Alternating copolymerization of CO2 and propylene oxide catalyzed by CoIII(salen)/Lewis base

Article abstract of DOI:10.1021/ma047551k

The use of Co^III(salen)/Lewis base catalyst system for copolymerization of CO₂ and propylene oxide was analyzed. The ratio of catalyst to Lewis base cocatalyst influenced the TOF. The use of Lewis base cocatalyst enables high catalytic activity at low CO₂ pressure and affords superior enantioselectivity. The results show that the use of cocatalyst produced a highly alternating monodisperse polycarbonate product without concomitant cyclic carbonate production.

Full text of DOI:10.1021/ma047551k

Macromolecules 2005, 38, 6251-6253
6251
III
Alternating Copolymerization of CO
2
and
our results employing the Co (salen)/Lewis base cata-
lyst system in the alternating copolymerization of CO2
and PO.
III
Propylene Oxide Catalyzed by Co (salen)/
Lewis Base
III
Initially, enantiomerically pure (R,R)-Co (salen) com-
Robert L. Paddock and SonBinh T. Nguyen*
plex 1a, in conjunction with N,N-dimethylaminoquino-
line (DMAQ), was investigated as a catalyst system for
the copolymerization of PO and CO2 over a wide range
of CO2 pressures (Table 1, entries 1-5). This catalyst
system was found to be active at low CO2 pressures,
with optimal activity achieved in the range of 150-350
Department of Chemistry and Institute for Environmental
Catalysis, Northwestern University, 2145 Sheridan Rd.,
Evanston, Illinois 60208
Received November 26, 2004
22
CO2 is naturally abundant, nontoxic, nonflammable,
inexpensive, and easy to store, transport, and handle,
making it an attractive alternative C1 feedstock. Un-
fortunately, despite a substantial amount of effort, few
efficient and economical synthetic processes have been
2 2
psig of CO . It is even active at CO pressures as low
as 50 psig, maintaining over 50% of its optimal turnover
frequency (TOF). Increasing the pressure above the
optimal CO pressure range resulted in a significant
2
drop-off in reaction rate.
1
developed which utilize CO2 as a reagent. One of the
The generally accepted mechanism for the copoly-
most promising methodologies being investigated in this
area, so-called chemical CO2 fixation, is the alternating
copolymerization of CO2 and epoxides (eq 1).2 This
reaction was first reported by Inoue and co-workers in
merization of CO and epoxides involves the alternate
2
enchainment of CO and the epoxide via insertion into
2
-4
either a metal alkoxide or carbonate bond (Scheme 1).
Scheme 1. Proposed Mechanism for the
1
969, employing a heterogeneous catalyst synthesized
2
Copolymerization of CO and PO
5
from ZnEt2 and water. In the years following this initial
report, a variety of heterogeneous catalyst systems have
been developed with limited success, generally suffering
from irreproducibility, low activity, high polyether
content in the polycarbonate product, and production
of significant amounts of the undesired cyclic carbonate
byproduct. In the past decade, several well-defined
single site catalysts have been reported which demon-
strate greatly improved activity and selectivity for
6
-16
reaction 1.
These catalysts are especially active in
the copolymerization of CO2 and alicyclic epoxides such
as cyclohexene oxide, which are sterically inhibited from
Recently, a number of elegant mechanistic studies have
been conducted to determine the possible role of a Lewis
base cocatalyst in the copolymerization of CO2 and
1
7
producing the cyclic carbonate byproduct. Unfortu-
nately, these same catalysts are much less effective
when aliphatic epoxides such as propylene oxide (PO)
are employed as substrates; even those which are active
generally suffer from concomitant production of cyclic
carbonate.
epoxides catalyzed by single site Cr and Al com-
plexes.1
4,23,24
These studies have suggested that the
Lewis base coordinates to the active metal center trans
to the propagating metal-polymer chain, thereby labi-
lizing the metal alkoxide bond and facilitating the
insertion of CO2. We believe that DMAQ functions in
an analogous manner in our catalyst system, dramati-
cally increasing the rate of CO2 enchainment, thereby
explaining the high copolymerization activity at low CO2
III
III
Recently, Coates et al. reported that Co (salen)
pressures for the Co (salen) complexes in the presence
complexes catalyze the copolymerization of CO2 and
propylene oxide to yield poly(propylene carbonate) (PPC)
with an unprecedented selectivity (>99%) for the poly-
of the Lewis base.
The ratio of catalyst to Lewis base cocatalyst was also
found to influence the TOF (Table 1, entries 2, 6-8).
Optimal activity occurred when 2 equiv of cocatalyst was
employed. Using only 1 equiv of DMAQ decreased the
activity by over 50%, whereas an increase beyond 2
equiv lead to a slight decrease in activity along with a
slight increase in the polymer dispersity. In general, 1a/
DMAQ was highly selective for the polycarbonate
product (>99%), which was completely alternating,
typically exhibiting >99% carbonate linkages. The poly-
carbonate products have very narrow polydispersity
indices (PDI <1.1); however, the observed number-
1
3
carbonate vs the cyclic carbonate product. However,
these catalysts were only active at very high CO2
pressures with optimal reaction conditions requiring 800
psig of CO2. Since 2001, our group has been investigat-
ing binary Lewis acid/Lewis base catalyst systems for
the coupling reaction of CO2 and three-member hetero-
1
8-21
cycles,
and at the time of Coates report we were
III
investigating Co (salen)/Lewis base catalyst systems
2
1
for the synthesis of cyclic carbonates. During the
course of those investigations, we were very pleased to
discover the formation of PPC can be induced to occur
at low temperatures and low CO2 pressure regimes
average molecular weight (M , vs polystyrene stan-
n
dards) was generally below the expected stoichiometric
III
where Coates et al. had reported that Co (salen)
complexes by themselves are inactive. Herein, we report
value, which has been suggested as being indicative of
chain transfer during the reaction.15
The nature of the catalyst’s counteranion and ligand
structure were found to influence both the activity and
stereoselectivity of the copolymerization. The TOF
*
Corresponding author: Fax 847-491-7713; Tel 847-467-3347;
e-mail stn@northwestern.edu.
1
0.1021/ma047551k CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/30/2005

