Ethylene Homopolymerizaton and Copolymerizaton by Vanadium(III) Complexes Containing Tridentate or Tetradentate Iminopyrrolyl Ligands

Iminopyrrolyl vanadium(III) complexes 2a-b bearing tridentate ligands [C4H3NCH=NC6H4L]VCl2(THF)[L = 2-P(C6H5)(2) (2a), 2-SMe (2b)] and complexes 2c-d with tetradentate ligands [(C4H3NCH = N)(2)R]VCl(THF) [R = 1,2-C6H4 (2c), 1,2-C2H4 (2d)] have been synthesized in high yields. With diethylaluminium chloride as a cocatalyst, complexes 2a-d were investigated as efficient catalysts for ethylene polymerization under various reaction conditions, and exhibited high catalytic activity and remarkable thermal stability. With these complexes, high molecular weight polymers with unimodal molecular weight distributions were obtained, indicating that the polymerization reaction took place in a single-site nature. Ethylene/1-hexene copolymerizations were also investigated in the presence of Et2AlCl. Both increasing ligand denticity and introducing softer atom into the sidearm of the ligands significantly influenced catalytic activity, comonomer incorporation, and the molecular weights of the resultant polymers, suggesting that both the steric and the electronic effects of the ligands played an important role in adjusting chain propagation and transfer rate. The chain transfer mechanisms involved in the copolymerization process were investigated by carefully analyzing the microstructure of the copolymers. The signals of vinyl, disubstituted and tri-substituted vinylene double bond end groups were detected in the copolymer obtained by 2a/Et2AlCl system but not in those by 2b-c/Et2AlCl systems, indicating that bulky electron- donating group, AP(C6H5)(2), may lead to those unusual transfer reactions. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 2700-2708, 2011