The invention relates to a process for preparing bromine-containing polymer, comprising a Friedel-Crafts alkylation reaction of tetrabromoxylylene dihalide, or tetrabromoxylylene dihalide in combination with pentabromobenzyl halide, with a reactant having one or more six-membered aromatic rings, wherein the reaction takes place in a solvent in the presence of a Friedel-Crafts catalyst, and isolating from the reaction mixture the bromine-containing polymer. The so-formed polymers and their use as flame retardants form additional aspects of the invention.