Polystyrenes (PS's) with different molecular weights were chemically modified with maleic anhydride (MA) with of benzoyl peroxide as an initiator or with a cationic catalyst such as FeCl3 in chloroform. The effect of the solvents (i.e.; chloroform, benzene, toluene, xylene, and tetrahydrofuran), PS and MA concentration, molecular weight of PS, and type of catalyst used in the chemical modification was studied. The rate of the reaction was different with different solvents because of chain transfer to the solvent. The IR spectra of the modified polystyrene (MPS) existing at the end of the chemical modification were taken to show the presence of a carboxyl group fixed to the aromatic ring of PS. The kinetics of the reaction of MPS with MA were also studied. MPS samples containing -CO-CH==CH-COOH fragments inside chains were crosslinked, as proven by differential thermal analysis. (c) 2008 Wiley Periodicals, Inc.