Q.What is Buna-N? How does its structure and monomer composition differ from Buna-S?

Both Buna-N and Buna-S are synthetic rubbers formed by free-radical copolymerization of buta-1,3-diene with a second monomer, but that second monomer differs: Buna-S uses styrene, $\text{C}_6\text{H}_5\text{--CH=CH}_2$, while Buna-N uses acrylonitrile, $\text{CH}_2{=}\text{CH--CN}$. This difference has a large practical consequence: acrylonitrile's polar nitrile ($-\text{C}{\equiv}\text{N}$) group gives Buna-N chains significant polarity, so Buna-N resists swelling in non-polar oils, greases and petrol much better than the largely non-polar Buna-S does. Buna-N is therefore chosen, despite being more expensive, for oil-resistant hoses, fuel-line seals and gaskets, while Buna-S (cheaper, and with better abrasion resistance from its hi …

Compare the identity of the second (non-butadiene) monomer in each copolymer and trace how its chemical character (polar nitrile vs non …

Mixing up which rubber uses which co-monomer (Buna-S = styrene, Buna-N = nitrile/acrylonitrile -- 'N' for nitrile is the memory …