Phosphazene-Catalyzed Alternating Copolymerization of Dihydrocoumarin and Ethylene Oxide: Weaker Is Better

Metal-free alternating copolymerization of 3,4-dihydrocoumarin (DHC) and ethylene oxide (EO) was realized by relatively mild phosphazene bases (t-BuP2 and t-BuP1), which unexpectedly outperformed previously employed t-BuP4 superbase in terms of polymerization rate, monomer conversion, and copolymer molar mass, though macrocycles were still generated when long chains were targeted. Such facts have indicated the occurrence of proton shuttling between phosphazenium cation and alkoxide which reduced chain-end nucleophilicity and thus alleviated side reactions such as backbiting. It was surprising that t-BuP1 whose basicity was too low for the homopolymerization of EO triggered alternating copolymerization, indicating that generation of anionic species (phenoxide) was essential for the epoxide-opening step. Well-defined short-chain diols were subjected to one-pot subsequent chain extension by addition of an aliphatic lactone or a diisocyanate leading to, respectively, block copolymer or polyurethane constituted by alternating segments. Poly(DHC-alt-EO) showed a better thermal stability than those of the substituted epoxides. This study has suggested that mild and non-nucleophilic organobases may be more suitable catalysts for epoxide-based metal-free alternating copolymerization toward well-defined macromolecular structures.