any of a class of resinous, fibrous, or rubbery substances belonging to the family of organic polymers based on acrylonitrile. Almost all polyacrylonitrile resins are copolymers (q.v.), made from mixtures of monomers, with acrylonitrile (see nitrile) usually making up the major portion; other monomers often present include butadiene, styrene, vinylidene chloride, or other vinyl compounds.

Acrylic fibres, such as Orlon, contain at least 85 percent acrylonitrile; modacrylics contain 35–85 percent. The inclusion of other monomers confers desirable properties upon the fibres, such as increased affinity for dyestuffs. Acrylonitrile, butadiene, and styrene polymerize together to form the so-called ABS resins, which are hard, tough, strong, and resistant to impact; they are easily fabricated into pipe, structural panels, housewares, and the like. Mixtures of butadiene and acrylonitrile, containing 15–40 percent of the latter, form elastomers called NBR, or Buna N rubber, which are highly resistant to abrasion and to attack by hydrocarbon solvents such as gasoline. Saran N is a resin made from acrylonitrile and vinylidene chloride (q.v.).