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Three new α-diimine Ni(II) complexes {bis[N,N′-(4-fluoro-2,6-diphenylphenyl)imino]acenaphthene}dibromonickel 4a, {bis[N,N′-(4-chloro-2,6-diphenylphenyl)imino]acenaphthene}dibromonickel 4b, and {bis[N,N′-(4-methyl-2,6-diphenylphenyl)imino]acenaphthene}dibromonickel 4c, were synthesized and characterized. The crystal structure of the complex 4a was determined by X-ray crystallography. Complex 4a has pseudo-tetrahedral geometry about the nickel center, showing C2v molecular symmetry. These complexes, activated by diethylaluminum chloride (DEAC) were tested in the polymerization of ethylene under mild conditions. Complex 4a bearing 2,6-diphenyl and strong electron-withdrawing 4-fluorine groups, activated by diethylaluminum chloride (DEAC) shows highly catalytic activity for the polymerization of ethylene [4.95 × 106 g PE/(mol Ni h bar)] and produced dendritic polyethylene (153.3 branches/1000 C). The dendritic polyethylene particle size obtained by 4a/DEAC can be controlled in the 1–20 nm under 0.2 bar ethylene pressure, and could be expected to become a nano-targeted drug carrier after modified with water-soluble oligo(ethylene glycol) (OEG).