Polymerization of 1,5-hexadiene (HD) by Cp* TiCl2(OAr) (1, Ar = 2,6-(Pr2C6H3)-Pr-i), [Me2Si(C5Me4)(N-t-Bu)] TiCl2 ( 2), [Me2Si(Ind)(2)] ZrCl2 ( 3), and Cp2ZrCl2 ( 4) was explored in the presence of methylaluminoxane (MAO). Both 1 and 3 exhibited the remarkable catalytic activities, and the resultant poly( HD)s were insoluble and possessed a sole glass transition temperature. On the basis of the DSC thermograms, C-13 CPMAS spectra, and the dynamic mechanical analysis(DMA) results, it turned out that 1 favored repeated 1,2-insertion rather than cyclization, affording a polymer-containing olefinic double bond ( butenyl group) in the side chain with uniform distribution, whereas the polymerization by metallocenes (3, 4) favored cyclization. Linked half-titanocene 2 showed lower catalytic activities with favored repeated insertion, affording high molecular weight poly( HD) s which possessed internal olefinic double bonds. ((BuC5H4)-Bu-t) TiCl2(OAr) ( 5) exhibited lower catalytic activities and favored the repeated insertion; the Cp*-ketimide analogue, Cp* TiCl2(N=(CBu2)-Bu-t) ( 6), also favored the repeated insertion, but the degree of the subsequent cross-linking was significantly higher than that by 1. Both cross-linking and cyclization occurred in the polymerization by CpTiCl2(N=(CBu2)-Bu-t) ( 7), and the distribution was not uniform. These results strongly suggest that nature of both cyclopentadienyl and anionic ancillary donor ligands directly affects selectivity for the repeated insertion as well as the microstructure in the resultant poly( HD)s.