Poly(phenylethene), commonly known as polystyrene, is the third most important polymer, in terms of the amount made from ethene. Polystyrene (PS) is a polymer made from the monomer styrene, a liquid hydrocarbon that is commercially manufactured from petroleum. At room temperature, PS is normally a solid thermoplastic but can be melted at higher temperatures for molding or extrusion, and then resolidified. Its physical properties can be adjusted to suit a range of everyday uses. Techniques have been developed that increase its mechanical strength, its ability to absorb shock, and its thermal insulating properties.

The largest use for Polystyrene is for packaging, particularly for foods such as poultry and eggs, cold drinks, and takeaway meals. It is also used in making appliances, including refrigerators, microwaves, and blenders. It is the leading choice for jewel boxes (cases for CDs and DVDs) and is also widely used for its insulating properties.