A series of α-diimine Ni(II) complexes containing bulky phenyl groups, [ArN = C(Naphth)C = NAr]NiBr2 (Naphth: 1,8-naphthdiyl, Ar = 2,6-Me2-4-PhC6H2 (C1); Ar = 2,4-Me2-6-PhC6H2 (C2); Ar = 2-Me-4,6-Ph2C6H2 (C3); Ar = 4-Me-2,6-Ph2C6H2 (C4); Ar = 4-Me-2-PhC6H3 (C5); Ar = 2,4,6-Ph3C6H2 (C6)), were synthesized and characterized. Upon activation with either diethylaluminum chloride (Et2AlCl) or modified methylaluminoxane (MMAO), all Ni(II) complexes showed high activities in ethylene polymerization and produced highly branched amorphous polyethylene (up to 145 branches/1000 carbons). Interestingly, the sec-butyl branches were observed in polyethylene depending on polymerization temperature. Polymerization of 1-alkene (1-hexene, 1-octene, 1-decene and 1-hexadecene) with C1-MMAO at room temperature resulted in branched polyolefins with narrow Mw/Mn values (ca. 1.2), which suggested a living polymerization. The polymerization results indicated the possibility of precise microstructure control, depending on the polymerization temperature and types of monomers.