Highly active, bifunctional Co(III)-salen catalyst for alternating copolymerization of CO2 with cyclohexene oxide and terpolymerization with aliphatic epoxides

Article abstract of DOI:10.1021/ma902321g

The cobalt(III) complex of a salen ligand bearing one quaternary ammonium salt on the three-position of one aromatic ring is a highly active catalyst for the alternating copolymerization of cyclohexene oxide (CHO) and CO₂ to afford the correspo

Full text of DOI:10.1021/ma902321g

1396 Macromolecules 2010, 43, 1396–1402
DOI: 10.1021/ma902321g
Highly Active, Bifunctional Co(III)-Salen Catalyst for Alternating
Copolymerization of CO2 with Cyclohexene Oxide and Terpolymerization
with Aliphatic Epoxides
Wei-Min Ren, Xiao Zhang, Ye Liu, Jian-Feng Li, Hui Wang, and Xiao-Bing Lu*
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
Received October 19, 2009; Revised Manuscript Received December 20, 2009
ABSTRACT: The cobalt(III) complex of a salen ligand bearing one quaternary ammonium salt on the
three-position of one aromatic ring is a highly active catalyst for the alternating copolymerization of
cyclohexene oxide (CHO) and CO2 to afford the corresponding poly(cyclohexene carbonate) (PCHC) at
various temperatures. The cobalt-based catalyst exhibited excellent activity and selectivity for polymer
formation at high temperatures up to 120 °C and even under low CO2 pressures of 0.1 MPa. Also, the
cobalt(III)-salen complex could operate very efficiently for the terpolymerization of CHO and aliphatic
epoxides with CO2 to provide selectively polycarbonates with a narrow polydispersity at high temperatures.
The polycarbonates resulting from the terpolymerization of equimolar CHO and propylene oxide (PO) with
CO2 have a close content for both cyclohexene carbonate and propylene carbonate units. This is ascribed to
the presence of CHO significantly inhibiting the reactivity of PO and thereby causing a matched reactivity for
both epoxides during the terpolymerization. The competition coordination of CHO and PO to the
electrophilic cobalt(III) ion has no effect on regioselective ring-opening of PO in the terpolymerization.
Introduction
(PPC) with >99% selectivity and 90-99% carbonate linkages at
a CO2 pressure of 55 bar and ambient temperature. Unfortunately,
The development of efficient catalytic processes for CO2
transformation to economically competitive products is of great
interest and has been a long-standing goal for chemists.1 One of
the most promising ways to utilize effectively CO2, the synthesis
of polycarbonates through the metal-catalyzed coupling reac-
tions of CO2 and epoxides, was first reported by Inoue et al. in
19692 and has attracted much attention to both academic and
industrial researchers. In recent decades, numerous catalyst
systems have been developed for this transformation.3 In parti-
cular, some well-defined homogeneous metal complexes have
been reported to be highly active and selective catalysts, most
notably zinc phenoxides,4 β-diiminate zinc alkoxides,5 and binary
or bifunctional catalyst systems based on metal-salen or -salan
complexes.6-9
low CO2 pressures or elevated temperatures (∼50 °C) led to the
nearly complete loss in activity. We delightedly found that the
addition of a nucleophilic cocatalyst such as quaternary ammo-
nium halides could significantly improve the activity of SalenCo-
(III)X (X = 2,4-dinitrophenoxy) 1 even at low CO2 pressures and/
or elevated temperatures.7a Further mechanistic studies demon-
strated that the nucleophilic cocatalyst functioned not only as an
initiator7d but also in another role for stabilizing the active
SalenCo(III) species against decomposition to SalenCo(II), which
was ineffective in producing copolymer.11 An elegant design of
bifunctional catalyst 2 that contains a Lewis acidic metal center
and quaternary ammonium salt units in a molecule was repo-
rted by Lee et al.9b,c The bifunctional catalyst exhibited excellent
activity and polymer selectivity, even at high temperatures and
high [epoxide]/[catalyst] ratios. Prior to this study, Nozaki et al.
utilized a salen-type cobaltate complex with a piperidinium end-
capping arm as catalyst for selectively synthesizing aliphatic
polycarbonates from CO2 and terminal epoxides and first suc-
ceeded in synthesizing a block polycarbonates terpolymer.9a
In 2003, Coates and coworkers were the first to report the use
of SalenCoX (salen, N,N0-bis(3,5-di-tert-butylsalicylidene)-1,2-
cyclohexanediamine) complexes with a nucleophilic axial X
group alone as catalysts for CO2/propylene oxide (PO) copolym-
erization.10 This approach produced poly(propylene carbonate)
Stimulated by the success with bifunctional cobalt-based
catalyst systems for the alternating copolymerization of CO2
and PO, herein we apply the bifunctional Co(III)-salen complex 3
*Corresponding author. E-mail: lxb-1999@163.com.
pubs.acs.org/Macromolecules
Published on Web 01/14/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 3, 2010 1397
Table 1. CHO/CO2 Copolymerization Resultsa
TOF (h-1 b
)
Mn (kg/mol)c
PDI Mw/Mn
c
entry
catalyst
[CHO]/[cat.]
