Nickel alpha-diimine complex [C6H5-N=C(CH3)C(CH3)=N-C6H5]NiCl2 was impregnated on SiO2/MgCl2 bisupport, and ethylene polymerizations were carried out with alkylaluminum compounds as cocatalyst. Branched polyethylenes were prepared when heptane is used as solvent. Polymerization conditions such as modified method of bisupport, cocatalyst, Al/Ni ratio, temperature and nickel concentration had a pronounced effect on catalytic activity and properties of polyethylenes. The activity of 2.9 x 10(5) g PE mol Ni h(-1) was obtained in the presence of AlEt2Cl as well as the bisupport without AlEt3 modification, Al/Ni ratio 80, nickel concentration 0.12 mmol/l and temperature 14 degreesC. Branching degree of polyethylenes increased with temperature, and molecular weight, melting point and crystallinity of polyethylenes decreased correspondingly. Polymerization temperature had a pronounced effect on branches distribution of polyethylenes. Content of methyl branch increased sharply with temperature, but long branches dropped quickly. (C) 2004 Elsevier Ltd. All rights reserved.