Incorporation of elastomers into polystyrene has been used in recent years to overcome its inherent brittleness. Mechanical mixing of rubber with polystyrene has been largely superseded by dissolving the elastomer in the styrene before polymerization. Impact-resistant polystyrene produced by either method is a two-phase system with polystyrene the continuous phase and elastomer the discrete phase. Polymerization of styrene in the presence of an unsaturated elastomer converts the elastomer to a graft copolymer. The graft copolymer is formed by reaction of the polymerization initiator with the rubber to form an active site thereon, which can initiate polymerization of styrene to form a side chain of polystyrene. The graft copolymer is crosslinked in the process. The size of the gel particles must not be too large for a satisfactory product, but can be controlled by shear and by chain transfer agents. The rubber chiefly used with polystyrene has been SBR, but use of polybutadiene is growing because of improved impact resistance at low temperatures. The resistance to impact increases with increasing amounts of elastomer, but surface gloss and hardness decrease. The chief advantage of graft copolymerization over poly-blending is more efficient utilization of the elastomer. The increase in resistance to impact has been attributed to the energy absorbing character of the elastomer. The two-phase system is necessary so as not to lose this basic property of elastomers. The rubber particles may also act to induce many small cracks rather than one large one, thus increasing the energy requirement for total failure. The greater efficiency obtained by a graft copolymer may be due to a more diffuse interface between rubber and polystyrene, leading to more absorption and less reflectance of a stress wave by the particle of rubber.