pressure (MPa)
temp (°C)
time (h)
1
2
3
4
5
6
7
8
3
2000
2000
5000
5000
5000
5000
5000
5000
5000
5000
5000
1.5
1.5
1.5
2.5
0.1
0.1
2.0
2.0
2.5
2.5
2.5
25
25
25
25
25
50
50
80
6.0
8.0
8.0
8.0
8.0
5.0
2.0
1.0
0.5
0.3
1.0
170
87
158
162
68
263
1018
1924
4509
6105
<5
52.0
19.1
62.9
64.1
24.7
48.2
68.7
52.7
43.8
29.3
1.08
1.17
1.06
1.04
1.08
1.12
1.10
1.14
1.20
1.23
1/nBu4NXd
3
3
3
3
3
3
3
3
9
10
11
100
120
100
1/nBu4NXd
a Reaction was performed in neat cyclohexene oxide (CHO) (9.8 mL, 100 mmol) in a 75 mL autoclave. The polymer selectivity and carbonate linkages
of the resulted polycarbonates are >99% based on 1H NMR spectroscopy. b Turnover frequency (TOF) = mole of product (PCHC)/mol of catalyst per
hour. c Determined by gel permeation chromatography in THF, calibrated with polystyrene standards. d Reaction was performed using binary catalyst
system of 1/nBu4NX (X = 2,4-dinitrophenoxy) with a molar ratio of 1/1.
bearing a quaternary ammonium salt on the three-position of one
aromatic ring to the copolymerization of CO2 and cyclohexene
oxide (CHO) as well as terpolymerization with aliphatic epoxides
at high temperatures. As distinct from the complex 2 containing
two quaternary ammonium salts on the five-position of the
aromatic rings, complex 3 present in this study concerns only a
pendant quaternary ammonium salt on the three-position of one
aromatic ring. This structure probably benefits to improving
intramolecular cooperation catalysis of the electrophilic cobalt
ion and the nucleophilic anion of the anchored quaternary
ammonium salt. Also, we are interested in exploring the regio-
chemistry with regard to carbonate units sequence in the ter-
polymers obtained at various temperatures.
for CHO/CO2 copolymerization, and an activity with a turn-
over frequency (TOF) of 24 h-1 was achieved at 0.1 MPa CO2
pressure and 90 °C.13
In sharp contrast with the copolymerization of CO2 with
aliphatic epoxides, the copolymerization of CO2 with alicyc-
lic CHO has great selectivity for polycarbonates formation,
even at elevated temperatures, because of the increased strain
in forming the five-membered carbonate ring imposed by the
conformation of the alicyclic group.14 Therefore, the deve-
lopment of a thermally robust catalyst is highly desirable. In
a recent mechanistic study regarding a highly active and
thermally stable cobalt(III)-salen catalyst with a sterically
hindered organic base anchored on the ligand framework, we
have demonstrated that the formed carboxylate intermediate
helped to stabilize the active Co(III) species against decom-
position to inactive Co(II) by reversibly intramolecular
Co-O bond formation and dissociation.11 Indeed, the addi-
tion of a nucleophilic cocatalyst could effectively stabilize the
active Co(III) species and thus significantly improve the
activity of Co(III)-salen complexes only at temperatures
<80 °C, whereas beyond this temperature, the binary cata-
lyst systems usually lose their activities and predominately
form the inactive Co(II)-salen. We were gratified to discover
that the bifunctional catalyst 3 exhibited highly thermal
stability and excellent activity at high temperatures even up
to 120 °C. As anticipated, the reaction rate is highly sensitive
to the reaction temperature (entries 4, 7-10). An increase in
temperature from 25 to 100 °C resulted in a dramatic increase
in TOF from 162 to 4509 h-1. Furthermore, a TOF up to
6105 h-1 was achieved at 120 °C, the highest record in this
copolymerization. In comparison, the binary catalyst system
shows complete loss in activity at 100 °C, and Co(II)
derivative as red solid precipitate was observed in the reac-
tion mixture (entry 11). It is generally known that the
homopolymerization of CHO easily takes place at elevated
temperatures and is normally observed at catalyst systems
with regard to much-studied zinc complexes.4 However, in
the present studies, the polycarbonates obtained at the range
of 25 to 120 °C all have >99% carbonate linkages; also, the
polymer selectivity of >99% does not change with the
reaction temperature. To the best of our knowledge, this is
the first example to be highly active and selective for alter-
nating copolymerization of CHO and CO2 at >100 °C. It is
Results and Discussion
Alternating Copolymerization of CO2 and CHO Catalyzed
by the Complex 3 at Various Temperatures. The Co(III)-salen
complex 3 bearing a quaternary ammonium salt on the three-
position of one aromatic ring was prepared by the reported
method.11 The synthesis of enantiopure Co(III)-salen com-
plex 3 is similar to the corresponding racemic complex only
concerning the use of enantiopure 1,2-diaminocyclohexane
mono(hydrogen chloride). Because complex 3 is easily dis-
solved in neat epoxides surveyed, the catalyzed coupling of
CO2 and epoxides does not require any organic cosolvent.