6252 Communications to the Editor
Macromolecules, Vol. 38, No. 15, 2005
Table 1. CO2/PO Copolymerization Resultsa
b
selectivityc
(% PPC)
Mnd
press.
(psig)
DMAQ
(equiv)
temp
(°C)
time
(h)
TOF
carbonate
linkages (%)
PDI
(Mw/Mn)
-
1
c
(g mol-1)
krele
entry
catalyst
(h
)
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1f
1g
2g
3g
3g
3g
3g
50
150
350
500
750
150
150
150
150
150
150
150
150
150
150
150
350
350
350
2
2
2
2
2
1
3
5
2
2
2
2
2
2
2
2
2
2
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
40
60
40
5
5
47
86
>99
>99
>99
>99
>99
>99
>99
>99
98
>99
>99
>99
86
>99
>99
>99
>99
>99
>99
>99
>99
99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
17.1
25.6
21.7
18.2
14.4
35.2
17.6
10.8
11.2
12.3
20.3
21.3
23.5
23.2
17.2
17.8
17.4
12.9
18.4
1.07
1.06
1.06
1.07
1.08
1.06
1.08
1.25
1.16
1.17
1.05
1.06
1.07
1.07
1.06
1.06
1.07
1.18
1.05
5.1
5.3
4.8
4.7
4.7
5.1
5.4
5.2
5.0
5.0
5.0
4.4
4.2
5.1
2.0
1.2
1.9
1.7
1.9
5
75
6
49
12.5
10
4.5
5
16
41
83
71
5
40
10
11
12
13
14
15
16
17
18
19
5
43
5
63
5
86
5
91
5
97
>99
>99
>99
>99
92
3
118
118
199
145
589
3.25
1.66
1.5
0.66
f
2
>99
a
Reaction conditions: catalyst (0.067 mol %, 3.33 × 10 mol), PO (1500 equiv, 3.5 mL). b Moles of product (polymer and cyclic carbonate)
-5
c
1
d
per mole of catalyst per hour. Determined using H NMR spectroscopy. Only side product is PC. Determined by gel permeation
e
chromatography at 35 °C relative to polystyrene standards. krel ) ln[1 - c(1 + ee)]/ln[1 - c(1 - ee)], where c ) conversion and ee )
2
8 f
enantiomeric excess of unreacted PO.
DMAP* was employed as the Lewis base cocatalyst.
changed with the identity of the counteranion (Table 1,
entries 2, 9-14), with the NO3- anion providing the
most active catalyst 1g. However, there is no obvious
trend in the relationship between counteranion identity
and catalyst activity. For the same counteranion, chang-
ing the salen ligand framework to incorporate noncyclic
backbones resulted in a slight improvement in TOF as
well as a significant drop-off in stereoselectivity, as
determined by the enantiomeric excess (ee) of the
remaining PO (Table 1, entries 14-16). A krel of 5.1 is
obtained with the (R,R)-cyclohexyl backbone, whereas
the (R,R)-diphenyl and (R)-propyl backbones of catalyst
2
g and 3g gave a krel of only 2.0 and 1.2, respectively.
(
In the absence of a cocatalyst, Coates et al. reported a
III
krel of only 2.8 for Co (salen) complexes with a (R,R)-
13
cyclohexyl backbone. ) We postulate that the improve-
ment in enantioselectivity observed with the addition
of the DMAQ cocatalyst may be a result of the steric
interactions between the coordinated amine and the
salen ligand framework. These interactions may lead
to an “improved” ligand geometry about the active metal
center that is better able to induce enantioselectivity
during the reaction. In addition to an increase in
enantioselectivity, the utilization of the DMAQ cocata-
lyst also lead to a considerable improvement in regio-
selectivity in regards to PO enchainment. Examination
Figure 1. 13C spectrum (100 MHz) of the carbonyl region for
PPC obtained using catalyst 1a/DMAQ.
III
of racemic PO and CO2 when employing Co (salen)
complexes as catalysts in the absence of a Lewis
base.13
Previous reports have demonstrated that increasing
the reaction temperature for the alternating copoly-
merization of CO2 and PO generally results in a
decrease in selectivity for PPC as more cyclic carbonate
1
3
of the carbonate region of the C NMR spectra of the
9,16,26
is produced.
For 3g/DMAQ, increasing the tem-
2
5
polycarbonate product resulting from the copolymer-
ization of racemic PO and CO2 shows that 1a/DMAQ is
highly regioselective, as the polycarbonate product
exhibits a microstructure with a head-to-tail content of
perature to 40 °C only had the positive effect of improv-
ing the TOF and did not lead to production of the cyclic
carbonate. An increase in temperature to 60 °C eventu-
ally led to slight decreases in TOF and selectivity for
PPC, as 8% of the product mixture was propylene
carbonate (Table 1, entries 15, 17-18).
9
6% (Figure 1). In comparison, Coates et al. obtained
only 80% head-to-tail linkages for the copolymerization

