A series of (imido)vanadium(V) dichloride complexes containing 1,3-imidazolin-2-iminato or 1,3-imidazolidin-2-iminato ligands of the type, V(NR')Cl-2(L) [R' = 2,6-Me2C6H3, L = 1,3-R-2(CHN)(2)C=N (1a-c,e) or 1,3-R-2(CH2N)(2)C=N (2a-d), R = Bu-t (a), 2,6-Me2C6H3 (b), 2,6-(Pr2C6H3)-Pr-i (c), C6H5 (d), 2,6-(Ph2CH)(2)-4-MeC6H2 (e); L = 1,3-(2,6-(Pr2C6H3)-Pr-i)(2)(CHN)(2)C=N, R' = 1-adamantyl (Ad, 3c), C6H5 (4c); L = 1,3-(2,6-(Pr2C6H3)-Pr-i)(2)(CH2N)(2)C=N, R' = Ad (5c)], were prepared and characterized. The molecular structures of 1a, 2a,c,d, 3c, 4c, and 5c were determined by X-ray crystallography. All complexes showed high catalytic activity for ethylene polymerization especially in the presence of Et2AlCl cocatalyst; the 2,6-R2C6H3 analogues (R = Me, Pr-i; 1b,c, 2b,c) exhibited higher catalytic activities than the Bu-t analogues (1a, 2a), which display rather unique (small) V-N-C(imido) bond angles in the solid state. A good correlation between the activity and the V-51 NMR chemical shift was found for the (arylimido)vanadium precatalysts (1a-c,e, 2a-d, and 4c). These complexes showed high catalytic activity for the copolymerization of ethylene with norbornene (NBE), affording ultrahigh molecular weight copolymers with uniform molecular weight distributions. The activities were affected by the imido ligand as well as by the substituents in the anionic ligand, and the 2,6-(Pr2C6H3)-Pr-i analogues (especially 2c and 4c) showed the higher activities. The complexes 2c and 4c also showed high activities with efficient comonomer incorporation for the ethylene copolymerization with 5-ethylidene-2-norbornene (ENB) in the presence of Et2AlCl; both the comonomer incorporation and the molecular weight in the resulting polymers were affected by the comonomer employed (NBE vs ENB).