Final answer:
The monomer used to synthesize polystyrene is styrene, which undergoes a process of radical polymerization involving initiation, propagation, and termination to form long polymeric chains.
Step-by-step explanation:
The monomer used to synthesize polystyrene is styrene (which is also known as ethenylbenzene, vinylbenzene, or phenylethene). During the radical polymerization process, the double bond in the styrene monomer opens, and these monomers link up to form long chains, creating the polymer polystyrene. This reaction involves the steps of initiation, propagation, and termination, leading to a polymer with a high molecular weight. Polystyrene is a type of addition polymer, which forms without the loss of any atoms or small molecules during the linking of monomers.