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The only commercially important form of Polystyrene, also known as General-Purpose Polystyrene, is atactic, which Polymerization is initiated with Free Radicals.

In atactic Polystyrene, the Phenyl Groups are randomly distributed on both sides of the Polymer Chain (Fig. 1). This random positioning prevents the Chains from aligning with sufficient regularity to achieve any crystallinity. The Plastic has a glass transition temperature T_g of ~90 °C.

Figure 1 - Polystyrene Tacticity

Source: Wikipedia, Polystyrene

GPPS is a clear, hard, and brittle homopolymer of Styrene. As a thermoplastic polymer, it is in a solid (glassy) state at room temperature but flows if heated above about 100 °C. It is a poor barrier to air and water vapor and has a relatively low melting point. Polystyrene is naturally transparent, but can be colored with colorants.

Polystyrene is commonly injection molded, vacuum formed, or extruded. Uses include protective packaging (such as packing peanuts and in the jewel cases used for storage of optical discs such as CDs and occasionally DVDs), containers, lids, bottles, trays, tumblers, disposable cutlery, in the making of models, and as an alternative material for phonograph records. Polystyrene is also used in some polymer-bonded explosives (PBX).

Source: Wikipedia, Polystyrene