure 2). It is clear that original
signals assigned to protons near trithiocarbonate moiety
completely disappear (e.g. e, f and g), while new signals
around δ=-0.6 and -1.4 ppm can be respectively assigned to -
CH2CH2CO2- and -CH2CH2CO2- according to the peak shapes
as well as their corresponding integral areas compared with
pyrrole protons of porphyrin ligand. The sharp shift to high field
could be ascribed to the strong magnetic shielding effect
induced by a porphyrin ring, confirming the direct bonding of -
CO2-TTC with Al center.[19] Morevoer, 1H NMR study of mixture
of (TPP2-Cl)Al(III)Cl, PPNCl and TTC-COOH has revealed that
TTC-COOH does not react with PPNCl. (Figure S9).
by (TPP2-Cl)Al(III)Cl at 65 C (Table 1). Higher loading of TTC-
o
COOH is beneficial for enhancing chain transfer efficiency and
producing PPC with smaller PDIs (Table 1, entries 1-3), however,
loss of catalytic activity accompanied and (TPP2-Cl)Al(III)Cl was
found to be completely inactive in the presence of 15 equivalent
TTC-COOH (Table 1, entry 5). Moreover, Molecular weights of
the products were tailorable by controlling the reaction time (Table
1, entries 3, 4, 7 and 8).
Figure 2. 1H NMR spectrum of equal molar mixture of (TPP2-Cl)Al(III)Cl and TTC-
COOH in d6-DMSO at room temperature after 15 min.
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10.1002/anie.201710734
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COMMUNICATION
Table 2. One-pot simultaneous ROCOP/RAFT for synthesis of PPC-b-PMMA.[a]
PO
(mL)
MMA
(mL)
Temp
(oC)
t
PPC yield[b]
(%)
PC yield[b]
(%)
PMMA yield[b]
(%)
Mn(GPC)[c]
Mn(theo)[d]
Entry
PDI[c]
(h)
(kg mol-1)
(kg mol-1)
1
2
3
4
5
6
7
5
5
5
5
6
7
8
5
5
5
5
4
3
2
65
70
75
75
65
65
65
8
8
2.5
4
8
31
39
19
22
37
37
35
11
19
49
49
20
18
19
48
80
44
74
71
66
59
14.4
21.6
11.8
17.4
17.7
14.5
11.9
16.2
22.2
12.5
18.6
19.1
15.1
13.0
1.09
1.12
1.14
1.13
1.09
1.09
1.10
8
8
[a] The reactions were carried out in autoclave pressured with 3.0 MPa CO2, mole ratio of [PO]/[(TPP2-Cl)Al(III)Cl]/[TTC-COOH]/[PPNCl]/[AIBN]
was set as 2500/1/10/0.8/2. [b] Determined by 1H NMR, PPC yield=A4.92/(A4.92+A4.57+A2.66), PC yield=A4.57/(A4.92+A4.57+A2.66), PMMA
yield=A3.56/(A3.56+A3.68). [c] Determined by GPC, calibrated by polystyrene standards. [d] Calculated by the equation:
Mn,theo=403.67+102.09×nPO/nRAFT-COOH×A 4.92/(A4.92+A4.57+A2.66)+100.12×nMMA/nRAFT-COOH×A 4.92/A4.57, where nPO, nMMA, nRAFT-COOH respectively
represent the amount of PO, MMA and TTC-COOH.
Furthermore,
compatibility
between
poly(propylene
distributions despite different monomer feed ratios and
conversions, indicating the formation of block copolymer instead
of blend of PPC and PMMA (Figure S35-41). Given that carboxylic
group and trithiocarbonate independently serve as chain transfer
agent for ROCOP of CO2/PO and RAFT of MMA, the theoretical
molecular weights (MW) of PPC and PMMA segments could be
easily calculated, the sum of which were found to be in good
agreement with the number average molecular weight (Mn) from
GPC curve. Taking entry 3 for example, the theoretical MW of
PPC and PMMA could be respectively determined as 4.7 kg mol-
carbonate) (PPC) and free radical has been investigated using
PPC-TTC obtained from ROCOP as macroinitiator for RAFT
polymerization of methyl methacrylate (MMA). Surprisingly, PPC
was found to be completely degraded into cyclic carbonate
(Figure S10) when PPC-TTC with catalyst residue was directly
explored as RAFT agent (Scheme 2, Route 1). However,
experiments with purified PPC-TTC led to formation of diblock
PPC-b-PMMA, suggesting that degradation of PPC was caused
by catalyst residue instead of radicals (Route 2). Moreover,
diblock copolymers were also obtained using PPC-TTC with
catalyst residue as macro RAFT agent when the reaction was
carried out in an autoclave pressured with 3.0 MPa CO2 (Route
1
and 7.3 kg mol-1, while the Mn(GPC) was 11.8 kg mol-1. More
direct evidence for the formation of PPC-b-PMMA came from the
1H DOSY NMR spectra of the resultant products, where all signals
shared the same diffusion coefficient (Figure S47), while that of a
blend of PPC/PMMA showed two different diffusion coefficients
(Figure S48).[8b,20]
3).