Our initial studies show that complex 3 could efficiently
catalyze CHO/CO2 copolymerization to afford selectively the
corresponding copolymer with >99% carbonate linkages at a
[CHO]/[catalyst] molar ratio of 2000 under a CO2 pressure of
1.5 MPa and at ambient temperature (Table 1, entry 1). The
catalytic activity is double that of the binary catalyst system of
complex 1 in conjunction with nBu4NX (X = 2,4-dinitrophe-
noxy) under the same conditions (entry 2). When [CHO]/
[catalyst] ratio was increased to 5000, no obvious change in
catalyst activity and polymer selectivity was observed in this
reaction (entry 3). Notably, the bifunctional catalyst proved to
be very active under 0.1 MPa CO2 pressure under ambient
temperature or elevated temperatures (entries 5 and 6). Indeed,
only limited catalysts exhibit certain activities for CHO/CO2
copolymerization at room temperature and relatively low CO2
pressures, probably because of the more sterically hindered, less
reactive CHO monomer.12-14 For example, Williams and co-
workers recently reported a novel macrocyclic dizinc catalyst

1398 Macromolecules, Vol. 43, No. 3, 2010
Table 2. Terpolymerization of CHO and Aliphatic Epoxides with CO2 Catalyzed by the Complex 3a
Ren et al.
CHC
linkages (mol %)d
c
entry
epoxide
CHO
CHO
PO
PO
CHO/PO
CHO/PO
CHO/PO
CHO/EOh
CHO/BuOi
CHO/HOj
temp (°C)
time (h)
TOF (h-1 b
)
Mn (kg/mol)c
Mw/Mn
Tg (°C)e
T50 (°C)f
1
2
25
90
25
90
25
60
90
90
90
90
8.0
0.5
4.0
0.5
8.0
1.5
0.5
0.5
0.5
0.5
158
3020
497
5160
202
1246
3590
4250
2564
1958
62.9
40.6
69.5
58.8
70.5
82.1
50.9
48.0
42.1
39.7
1.06
1.15
1.09
1.22
1.04
1.14
1.12
1.18
1.10
1.15
100
100
0
118
310
3
42
252
4g
5
0
48
49
52
37
56
65
77
78
79
32
68
72
285
290
292
289
298
302
6
7
8
9
10
a Reaction was performed in neat epoxide (3/epoxide = 1/5000, molar ratio) in a 75 mL autoclave. The molar ratio of CHO/aliphatic epoxide in
terpolymerization is 1/1. When the reaction temperatures are 25, 60, and 90 °C, the CO2 pressures in the corresponding systems are 1.5, 2.0, and 2.5 MPa,
respectively. The resulting polymers all have >99% carbonate linkages. The selectivity for polymer formation in the CHO/aliphatic epoxide/CO2
terpolymerization is all >99%. b Turnover frequency (TOF) = mole of product (polycarbonates)/mol of catalyst per hour. c Determined by GPC
against polystyrene standards. d Proportion of cyclohexene carbonate linkages in the resulting polymers. e Glass-transition temperature determined by
DSC. f Temperature at polymer decomposition of 50% measured by TGA. g About 6% of cyclic propylene carbonate was also observed. h EO, ethylene
oxide. i BuO, 1,2-butene oxide. j HO, 1,2-hexene oxide.
worth noting here that a decrease in polycarbonate molecu-
lar weight and a slight increase in polymer molecular-weight
distribution were observed at enhanced reaction tempera-
tures.
Terpolymerization of CO2, CHO, and Aliphatic Epoxides
Catalyzed by the Complex 3. Although aliphatic epoxides are
inexpensive or easily accessible from inexpensive commercial
starting materials, the resulting polycarbonates by the co-
polymerization with CO2 have relatively low glass-transition
temperatures (Tg) <50 °C, which limits their use as structur-
al materials in various fields. As a result, some attention was
paid to the terpolymerization of aliphatic epoxides, CHO,
and CO2 for adjusting Tg of the polycarbonates, but only
limited success was achieved.15 The main problem is that the
great difference in reactivity of CHO and aliphatic epoxides
during the terpolymerization with CO2 leads to the difficulty
in controlling the composition and the alternating nature of
the resulting copolymer.15 For example, bis-2,6-difluoro-
phenoxide dimeric complexes of zinc have been reported as
catalysts for the terpolymerization of PO (50 mol %) and
Figure 1. Plot of the Tg versus CHC units content in the CHO/PO/CO2
terpolymers.
CHO (50 mol %) with CO2 at 55 °C and 4 to 5 MPa pressure,
but the resulting polymer had 84.76 mol % cyclohexene
(Mw/Mn ratio) (Table 2, entries 5-7). In the 1H NMR of the
resulting terpolymers in CDCl3, one (δ = 4.65) of the two
peaks was attributed to CH in cyclohexene carbonate unit
and the other (δ = 4.99) was attributed to CH in propylene
carbonate unit (Supporting Information, Figure S1). No
ether linkage signal at δ = 3.4 to 3.5 assignable to the repeat-
ing oxy(1,2-cyclohexene) or oxy(1,2-propylene) units was
carbonate linkages, only 11.90 mol % propylene carbonate
linkages, and 3.34 mol % propylene ether linkages, with
concomitant production of cyclic propylene carbonate.15a
Inspired by our success in the use of binary cobalt-based
catalyst systems for the alternating copolymerization of both
PO and CHO with CO2 under mild conditions, we also
applied these catalyst systems to the terpolymerization of
PO, CHO, and CO2 and first succeeded in synthesizing the
CO2/PO/CHO terpolymer with only one Tg and one thermo-
lysis peak.17 Notably, the Tg is adjustable between 50 and
100 °C by controlling the relative proportions of PO and
CHO. The main drawback is that the catalytic efficiency and
the resulting polymer molecular weight are relatively low
because the binary catalyst systems could not perform at
high temperatures. Therefore, the development of a highly
active, thermally stable catalyst is highly desirable for this
terpolymerization.
