Aqueous emulsion polymerization of styrene in the presence of half-titanocenes which CpTiCl3, (CpCH2CH2OCH3)TiCl3, and of the titanocene (Ph)(2)C[(Cp)(Flu)Ti]Cl-2 is effected. The polymer features are compared with those of polystyrene obtained in the same reaction conditions, by using Cp2TiCl2 as initiator. Titanocene (Ph)(2)C[(Cp)(Flu)Ti]Cl-2 does not show any activity in styrene polymerization. On the contrary, by using both half-titanocenes CpTiCl3 and (CpCH2CH2OCH3)TiCl3, stereoirregular polystyrene is produced. Aqueous emulsion polymerization of substituted styrenes (3-methyl-styrene, 4-methyl-styrene, 3-chloro-styrene, 4-chloro-styrene) is also performed in the presence of Cp2TiCl2. From all monomers, stereoirregular polymers are obtained. Monomer reactivity results dependent on the substituent on the aromatic ring. Aqueous emulsion styrene - 4-chloro-styrene copolymerizations are performed by using Cp2TiCl2, too. Copolymer composition totally reflects that of comonomers used in the feeding. According to a statistical distribution of comonomers, the product of the reactivity ratios is 1.1. (C) 2013 Elsevier Ltd. All rights reserved.