An Ultrafast Benchmark for Ring Opening Metathesis Polymerization Catalysts: A Ruthenium Phosphinimine System Approaching the Diffusion Rate Limit

A ruthenium phenylidene complex bearing a monodentate phosphinimine ligand (Ru1) was investigated as a ring-opening metathesis polymerization (ROMP) catalyst. Building on prior studies showing ultrafast initiation and a bimolecular mechanism of decomposition, Ru1 was evaluated using norbornene (NBE), cyclooctene (COE), and cyclooctadiene (COD) as model substrates. Kinetic analysis revealed first-order dependence on monomer concentration, consistent with catalyst activation proceeding through a rate-limiting interchange ligand substitution reaction. Ru1 displayed exceptional activity, achieving turnover frequencies (TOFs) up to 4.00 × 10⁴ s^-1 and turnover numbers (TONs) up to 232,000 for NBE, TOFs up to 9.88 × 10³ s^-1 and TONs up to 240,000 for COE, and TOFs up to 1.80 × 10² s^-1 with TONs up to 2760 for COD. Experimentally determined rate constants reached 7.9 × 10⁸ M^-1·s^-1 (NBE), 9.6 × 10⁷ M^-1·s^-1 (COE), and 1.1 × 10⁵ M^-1·s^-1 (COD); values that approach the diffusion limit and rival the fastest enzymatic systems. To the best of our knowledge, these are the highest catalytic rates reported for ROMP to date, establishing Ru1 as a benchmark system for ultrafast metathesis polymerization.