A polymer made up of long chains of 2 repeating monomers such as styrene acrylonitrile.
copolymer is a polymer derived from two (or more) monomeric species, as opposed to a homopolymer where only one monomer is used. Copolymerization refers to methods used to chemically synthesize a copolymer.
Commercially relevant copolymers include ABS plastic, SBR, Nitrile rubber, styrene-acrylonitrile, styrene-isoprene-styrene (SIS) and ethylene-vinyl acetate.
Copolymerization is used to modify the properties of man-made plastics to specific needs, for example to reduce crystallinity, modify glass transition temperature or to improve solubility. It is a way of improving mechanical properties, in a technique known as rubber toughening. Elastomeric phases within a rigid matrix act as crack initiators, and so increase the energy absorption when the material is impacted for example. Acrylonitrile butadiene styrene is a common example.