Q.Explain vulcanisation of rubber. Which vulcanizing agents are used for the following synthetic rubber.
a. Neoprene b. Buna-N

Raw natural rubber alone has limited practical usefulness -- it softens when warm and stiffens when cold -- so its physical properties are dramatically improved by vulcanization, a process invented by Charles Goodyear in 1839. Vulcanizing means heating raw rubber with sulfur powder together with accelerators (the most common being zinc butyl xanthate, ZBX) at around 150 degC. During this process, sulfur atoms open the double bonds along the polyisoprene chain and form sulfur cross-links (bridges) directly between neighbouring chains, converting the originally loose, independently-stretchable linear rubber into a three-dimensional, cross-linked network. This cross-linking is exactly what lets a stretched vulcanized rubber snap back to its original shape once the applied force is removed, dramatically improving tensile strength, stiffness, elasticity and toughness compared with unvulcanized rubber. The amount of sulfur used controls the degree of hardening: more sulfur gives a harder product, and raising the sulfur content to 40-45% converts rubber into ebonite, a hard, non-elastic material rather than a stretchy one. The same principle of vulcanization -- introducing a network of cross-links into an elastomer -- also applies to synthetic rubbers: Buna-S is vulcanized with sulfur just like natural rubber, while neoprene is instead vulcanized using magnesium oxide (MgO).