Copolymerizations of ethylene with norbornene (NBE), cyclopentene (CPE) and 1-hexene by (arylimido)(aryloxo)vanadium(V) complexes of the type VCl2(N-2,6-Me2C6H3)(OAr) [Ar = 2,6-Me2C6H3 (1), 2,6-(Pr2C6H3)-Pr-i (2), 2,6-Ph2C6H3 (3)] with cocatalyst [methylaluminoxane (MAO) or Et2AlCl] catalyst systems have been explored. These complexes exhibited remarkable catalytic activities for ethylene polymerization in the presence of Et2AlCl cocatalyst; in particular, the resultant polyethylene prepared by 1 possessed narrow molecular weight distributions and the M. value increased linearly upon increasing the polymer yields (TON values). Notable effects of Al cocatalyst have been observed in the copolymerization of ethylene with NBE: the catalytic activities, the NBE incorporation, dominant chain transfer reactions, and the M. values for resultant copolymers were highly dependent upon the Al cocatalyst employed (MAO or Et2AlCl). The 1-Et2AlCl catalyst system exhibited remarkable catalytic activities, affording ultrahigh molecular weight poly(ethylene-co-NBE)s with narrow molecular weight distributions (M-n = 2.12-2.70 x 10(6), M-w/M-n = 1.29-1.53), and the M-n values were independent upon the NBE contents but were dependent upon the Al/V molar ratios. In contrast, 1-MAO catalyst exhibited better NBE incorporation than the 1-Et2AlCl catalyst, and the Mn value was independent of the Al/V molar ratio but was dependent upon the NBE contents. Modification of the aryloxide ligand affected the catalytic activity, but it did not strongly affect the NBE incorporation, the M values, and microstructure in the resultant copolymers. However, 1 exhibited a negligible amount of CPE incorporation in the ethylene/CPE copolymerization and also exhibited inefficient 1-hexene incorporation in the ethylene/1-hexene copolymerization. Since the resultant polymer obtained by the attempted copolymerization of ethylene with styrene was polyethylene, the effect of comonomer toward both the catalytic activity and the comonomer incorporation has been demonstrated.