Abstract Click to copy section linkSection link copied! 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.