Four α-diimine nickel complexes Ar-N=C(R1)C(R2)=N-ArNiBr2 with different alkyl substituents on backbone of ligand ［Ar=2

6-dimethylphenyl，R1=CnH2n+1，R2=CmH2m+1; Cat 1: m=1，n=1; Cat 2:m=2

n=1; Cat 3:m=3

n=1; Cat 4: m=2

n=2］ were synthesized and used to catalyze ethylene polymerization in toluene. The influences arousing from changes in alkyl substituents on the backbone carbon of the catalyst ligand on the ethylene polymerization activities

microstructures and semi-crystalline properties of polyethylene were evaluated under varied polymerization temperatures and catalyst concentrations. It was showed that the catalysts with asymmetric alkyl substituents

i.e. R1 and R2 were different

had higher activity and produced polyethylenes having higher molecular weight as compared with those with symmetric substituents. With Cat 2 and Cat 3

the ethylene polymerization activities reached 1.86×103kg PE/(mol Ni#12539;h) and 1.92×103kg PE/(mol Ni#12539;h)

and molecular weights of polymers achieved 68.2×104 and 101.9×104

respectively under the condition of 20℃

atmosphere and 5.8 mmol/L of catalyst concentration. Analysis of polyethylene chain structures indicated that the main branching chain structure in polyethylene was methyl groups

and the branching degree was dependent mainly on polymerization temperature. Meanwhile

it was also showed that the polyethylene synthesized with α-diimine nickel complexes containing diversed alkyl substituents on ligand backbone had comparatively high proportion of hexyl and longer alkyl branching chains in polymer molecules

especially for the cases when polymerization was conducted at 40℃.