Simple/commercially-available Lewis acid in anionic ring-opening polymerization: powerful compounds with multiple applications in macromolecular engineering

Simple and commercially-available Lewis acids (LA) are commonly used catalysts in anionic ring-opening polymerization (AROP) reactions. In particular, for the AROP of epoxides, the addition of a Lewis acid allows the transition from a so-called end-chain mechanism to a monomer-activated mechanism. The presence of the LA simultaneously leads to a decrease in the reactivity of active centers through the formation of a three-species ate complex and to the activation of the monomer by LA coordination to the oxygen atom of the oxirane ring. These two effects result in both an increase in propagation kinetics and a decrease in transfer reactions, which has enabled the synthesis of high molecular weight polyethers. However, the impact of Lewis acids goes far beyond these classic effects. They have indeed enabled the polymerization of new functional monomers as well as the synthesis of heterotelechelic macromolecules. Also widely used as catalysts in copolymerization reactions (statistical, sequential, or alternating) Lewis acids can strongly influence the composition and sequence of monomer units in macromolecules. Finally, Lewis acids can also significantly influence the architecture of the obtained macromolecules. This review aims to list the various contributions of Lewis acids to macromolecular engineering and illustrate them with well-chosen examples.