Ring-opening metathesis polymerization (ROMP) of norbornene (NBE) using a catalyst system consisting of V(N-2,6-Me2C6H3)Cl-2(N = (CBu2)-Bu-t) (1a), MeMgBr, and PMe3 proceeded at a remarkable rate, and the catalytic activities (TON values, TON = turnover number) were low when MeLi or AlMe3 was used in place of MeMgBr. The activities increased upon increasing the polymerization temperature, and were dependent upon the molar ratio(s) of the catalyst component(s) employed. The resultant polymers prepared at 25 degrees C under the optimized conditions possessed high molecular weights with unimodal molecular weight distributions (M-n = 1.31-1.66 x 10(6), M-w/M-n = 1.40-1.59), and contained a mixture of cis/trans olefinic. double bonds. The effects of anionic donor ligands (L) and substituents on the arylimido ligands on the activity in the ROMP using V(NAr)C-12(L) [Ar = 2,6-Me2C6H3 (1), 2,6-(Pr2C6H3)-Pr-i (2); L = N = (CBu2)-Bu-t (a), N = QCH(2)SiMe(3))Bu-t (b), O-2,6-Me2C6H3 (c), O-2,6-(PrC6H3)-Pr-i (d), O-2,6-Bu-t(2)-4-MeC6H2 (e)]-MeMgBr-PMe3 were explored; both la and le showed the remarkable activities, and the observed trends were similar to those observed in ethylene polymerization using V(NAr)Cl-2(L)-MAO catalyst systems. (c) 2007 Elsevier B.V. All rights reserved.