Chloromethylstyrene(CMS) was synthesized as the reactive comonomer to modify the various kinds of polymers. Styrene monomer was reacted with paraformaldehyde and hydrochloric acid in the presence of sulphur to give CMS. The retain of double bond was confirmed by infrared spectroscopy. The maximum yield was obtained at 5 hours reaction, where the yield starts to decrease due to the polymerization reaction. The crude CMS was purified by distilling at 72$^{circ}C$ under 2mmHg. The refractive index and the density of the distilled one were found nD20=1.5771 and d20=1.0832, respectively. ZnCl$_2$ was used as a catalyst of the chloromethylation. Although the reaction was completed in between 1.5 hours to 2 hours, the yield was low due to the polymerization of CMS and styrene. ZnCl$_2$is expected to initiate polymerization. When CMS was emulsion-, solution-, and bulk polymerized, the molecular weight of the polymer was very low. When CMS, however, was copolymerized with styrene and acrylonitrile, the molecular weight of the copolymer was sufficiently high.