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Ethylene/1-hexene was copolymerized by an unbridged zirconocene, (2-PhInd)2ZrCl2/MAO (methyl aluminoxane) at 0°C and 50°C respectively. High copolymerization activity and 1-hexene incorporation were observed at 0°C, with the copolymer formed having random sequence distribution and narrow molecular weight distribution. Ethylene polymerization at 50°C showed high activity, but copolymerization at 50°C showed much lower activity, which decreased sharply with increasing 1-hexene concentration in the monomer feed. Copolymer formed at 50°C showed blocky sequence distribution and broad molecular weight distribution. A mechanism model based on ligand rotation hindered by the propagation chain has been proposed to qualitatively explain the observed phenomena. [ABSTRACT FROM AUTHOR]