The oxidative addition of benzyl chloride to Ni(cod)(2) in the presence of 1,4-bis-2,6-diisopropylphenyl)-acenaphthenediimine followed by chloride abstraction affords [(eta(3)-CH2C6H5)Ni(alpha-diimine)][PF6] (alpha-diimine-1,4-bis(2,6-diisopropylphenyl)acenaphthenediimine) in 70% yield. The complex is active in ethylene polymerization in the presence of methylaluminoxane and under mild reaction conditions. The polyethylenes obtained are highly branched, have very low densities, do not show T-m or measurable crystallinity and have molecular weights ranging from 80 x 10(3) to 290 x 10(3) g.mol(-1).