Catalyst 3 bearing a quaternary ammonium salt on the
three-position of one aromatic ring exhibited very excellent
activities for both PO/CO2 and CHO/CO2 copolymerization
with high polymer selectivity at high temperatures (Table 2,
entries 1-4). As expected, complex 3 could operate effi-
ciently for the terpolymerization of CHO (50 mol %) and PO
(50 mol %) with CO2 at various temperatures to pro-
vide selectively polycarbonates with a narrow polydispersity
1
observed in the H NMR spectra of the resulting terpoly-
mers. This verified that the terpolymers have >99% carbo-
nate linkages. A TOF up to 3590 h-1 was observed for this
terpolymerization at 90 °C. Although PO is a more active
epoxide than CHO with copolymerization with CO2 under
the same reaction conditions, the resulting terpolymers have
>48 mol % cyclohexene carbonate (CHC) units after ∼20%
conversion of all epoxides.
Surprisingly, the selectivity for polymer formation in the
CHO/PO/CO2 terpolymerization is >99% at 90 °C, whereas
∼6% cyclic propylene carbonate was observed at mixture
products during CO2/PO copolymerization at the same
temperature. This result indicates that the presence of
CHO significantly inhibits the cyclic carbonate formation.
Also, we got CHO/PO/CO2 terpolymers with various CHC
linkage content by altering the molar fraction of CHO in the
mixture epoxides and found that the Tg was directly propor-
tional to the CHC unit content (Figure 1). Usually, high
CHC unit content in the CHO/PO/CO2 terpolymers results

Article
Macromolecules, Vol. 43, No. 3, 2010 1399
Scheme 1. Pathways of Epoxide Ring-Opening during CHO/PO/CO2
Terpolymerization
Figure 2. Plot of the CHC unit content in the resulting terpolymers
versus the conversion of all epoxides in the CHO/PO/CO2 terpolymer-
ization catalyzed by the complex 3 (epoxides/3 = 5000/1, CHO/PO =
1/1, molar ratio) at ambient temperature.
in enhanced thermal stability. Furthermore, the bifunctional
catalyst 3 was found to be applicable to the terpolymeriza-
tion of CO2 with CHO and other aliphatic epoxides, provid-
ing the corresponding terpolymers in high activity and
excellent selectivity (Table 2, entries 8-10). No ether linkage
signal at δ = 3.4-3.6 was observed in the 1H NMR spectra
of the resulting terpolymers (Supporting Information, Fig-
ures S2-S4). Ethylene oxide was found to be the most
reactive epoxide, so the relative low CHC unit content was
observed in the resulting terpolymer. On the contrary, 1,2-
hexene oxide exhibited relatively low reactivity among the
epoxides surveyed, and the resulting terpolymer has a CHC
unit content up to 65%. These terpolymers all showed highly
thermal stability. Also, the resulting terpolymers have only
one Tg and one thermolysis peak (Supporting Information,
Figure S5-S8). These results are similar to our previously
reported binary catalyst systems at ambient temperature.17
Regiochemistry in the CHO/PO/CO2 Terpolymerization.
We are interested in exploring what factors lead to the close
content for both cyclohexene carbonate and propylene
carbonate units in the resulting terpolymer from the mixture
epoxides of equimolar CHO and PO, whereas the reactivity
of PO is about 3 times that of CHO using the complex 3 as
catalyst at ambient temperature. We performed the competi-
tion polymerization of CHO and PO with CO2 using equi-
molar CHO and PO. Figure 2 shows the plot of the CHC unit
content of the resulting terpolymers at different time points
versus the conversion of epoxides at ambient temperature. In
the system of equimolar CHO and PO, the CHC unit content
in the resulting terpolymers at different time points is very
close to 50%. For a comparison purpose, we further per-
formed the terpolymerization in the system of low CHO
loading (CHO/PO = 1/2, molar ratio) and found that the
CHC unit content in the terpolymers was 32% after ∼20%
conversion of all epoxides. These results indicate that the
presence of CHO significantly inhibits the reactivity of PO,
causing a matched reactivity for both epoxides during the
terpolymerization.
the relatively high reactivity of PO probably lead to the
enhanced content of PC-CHC or CHC-PC linkages in the
terpolymer. Obviously, the relatively high basicity and co-
ordination ability of CHO result in the concentration of the
bound CHO (B) being higher than that of the bound PO (A)
(Scheme 1). Because of the steric hindrance of CHO and the
CHC end of the species (D), the ring-opening of the bound
CHO (B) attacked by (D) is more difficult than that by the
species (C). These will result in the ring-openings of the
bound PO (A) attacked by the species (D) and the bound
CHO (B) attacked by the species (C) being main reaction
pathways. This arrangement will cause PC-CHC or CHC-
PC linkage content to be higher than the total content of PC-
PC and CHC-CHC linkages in the resulting terpolymer.
Indeed, we have investigated ESI-Q-TOF mass spectrum of
the mixture resulting from the PO/CHO/CO2 terpolymeriza-
tion catalyzed by binary SalenCoX/MTBD system in the
initial stage and clearly found that the two different carbo-
nate units in the resulting terpolymer mainly arrange in
alternating fashion.18 Of course, the PC-CHC or CHC-PC
linkages distribute in random nature rather than perfect
alternating mode. Unfortunately, because of the very poor
ion intensity of the propagating polymer chains based on 2,4-
dinitrophenoxy in ESI-Q-TOF mass spectroscopy, we failed
to determine the sequence nature of carbonate units in the
terpolymer with the complex 3 as catalyst.
