Butyl rubber (IIR), the chemical name of which is Isobutylene-isoprene copolymer is actually the copolymer of isobutylene and a small amount of isoprene. Also known as polyisobutylene, this rubber was first commercialized in 1943. Resulting from low levels of unsaturation between long polyisobutylene segments, the primary quialities of butyl rubber are excellent impermeability or air retention and good flexible properties. The first major use of butyl rubber was Tire innertubes, and this continues to be a significant market today.