Macromolecules, Vol. 38, No. 15, 2005
Communications to the Editor 6253
III
In our prior investigations of the Co (salen)/Lewis
base catalyst system for the synthesis of cyclic carbon-
(3) Darensbourg, D. J.; Holtcamp, M. W. Coord. Chem. Rev.
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2
1
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TOF than that obtained with DMAQ, perhaps as a
(
5) Inoue, S.; Koinuma, H.; Tsuruta, T. J. Polym. Sci., Part B
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(
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2
7
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(
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III
In summary, we have demonstrated that Co (salen)/
(10) Mang, S.; Cooper, A. I.; Colclough, M. E.; Chauhan, N.;
Lewis base catalyst systems are highly active and
selective in the copolymerization of CO2 and propylene
oxide. The use of the Lewis base cocatalyst enables high
catalytic activity at low CO2 pressures, affords superior
regio- and enantioselectivity, and produces a highly
alternating, monodisperse polycarbonate product with-
out concomitant cyclic carbonate production. Future
work will be directed toward obtaining mechanistic
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and stereoselectivity via optimization of the catalyst and
cocatalyst structure through perturbation of the steric
and electronic parameters.
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(
(
(
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Acknowledgment. We thank Ms. So-Hye Cho for
helpful discussions. This work was supported by the
EMSI program of the NSF and the DOE (NSF Grant
CHE-9810378) at the Northwestern University Institute
for Environmental Catalysis. Additional support from
the DuPont and Union Carbide Companies and the
Packard Foundation is appreciated. S.T.N. is an Alfred
P. Sloan Research Fellow.
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(
(
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(
salen)/ammonium salt catalyst system is active for the
Supporting Information Available: General experimen-
tal procedures. This material is available free of charge via
the Internet at http://pubs.acs.org.
alternating copolymerization of PO and CO2 at low pres-
sures; see ref 15.
(
(
(
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MA047551K