Synthesis and characterization of the titanium complexes bearing two β-enaminoketonato ligands with electron withdrawing groups/modified phenyls and their behaviors for ethylene (co-)polymerization

摘要

A series of new titanium complexes with two asymmetric bidentate β-enaminoketonato [N,O] ligands (2b-t), [PhNC(CF_3)CHC(Ar)O] _2TiCl_2 (2b, Ar = -C_6H_4F(o); 2c, Ar = -C_6H_4F(m); 2d, Ar = -C_6H_4F(p); 2e, Ar = -C_6H_4Cl(p); 2f, Ar = -C_6H _4OMe(p); 2g, Ar = -C_6H_4CF_3(p); 2h, Ar = -C_6H_4CF_3(m); 2i, Ar = -C_6H _4CF_3(o); 2j, Ar = -C_6H_4Cl(o); 2k, Ar = -C_6H_4Br(o); 2l, Ar = -C_6H_4I(o); 2m, Ar = -C_6H_3F_2(2,4); 2n, Ar = -C _6H_3F_2(2,6); 2o, Ar = -C_6H _3F_2(3,4); 2p, Ar = -C_6H_3F _2(3,5); 2q, Ar = -C_6F_5; 2r, Ar = C _6F_4OMe; 2s, Ar = -C_6H_3Cl _2(2,6); 2t, Ar = -C_6H_3Cl_2(2,5)), have been synthesized based on substituted acetophenones. X-Ray analyses reveal that complexes 2h, 2k, 2m, and 2n adopt distorted octahedral geometry around the titanium center, in which the two chloride ligands are situated in the cis-orientation. 2s also adopts distorted octahedral geometry, but the two chloride ligands in it are situated in the trans-orientation due to the increase of the steric effect of the phenyl derived from the acetophenone. The influence of the substituent effects on catalyst performance, including catalytic activities and the molecular weight distribution of the polymers obtained, was investigated in detail. With modified methylaluminoxane (MMAO) as a cocatalyst, complexes 2b-r and 2t are active catalysts for ethylene polymerization at room temperature, and produce high molecular weight polymers. It is observed that the catalytic activities are significantly enhanced by introducing some electron-withdrawing groups, such as -F, -Cl and -CF_3, into the suitable positions of the phenyl ring close to the oxygen donor. It should be noted that complexes 2c-i, 2p, 2n and 2t are also capable of promoting the living copolymerization of ethylene with norbornene at room temperature, yielding high molecular weight copolymers with narrow molecular weight distributions (PDI = 1.05-1.30).