4-Methyl-1-pentene (4MP) was polymerized with a classical α-diimine nickel complex [(2,6-(ⁱPr)₂C₆H₃)NC(acenaphthene)CN(2,6-(ⁱPr)₂C₆H₃))NiBr₂1] in the presence of various alkylaluminium compounds. Influences of cocatalysts on 4MP polymerization behavior were evaluated in detail. The different effects of trialkylaluminium cocatalysts between ethylene polymerization and 4-methyl-1-pentene polymerization were observed. Inexpensive diethylaluminium chloride (DEAC) compound could replace methylaluminoxane (MAO) as a more active cocatalyst for 4MP polymerization, and the influences of polymerization parameters including temperature and [Al]/[Ni] mole ratio were examined. At 0 °C, living/controlled polymerization of 4-methyl-1-pentene (4MP) was also achieved using inexpensive DEAC as cocatalyst, and trialkylaluminium compounds as chain transfer agents were closely relevant to achieve living/controlled polymerization. The obtained poly(4-methyl-1-pentene)s are amorphous elastomers with low glass transition temperature (T_g). Nuclear magnetic resonance (NMR) analyses showed that various branches such as methyl, isobutyl, long 2-methylalkyl branches are present in the polymer.