A type of synthetic rubber based in polymerization of butadiene (buta-1,3-diene). The name comes from Bu (for butadiene) and Na (for sodium, which was used as a catalyst in the original polymerization reaction). An improved form, known as Buna-S was developed by copolymerizing butadiene with styrene. In 1934, Buna-N was invented, in which the styrene was replaced by acrylonitrile, giving a product with better oil resistance (see nitrile rubber).