This
interesting
phenomenon
comfirms
the
polycarbonate/radical compatibility and indicates the viability of
one-step synthetic prototol for CO2-based block copolymers.
Table 3. Block copolymers prepared from other epoxides (HO, CHO) and
vinyl monomers (styrene, benzyl methacrylate). [a]
Mn(GPC) [b]
(g mol-1)
Mn(NMR) [c]
(g mol-1)
Vinyl
monomer
Entry
epoxide
PDI
1
2
3
HO
CHO
PO
MMA
MMA
styrene
benzyl
methacrylate
16.4
19.2
13.8
17.5
18.7
12.7
1.09
1.10
1.14
4
PO
17.2
18.6
1.14
Scheme 2. RAFT polymerization of MMA with PPC-TTC as RAFT agent at
various conditions.
[a]The reactions were carried out in autoclave pressured with 3.0 MPa CO2
at 65 oC for 6-8 h, mole ratio of [Epoxide]/[Vinyl monomer]/[(TPP2-
Cl)Al(III)Cl]/[TTC-COOH]/[PPNCl]/[AIBN] was set as 2500/1650/1/10/0.8/2.
[b] Determined by GPC, calibrated by polystyrene standards. [c] Determined
by 1H NMR.
One-pot terpolymerization of epoxide, CO2 and vinyl
monomer was conducted using PO and MMA as modal
substrates in autoclave under 3.0 MPa, the feed ratio of
[PO]/[(TPP2-Cl)Al(III)Cl]/[TTC-COOH]/[PPNCl]/[AIBN] was set as
2500/1/10/0.8/2 to balance the catalytic activity and chain transfer
efficiency for both ROCOP and RAFT polymerization (Table 2).
According to 1H NMR spectra of the reaction mixture, both PPC
with completely alternating structure and PMMA formed in the
one-step process, moreover, none signals corresponding to
junction units were found. At 65 oC, the yields of PPC and PMMA
were respectively 32% and 49%, indicating that ROCOP of
CO2/PO and RAFT of MMA shared close polymerization rates
under experimental conditions. The polymerization rates for both
ROCOP and RAFT increased steadily with temperature, however,
Having demonstrated the viability of this one-step protocol,
we embarked on further evaluation of its generality among various
epoxides and vinyl monomers (Table 3). In all cases, we
successfully obtained the target block copolymers with completely
alternating polycarbonate segments. It is notable that the
resultant block copolymers all displayed narrow polydispersity
distributions (<1.14) owing to the double chain transfer effect,
which is remarkable for block copolymer synthesis. Based on the
experimental results, mechanism has been proposed for ROCOP
(Scheme 3), wherein fast axial group exchange reaction between
(TPP2-Cl)Al(III)Cl and TTC-COOH before initiation allows
formation of homogenous α-TTC-ω-hydroxyl polycarbonates
instead of α-Cl-ω-hydroxyl polycarbonates. As α-Cl-ω-hydroxyl
polycarbonates can not serve as CTA for RAFT polymerization,
o
the content of propylene carbonate (PC) reached 49% at 75 C
(Table 2, entry 3). All the GPC curves showed monomodal
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10.1002/anie.201710734
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COMMUNICATION
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block copolymer synthesis.
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Scheme 3. Mechanistic aspects for ROCOP in the presence of (TPP2-Cl)Al(III)Cl
and TTC-COOH.
In summary, novel one-step synthesis of CO2-based block
copolymers via simultaneous RAFT polymerization and ROCOP
of CO2/epoxides in the presence of bifunctional chain transfer
agent is described. This protocol has demonstrated its generality
among various epoxides and vinyl monomers, enabling facile
synthesis of well-controlled CO2-based block copolymers with
different functionalities by taking the advantage of RFAT
technique. Fast axial group exchange reaction between catalyst
and chain transfer agent for ROCOP has been observed for the
first time, which impedes the formation of homopolycarbonates.
Future efforts will be directed towards obtaining CO2-based block
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Acknowledgements
Financial support from NSFC (No. 21604027) and National Key
R&D Plan (No. 2016YFB0302400) as well as and the analytical and
testing assistance from the Analysis and Testing Center of HUST are
gratefully acknowledged.
Keywords: one-step • block copolymers • carbon dioxide •
coordination copolymerization • RAFT
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10.1002/anie.201710734
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
One-step synthesis of CO2-based
block copolymers with completely
alternating polycarbonate segments
was achieved via simultaneous ring
Yong Wang,[a][b] Yajun Zhao,[a] Yunsheng
Ye,[a] Haiyan Peng,[a] Xingping Zhou,[a]
Xiaolin Xie,*[a] Xianhong Wang,*[b] and
Fosong Wang[b]
opening
CO2/epoxides
copolymerization
and
of
RAFT
Page No. – Page No.
One-Step Route to CO2-based Block
Copolymers via Simultaneous
ROCOP of CO2/Epoxides and RAFT
Polymerization of Vinyl Monomers
polymerization of vinyl monomers.
Double chain transfer effect ensures
narrow polydispersities and axial group
exchange reaction impedes the
formation of homopolycarbonates.
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