Polystyrene (PS): -(CH2-CH(C6H5))n-. Monomer = styrene (CH2=CHC6H5, also called vinylbenzene). Addition polymerization via free radical mechanism.

Regular PS: Clear, rigid, brittle, glassy appearance. Good electrical insulator. Used in CD cases, disposable cutlery, laboratory containers, Petri dishes.

Expanded polystyrene (EPS, Thermocol/Styrofoam): PS beads containing pentane as blowing agent are heated with steam → pentane vaporizes → beads expand to 40x volume → fused together. Result: 95-98% air by volume → excellent thermal insulation, very lightweight, shock-absorbing. Uses: packaging, disposable cups, insulation boards, ice boxes.

Environmental concern: PS is non-biodegradable and takes 500+ years to decompose. Expanded PS is especially problematic — lightweight pieces spread easily, animals ingest them. Recycling is possible (symbol 6 PS) but not widely practiced due to low density (expensive to transport).