Bifunctional Catalysis Prevents Inhibition in Reversible-Deactivation Ring-Opening Copolymerizations of Epoxides and Cyclic Anhydrides.

Abstract

Reversible-deactivation chain transfer is a viable strategy to increase the catalytic efficiency of ring-opening polymerizations, such as the alternating copolymerization of epoxides and cyclic anhydrides. In conjunction with the catalyst, protic chain transfer agents (CTAs) initiate polymerization and facilitate rapid proton transfer between active and dormant chains. Functional-group-tolerant Lewis acid catalysts are therefore required to successfully apply protic CTAs in reversible-deactivation ring-opening copolymerizations (RD-ROCOP), yet the predominant binary Lewis acid catalyst/nucleophilic cocatalyst systems suffer lower polymerization rates when used with protic CTAs. New mechanistic insight into the inhibition pathways reveals that the alcohol chain ends compete with epoxide binding to the Lewis acid and hydrogen-bond with anionic chain ends to impede epoxide ring opening. We report that a bifunctional aminocyclopropenium aluminum salen complex maintains excellent activity in the presence of protic functionality, exhibiting resilience against these inhibition pathways, even at high CTA concentrations. We apply reversible-deactivation chain transfer in the bifunctional ROCOP system to demonstrate precise molecular-weight control, CTA functional group scope, and accessible polymer architectures.