High-activity, single-site catalysts for the alternating copolymerization of CO2 and propylene oxide

Article abstract of DOI:10.1021/ja028071g

Despite recent advances regarding catalysts for CO₂/epoxide copolymerization, the development of high-activity catalysts for alternating polymerization of CO₂ and commodity epoxides, such as propylene oxide, remains a challenge. A new class of unsymmetrically substituted β-diiminate zinc complexes is reported that exhibits unprecedented activity for CO₂/propylene oxide copolymerization. The polymers formed are of high molecular weight (M_n ≈ 35 kg/mol) and have narrow polydispersities (PDI ≈ 1.1), consistent with a living polymerization. Copyright

Full text of DOI:10.1021/ja028071g

Published on Web 11/08/2002
High-Activity, Single-Site Catalysts for the Alternating Copolymerization of
CO2 and Propylene Oxide
Scott D. Allen, David R. Moore, Emil B. Lobkovsky, and Geoffrey W. Coates*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca, New York 14853
Received August 8, 2002
Scheme 1. Copolymerization Scheme (P ) Polymer Chain)
Because CO2 is a nontoxic, nonflammable, and inexpensive
substance, there is continued interest in its activation with transition-
metal complexes and its subsequent use as a viable C1 feedstock.1,2
One area that has seen considerable recent success is copolymer-
ization of CO2 and epoxides.3
Several efficient catalyst systems for the copolymerization of
CO2 and cyclohexene oxide (CHO) have been recently reported.3-9
Interestingly, these catalysts do not readily polymerize related
epoxides, such as propylene oxide (PO), with CO2. Inoue first
showed that partially hydrolyzed ZnEt2 initiated the copolymeri-
zation of PO and CO2.10,11 Since this discovery, a wide variety of
initiating systems have been developed by mixing zinc sources (e.g.,
ZnO) with diprotic activators (e.g., glutaric acid).3 The resultant
systems, such as zinc glutarate, are structurally complex, insoluble
mixtures. High catalyst loadings, elevated CO2 pressures, and long
reaction times must be employed to obtain appreciable amounts of
polymer, which are often not perfectly alternating and exhibit broad
molecular weight distributions. Currently, the preferred catalyst for
PO/CO2 copolymerization, zinc glutarate, exhibits activities as high
as 4 turnovers of propylene oxide/CO2 per mole of zinc per hour.12,13
Because of the complex nature of this heterogeneous system, the
active catalytic species are unknown; thus, rational modification
of the catalyst is difficult. Discrete molecular catalysts provide an
excellent opportunity to understand the mechanism of catalysis and
to produce polymers with defined architectures.14
Scheme 2. Synthesis of [(BDI)ZnOAc] Complexes
Part of the difficulty in developing well-defined PO/CO2 po-
lymerization catalysts results from the complexity of the copolym-
erization cycle. In a model copolymerization, the enchainment steps
of both CO2 and PO would have low activation barriers and similar
negative ∆G° values. Moreover, pathways to cyclic carbonates15
would be inaccessible (Scheme 1). Despite the ring-strain of
epoxides, the rate-determining step of CHO/CO2 polymerizations
using â-diiminate (BDI) zinc complexes such as [(BDI-1)ZnOAc]
involves CHO enchainment. Unexpectedly, CO2 enchainment is
rapid and not generally rate-limiting. We attribute the facile CO2
enchainment step to a weakly coordinated monomeric zinc alkoxide
[(BDI-1)ZnOR] being converted to a stable four-coordinate zinc
carbonate [(BDI-1)Zn(η2-OCO2R)]. If this is indeed the case, ligand
modifications that alter the coordination state of the alkoxide would
be expected to cause significant changes in polymerization activity.
With studies underway to determine the precise energetics of these
copolymerizations, we embarked on empirical modifications of the
complexes to discover improved catalysts. Herein we communicate
our progress in the development of homogeneous catalysts for the
copolymerization of PO and CO2.
that unsymmetrical â-diiminate zinc initiators bearing electron-
withdrawing groups show unprecedented activities for the copo-
lymerization of CHO and CO2.9 Screening these catalysts for PO/
CO2 activity revealed that [(BDI-2)ZnOAc] produces propylene
carbonate (PC) at 50 °C (Table 1, entry 3). Simply reducing the
temperature from 50 to 25 °C (Table 1, entry 4) suppressed the
formation of PC and allowed for the synthesis of poly(propylene
carbonate) (PPC) with 85% selectivity.17 To our surprise, complexes
with symmetrical ligands (R1, R2 are all Et or iPr) are inactive for
either PC or PPC formation. Based on the success of unsymmetrical,
electron-deficient [(BDI-2)ZnOAc], we synthesized related com-
plexes with electron-withdrawing substituents and different aryl
groups on the BDI ligand (Scheme 2). Reaction of ZnEt2 with the
BDI ligand followed by addition of HOAc at 0 °C yields the
crystalline [(BDI)ZnOAc] complexes in 38-65% yield. Slow
