A series of mononuclear -M1 and M2- and dinuclear -C1 –C6- Ni α‐diimine catalysts activated by modified methylaluminoxane were used in polymerization of ethylene. Catalyst C2 bearing the optimum bulkiness showed the highest activity -1.6 × 106 g PE -mol Ni-−1 h−1- and the lowest short‐chain branching -32.5/1000 C- in comparison to the dinuclear and mononuclear analogues. Although the mononuclear catalysts M1 and M2 polymerized ethylene to a branched amorphous polymer, the dinuclear catalysts led to different branched semicrystalline polyethylenes. Homogeneity and heterogeneity in the microstructure of the polyethylene samples was observed. Different trends for each catalyst were assigned to syn and anti stereoisomers. In addition, thermal behavior of the samples in the successive self‐nucleation and annealing technique exhibited different orders and intensities from methylene sequences and lamellae thickness in respect of each stereoisomer behavior. Higher selectivity of hexyl branches obtained by catalyst C2 showed a cooperative effect between the centers. The results also revealed that for catalysts C5 and C6, selectivity of methyl branches led to very high endotherms and crystalline sequences with melting temperatures higher than that of 100% crystalline polyethylene indicating ethylene/propylene copolymer analogues. For catalysts C3 and C4, more vinyl end groups were a result of the long distance between the Ni centers. Kinetic profiles of polymerization along with a computational study of the precatalysts and catalysts demonstrated that there is a direct relation between rate constant, energy interval of catalyst and precatalyst, and interaction energy of Et···methyl cationic active center -Et···MCC or π–Comp.-. Based on this, narrow energy interval -activation energy- of precatalyst and catalyst leads to fast and higher activation rate -catalyst M2-, and strong interaction of ethylene and catalyst leads to high monomer uptake and productivity -catalyst C2-. Moreover, theoretical parameters including electron affinity, Mulliken charge on Ni, chemical potential and hardness, and global electrophilicity showed optimum values for C2.

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