In previous regio- and stereoselective copolymerization of
CO2 and PO, we found that the enhanced reaction tempera-
tures caused a significant decrease in regioselective ring-
opening of PO and thus had a negative effect on the stereo-
regularity of the resulting polycarbonates.7d With complex 3
as catalyst, the reaction temperature increases from 25 to
90 °C, the head-to-tail linkage content of the resulting PPC
decreases from 92 to 77% (Figure 3A,B). On the contrary,
the 13C NMR spectra at carbonyl region of the terpolymers
obtained at various temperatures are very similar (Figure 3C,
D), but the reason is not clear.
To investigate the effect of the competition copolymeri-
zation of CHO on the regioselective ring-opening of PO, we
performed the terpolymerization of CHO and (R)-PO with
CO2 catalyzed by the enantiopure complex (1R,2R)-3. For
the copolymerization or terpolymerization concerning chiral
PO, regioselective ring-opening of PO significantly affects
the resulting polymer enantioselectivity because head-to-tail
carbonate linkages normally retain the stereochemistry
at the methine carbon as a consequence of the preferential
In previous studies, we have demonstrated that the pro-
pagating polymer chains easily dissociated from the central
cobalt ion and thus left the vacant coordination site for
epoxide activation.7d,11 Because a matched reactivity for
both epoxides was observed in the terpolymerization, usually
the resulting terpolymer from the system at the feed ratio
of [CHO]/[PO] = 1 has ∼50% PC-CHC (or CHC-PC place-
ment) linkages in random distribution. However, the rela-
tively high basicity and steric hindrance of CHO as well as

1400 Macromolecules, Vol. 43, No. 3, 2010
Ren et al.
Notably, the bifunctional catalyst exhibits active under 0.1
MPa CO2 pressure at ambient temperature or elevated tempera-
tures, selectively affording the corresponding copolymer >99%
carbonate linkages. The functionalized Co(III)-salen complex
could operate very efficiently for the terpolymerization of CHO
and aliphatic epoxides with CO2 to provide selectively polycar-
bonates with a narrow polydispersity at various temperatures.
The resulting terpolymers have only one thermolysis peak and
one adjustable glass-transition temperature dependable on cyclo-
hexene carbonate unit content. The close content for both
cyclohexene carbonate and propylene carbonate units was ob-
served in the polymers from the terpolymerization of equimolar
CHO and PO with CO2 at various temperatures. This is ascribed
to the presence of CHO significantly inhibiting the reactivity of
PO and thereby causing a matched reactivity for both epoxides
during the terpolymerization.
Experimental Section
Synthesis of Chiral Salen Ligand Bearing a Quaternary Am-
monium Salt. A flask was charged with (R,R)-1,2-cyclohexane-
diamine mono(hydrogen chloride) (0.30 g, 2.0 mmol), activated
˚
5 A molecular sieve (3.00 g), and anhydrous methanol (20 mL).
3,5-Di-tert-butyl-2-hydroxybenzaldehyde (0.57 g, 2.4 mmol)
was added in one portion, and the reaction mixture was stirred
at room temperature for 2 h. A solution of 5-tert-butyl-
3-(3-(diethylamino)propyl)-2-hydroxybenzaldehyde (0.58 g,
2.0 mmol) in anhydrous ethanol (20 mL) was then added to
the reaction system, followed by the slow addition of triethyl-
amine (0.42 mL, 3.0 mmol). After stirring at room temperature
for an additional 4 h, the reaction mixture was filtered through a
short pad of dry silica gel. Then, all solvents and the excessive
triethylamine were removed in vacuo. The residue was purified
by column chromatography on silica gel using petrol ether/ethyl
acetate (5:1, 1% Et3N) as the mobile phase to afford 3-(diethyl-
amino)propyl appended Salen ligand as a yellow compound
(0.65 g). 1H NMR (400 MHz, CDCl3, δ): 13.70 (s, 1H), 13.40 (s,
1H), 8.29 (s, 1H), 8.27 (s, 1H), 7.30 (s, 1H), 7.16 (s, 1H), 6.98 (s,
1H), 6.96 (s, 1H), 3.30-3.33 (m, 2H), 2.48-2.73 (m, 8H),
1.67-1.96 (m, 10H), 1.41 (s, 9H), 1.24 (s, 9H), 1.22 (s, 9H),
1.00 (t, J = 7.2 Hz, 6H). HRMS (m/z) Calcd for [C39H62N3O2]þ,
604.4842; found, 604.4846.
Figure 3. Carbonyl region of the 13C NMR spectra of polycarbonates
resulted from (A) the PO/CO2 copolymerization at 25 °C; (B) the PO/
CO2 copolymerization at 90 °C; (C) the PO/CHO/CO2 terpolymeriza-
tion at 25 °C; and (D) the PO/CHO/CO2 terpolymerization at 90 °C.
