The effect of prepolymerization and hydrogen pressure on ethylene/1-hexene copolymerization was investigated with two catalysts having different Ti 3 + structure, i.e., isolated Ti 3 + and multinuclear Ti 3 + species. After prepolymerization with 1-hexene under mild conditions in the absence of hydrogen, the polymerization rates of both catalysts were enhanced. However, in the presence of hydrogen, both catalysts showed a decrease in the polymerization rate. From microstructure analyses with carbon-13 nuclear magnetic resonance ( 1 3 C-NMR), gel permeation chromatography and crystallization analysis fractionation, prepolymerization for both catalysts increased the reactivity of 1-hexene and changed the short chain branching distribution (SCBD) of the produced polymer but did not alter the molecular weight distribution (MWD). In the presence of hydrogen, isolated Ti 3 + species showed a remarkable increase in the 1-hexene reactivity and produced copolymers having broad MWDs and SCBDs. Multinuclear Ti 3 + species did not show significant changes in 1-hexene reactivity and SCBD of the produced copolymer when exposed to hydrogen.