Catalytic Coupling of Cyclopentene Oxide and CO₂ Utilizing Bifunctional (salen)Co(III) and (salen)Cr(III) Catalysts: Comparative Processes Involving Binary (salen)Cr(III) Analogs

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Abstract

Previously, the polycarbonate produced from the alternating copolymerization of cyclopentene oxide and carbon dioxide has been demonstrated to readily be recyclable to its monomers, hence affording a sustainable route to the production of this polymeric material. This, in turn, necessitates the development of effective synthetic methods for this copolymer. Coupling reactions of the alicyclic epoxides, cyclohexene oxide (CHO) and cyclopentene oxide (CPO), with CO2 were shown to provide grossly different product selectivities utilizing binary (salen)CrX/PPNX catalyst systems. That is, whereas the coupling of CHO/CO2 affords >99% selectivity for completely alternating copolymers, under identical conditions, CPO/CO2 yields >99% selectivity for cis-cyclopentene carbonate. By way of contrast, employing bifunctional (R,R)-(salen)M(III) catalysts (M = Cr and Co) for the coupling reactions of cyclopentene oxide and carbon dioxide results in the highly selective synthesis of well-controllable, narrowly distributed molecular weights copolymers. Although the chromium catalyst was found to be less active than its cobalt analog, it was shown to be thermally more stable, and hence, the reaction could be carried out at higher temperatures with little sacrifice in selectivity for copolymer production. The Tg of the resulting poly(cyclopentene carbonate) was determined to be 84.5 °C.

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