The others include butane, propylene, butadiene, butylene, isobutylene, and mixtures thereof. The catalyst protonates the alkenes (propylene, butylene) to produce reactive carbocations, which alkylate isobutane. If styrene-butadiene copolymer has a high butylene content, styrene-butadiene rubber (SBR) is formed. PBS consists of polymerized units of butylene succinate, with repeating CHO units. It is a four-carbon branched alkene (olefin), one of the four isomers of butylene. It has also been used as a precursor to polypropylene resins, butylene oxide, and butanone. Butene, also known as butylene, is a series of alkenes with the general formula CH. PBAT is fully biodegradable when composted due to the presence of butylene adipate groups.