Aryloxo-modified vanadium(V) dichloride complexes containing a dichloro- or difluoro-phenylimido ligand of the type, V(N-2,6-X₂C₆H₃)Cl₂(O-2,6-R₂C₆H₃) [X = Cl (1), F (2); R = Me (a), F (b)], showed remarkable catalytic activity in ethylene polymerisation and the copolymerisation with norbornene (NBE) in the presence of Et₂AlCl; the activities were higher than that of V(N-2,6-Me₂C₆H₃)Cl₂(O-2,6-Me₂C₆H₃) reported previously. Complex 1a is a suitable catalyst precursor in terms of not only the activity, but also synthesis of high molecular weight copolymers with both unimodal molecular weight distributions and uniform compositions.