an important member of the class of synthetic organic polymers, composed of long-chain molecules prepared by a chemical reaction in which many (usually 2,000–3,000) molecules of the aromatic hydrocarbon styrene become linked together.

Large-scale manufacture of polystyrene was begun in the late 1930s; usually the styrene is dispersed in water with the aid of soap, and the polymerization reaction is initiated by free-radical catalysts. The product, an inexpensive, strong, thermoplastic (softened by heat) resin that resists attack by acids, alkalies, and many solvents, does not absorb water and is an excellent electrical insulator.

Addition of rubber latex to the suspension in which polystyrene is made greatly increases its resistance to impact. By the mid-1960s, more than half the polystyrene made in the United States was produced by this process. Polystyrene has numerous uses, as in making housings for such large household appliances as refrigerators and air conditioners.