Natural Rubber:
Rubber is a natural polymer and possesses elastic properties. It is also termed as elastomer and has a variety of uses. It is manufactured from rubber latex which is a colloidal dispersion of rubber in water. This latex is obtained from the bark of rubber tree and is found in India, Sri Lanka, Indonesia, Malaysia and South America.
Chemically natural rubber is a hydrocarbon polymer. It is a linear 1, 4-addition polymer of isoprene i.e., 2-methyl- 1,3-butadiene.
Since each repeating unit in polyisoprene contains a double bond, it may have either a cis- or trans-orientation. Actually a natural rubber is cis-polyisoprene.
On the other hand, synthetic rubber (gutta percha) obtained by free radical polymerisation of isoprene is trans-
polyisoprene.
Vulcanization of Rubber:
Vulcanization is a chemical process in which the rubber is heated with sulphur, accelerator and activator at 140 -160°C. The process involves the formation of cross-links between long rubber molecules so as to achieve improved elasticity, resilience, tensile strength, viscosity, hardness and weather resistance.
synthetic rubber:
Majority of these rubbers are derived from butadiene derivatives and contain C = Cbonds so that they can also be vulcanized. Thus, synthetic rubbers are either homopolymers of 1,3-butadiene and its derivatives or are copolymers of 1,3-butadiene or its derivatives with another unsaturated monomer.
Some important synthetic rubbers are: cis-polybuta-1, 3-diene, Neoprene, Buna-S, Buna-N and thiokol.
(1) Cis-polybuta-1, 3-diene: Buta-1, 3-diene on polymerization in presence of Ziegler-Natta catalyst (a mixture of triethylaluminium and titanium tetra chloride) gives cis-polybuta-1, 3-diene in which 1, 4-addition takes place in each butadiene molecule and the remaining double bonds are in cis-orientation.
(2) Neoprene: It was the first synthetic rubber manufactured on large scale. It is a polymer of chloroprene (2-chloro-1,3-butadiene) and is also called polychloroprene. Its monomer chloroprene is prepared from acetylene as follows:
