The reactivity of the alkyl complexes [Cp*Ti(III)(CH3)][RB(C6F5)(3)] (Cp* = eta(5)-C5Me5; R = CH3, -C6F5) with alpha-C-13-enriched p-chlorostyrene has been investigated by means of ESR spectroscopy in order to gain insight into the chemical structure of the active species in syndiospecific polymerization of styrene and substituted styrenes promoted by "half-titanocene"-based catalysts. Secondary (2,1) polyinsertion of p-chlorostyrene into a Ti(III)-alkyl bond has been observed under polymerization conditions. The role of Cp*(TiR+)-R-III complexes in syndiospecific styrene polymerization has conclusively been confirmed. Furthermore, the reaction of the C-13-enriched complex [Cp*Ti((CH3)-C-13)][RB(C6F5)(3)] with PMe3 and Al((CH3)-C-13)(3) has also been investigated by ESR spectroscopy. PMe3 displaces the methylborate anion from the coordination sphere of the Cp*(TiCH3)-C-13(mu-(CH3)-C-13)B(C6F5)(3) ion pair and produces the corresponding phosphine adduct Cp*Ti(CH3)(PMe3)(2)(+). In the same manner, the reaction of [Cp*Ti((CH3)-C-13)][B(C6F5)(4)] with Al((CH3)-C-13)(3) affords the [Cp*Ti(mu-(CH3)-C-13)(3)Al((CH3)-C-13)](+) adduct. The increase of the intensity of the Ti(III) signals observed in the ESR spectra of these catalytic systems after addition of the monomer has been explained on the basis of the ESR results reported above.