Diment WT, Lindeboom W, Fiorentini F, Deacy AC, Williams CK. Synergic Heterodinuclear Catalysts for the Ring-Opening Copolymerization (ROCOP) of Epoxides, Carbon Dioxide, and Anhydrides.
Acc Chem Res 2022;
55:1997-2010. [PMID:
35863044 PMCID:
PMC9350912 DOI:
10.1021/acs.accounts.2c00197]
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Abstract
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The development of sustainable
plastic materials is an essential
target of chemistry in the 21st century. Key objectives toward this
goal include utilizing sustainable monomers and the development of
polymers that can be chemically recycled/degraded. Polycarbonates
synthesized from the ring-opening copolymerization (ROCOP) of epoxides
and CO2, and polyesters synthesized from the ROCOP of epoxides
and anhydrides, meet these criteria. Despite this, designing efficient
catalysts for these processes remains challenging. Typical issues
include the requirement for high catalyst loading; low catalytic activities
in comparison with other commercialized polymerizations; and the requirement
of costly, toxic cocatalysts. The development of efficient catalysts
for both types of ROCOP is highly desirable. This Account details
our work on the development of catalysts for these two related polymerizations
and, in particular, focuses on dinuclear complexes, which are typically
applied without any cocatalyst. We have developed mechanistic hypotheses
in tandem with our catalysts, and throughout the Account, we describe
the kinetic, computational, and structure–activity studies
that underpin the performance of these catalysts. Our initial research
on homodinuclear M(II)M(II) complexes for cyclohexene oxide (CHO)/CO2 ROCOP provided data to support a chain shuttling catalytic
mechanism, which implied different roles for the two metals in the
catalysis. This mechanistic hypothesis inspired the development of
mixed-metal, heterodinuclear catalysts. The first of this class of
catalysts was a heterodinuclear Zn(II)Mg(II) complex, which showed
higher rates than either of the homodinuclear [Zn(II)Zn(II) and Mg(II)Mg(II)]
analogues for CHO/CO2 ROCOP. Expanding on this finding,
we subsequently developed a Co(II)Mg(II) complex that showed field
leading rates for CHO/CO2 ROCOP and allowed for unique
insight into the role of the two metals in this complex, where it
was established that the Mg(II) center reduced transition state entropy
and the Co(II) center reduced transition state enthalpy. Following
these discoveries, we subsequently developed a range of heterodinuclear
M(III)M(I) catalysts that were capable of catalyzing a broad range
of copolymerizations, including the ring-opening copolymerization
of CHO/CO2, propylene oxide (PO)/CO2, and CHO/phthalic
anhydride (PA). Catalysts featuring Co(III)K(I) and Al(III)K(I) were
found to be exceptionally effective for PO/CO2 and CHO/PA
ROCOP, respectively. Such M(III)M(I) complexes operate through a dinuclear
metalate mechanism, where the M(III) binds and activates monomers
while the M(I) species binds the polymer change in close proximity
to allow for insertion into the activated monomer. Our research illustrates
how careful catalyst design can yield highly efficient systems and
how the development of mechanistic understanding aids this process.
Avenues of future research are also discussed, including the applicability
of these heterodinuclear catalysts in the synthesis of sustainable
materials.
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