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.

Advances in catalysis have enabled the ring-opening copolymerization of epoxides and cyclic anhydrides to afford structurally and functionally diverse polyesters with controlled molecular weights and dispersities. However, the most common systems employ binary catalyst/cocatalyst pairs which suffer from slow polymerization rates at low loadings. Inspired by new mechanistic insight into the function of binary metal salen/nucleophilic cocatalyst systems at low concentrations, we report a bifunctional complex in which the salen catalyst and an aminocyclopropenium cocatalyst are covalently tethered. A modular ligand design circumvents the extended linear syntheses typical of bifunctional catalysts, enabling systematic variation to understand and enhance catalytic activity. The optimized bifunctional aluminum salen catalyst maintains excellent activity for the ring-opening copolymerization of epoxides and cyclic anhydrides at low concentrations (≥0.025 mol %), and the aminocyclopropenium cocatalyst suppresses undesirable transesterification and epimerization side reactions, preserving the integrity of the polymer backbone.

The ring-opening copolymerization (ROCOP) of epoxides with CO2 or cyclic anhydrides is a versatile route toward synthesizing a wide range of polycarbonate and polyester copolymers. ROCOP most commonly uses binary catalyst systems comprising separate Lewis acid and nucleophilic cocatalyst components. However, the dependence on two discrete catalyst components leads to low activities at low loadings, and binary catalyst systems are prone to numerous side reactions. It was therefore proposed that covalently tethering the Lewis acid catalyst and cocatalyst together would increase both catalyst activity and selectivity in epoxide ROCOP. Since these initial efforts, many multifunctional catalysts featuring covalently tethered cationic or Lewis base cocatalyst(s) have been developed for epoxide ROCOP. This review examines multifunctional catalysts that have been developed for copolymerization of epoxides with CO2, cyclic anhydrides, carbonyl sulfide (COS), and cyclic thioanhydrides. In particular, we will assess how multifunctional catalysts' mechanisms of operations lead to improved activity and selectivity in ROCOP.