Styrene

Styrene is a clear, oily liquid with a sweet smell. Each molecule is a benzene ring with a small two-carbon vinyl group hanging off one side. That vinyl group has a double bond, and double bonds are what let small molecules snap together into long plastic chains.

Styrene is made in two steps. First, benzene and ethylene are passed over an acid catalyst, often a zeolite, and the ethylene attaches to the ring to make ethylbenzene. Then hot steam carries the ethylbenzene over an iron oxide catalyst at around 600 °C. Two hydrogen atoms are stripped off, which creates the double bond, and the result is styrene plus hydrogen gas. The mixture is distilled to purify it.

Linked into long chains on its own, styrene becomes polystyrene, used for the diffuser plate behind a TV's screen. Combined with acrylonitrile and butadiene, it becomes ABS, where it brings stiffness and a glossy finish. It also goes into the styrene-butadiene rubber used in car tires. Pure styrene slowly links up by itself, so it's shipped with a little inhibitor added.