NHC-Supported (Arylimido)niobium(V)–Alkylidene Catalysts Exhibiting Exceptionally High Catalytic Activities for ROMPs

The NHC-supported (arylimido)niobium(V)–alkylidene catalysts containing perhalophenoxide ligands, especially Nb(CHR)(NC6F5)(OC6X5)(NHC)(PMe3) [R = SiMe3 or CMe2Ph; X = Cl, Br; NHC = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene], display remarkable catalytic activities for ring-opening metathesis polymerization (ROMP) of norbornene (NBE), affording ultrahigh molar mass polymers with unimodal molecular weight distributions (Mn = 2.02–3.84 × 106). In particular, the ROMP by Nb(CHSiMe3)(NC6F5)(OC6Cl5)(NHC)(PMe3) reached completion within 5 s at 25 °C with a turnover frequency (TOF) of >4260 s–1. The ROMP by Nb(CHSiMe3)(N-2,6-F2C6H3)[OC(CF3)3](NHC) gave the cis-syndiotactic polymers, and the polymerization proceeded in a controlled manner without catalyst decomposition.