Self-AcceleratingRing 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 g_polymer mol_cat^–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.