Catalytic Synergy Using Al(III) and Group 1 Metals to Accelerate Epoxide and Anhydride Ring-Opening Copolymerizations

The controlled synthesis of polyesters via epoxide/anhydride ring-opening copolymerization is a versatile and generally applicable method to make many sustainable polymers, but catalyst activities are limited and the required catalyst loadings are typically high. Here, novel heterodinuclear complexes, featuring Al­(III)/M­(I) (M = Na, K, Rb, Cs), show exceptional activities for phthalic anhydride and cyclohexene oxide copolymerization (catalyst = Al­(III)/K­(I), turnover frequency = 1072 h^–1, 0.25 mol % catalyst loading vs anhydride, 100 °C). The Al­(III)/K­(I) catalyst is also tolerant to low loadings, maintaining a good performance at 0.025 mol % catalyst vs anhydride loading and 0.005 mol % vs epoxide. It rapidly polymerizes other epoxide/anhydride combinations yielding various semi-aromatic, rigid, and/or functionalizable polyesters and also shows activity in carbon dioxide/epoxide copolymerizations. The results of structure–activity, X-ray crystallography, polymerization kinetics, and density functional theory investigations support a mechanism with chain growth alternation between the metals. The rate-limiting step is proposed to involve epoxide coordination at Al­(III) with K­(I) carboxylate attack. Future exploitation of abundant and inexpensive Group 1 metals to deliver synergic polymerization catalysts is recommended.