Bis(beta-enaminoketonato) vanadium(III) complexes (2a-c) [O(R-1)C=C(H)(x)C(R-2)=NC6H5](2)VCI(THF) and the corresponding vanadium(IV) complexes (3a-c) (0(Fl1)C-=C(H)C(R2)= NC6H5](2)VO (R-1 = (CH2)(4)-, R-2 = H, x = 0, a; R-1 = C6H5, R-2 = H, x 1, b; R1 = C6H5, R-2 = -C6H5, X 1, C) have been synthesized from VCI3(THF)3 and VOCl2(THF)(2), respectively, by treating with 2.0 equivalent fl-enaminoketonato ligands in tetrahydrofuran. Structures of 2b and 3a-c were further confirmed by X-ray crystallographic analysis. The complexes were investigated as the catalysts for ethylene polymerization in the presence of Et2AlCl. Complexes 2a-c and 3a-c exhibited high catalytic activities (up to 23.76 kg of PE/mmolv h bar), and afforded polymers with unimodal molecular weight distributions at 70 'C indicating the good thermal stability. The cata- lytic behaviors were influenced not only by the oxidation state of the catalyst precursors but also by the ligand structures. Complexes 2a-c and 3a-c were also effective catalyst precursors for ethylene/1-hexene copolymerization. The influence of polymerization parameters such as reaction temperature, AIN molar ratio and hexene feed concentration on the ethylene/hexene copolymerization behaviors have bee also investigated in detail. In addition, the agents such as AlMe3, AliBu(3), MeMgBr, MgCl2, and ZnEt2 were applied to control the molecular weight and molecular weight distribution modal. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3062-3072, 2010