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Syndiotactic polystyrene samples end capped with bromine atoms in benzylic position have been synthesized by terminating the syndiospecific styrene polymerization catalyzed by Cp*TiBn3/B(C6F5)3/Al-Oct3(Cp* = η5-C5(CH3)5; Bn = CH2Ph, Oct = (CH2)7CH3) with N-bromosuccinimide (NBS) and 2-bromo-2-phenylethylisocyanate (BPEI). The chemical structure of the different polymer end groups was assessed by mono and bidimensional NMR techniques. The evaluation of the relative concentrations of the end groups permitted to give new insights into the bromination reaction and syndiospecific styrene polymerization mechanism. Aspecific bromination of the methine carbon of the growing polymer chain bound to the titanium catalyst was observed when NBS was used as terminating agent. The formation of Ti(III) hydrido complexes in the activation reaction was proved as well as the role of these species as active catalyst in syndiospecific styrene polymerization. [ABSTRACT FROM AUTHOR]