Relationships between structures and polymerizabilities of mono-substituted styrenes (o- and p-Cl; m- and p-OCH3; m- and p-CN; m- and p-Br; p-CH3) have been investigated. However, there have been no reports on o-, m-, and p-substituent effects on the polymerization under the same conditions. Poly(bromostyrene) and its copolymers are interesting because of their flame retarding properties. There have been many reports on polymerizations of brominated styrenes. Kinoshita et al. describe the monomer reactivity ratios of m- and p-monobrominated styrenes (MBS) at 30°C in bulk condition. Cubbon et al10,11 and Koton et a.12 reported the monomer reactivity ratios and time-conversion for dibromo- and 2,4,5-tribromostyrenes in bulk and benzene. Konigsberg et al.13 investigated the anionic polymerization of p-MBS. The overall radical polymerization rate equations for MBS have not been described except for only one report on p-MBS at 30°C in bulk condition.14 This paper describes the differences of radical polymerizabilities and properties of the polymers among positional isomers for MBS.