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A nickel-diimine catalyst (1,4-bis(2,6-diisopropylphenyl) acenaphthene diimine nickel(II) dibromide, DMN) was supported on mesoporous particles having parallel hexagonal nanotube pore structure (MCM-41 and MSF) for ethylene polymerization. The effects of supporting methods and particle morphological parameters, such as pore size and length, on the catalyst impregnation were systematically investigated. Pretreating the supports with methylaluminoxane (MAO) followed by DMN impregnation gave much higher catalyst loading and higher catalytic activity than the direct impregnation of DMN. The particle structure significantly affected the catalyst impregnation and this effect was explained with a semi-quantitative molecular diffusion model. Compared to homogeneous catalysts, significant reduction in activity was observed with the supported systems in ethylene polymerization. Extraction of active sites from the supports during polymerization was observed. The mesoporous supports exerted steric effects on unleached active sites, lowering chain walking ability, and producing polymers having lower short chain branch density. Replication of the particle morphology was observed in some polymer samples.