A series of alkoxyl substituted half-sandwich titanocenes [CpTiCl2(OR), CpTi(OR)(3) and IndTiCl(2)(OR)] were treated with BF3.Et2O, which were employed in situ as catalyst precursors for the syndiospecific polymerization of styrene. When activated with methylaluminoxane (MAO), the catalyst precursors obtained in situ from the reaction of methoxyl substituted complexes [CpTiCl2(OMe), CpTi(OMe)(3)] and BF3.Et2O exhibited increased activities by factors of 2-4 compared with the corresponding original complexes, and provide improved syndiotacticity and higher activity up to a temperature of 70 degreesC. The catalyst precursor may consist of [CpTiClF2 and/or CpTiCl2F] and [CpTiF2(OMe).BF3 and/or CpTiF(OMe)(2).2BF(3)], through conversion from BF3-coordinated intermediates to fluorinated complexes, respectively, which has properties in polymerization similar to that of CpTiF3. But in the case of IndTiCl(2)(OR) and alkoxyl substituted half-sandwich titanocene with bulky OR groups, the activity considerably decrease probably due to the bulky group retarding the conversion from BF3-coordinated intermediates to fluorinated complexes, which can stabilize catalytic precursor and inhibit reduction from Ti(IV) to Ti(III). (C) 2002 Elsevier Science B.V. All rights reserved.