Polystyrene

Polystyrene is produced by the free-radical polymerization of styrene, a monomer derived from petroleum. The reaction typically occurs in bulk, suspension, or emulsion processes, with initiators such as benzoyl peroxide. The resulting polymer has a linear backbone of carbon atoms, each bearing a phenyl group, giving it rigidity and a high glass transition temperature (about 100 °C for atactic polystyrene).¹ The tacticity—whether phenyl groups are arranged randomly (atactic) or in a regular pattern (syndiotactic or isotactic)—affects crystallinity and properties. Commercial polystyrene is mostly atactic, amorphous, and transparent, while syndiotactic polystyrene is semi-crystalline and has a higher melting point.²

Solid polystyrene is used in disposable cutlery, CD cases, and laboratory ware, while expanded polystyrene (EPS) is a foam made by blowing gas into the polymer, trapping air and creating a lightweight insulator. EPS is widely used in packaging (e.g., foam peanuts, food containers) and building insulation. Extruded polystyrene (XPS) is a denser foam with closed cells, used in rigid insulation boards. Polystyrene is also a component of copolymers, such as styrene-butadiene rubber and acrylonitrile-butadiene-styrene (ABS), which enhance impact resistance. In the medical field, polystyrene is used in petri dishes and tissue culture plates due to its optical clarity and biocompatibility.³

Polystyrene is notoriously resistant to biodegradation, persisting in the environment for centuries. It is a major component of marine litter, and its fragments can be ingested by wildlife, entering the food chain. Styrene, the monomer, is classified as a possible human carcinogen (Group 2B) by the International Agency for Research on Cancer, though the risk from finished products is considered low.⁴ Recycling is challenging because EPS is bulky and often contaminated with food; only about 30% of polystyrene was recycled in the US in 2018, with the rest landfilled or incinerated.⁵ Several cities and countries have banned or restricted polystyrene food containers, prompting development of biodegradable alternatives.⁶

Polystyrene was first accidentally discovered in 1839 by German apothecary Eduard Simon, who isolated a substance from storax resin and called it "styrol oxide." It was not until 1922 that Hermann Staudinger proposed its polymeric nature, earning him the Nobel Prize in Chemistry in 1953.⁷ In the 1950s, Dow Chemical developed a method to produce EPS foam, leading to the iconic Styrofoam brand. A niche application is in the production of "plastic wood" composites, where polystyrene is mixed with wood fibers for outdoor decking. Additionally, polystyrene is used in the fabrication of microfluidic devices for lab-on-a-chip research, exploiting its ease of molding and surface modification.⁸ The polymer also serves as a calibration standard in gel permeation chromatography.

Atactic

Random arrangement of side groups along the polymer chain, resulting in an amorphous material.

Glass transition temperature (Tg)

The temperature at which an amorphous polymer changes from a hard, glassy state to a rubbery state.

Expanded polystyrene (EPS)

A foam form of polystyrene produced by expanding beads with a blowing agent, containing about 98% air.

Extruded polystyrene (XPS)

A dense, closed-cell foam made by extruding polystyrene through a die, often used for insulation.