Detailed investigations of the polymerization of ethylene by (alpha-diimine)nickel(II) catalysts are reported. Effects of structural variations of the diimine ligand on catalyst activities, polymer molecular weights, and polymer microstructure are described. The precatalysts employed were [{(2,6-RR＇C6H3)-N=C(Nap)-C(Nap)=N-2,6-RR＇C6H3)}NiBr2] (Nap = 1,8-naphthdiyl) (4a, R = CF3, R＇ = H; 4b, R = CF3, R＇ = CK3; 4c, R = C6F5, R＇ = H, 4c, R = C6F5, R＇ = H: 4d, R = C6F5, R＇ = CH3; 4e, R = CH3, R＇ = H, 4f, R = R＇ = CH3; 4g, R = R＇ = CH(CH3)(2)), [{(2,6-C6H3(i-Pr)(2))-N=C(CH2CH2CH2CH2)C=N-(2,6-C6H3(i-Pr)(2))}NiBr2] (5), and [{(2,6-C6H3(i-Pr)(2))-N=C(Et)C(Me)=N-(2.6-C6H3(i-pr)(2))}NiBr2] (6). Active polymerization catalysts were formed in situ by combination of 4-6 with modified methylaluminoxane. In general, as the bulk and number of ortho substituents increase, polymer molecular weights, turnover frequencies and extent of branching in the homopolyethylenes all increase. Effects of varying ethylene pressure and temperature on polymerizations are also reported. The degree of branching in the polymers rapidly decreases with increasing ethylene pressure but molecular weights are not markedly affected. Temperature increases result in more extensive branching and moderate reductions in molecular weights. Catalyst productivity decreases above 60 degrees C due to catalyst deactivation.