Polyisobutene (polyisobutylene) is a class of organic polymers prepared by polymerization of isobutene.

Cationic polymerization, initiated with a strong Brønsted or Lewis acid, is the typical method for its production.

The molecular weight (MW) of the resulting polymer determines the applications.

Low MW polyisobutene, a mixture of oligomers with Mns of about 500, is used as plasticizers.

Medium and high MW polyisobutenes, with Mn ≥ 20,000, are components of commercial adhesives.