Polyurethane is a commonly encountered polymer composed of organic units joined by carbamate links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane is produced from a wide range of starting materials and is therefore a class of polymers, rather than a distinct compound. This chemical variety allows for polyurethanes with very different physical properties, leading to an equally wide range of different applications including: rigid and flexible foams, varnishes and coatings, adhesives, electrical potting compounds, and fibres such as spandex and PUL. Of these, foams are the largest single application, accounting for 67% of all polyurethane produced in 2016.
Polyurethane polymers are traditionally and most commonly formed by reacting a di- or triisocyanate with a polyol. Since polyurethanes contain two types of monomers, which polymerise one after the other, they are classed as alternating copolymers. Both the isocyanates and polyols used to make polyurethanes contain, on average, two or more functional groups per molecule.