Five new CpTiCl2(OR) alkorxl-substituted half-sandwich complexes, where R was methoxyethyl(1), methoxypropyl (2), methoxyisopropyl (3), o-methoxyphenyl (4), or tetrahydrofurfuryl (5), were synthesized, characterized, and tested as catalyst pre cursors for the syndiospecific polymerization of styrene. These precursors were more active than (eta (5)-cyclopentadienyl)trichlorotitanium (CpTiCl3). The different structures of the alkoxyl ligands affected the activity slightly. When the polymerization was carried out in bulk, all the complexes (1-5) exhibited high activities, even at the low molar ratio of Al/Ti = 300. The syndiotactic polystyrene (s-PS) percentage of the polymer produced by alkoxyl-substituted complexes was much higher than that of CpTiCl3. The really active center might be described as [CpTiMe](+.)[MAOX](-.)nMao (where MAO is methylaluminoxane). The normal active species [CpTiMe](+) made up the core and the anion mass [MAOX](-).nMAO surrounded the core and constituted the outer shell circumstance. They activated the syndiospecific polymerization of styrene as a whole. For a high concentration of MAO, the function of the alkoxyl group was weak because of the limited proportion in the outer shell. For a low concentration of MAO, the proportion of alkoxyl ligands in the outer shell increased greatly, and their influence also became significant, as reflected in a higher s-PS percentage of the obtained polymer. The existence of the additional oxygen atom in the alkoxyl ligand stabilized the active species more effectively; this was reflected in the higher temperature of the maximum activities.