In this study, a new synthetic methodology for the functionalization of polystyrene (PS) has been developed, in which maleic anhydride was used in the presence of zinc oxide as catalyst and microwave energy was used as power source. A mechanism for the acylation reaction is proposed. The effect of reaction conditions on the degree of functionalization of PS was investigated. A subsequent reaction with diisopropyl fumarate/benzoyl peroxide was used to produce a graft copolymer. The polymers were identified and characterized by Fourier transform infrared spectroscopy, ultraviolet (UV) spectroscopy, proton nuclear magnetic resonance and size exclusion chromatography using double detection to confirm the structure of the macromolecules. In addition, the effect of acylation and grafting into PS on the corrosion resistance of steel surfaces were studied. The results show high efficiency of the acylation and grafting reactions under microwave conditions, with no PS degradation. Improvement in the corrosion resistance of both acylated and graft polymers suggest that they are suitable for use as protective coatings.