Cocatalyst effects in α-diimine nickel catalyzed ethylene polymerization

Two α-diimine nickel dimethyl complexes bearing substituents with different steric hindrance effects were synthesized and characterized. In the absence of any organoaluminium cocatalysts or scavengers, commercially available organoboron cocatalysts B(C6F5)3 (B1) or [Ph3C][B(C6F5)4] (B2), can activate these α-diimine nickel dimethyl complexes to catalyze ethylene polymerization. Compared with B2, cocatalyst B1 led to significantly increased catalytic activities, resulting in polyethylenes with slightly lower molecular weight, higher branching densities and lower melting temperatures. Furthermore, the resulting polyolefin materials exhibited good tensile and elastic properties. Density functional theory (DFT) calculations indicated that the counter anion derived from B1 can form a secondary interaction with the Ni center in the active species, which may contribute to the increased stability and activity.