PURPOSE: To facilitate the formation of a brominated polystyrene substantially free from an emulsifier, a dispersant, etc., by polymerizing a brominated styrene in an alcoholic solvent.

CONSTITUTION: A brominated styrene is polymerized in an alcoholic solvent to obtain a polymer in the form of a dispersion. Although not particularly limited, the alcoholic solvent used is desirably exemplified by methyl alcohol, ethyl alcohol, isopropyl alcohol, or n-butyl alcohol. In order to obtain a polymer in a good dispersed state, it is desirable that the monomer concentration is 15wt.% or above. When the monomer concentration is 15wt.% or above, it is desirable to add a dispersant to the reaction system. To effect the molecular weight control and the prevention of gelation of the obtained brominated polymer, it is also possible to add a chain transfer agent to the reaction system. According to this process, a brominated polystyrene having a weight-average molecular weight of 10000 or above (in terms of polystyrene) is obtained.