Ni(II) complexes bearing an o-bis(aryl)phosphinophenolate ligand were synthesized as catalysts for copolymerization of ethylene and alkyl acrylates. When the P-bound aryl group was 2,6-dimethoxyphenyl group, one of the oxygen atoms in the methoxy groups coordinated to the nickel center on its apical position. This complex was a highly active catalyst without any activators to give highly linear and high molecular weight copolymers. The structures of the copolymers were determined by H-1 and C-13 NMR to clarify that the alkyl acrylate comonomers were incorporated in the main chain and that the structures of the copolymers were significantly influenced by the structure of the aryl group in the ligand.