Long-chain-branched polyethylene with a broad or bimodal molecular weight distribution was synthesized by ethylene homopolymerization via a novel nickel(II) alpha-diimine complex of 2,3-bis(2-phenylphenyl)butane diimine nickel dibromide ({[2-C6H4(C6H5)]-N=C-(CH3)C(CH3)=N-[2-C6H4(C6H5)]}NiBr2) that possessed two stereoisomers in the presence of modified methylaluminoxane. The influences of the polymerization conditions, including the temperature and Al/Ni molar ratio, on the catalytic activity, molecular weight and molecular weight distribution, degree of branching, and branch length of polyethylene, were investigated. The resultant products were confirmed by gel permeation chromatography, gas chromatography/mass spectrometry, and C-13 NMR characterization to be composed of higher molecular weight polyethylene with only isolated long-branched chains (longer than six carbons) or with methyl pendant groups and oligomers of linear alpha-olefins. The long-chain-branched polyethylene was formed mainly through the copolymerization of ethylene growing chains and macromonomers of alpha-olefins. The presence of methyl pendant groups in the polyethylene main chain implied a 2,1-insertion of the macromonomers into [Ni]-H active species. (c) 2005 Wiley Periodicals, Inc.