Bromine or chlorine can be added to the small isoprene fraction of IIR to make BIIR or CIIR (known as halobutyls). The properties of these polymers are similar to those of IIR, but they can be cured more rapidly and with different and smaller amounts of curative agents. As a result, BIIR and CIIR can be cocured more readily in contact with other elastomers making up a rubber product.
Butyl rubber was first produced by American chemists William Sparks and Robert Thomas at the Standard Oil Company of New Jersey (now Exxon Corporation) in 1937. Earlier attempts to produce synthetic rubbers had involved the polymerization of dienes (hydrocarbon molecules containing two carbon-carbon double bonds) such as isoprene and butadiene. Sparks and Thomas defied convention by copolymerizing isobutylene, an olefin (hydrocarbon molecules containing only one carbon-carbon double bond) with small amounts—e.g., less than 2 percent—of isoprene. As a diene, isoprene provided the extra double bond required to cross-link the otherwise inert polymer chains, which were essentially polyisobutylene. Before experimental difficulties were resolved, butyl rubber was called “futile butyl,” but with improvements it enjoyed wide acceptance for its low permeability to gases and its excellent resistance to oxygen and ozone at normal temperatures. During World War II the copolymer was called GR-I, for Government Rubber-Isobutylene.