Harnessing borane-potassium cooperativity for sulfurated ring-opening copolymerisation

Sulfur-containing polymers, such as thioesters and thiocarbonates, can exhibit improved thermal properties and degradability compared to their all-oxygen analogues, yet their synthesis remains challenging. In this respect, ring-opening copolymerization (ROCOP) offers access to sulfur-containing polymers; however, the catalysts used for this process often rely on toxic, expensive or synthetically complex components. Here, we demonstrate that combining commercial borane Lewis acids with easily accessible potassium acetate crown ether complexes highly selectively mediates the ring-opening copolymerization of oxetanes with a wide range of sulfur-containing monomers. Mechanistic investigations clearly indicate a cooperative mode of action between boron and potassium, yielding high-melting, semicrystalline materials that exhibit improved thermal stability compared to those generated via chromium catalysis. Our study establishes new concepts in cooperative catalysis to produce sustainable materials that are otherwise difficult to access.