Controlled ring-opening copolymerization of epoxides and cyclic anhydrides through bifunctional thiourea–quaternary ammonium salt catalysts

The use of bifunctional catalysts (bifunctional initiators) to achieve controllable alternating copolymerization of epoxides and anhydrides is an attractive field worthy of exploration. Herein, we discussed a type of metal-free, structurally diverse and efficient bifunctional thiourea–quaternary ammonium salt catalyst to prepare perfectly alternating copolymers of polyesters. The structural framework of the bifunctional catalyst, including the length of the carbon chain connecting the nucleophilic/electrophilic centers, the electronic effect and steric hindrance of substituents, and the regulation of halogen anions on the nucleophilic center were all considered to optimize the catalyst structure. Compared to a two-component system with a similar structure, the bifunctional catalyst exhibited significant advantages, especially at a low catalyst loading (<0.1%). The GPC characterization of the prepared polyesters showed a unimodal and narrow molecular weight distribution (Đ < 1.2) at a low feed ratio. At a feed ratio of CHO : PA : Cat. = 8000 : 4000 : 1, a high molecular weight (Mn) of 384.3 kDa was obtained. Kinetic research and polymer characterization and analysis provided a mechanistic basis for the effectiveness of the catalyst system.