Selective activation of metallic center in heterobinuclear cobalt and nickel complex in ethylene polymerization

摘要

Heterobinuclear cobalt and nickel complex {2-[2,6-R_2-C_6H3NC(CH_3)-(CH3)CN-(3,5-R_2')C_6H_2-CH_2-(3',5'-R_2)C6H2NC(CH_3)]-6-[2,6-R_2-C_6H_3NC(CH_3)] pyridine}CoCl_2NiBr_2 (R = isopropyl) (N_5CoNi) was prepared by reaction of pentadentate nitrogen ligand containing 2,6-bis(imino)-pyridine and α-diimine moieties with CoCl_2 and NiBr_2(DME) in turn. The complex was applied as catalyst for ethylene polymerization activated by AlEt_3, MMAO and AlEt3/[PhMe_2NH] [B(C_6F_5)_4] respectively. The performance of the heterobinuclear complex in ethylene polymerization was compared with corresponding mononuclear complexes (α-diimine nickel bromide and 2,6-bis(imino)-pyridine cobalt chloride) and their equivalent mixture (binary complexes). When the complex N_5CoNi was activated by AlEt_3 or MMAO, its ethylene polymerization activity was lower than its control, the binary complexes. Both heterobinuclear complex and binary complex produced PE with bimodal molecular weight distribution. The amount of high-molecular-weight polyethylene produced by nickel center of N_5CoNi was less than the binary complexes, which reveals that productivity of nickel center of N_5CoNi is selectively suppressed. When the heterobinuclear complex N_5CoNi is activated by AlEt_3/[PhMe_2NH][B(C_6F_5)_4], the relative productivity of nickel center increased, although the total activity of catalyst decreased compared with AlEt_3 as cocatalyst. With respect to AlEt_3, [PhMe_2NH][B(C_6F_5)_4] can preferably activate nickel center of heterobinuclear complex. The results suggest that metal site in the heterobinuclear complex is selectively activated by cocatalyst.