The isospecific polymerization of several para-substituted styrenes was performed in the presence of the catalyst dichloro[1,4-dithiabutanediyl-2,2'-bis(4,6-di-tert-butyl-phenoxy)]titani um activated by methylaluminoxane. All the polymers were highly regioregular and isotactic with narrow molecular weight distributions. The presence of electron-donating substituents on the aromatic ring had a positive effect on the catalyst activity, whereas electron-withdrawing substituents affected the polymerization activity negatively. Binary copolymerizations of the various substituted styrenes showed an inversion of the reactivity with respect to that observed in the homopolymerization. These results suggested that the last monomer unit of the polymer chain coordinated to the metal center, influencing the reactivity of the catalyst with respect to the incoming monomer. (c) 2006 Wiley Periodicals, Inc.