A R T I C L E S
Ren et al.
CO2 pressure (up to 5.5 MPa), these cobalt complexes were
proved to be inactive under low CO2 pressures or at elevated
temperature (∼50 °C) and unexpectedly reduced to Co(II)
derivatives as red solid precipitates, which were ineffective in
producing copolymer. The addition of a nucleophilic cocatalyst
such as quaternary ammonium halides or sterically hindered
strong organic bases could significantly improve the activity
even at low CO2 pressures.8-10 The nucleophilic cocatalysts
were demonstrated to function as an initiator, as well as
tentatively thought to stabilize the active SalenCo(III) against
decomposition to SalenCo(II).8,9 In our opinion, the latter role
may be more important in maintaining high activity of these
Co(III) complexes. Except for this experimental observation,
we did not have any direct evidence concerning the role of these
nucleophilic cocatalysts in stabilizing active Co(III) species
during the copolymerization of CO2 and epoxides.
SalenCoX in conjunction with a quaternary ammonium salt for
CO2/epoxides copolymerization, Lee et al. reported an elegant
design of catalyst system 1. This system contains a Lewis acidic
metal center and quaternary ammonium salt units in a molecule
that has the highest activity for polycarbonoate synthesis even
at high temperatures and high [epoxide]/[catalyst] ratios.15 The
high catalytic activity and polymer selectivity at high temper-
atures was thought to result from the chain-growing carbonate
unit attracted to the metal center through Coulombic interaction
between the quaternary ammonium cation anchored on the
ligand framework and the chain-growing anion. Further sub-
stitution of a small methyl group for the bulky tert-butyl group
on the 3-position of the aromatic rings significantly enhanced
activity, and thereby resulting in the discovery of the fastest
catalyst to date.15b Prior to this study, Nozaki and co-workers
utilized a Salcy-type cobaltate complex 2 with a piperidinium
end-capping arm as catalyst for selectively synthesizing aliphatic
polycarbonates from CO2 and terminal epoxides.16 The high
polymer selectivity was achieved based on the proposal that
the piperidinium arm controls the formation of cyclic carbonates
by protonating the anionic propagating species when they
dissociate from the cobalt center. Notably, complete consump-
tion of the epoxide was accomplished with high copolymer
selectivity, which allowed for the production of block terpoly-
mers by stepwise addition of two different aliphatic epoxides.
Although this cobalt complex was active at 60 °C, there was a
concomitant production of cyclic carbonates in the final products
(up to 10-40%).15,16
Since the coupling reaction of CO2 with epoxides is exo-
thermic, the development of a thermally robust catalyst is
industrially desirable. Five-membered cyclic carbonates are more
thermodynamically stable than the corresponding polycarbon-
ates. In addition, the difference in the energies of activation for
cyclic carbonate versus copolymer formation in the terminal
epoxides such as PO is very small.14 As a result, the concomitant
production of cyclic carbonate easily occurs in the coupling
reaction carried out at elevated temperatures. Recently, based
on the mechanistic understanding of binary catalyst systems of
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1997, 30, 368–372. (b) Super, M.; Beckman, E. J. Macromol. Symp.
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11009. (e) Nakano, K.; Nozaki, K.; Hiyama, T. J. Am. Chem. Soc.
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We have reported that the addition of a nucleophilic cocatalyst
could significantly improve the activity of Co(III)-Salen
complexes even at low CO2 pressures and have confirmed its
initiator role during CO2/epoxides copolymerization.8 Although
these studies resulted in the discovery of some highly active
catalyst systems,15 no attempt was made to investigate the
mechanistic properties of these nucleophilic cocatalysts for
stabilizing active Co(III) species during the copolymerization.
Herein, we report a highly active and thermally stable cobalt-
based catalyst with 1,5,7-triabicyclo[4,4,0] dec-5-ene (designated
as TBD, a sterically hindered organic base) anchored on the
ligand framework. This report will provide details of mechanistic
studies on the role of the anchored TBD in maintaining high
activity and thermal stability of the catalyst during the copo-
lymerization of CO2 and epoxides. These mechanistic studies
are beneficial to elucidating the propagating polymer-chain anion
with regard to nucleophilic cocatalysts in previously much-
studied binary catalyst systems for stabilizing active Co(III)
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