cooling of a concentrated toluene solution of the resulting complexes
gives X-ray quality crystals. The molecular structures of [(BDI-
3)ZnOAc], [(BDI-4)ZnOAc], and [(BDI-5)ZnOAc] are similar to
other [(BDI)ZnOAc] complexes previously reported.7,8,18 The
compounds are acetate bridged dimers in the solid state, with
distorted tetrahedral zinc centers separated by ∼4.2 Å. The
Although [(BDI-1)ZnOAc] rapidly polymerizes CHO and CO2,
it only produces traces of cyclic material and virtually no polymer
from PO and CO2 (Table 1, entry 2).16 We have recently reported
* To whom correspondence should be addressed. E-mail: gc39@cornell.edu.
9
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J. AM. CHEM. SOC. 2002, 124, 14284-14285
10.1021/ja028071g CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Propylene Oxide/CO2 Copolymerization Resultsa
b
time
(h)
temp
(°C)
pressure
(psi)
TOF
selectivityc
PPC:PC
M
PDI
n
entry
catalyst
(mol PO/(mol Zn‚h))
(×10-3 d
)
(M /M )
w
n
1e
2
3
4
5
6
7
8
9
Zn-glutarate
40
8
8
8
8
2
8
2
2
2
2
50
25
50
25
25
25
25
25
25
40
10
758
300
300
300
300
300
300
100
500
300
300
3.4
3
50
47
26
212
0
235
138
215
37
NRf
143
2.4
[(BDI-1)ZnOAc]
[(BDI-2)ZnOAc]
[(BDI-2)ZnOAc]
[(BDI-3)ZnOAc]
[(BDI-4)ZnOAc]
[(BDI-5)ZnOAc]
[(BDI-4)ZnOAc]
[(BDI-4)ZnOAc]
[(BDI-4)ZnOAc]
[(BDI-4)ZnOAc]
<1:100
<1:100
85:15
72:28
87:13
NAg
75:25
93:7
20:80
93:7
NAg
NAg
43.3
21.9
35.9
NAg
36.7
30.6
16.6
10.0
NAg
NAg
1.09
1.10
1.11
NAg
1.13
1.15
1.27
1.16
10
11
a All of the polymerizations were carried out in neat PO (4.9 mL) with [monomer]/[Zn] ) 2000 unless otherwise noted. b Turnover frequency of PO to
products. c Based on the 1H NMR integration of PPC and PC resonances at the end of the reaction. d Determined by gel permeation chromatography relative
to polystyrene standards in tetrahydrofuran. e Reference 12. f Not reported. g Not applicable.
complexes crystallize to alleviate steric repulsions, positioning the
isopropyl substituted aryl groups anti to each other.
the Packard and Beckman Foundations. This work made use of
CCMR Shared Experimental Facilities, supported through the NSF
MRSEC program (DMR-0079992).
Subtle changes in the ligand architecture drastically affect the
catalyst activity for PPC formation (Table 1, entries 4-7).19 Shift-
ing the CF3 group from the side of the diethyl aniline to the side of
the diisopropyl aniline yields a nearly 10-fold increase in activity.
On the other hand, placing both a CN group and a CF3 group on
the backbone of the ligand results in complete deactivation of the
complex, yielding neither polymer nor cyclic carbonate. The
intriguing sensitivity of this reaction to steric and electronic
perturbations of the BDI ligand is currently being studied and further
exploited.
As anticipated, varying the reaction temperature and CO2 pressure
affects both the selectivity and the activity of the superior catalyst,
[(BDI-4)ZnOAc]. As seen in entries 6, 8, and 9, increasing the
pressure of CO2 from 100 to 500 psi suppresses cyclic carbonate
formation, increasing the selectivity for polymer from 75 to 93%
while moderately decreasing the catalyst activity for PPC formation.
This suggests that PC is formed by a backbiting reaction of the
metal alkoxide intermediate (Scheme 1). Entries 6, 10, and 11 show
that lowering the reaction temperature dramatically increases
selectivity for polymer formation, although catalyst activity for PPC
formation is optimum at 25 °C.
All of the isolated polymers have narrow polydispersity indices
and Mn values close to the predicted values, hallmarks of living
polymerizations. The 1H NMR spectra of all PPCs produced show
>99% carbonate linkages, and the 13C NMR spectra of the PPCs
show head/head, tail/tail, and head/tail linkages in the ratio 23:23:
54. Such a microstructure is consistent with a near regiorandom
ring-opening of PO and is similar to polymers made using zinc
glutarate (∼60% head/tail linkages).20 Notably, the Tg of the
polymer (38 °C) is the same as that for PPC made using zinc
glutarate.12
In conclusion, we report several [(BDI)ZnOAc] complexes that
are active for PO/CO2 copolymerization. The activity observed for
[(BDI-4)ZnOAc] is significantly higher than any other reported
catalyst. The alternating polymers produced are regioirregular and
exhibit narrow molecular weight distributions. Future efforts are
directed toward understanding the mechanism of catalysis and
controlling the regio- and stereochemistry of the copolymerization.
Supporting Information Available: Synthesis and characterization
of complexes and polymers (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
References
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(18) See Supporting Information.
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Acknowledgment. G.W.C. gratefully acknowledges an NSF
CAREER Award, support from Eastman Chemical, and grants from
JA028071G
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