Polymerization :

Polystyrene results when styrene monomers interconnect. In the polymerization, the carbon-carbon pi bond (in the vinyl group) is broken and a new carbon-carbon single (sigma) bond is shaped, attaching another styrene monomer to the chain. The newly formed sigma bond is much durable than the pi bond that was broken, thus it is very difficult to depolymerize polystyrene. About a few thousand monomers normally comprise a chain of polystyrene, giving a molecular weight of 100,000–400,000.

Atactic Polystyrene :

The only main commercially form of polystyrene is atactic, in which the phenyl groups are accidentally distributed on both sides of the polymer chain. This random positioning avoids the chains from aligning with sufficient regularity to achieve any crystallinity. The plastic has a glass transition temperature Tg of ~90 °C. Polymerization is initiated with free radicals.

Syndiotactic Polystyrene :

Ziegler-Natta polymerization could produce ordered syndiotactic polystyrene with the phenyl groups located on alternating sides of the hydrocarbon backbone. This form is highly crystalline with a Tm of 270 °C (518 °F). Such materials are not commercially produced .because the polymerization is slow.