TT, tail-to-tail carbonate linkage; HT, head-to-tail carbonate linkage;
HH, head-to-head carbonate linkage.
ring-opening at the methylene carbon.8a,19 On the contrary,
the ring-opening at the methine carbon occurs by backside
attack, usually leading to the formation of head-to-head or
tail-to-tail carbonate linkages; this may cause a change in
stereochemistry with inversion. The degradation of linear
PPC to cyclic propylene carbonate by intramolecular cyclic
elimination normally retains the stereochemistry at the
methine carbon of PPC because the reaction involves alk-
oxide ion attack on the carbonate carbon rather than the
methine or methylene carbon. As a result, we can easily
evaluate the relationship of polymer enantioselectivity and
its head-to-tail linkages in relation to regioselective ring-
opening of the epoxide by determining the enantiomeric
excess of the cyclic carbonate produced from the degradation
of the resulting polycarbonates. Interestingly, the enantio-
meric excess of cyclic propylene carbonate given after com-
plete degradation of the polycarbonates from the terpoly-
merization of CHO and (R)-PO with CO2 at ambient
temperature is 79%, which is the same as that from (R)-PO/
CO2 copolymerization under the same conditions. This result
clearly demonstrates that the competition coordination of
CHO and PO to the electrophilic metal ion has no effect on
regioselective ring-opening of PO in the terpolymerization.
To a stirred mixture of the resulting 3-(diethylamino)propyl-
appended Salen ligand (0.64 g, 1.0 mmol) in acetonitrile (20 mL)
in a 50 mL flask wrapped in aluminum foil was added iodo-
methane (0.075 mL, 1.2 mmol). The resulting mixture solution
was stirred for 24 h. The solvent was removed in vacuo; then, the
residue was dissolved in ethanol (40 mL) and AgBF4 (0.21 g,
1.1 mmol) was added. The mixture was stirred for 1 h and then
filtered to remove the Ag byproduct. The solvent was removed
in vacuo; then, the residue was purified by column chromato-
graphy on silica gel using ethyl acetate (1% Et3N) as the mobile
phase to give the afforded quaternary ammonium salt-appended
salen ligand (0.58 g, 41%) as a bright-yellow solid. [R]2D0 - 312
1
(c 1.0, CHCl3). H NMR(400 MHz, CDCl3, δ): 13.70 (s, 1H),
13.50 (s, 1H), 8.35 (s, 1H), 8.32 (s, 1H), 7.30 (s, 1H), 7.17 (s, 1H),
7.05 (s, 1H), 7.01 (s, 1H), 3.13-3.36 (m, 8H), 2.97 (s, 3H), 2.69
(m, 2H), 1.45-1.98 (m, 10H), 1.38 (s, 9H), 1.20-1.28 (m, 24H).
13C NMR (100 MHz, CDCl3, δ): 165.6, 165.2, 158.1, 157.0,
141.3, 140.2, 136.6, 130.3, 126.9, 126.8, 126.3, 126.1, 117.9,
117.8, 73.2, 71.7, 59.9, 56.8, 47.4, 35.1, 34.2, 34.0, 33.6, 33.1,
32.6, 31.5, 29.5, 27.3, 24.5, 24.4, 22.1, 7.8. HRMS (m/z) Calcd
for [C40H64N3O2]þ, 618.4999; found, 618.4987.
Conclusions
We have developed a highly active, bifunctional Co(III)-salen
catalyst bearing a quaternary ammonium salt on the three-
position of one aromatic ring for the copolymerization of CO2
and CHO as well as terpolymerization with aliphatic epoxides at
high temperatures. The catalytic activity is highly sensitive to the
reaction temperature, and the highest TOF up to 6105 h-1 was
obtained at 120 °C, without sacrificing polymer selectivity.
Synthesis of the Complex (R,R)-3. Cobalt(II) acetate (0.18 g,
1.0 mmol) and the quaternary ammonium salt-appended Salen
ligand (0.35 g, 0.5 mmol) were dissolved in methanol (10 mL).
After stirring for 12 h at room temperature, LiCl (0.04 g, 1.0
mmol) was added, and the mixture was further stirred for 12 h
while oxygen was bubbled through the reaction mixture. The
solvent was removed in vacuo, and the residue was extracted

Article
Macromolecules, Vol. 43, No. 3, 2010 1401
with methylene chloride. The organic layer was washed with
saturated aq NaHCO3 and brine and then dried over anhydrous
sodium sulfate, filtered, and concentrated in vacuo. After the
removal of the solution, the residue was dissolved in methylene
chloride (20 mL) in a 80 mL Schlenk vial wrapped in aluminum
foil, and AgBF4 was added (0.20 g, 1.0 mmol). The mixture was
stirred for 24 h and then filtered to remove the Ag byproduct. To
the filtrate was added sodium 2,4-dinitrophenolate (0.21 g, 1.0
mmol), and the solution was stirred for another 2 h and then
filtered to remove inorganic salt. The solvent was removed in
vacuo. The resulting solid was further treated with a mixture of
methylene chloride and hexane to afford the enantiopure com-
plex (R,R)-3 as a brown solid (0.47 g, 90%). 1H NMR (DMSO-
d6, δ): 8.60 (s, 2H), 7.98 (s, 1H), 7.90 (s, 1H), 7.81 (d, J = 8.4 Hz,
2H,), 7.51 (s, 1H), 7.47 (s, 1H), 7.45 (s, 1H,), 7.41 (s, 1H), 6.35 (d,
J = 8.4 Hz, 2H), 3.55-3.58 (m, 2H), 3.28-3.41 (m, 6H), 3.03 (t,
J = 6.8 Hz, 2H), 2.98-3.01 (m, 2H), 2.90 (s, 3H), 2.16-1.49 (m,
8H), 1.75 (s, 9H), 1.30 (s, 9H), 1.28 (s, 9H), 1.22 (t, J = 7.2 Hz,
6H). 13C NMR (DMSO-d6, δ): 169.8, 164.4, 164.1, 162.0, 160.9,
141.5, 136.9, 135.8, 132.8, 131.8, 128.8, 128.5, 127.7, 127.4,
124.7, 118.6, 118.4, 69.4, 69.0, 59.8, 55.6, 46.6, 35.4, 33.4, 33.3,
31.3, 31.2, 30.2, 29.3, 29.2, 26.8, 24.1, 22.2, 7.9. HRMS (m/z):
Calcd for [C46H65CoN5O7]þ, 858.4216; found, 858.4235.
