Polybutene is an organic polymer made from a mixture of 1-butene, 2-butene, and isobutylene. C4 hydrocarbons, produced during ethylene steam cracking in oil processing, are also used as supplemental feed for polybutene. It is similar to polyisobutylene (PIB), which is produced from essentially pure isobutylene made in a C4 complex of a major refinery. The presence of isomers other than isobutylene can have several effects including: lower reactivity due to steric hindrance at the terminal carbon in, for example in polyisobutenylsuccinic anhydride (PIBSA) dispersant manufacture; and in the molecular weight–viscosity relationships of the two materials may also be somewhat different.