Availability of Other Aliphatic Polycarbonates Derived from Geometric Isomers of Butene Oxide and Carbon Dioxide Coupling Reactions

In this study, we investigate the selectivity for copolymer versus cyclic carbonate production from the coupling of isomeric forms of butene oxide with carbon dioxide in the presence of binary and bifunctional cobalt­(III) and chromium­(III) salicylaldimine catalysts. Use of the less problematic 1-butene oxide has previously been reported to preferentially afford copolymer from its coupling with CO₂. Of the epoxides, cis- and trans-2-butene oxide and isobutene oxide, only cis-2-butene oxide was shown to selectively provide polycarbonate, with both cobalt­(III) catalysts being more effective than their chromium­(III) analogues. The binary chromium catalyst system produced both cis- and trans-cyclic carbonates from the cycloaddition of CO₂ and cis-2-butene oxide, whereas, the corresponding cobalt­(III) catalyst selectively yielded 75.4% copolymer at 40 °C with the remaining product being trans-cyclic carbonate. In this instance, the trans-cyclic carbonate results from copolymer degradation, consistent with the observation that depolymerization of the copolymer derived from CO₂ and cis-2-butene oxide affords trans-cyclic carbonate exclusively. By way of contrast, both bifunctional catalysts were efficient at producing copolymers with selectivities of 100% (40 °C) and 79% (70 °C) for the cobalt and chromium catalysts, respectively. The glass transition temperature (T_g) of poly­(trans-2-butene carbonate) derived from the completely alternating copolymerization of CO₂ and cis-2-butene oxide was found to be 68 °C, some 30 deg higher than poly­(propylene carbonate). Furthermore, it was shown to have a significantly lower % elongation-to-break value than poly­(propylene carbonate).