In this work, ethylene polymerization was investigated by using homogeneous and heterogenouse nickel α-diimine catalysts [1,4-bis(2,6-diisopropylphenyl) acenaphthene diimine nickel(II) dibromide]. Methyl aluminoxane (MAO) and triethyl aluminum (TEA) were used as cocatalysts in homogenous and heterogeneous polymerizations, respectively. The heterogeneous catalyst showed lower activity than its homogeneous equivalent. The influence of polymerization temperature and heterogenization conditions was studied on the microstructure properties of the prepared polymers. Increasing polymerization temperature (T P) up to 50 °C decreased the activity of both homogenous (LN) and heterogeneous (LNS) nickel α-diimine catalysts. The highest activities were 1286 and 982 kg PE (mol Ni bar h)−1 obtained at T P = 30 °C for LN and LNS catalysts, respectively. The polymer samples obtained by supported catalyst (LNS) showed lower unsaturation contents. Moreover, DSC analysis did not show any melting peaks for polymers obtained by LN catalyst due to their amorphous structure, which was confirmed by XRD analysis. The microstructure of the prepared polymers was completed by successive self-nucleation annealing (SSA) and was investigated by 13C NMR studies. The SSA thermogram of samples made by LNS catalyst exhibited several crystal types with different lamella thicknesses. The branches in polyethylene samples produced by homogenous catalyst were higher and showed more diversity. The total methyl branch percentages for both LN and LNS catalysts were 13.1 and 3.4%, respectively.