Copolymerization and Cycloaddition Products Derived from Coupling Reactions of 1,2-Epoxy-4-cyclohexene and Carbon Dioxide. Postpolymerization Functionalization via Thiol–Ene Click Reactions

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Abstract

The coupling reaction of 1,2-epoxy-4-cyclohexene with CO2 in the presence of a ZnCl2/nBu4NI catalyst system was shown to provide the naturally occurring cis-cyclohexadiene carbonate. An alternative synthesis of this compound, which was characterized by X-ray structural analysis, was carried out from the cis-diol and triphosgene. Upon utilizing binary or bifunctional (salen)CrX catalysts, this coupling process resulted in the selective formation of completely alternating copolymer of 1,2-epoxy-4-cyclohexene and carbon dioxide. In the case involving the binary chromium(III)/onium salt catalyst, small quantities of both the cis and trans cyclic carbonates were also produced. The (salen)CoDNP/PPNDNP (DNP = 2,4-dinitrophenolate) catalyst system was most effective at producing high molecular weight copolymer with 100% selectivity. The Tg of this polymer (Mn = 35.9 kDa) was determined to be 123 °C, which is higher than the Tg (116 °C) of the corresponding saturated copolymer. Depolymerization of poly(cyclohexadiene carbonate) to trans-cyclohexadiene carbonate occurred slowly and cleanly at 110 °C following deprotonation of the terminal hydroxyl group. The trans-cyclohexadiene carbonate was independently synthesized via the carbonylation of the trans-diol with ethyl chloroformate. The hydrophobic 1,2-epoxy-4-cyclohexene/CO2 derived copolymer was modified by the quantitative addition of thioglycolic acid by way of the thiol–ene click reaction to afford an amphiphilic copolymer. Upon deprotonation of this functionalized polycarbonate with ammonium hydroxide, the production of a water-soluble polymeric material was achieved which displayed a Tg of 120 °C.

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