The polymerization of halide-substituted styrenes (XSt) and their copolymerization with styrene by half-sandwich scandium catalysts with and without a THE ligand have been examined. The THE-free aminobenzyl scandium complex (C5Me4SiMe3)Sc(CH2C6H4NMe2-o)(2) (2) in combination with 1 equiv of [Ph3C] [B(C6F5)(4)] showed high activity and excellent syndiotacticity for the polymerization of the halide-substituted styrenes (XSt = p-, o-, and m-ClSt, p-FSt, p-BrSt, and p-ISt). Despite the presence of the electron-withdrawing halogen substituent on the aromatic ring, the polymerization activity reached up to 10(5) g of polymer mol(sc)(-1) h(-1) with syndiotacticity (rrrr) up to 99%. This constitutes the first example of the syndiospecific polymerization of halogenated styrenes with high activity and high stereoselectivity. Moreover, the syndiospecific copolymerization of XSt with styrene has also been achieved by this catalyst, which afforded the corresponding syndiotactic copolymers with high molecular weight and a controllable amount of the halogenated styrene units. In contrast, the THF-containing trimethylsilylmethyl scandium complex (C5Me4SiMe3)Sc(CH2SiMe3)(2)(THF) (1) showed high activity and high syndiotacticity for the polymerization of m-ClSt but gave atactic polymers in the polymerization of p- and o-ClSt, p-BrSt, and p-ISt, while a mixture of syndiotactic and atactic polymers was yielded in the case of p-FSt. The copolymerization of XSt with styrene by complex 1 was also achieved, but the stereoselectivity and comonomer distribution sequences in the resulting copolymers were significantly different from those obtained by using complex 2.