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 (<b>Ru1</b>) was investigated as a ring-opening metathesis polymerization (ROMP) catalyst. Building on prior studies showing ultrafast initiation and a bimolecular mechanism of decomposition, <b>Ru1</b> 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. <b>Ru1</b> 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 <b>Ru1</b> as a benchmark system for ultrafast metathesis polymerization.