CO2/CHO Copolymerization Procedure. A stirred mixture of
complex 3 (0.021 g, 0.020 mmol, 1 equiv) was dissolved in CHO
(9.8 mL, 100 mmol, 5000 equiv) to form red-brown solution
under a nitrogen atmosphere. The mixture solution was charged
into a predried 75 mL autoclave equipped with a magnetic
stirrer under a CO2 atmosphere. The autoclave was put into a
bath, which had been set at a given temperature. Then, the
system was pressurized to appropriate pressure with CO2. After
the allotted reaction time, the CO2 pressure was released, and a
small amount of the residue was removed for 1H NMR analysis
to give quantitatively the activity and selectivity of PCHC and
also used for GPC analysis. The unreacted CHO was isolated
under ambient temperature by vacuum (3 mmHg) transfer into a
cooled (-20 °C) receiving flask. The crude polymer was dis-
solved in 10 mL of CHCl3/MeOH (5/1, v/v) mixture and
precipitated from methanol or diethyl ether. This process was
repeated 3-5 times to remove completely the catalyst, and white
polymer was obtained by vacuum drying.
CO2/CHO/Aliphatic Epoxides Terpolymerization Procedure
at High Temperatures. A stirred mixture of complex 3 (0.021 g,
0.020 mmol, 1 equiv.) was dissolved in CHO/aliphatic epoxide
(CHO/aliphatic epoxide/catalyst, 2500/2500/1, molar ratio) to
form a red-brown solution under a nitrogen atmosphere at 0 °C.
The mixture solution was charged into a predried 75 mL
autoclave equipped with a magnetic stirrer under a CO2 atmo-
sphere. After it was pressurized to 1.0 MPa pressure with CO2,
the autoclave was put into a bath whose temperature had been
set at a given temperature, and the solution was stirred for 15
min to allow the solution temperature to reach the bath tem-
perature. Then, the system was pressurized to 2.5 MPa pressure
with CO2. After the allotted reaction time, the autoclave was
cooled to ambient temperature by immersion in an ice bath.
After CO2 was released, a small amount of the residue was taken
and dissolved in CDCl3 for 1H NMR analysis to give quantita-
tively the activity and selectivity of polymer as well as carbonate
linkage. The unreacted epoxides were isolated at ambient tem-
perature by vacuum (3 mm Hg) transfer into a cooled (-20 °C)
receiving flask. The crude terpolymer was dissolved in 10 mL of
CHCl3/MeOH (5/1, v/v) mixture and precipitated from metha-
nol or diethyl ether. This process was repeated three to five times
to remove completely the catalyst, and white polymer was
obtained by vacuum drying.
Outstanding Young Scientist Foundation of NSFC (grant
20625414).
Supporting Information Available: General information,
characterization of various terpolymers, and NMR spectra of
chiral ligand and complex. This material is available free of
charge via the Internet at http://pubs.acs.org.
References and Notes
(1) Beckman, E. J. Science 1999, 283, 946–947.
(2) Inoue, S.; Koinuma, H.; Tsuruta, T. J. Polym. Sci., Polym. Lett.
1969, 7, 287–292.
(3) For recent reviews on CO2/epoxides copolymerization, see:
(a) Darensbourg, D. J. Chem. Rev. 2007, 107, 2388–2410.
(b) Coates, G. W.; Moore, D. R. Angew. Chem., Int. Ed. 2004, 43,
6618–6639. (c) Darensbourg, D. J.; Mackiewicz, R. M.; Phelps, A. L.;
Billodeaux, D. R. Acc. Chem. Res. 2004, 37, 836–844. (d) Sugimoto, H.;
Inoue, S. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 5561–5573.
(e) Chisholm, M. H.; Zhou, Z. J. Mater. Chem. 2004, 14, 3081–3092.
(4) (a) Darensbourg, D. J.; Holtcamp, M. W. Macromolecules 1995,
28, 7577–7579. (b) Darensbourg, D. J.; Lewis, S. J.; Rodgers, J. L.;
Yarbrough, J. C. Inorg. Chem. 2003, 42, 581–589. (c) Van Meerendonk,
W. J.; Duchateau, R.; Koning, C. E.; Gruter, G. M. Macromolecules
2005, 38, 7306–7313. (d) Darensbourg, D. J.; Holtcamp, M. W.; Struck,
G. E.; Zimmer, M. S.; Niezgoda, S. A.; Rainey, P.; Roberston, J. B.;
Draper, J. D.; Riebenspies, J. H. J. Am. Chem. Soc. 1999, 121, 107–
116. (e) Darensbourg, D. J.; Wildeson, J. R.; Yarbrough, J. C.;
Riebenspies, J. H. J. Am. Chem. Soc. 2000, 122, 12487–12496.
