A poly(p-methylstyrene)-block-polyisoprene-block-poly(p-methyl styrene) thermoplastic elastomer was synthesized via anionic polymerization using n-butyllithium as an initiator. The sequential method used for the synthesis has resulted in a nearly monodispersed polymer with a polydispersity of 1.02. Chlorination of such formed copolymer using aqueous sodium hypochlorite was then conducted in a variety of solvents. At a 6.9 mol ratio of sodium hypochlorite Do monomer unit, chlorination occurred via a substitution reaction instead of an addition reaction, regardless of the type of solvent used. Nevertheless, the location at which the chlorine was incorporated into the polymer varied with the type of solvent used. The chlorination occurred primarily in the two poly(p-methylstyrene) end blocks when conducted in n-hexane solvent. However, only the polyisoprene middle block was chlorinated in chloroform. All three blocks could be chlorinated when the reaction was carried out in methylene chloride. The microstructure of the chlorinated molecules were analyzed using H-1-NMR and C-13-NMR, and the degree of chlorination varied from 7 to 50% of constituting monomer units. A significantly higher degree of chlorination occurred when the reaction was conducted in methylene chloride due to its high dielectric constant.