Polyisobutylene is a synthetic rubber, or elastomer. It is special because it is that only rubber that is gas impermeable, that is, it is the only rubber which can hold air for long periods of time. Polyisobutylene, sometimes called butyl rubber is a vinyl polymer, and is very similar to polyethylene and polypropylene in structure.

A butyl compound typically the primary seal in a dual seal Insulating Glass Unit and the key component in restricting moisture vapour transmission.

(1) A synthetic classified as an uncured elastomer composed of isoprene, highly-molecular isobutylene, carbon black and aging protectors; (2) A natural or synthetic chemical compound of high molecular weight or a mixture of such compounds, formed when monomers (small individual molecules) are combined to from large long-chain molecule; (3) A product formed by the polymerization of isobutylene. May be compounded for use as a roof membrane material.