Butyl rubber

Butyl rubber, sometimes just called butyl, is a synthetic rubber, a copolymer of isobutylene with isoprene. The abbreviation IIR stands for isobutylene isoprene rubber. Polyisobutylene, also known as "PIB" or polyisobutene, (C4H8)n, is the homopolymer of isobutylene, or 2-methyl-1-propene, on which butyl rubber is based.

Butyl rubber is produced by polymerization of about 98% of isobutylene with about 2% of isoprene.
Structurally, polyisobutylene resembles polypropylene, but has two methyl groups substituted on every other carbon atom, rather than one.
Polyisobutylene is a colorless to light yellow viscoelastic material.

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Source: Wikipedia · Image: Wikimedia Commons · CC BY 3.0

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