We report a new semiaromatic poly­(thioester) obtained through the copolymerization of phthalic thioanhydride and propylene oxide. The reaction was catalyzed by a chromium-based complex in conjunction with [PPN]­Cl, where PPN = bis­(triphenyl­phosphine)­iminium. The reaction temperature exerted a critical influence over catalytic activity as well as the structure of the resulting polymer chain. NMR spectroscopy revealed that the resultant copolymers contained multiple repeating units, including thioester, ester, and thioether–ester linkages, in their main chains due to transesterification, particularly when they were produced at elevated reaction temperatures. The thioester linkage content affected the thermal properties of the polymers. A relatively high glass transition temperature of 69.5 °C was observed in the copolymer containing a large number of thioester linkages obtained at 25 °C. In addition, this sulfur-containing polymer exhibited desirable optical properties, with a refractive index of 1.60.

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.

The precise synthesis of poly(thioester)s with diverse structures is still a significant challenge in the polymeric materials field. Herein, we report a novel approach to the synthesis of well‐defined poly(thioester)s by the controlled alternating copolymerization of cyclic thioanhydrides and episulfides induced by simple organic ammonium salts. Both the cation and anion have strong effects on the copolymerization. [PPN]OAc ([PPN]=bis(triphenylphosphine)iminium) with a bulky cation was proven to be efficient in initiating this polymerization, yielding poly(thioester)s with a completely alternating structure, controlled molecular weight, and narrow polydispersity. The poly(thioester) obtained from succinic thioanhydride and propylene sulfide is a typical semicrystalline material, possessing a high refractive index of up to 1.78. Because it uses readily available monomers, this method is expected to open up a new route to poly(thioester)s with diverse structures and properties.