High activity and low polymer polydispersity are found in ring-opening metathesis polymerization (ROMP) of norbornene by ruthenium complexes containing chelating diphosphine ligands. Treatment of compounds with general formula RuCl2(PP)(PPh3) (PP dppb (3b), binap (3c), dcypb (3d)) or [RuCl2(PP)](2) (PP = dppb (4b)) with PhCHN2 generates carbene complexes of the type RuCl2(PP)-(CHPh), which exhibit high ROMP activity without halide or phosphine abstraction. Investigations into the role of dissociated PPh3 have shown that free phosphine acts as a poison, dramatically retarding the rate of polymerization via 3 relative to catalysts derived from 4.