Ethylene homopolymerization and ethylene/␣-olefin copolymerization were carried out using unbridged and 2-alkyl substituted bis(indenyl)zirconium dichloride complexes such as (2-MeInd) 2 ZrCl 2 and (2-BzInd) 2 ZrCl 2 . Various concentrations of 1-hexene, 1-dodecene, and 1-octadecene were used in order to find the effect of chain length of ␣-olefins on the copolymerization behavior. In ethylene homopolymerization, catalytic activity increased at higher polymerization temperature, and (2-MeInd) 2 ZrCl 2 showed higher activity than (2-BzInd) 2 ZrCl 2 . The increase of catalytic activity with addition of comonomer (the synergistic effect) was not observed except in the case of ethylene/1-hexene copolymerization at 40°C. The monomer reactivity ratios of ethylene increased with the decrease of polymerization temperature, while those of ␣-olefin showed the reverse trend. The two catalysts showed similar copolymerization reactivity ratios. (2-MeInd) 2 ZrCl 2 produced the copolymer with higher M w than (2-BzInd) 2 ZrCl 2 . The melting temperature and the crystallinity decreased drastically with the increase of the ␣-olefin content but T m as a function of weight fraction of the ␣-olefins showed similar decreasing behavior.