A series of 2-aminopyridine Ni(II) complexes bearing different substituent groups [(2-PyCH2NAr)NiBr, Ar = 2,4,6-trimethylphenyl (3a), 2,6-dichlorophenyl (3b), 2,6-dimethylphenyl (3c), 2,6-diisopropylphenyl (3d), 2,6-difluorophenyl (3e); (2PyCH(2)NHAr)(2)NiBr2, Ar = 2,6-diisopropylphenyl (4a)] have been synthesized and investigated as precatalysts for ethylene polymerization in the presence of methylaluminoxane (MAO). High molecular weight branched polymers as well as short-chain oligomers were simultaneously produced with these complexes. Enhancing the steric bulk of the ortho-aryl-substituents of the catalyst resulted in higher ratio of solid polymer to oligomer and higher molecular weight of the polymer. With ortho-haloid-substitution, the catalysts afforded a product with low polymer/oligomer ratio (3b) and even only oligomers (3e) in which C14H28 had the maximum content. Compared with complex 3d containing ionic ligand, complex 4a containing neutral ligand exhibited obviously low catalytic activity for ethylene polymerization. The molecular weight, molecular weight distribution, and microstructure of the resulted polymer were characterized by gel permeation chromatography and C-13 NMR spectrogram. (C) 2008 Wiley Periodicals, Inc.