Polystyrene IPA: /ˌpɒliˈstaɪriːn/ (IUPAC Polyphenylethene) is an aromatic polymer mae from the aromatic monomer styrene, a liquid hydrocarbon that is commercially manufactured from petroleum by the chemical industry. Polystyrene is a thermoplastic substance, normally existing in solid state at room temperature, but melting if heated (for molding or extrusion), and becoming solid again when cooling off.