Polystyrene is formed from the polymerization of the monomer vinyl benzene (also known as styrene).

You can see that styrene is composed of two parts. You may even recognize both . On the top is a benzene ring (1,3,5-cyclohexene) that is minus one hydrogen. On the bottom is an ethene (also minus one hydrogen). Chemist call this HC=CH2 structure vinyl. When these two bond together they make the monomer vinyl benzene - commonly called styrene.

During polymerization, one covalent bond between carbon atoms at the vinyl end of the monomer will break and form a bond to another styrene monomer.

This long string of styrene molecules all bonded together is the polymer polystyrene.

Polystyrene is used in a wide variety of plastic products. Compact disc "jewel cases", videocassette and audiocassette cases are made of polystyrene. Polystyrene can be injected with air to make Styrofoam. This common plastic is used to make styrofoam cups, plates, egg cartons, meat trays, coolers and "peanut" shipping packaging.

More Information: Polystyrene Facts from the Polystyrene Packaging Council.