Q7(c). Synthetic rubbers: Buna-S and neoprene

Introduction. Synthetic rubbers are elastomers: long, flexible chains with weak interchain forces, cross-linked lightly (vulcanisation) so that they stretch and recover. Most are polymers or copolymers of 1,3-dienes, made by radical (emulsion) polymerisation.

1. Buna-S (styrene–butadiene rubber, SBR)

Scheme: Butadiene is the major monomer; it enters the chain mainly by 1,4-addition.

1. Buta-1,3-diene: catalytic dehydrogenation of butane or butenes from petroleum cracking (Cr₂O₃/Al₂O₃, high temperature): CH₃CH₂CH₂CH₃ → CH₂=CH-CH=CH₂ + 2H₂. (Older route: from ethanol over a catalyst.)
2. Styrene: Friedel–Crafts alkylation of benzene with ethene (AlCl₃/HCl) to ethylbenzene, then dehydrogenation over hot iron oxide catalyst: C₆H₅CH₂CH₃ → C₆H₅CH=CH₂ + H₂.
3. Polymer: [-CH₂-CH=CH-CH₂⁻]ₓ[-CH₂-CH(C₆H₅)⁻]_y; vulcanised with sulfur. Used for tyres and footwear.

2. Neoprene (polychloroprene)

Scheme: Chloroprene is 2-chlorobuta-1,3-diene; it polymerises mainly by 1,4-addition (trans).

1. Vinylacetylene: dimerisation of acetylene with CuCl/NH₄Cl: 2HC#CH → CH₂=CH-C#CH.
2. Chloroprene: addition of HCl to the triple bond of vinylacetylene (CuCl catalyst, Markovnikov) gives CH₂=C(Cl)-CH=CH₂. (Present route: chlorination of butadiene to 3,4-dichlorobut-1-ene, then dehydrochlorination with NaOH.)
3. Polymer: [-CH₂-C(Cl)=CH-CH₂⁻]ₙ. The polar C–Cl gives resistance to oil, petrol, heat and flame: hoses, gaskets, conveyor belts.

Conclusion. Buna-S from butadiene (dehydrogenation of butane) and styrene (benzene + ethene, then dehydrogenation); neoprene from chloroprene (acetylene → vinylacetylene → + HCl). Other examples: Buna-N (butadiene + acrylonitrile), butyl rubber (isobutylene + a little isoprene).

Application. Vulcanisation with sulfur cross-links the chains through -Sₓ⁻ bridges, removing tackiness and raising tensile strength; SBR is the main tyre rubber worldwide.