Polyisobutene (polyisobutylene) is a class of organic polymers prepared by polymerization of isobutene. The polymers often have the formula Me₃C[CH₂CMe₂]_nH (Me = CH₃). They are typically colorless gummy solids.

Polyisobutene

Sample of polyisobutene with a molecular weight of ~1,000,000[1]
Names
Other names Polyisobutylene; Poly(isobutene); Poly(isobutylene); PIB
Identifiers
CAS Number	9003-27-4 Y
ChEBI	CHEBI:53725
ECHA InfoCard	100.108.750
UNII	241BN7J12Y Y
CompTox Dashboard (EPA)	DTXSID2036451
Properties
Chemical formula	(C4H8)n
Molar mass	Variable
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Polymerization is typically initiated with a strong Brønsted or Lewis acid. The molecular weight (MW) of the resulting polymer determines the applications. Low MW polyisobutene, a mixture of oligomers with M_ns of about 500, is used as plasticizers. Medium and high MW polyisobutenes, with M_n ≥ 20,000, are components of commercial adhesives.^[2]