Self-Accelerating Ring Expansion Metathesis Polymerization

We report the one-step synthesis of a double-tethered metallacyclobutane molybdenum catalyst for ring expansion metathesis polymerization, a method increasingly popular for cyclic polymer production. The metallacyclobutane’s geometry, based on 13C NMR and DFT analysis, suggests it should not participate in metathesis. However, polymerization of norbornene showed high activity (an upper limit of 1,000,000,000 gpolymer molcat–1 h–1). DFT studies reveal that the catalyst’s slow initiation and fast propagation arise from a self-accelerating effect triggered by increasing steric demand, causing a geometry shift from square-pyramidal to trigonal bipyramidal. This insight will advance catalyst design principles and, in the future, allow for more precise control of the molecular weight, dispersity, and tacticity.