The effect of the time of styrene addition on the composition of SBS block copolymers synthesized using a dicumyl chloride/TiCl 4 /pyridine initiating system and the sequential addition of monomers was inveatigated at-80°C. It was found that the addition of styrene at an isobutylene (IB) reaction time significantly exceeding the time necessary for complex IB consumption results in contamination of the material with a substantial amount of homo-PS. Conversely, the addition of styrene to PIB chains formed under conditions of intermediate IB conversion resulted in a very slow buildup in molecular weight after the addition of styrene