Styrene
= Styrene
<Styrene> is vinylbenzene, $\mathrm{C_6H_5CH{=}CH_2}$. The vinyl double bond is conjugated with the aromatic ring.
Addition polymerization consumes that double bond to form polystyrene while leaving phenyl groups as side substituents. This makes styrene a useful example of how a small molecular change, converting a pi bond into backbone sigma bonds, creates a macromolecular material.