Copolymerization of styrene (S) with p-methylstyrene (PMS) and p-chlorostyrene (PCS), catalyzed by half-titanocenes (CpCH2CH2O)TiCl2 (1) and (CpCH2CH2OCH3)TiCl3 (2) activated by methylaluminoxane (MAO), was investigated from both experimental and computational points of view. The reactivity ratios and syndioselectivity of the reaction are affected by the styrene derivative and the catalytic system, leading to the formation of block or random copolymers. Using both catalytic systems, styrene, p-methylstyrene, and p-chlorostyrene show the same reactivity trend (PMS > S > PCS) already reported in the presence of the CpTiCl3/MAO catalyst and in the syndiotactic and isotactic polymerization of styrene derivatives with other Ti-based catalysts. According to density functional theory (DFT) calculations, extended also to the CpTiCl3/MAO catalyst, the reactivity of the employed comonomers is mainly due to the different electron- donating character of monomer para-substituents. As for stereoselectivity, (1) and (2) lead to syndiotactic copolymer sequences with PMS and S, whereas a loss of selectivity is observed in the poly-p-chlorostyrene homosequences of PCS/S copolymers. According to DFT studies, this lack of selectivity can be attributed to the presence of a sigma terminal growing chain in the insertion transition states when PCS is involved in the polymerization.