Also known as polyisobutylene (C4H8)—is a synthetic rubber, a homopolymer of 2-methyl-1-propene. Polyisobutylene is produced by polymerization of about 98% of isobutylene with about 2% of isoprene. Structurally, polyisobutylene resembles polypropylene, having two methyl groups substituted on every other carbon atom. It has excellent impermeability, and the long polyisobutylene segments of its polymer chains give it good flex properties. Polyisobutylene is a colorless to light yellow elastic semi-solid or viscous substance. It is generally odorless and tasteless, though it may exhibit a slight characteristic odor.A synthetic rubber, or elastomer, polyisobutylene is impermeable to air and used in many applications requiring an airtight rubber. Polyisobutylene is used in making adhesives, agricultural chemicals, fiber optic compounds, caulks and sealants, cling film, electrical fluids, lubricants (2 cycle engine oil), paper and pulp, personal care products, pigment concentrates, for rubber and polymer modification, and as a gasoline/diesel fuel additive. The first major application of butyl rubber was tire inner tubes. This remains an important segment of its market even today.