Abstract: Disclosed herein are modified diene-containing (co)polymers, in particular, modified styrene-butadiene copolymer, which can be used as flame retardants for expandable polystyrene polymer compositions. In particular, disclosed is a modified diene-containing (co)polymer, a method for preparing it, and use as a flame retardant for expandable polystyrene. The modified diene-containing (co)polymer claimed in the invention is characterized by high thermostability, namely, a 5% weight loss temperature of at least 180° C., a molecular weight of at least 1500 g/mol and a halogen content of at least 35 wt. %; the content of tertiary and/or allyl halogenides is in the range from 0 to less than 1.5 wt. %, and also does not affect the polymerization process and the formation of polystyrene granules either, and allows to obtain polystyrene granules with a yellowness index of 2 to 6 units, comparable to the yellowness index of polystyrene granules containing HBCD (from 0 to 3 units).