Copolymerisation: Buna-S and Buna-N

Copolymerisation is addition polymerisation carried out with TWO (or occasionally more) different monomers together in the same reaction, so that the resulting chain -- a copolymer -- contains both kinds of monomer unit distributed along its backbone, in contrast to a homopolymer (such as polythene or PVC), which is built from only one monomer. Copolymerisation is especially important for making synthetic rubbers (elastomers) whose properties can be tuned by choosing which monomers, and in what ratio, to combine.

Buna-S (also called SBR, styrene-butadiene rubber) is a copolymer of buta-1,3-diene, $\text{CH}_2{=}\text{CH--CH=CH}_2$, and styrene, $\text{C}_6\text{H}_5\text{--CH=CH}_2$, polymerised together by a free-radical addition mechanism (the 'Buna' name itself comes from Butadiene and Na, sodium, the original catalyst used; the 'S' denotes styrene as the co-monomer). Buta-1,3-diene contributes most of the chain's elastic character (its two double bonds allow the chain to add in a way that leaves one residual double bond per unit, giving the flexible, rubbery backbone), while the bulkier aromatic styrene units are interspersed along the chain and toughen the material. Buna-S is the synthetic rubber used in the largest tonnage worldwide, mainly for vehicle tyre treads, where its abrasion resistance is prized.

Buna-N (nitrile rubber, NBR) is likewise a copolymer of buta-1,3-diene, but with acrylonitrile, $\text{CH}_2{=}\text{CH--CN}$, as the second monomer in place of styrene. The polar nitrile ($-\text{C}{\equiv}\text{N}$) group introduced by acrylonitrile gives Buna-N markedly better resistance to swelling in non-polar oils, greases and petrol than Buna-S has (whose non-polar, all-hydrocarbon composition swells readily in oil) -- so Buna-N, rather than Buna-S, is chosen for oil-resistant hoses, fuel-line seals, and gaskets in automotive and industrial applications, even though it is generally more expensive. …