(5) (a) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc.
1998, 120, 11018–11019. (b) Cheng, M.; Moore, D.; Reczek, J.;
Chamberlain, B.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem.
Soc. 2001, 123, 8738–8749. (c) Moore, D. R.; Cheng, M.; Lobkovsky,
E. B.; Coates, G. W. Angew. Chem., Int. Ed. 2002, 41, 2599–2602.
(6) (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.; Phelps, A. L. Inorg. Chem. 2005, 44, 4622–4629. (d) Li, B.; Zhang,
R.; Lu, X. B. Macromolecules 2007, 40, 2303–2307.
(7) (a) Lu, X. B.; Wang, Y. Angew. Chem., Int. Ed. 2004, 43, 3574–
3577. (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. (d) 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. (e) Cohen, C. T.; Coates, G. W. J. Polym. Sci., Part
A: Polym. Chem. 2006, 44, 5182–5191. (f) Niu, Y.; Zhang, W.; Pang,
X.; Chen, X.; Zhuang, X.; Jing, X. J. Polym. Sci., Part A: Polym.
Chem. 2007, 45, 5050–5056.
(8) (a) 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.
(b) Rao, D. Y.; Li, B.; Zhang, R.; Wang, H.; Lu, X. B. Inorg. Chem.
2009, 48, 2830–2836. (c) Nakano, K.; Nakamura, M.; Nozaki, K.
Macromolecules 2009, 42, 6972–6980. (d) Darensbourg, D. J.; Ulusoy,
M.; Karroonnirum, O.; Poland, R. R.; Reibenspies, J. H.; C-etinkaya, B.
Macromolecules 2009, 42, 6992–6998.
(9) (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, 7306–7309.
(10) Qin, Z.; Thomas, C. M.; Lee, S.; Coates, G. W. Angew. Chem., Int.
Ed. 2003, 42, 5484–5487.
(11) Ren, W. M.; Liu, Z. W.; Wen, Y. Q.; Zhang, R.; Lu, X. B. J. Am.
Chem. Soc. 2009, 131, 11509–11518.
(12) (a) Xiao, Y.; Wang, Z.; Ding, K. Macromolecules 2006, 39, 128–
137. (b) Xiao, Y.; Wang, Z.; Ding, K. Chem.;Eur. J. 2005, 11, 3668–
3678. (c) Sugimoto, H.; Kuroda, K. Macromolecules 2008, 41, 312–
317. (d) Sugimoto, H.; Ohshima, H.; Inoue, S. J. Polym. Sci., Part A:
Polym. Chem. 2003, 41, 3549–3555.
(13) Kemper, M. R.; Knight, P. D.; Reung, P. T. R.; Williams, C. K.
Angew. Chem., Int. Ed. 2009, 48, 931–933.
(14) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am.
Chem. Soc. 2003, 125, 11911–11924.
Acknowledgment. This work was supported by National
Natural Science Foundation of China (NSFC) program (grant
20634040) and National Basic Research Program of China (973
program: 2009CB825300). X.-B.L. gratefully acknowledges the
(15) (a) Darensbourg, D. J.; Wildeson, J. R.; Yarbrough, J. C.;
Reibenspies, J. H. J. Am. Chem. Soc. 2000, 122, 12487–12496.
(b) Quan, Z.; Min, J.; Zhou, Q.; Xie, D.; Liu, J.; Wang, X.; Zhao, X.;
Wang, F. Macromol. Symp. 2003, 195, 281–286.

1402 Macromolecules, Vol. 43, No. 3, 2010
Ren et al.
(16) Darensbourg, D. J.; Yarbrough, J. C.; Ortiz, C.; Fang, C. C. J. Am.
Chem. Soc. 2003, 125, 7586–7591 .
(17) Shi, L.; Lu, X. B.; Zhang, R.; Peng, X. J.; Zhang, C. Q.; Li, J. F.;
are attributed to the species of [(HþþMTBDþPO)þPC],
[(HþþMTBDþPO)þCHC], [(HþþMTBDþPO)þPCþCHC] or
[(HþþMTBDþPO)þCHCþPC], [(HþþMTBDþPO)þ2PCþCHC],
[(HþþMTBDþPO)þPCþ2CHC], and [(HþþMTBDþPO)þ2(PCþ
CHC)], respectively. (See Figure S9 of the Supporting In-
formation.) This result indicates that the two different carbonate
units in the resulting terpolymer mainly arrange in alternating
fashion (PC-CHC or CHC-PC linkage).
Peng, X. M. Macromolecules 2006, 39, 5679–5685.
(18) We chose 7-methyl-1,5,7-triazabicyclo[4.4.0] dec-5-ene (MTBD, a
sterically hindered strong organic bases) as a cocatalyst to be
consistent with the complex 1 for the terpolymerization of CHO/
PO/CO2. In the ESI-Q-TOF mass spectra in positive ion mode, we
observe the strong signals of 314, 354, 456, 558, 598, and 700, which
(19) Chisholm,M.H.;Zhou, Z. P. J. Am. Chem. Soc. 2004,126,11030–11039.