Why is Buna-S called so? Styrene Butadiene synthetic rubber is also called Buna-S. It is the abbreviation of Butadiene (Bu) Sodium (Na) Styrene (S). 
Why is Sodium included?
Sodium is included in the name because it was essential in the chain polymerization that forms the rubber.
Sodium served as the initiator for the anionic polymerization.