A binaphthyl-bridged salen dichlorozirconium (iv) complex that displays an octahedral structure with a trans-O, cis-N, and cis-Cl arrangement was synthesized and tested as a precatalyst for ethylene and alpha-olefin polymerization. While use of methylaluminoxane (MAO) cocatalyst afforded poor catalytic activity, activation by mixtures of aluminium alkyls such as (AlBu3)-Bu-i and either MAO or [CPh3][B(C6F5)(4)] resulted in reasonable polymerization activities for ethylene, propene, and higher alpha-olefins. Quite unexpectedly, while the polymerization of propene results in the production of a high-molecular-weight stereoirregular polymer, highly isotactic polymers are obtained under similar conditions from polymerization of 1-butene, 1-pentene, and 1-hexene. Polymerization employing the binaphthyl-bridged salen dichlorozirconium (iv) complex gave unexpected different stereospecificities for the polymerization of propene and higher alpha-olefins, to yield ultrahigh-molecular-weight atactic poly(propylene) and highly isotactic polymers, respectively.