[ONN]-type amine pyridine(s) phenolate-based oxovanadium(V) catalysts for ethylene homo- and copolymerization
A series of oxovanadium(V) complexes containing amine pyridine(s) phenolate ligands [ONN] (2a–f) have been synthesized in high yields (68–83%) by reacting VO(OnPr)3 with 1.0 equiv. of the ligands in CH2Cl2. These complexes were characterized by 1H, 13C and 51V NMR spectroscopy and elemental analysis. X-ray structural analysis for 2a, 2c and 2d revealed that these complexes adopt a six-coordinate distorted octahedral geometry around the vanadium center in the solid state. Upon treatment with Et2AlCl and CCl3COOEt, these complexes displayed high catalytic activities for ethylene polymerization even at elevated reaction temperatures, depending on ligand structures. The resulting polymers possessed high molecular weight and unimodal molecular weight distributions, indicative of the formation of a single catalytically active species during the polymerization catalysis. Excitingly, these vanadium(V) complexes could efficiently promote ethylene/norbornene copolymerization. The observed catalytic activity for the copolymerization was higher than that for ethylene homopolymerization. Moreover, the molecular weights of the resulting copolymers increased upon increasing the norbornene feed. These results indicated that introducing a suitable amount of norbornene into the system not only could accelerate the polymerization rate but also could restrain chain transfer reactions to some extent.