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Ethylene polymerization was carried out using three nickel α-diimine catalysts ((ArNC(An)–C(An)NAr)NiBr2 (1), (ArNC(CH3)–C(CH3)NAr)NiBr2 (2) and (ArNC(H)–C(H)NAr)NiBr2 (3); where An=acenaphthene and Ar=2,6-(i-Pr)2C6H3) activated with modified methylaluminoxane (MMAO) in a slurry semi-batch reactor. We investigated the effects of ethylene pressure, reaction temperature, and α-diimine backbone structure variation on the catalyst activity and polymer properties. Changes in the α-diimine backbone structure had remarkable effect on the polymer microstructure as well as the catalyst activity. Catalyst 2 produced polymer with the highest molecular weight, while Catalyst 3 produced polymer with the lowest molecular weight. In addition, Catalyst 2 produced polymer with the lowest melting point, while Catalyst 3 produced the highest melting level exhibiting a melting behavior typical of high-density polyethylene (HDPE). With all the three catalysts, polymer molecular weight tended to decrease with increasing polymerization temperature due to the increase in chain transfer rates. In general, there was no clear and consistent trend observed for the effects of ethylene pressure on the polymer molecular weight. However, in polyethylene produced with Catalyst 2, the molecular weight was independent of ethylene pressure suggesting that chain transfer to ethylene may be a dominant mechanism for this catalyst.