Intramolecularly Cooperative Catalysis for Copolymerization of Cyclic Thioanhydrides and Epoxides: A Dual Activation Strategy to Well-Defined Polythioesters

The copolymerization of epoxides and cyclic thioanhydrides provides a promising method to produce polythioesters and represents an alternative route to the step-growth condensation of dithiols and diacyl chlorides. However, typical binary metal–salen complex/ionic salt systems suffer from an unavoidable transesterification reaction, which degrades the polymer backbone and changes the polymer properties. We herein describe the precise synthesis of polythioesters from the copolymerization of cyclic thioanhydrides and epoxides by an intramolecular acid–base cooperative catalysis strategy, which can significantly suppress side reactions. The bifunctional catalyst composed of Lewis acidic Cr–salen complex with a covalently tethered 1,5,7-triazabicyclo[4,4,0]dec-5-ene (TBD, a sterically hindered strong Lewis base) successfully suppressed the transesterification reaction even at low concentrations, yielding polythioesters with completely alternating structures, controlled molecular weights, and low dispersities. Mechanistic studies revealed that polymer chain propagation was predominantly proceeded by intramolecular alternating enchainment between the Lewis acid-activated epoxide and the Lewis base-mediated cyclic thioanhydride.