What is the difference between Buna S and Buna N?

What is the difference between Buna S and Buna N?

Styrene-Butadiene or Styrene-Butadiene-Rubber (SBR) is a Synthetic rubber copolymer consisting of styrene and butadiene. It has good abrasion resistance and good aging stability when protected by additives, and is widely used in car tires, where it may be blended with natural rubber.
The material was initially marketed with the brand name Buna S. It derives this name from: But for butadiene Na for sodium
Nitrile rubber, also known as Buna-N, Perbunan, or NBR, is a Synthetic rubber of copolymer (ACN) and butadiene