Syndiospecific Lewis Pair Polymerization of Methacrylates by Rare-Earth/Phosphine Combinations

Achieving precise stereocontrol in macromolecular synthesis under mild conditions remains a fundamental challenge in polymer chemistry. Herein, an efficient and highly syndiospecific polymerization of methyl methacrylate (MMA) is realized by using Lewis pairs composed of rare-earth (RE) aryloxides and phosphines, affording syndiotactic PMMA (up to 90.3% rr at 25 °C) with well-controlled molecular weight (near-quantitative initial efficiency, Mn up to 21.0 × 104 g/mol). This methodology exhibits broad applicability, extending to diverse methacrylate monomers such as allyl, benzyl, di(methylamino)ethyl, and methoxyethyl derivatives (80.7–90.0% rr). Mechanistic studies, including kinetic analysis and chain-end characterization, support a bimolecular, activated-monomer propagation pathway. Crystallographic topographic maps reveal that moderate steric hindrance in RE Lewis acid phenolate ligands is crucial for the optimal polymerization stereoselectivity.