Chemistry Copolymerisation and Rubbers
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### Topics Covered :

● Copolymerisation
● Natural Rubber
● Synthetic Rubbers
● Prepparation of Synthetic Rubbers
● Neoprene
● Buna-N

### Copolymerisation :

color{green}("Copolymerisation ") : It is a polymerisation reaction in which a mixture of more than one monomeric species is allowed to polymerise and form a copolymer.

=> The copolymer can be made not only by chain growth polymerisation but by step growth polymerisation also.

=> It contains multiple units of each monomer used in the same polymeric chain.

●color{red}("Example ") : A mixture of 1, 3 – butadiene and styrene can form a copolymer.

=> Copolymers have properties quite different from homopolymers.

●color{red}("Example ") : Butadiene - styrene copolymer is quite tough and is a good substitute for natural rubber. It is used for the manufacture of autotyres, floortiles, footwear components, cable insulation, etc.

### 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, Srilanka, Indonesia, Malaysia and South America.

=> Natural rubber may be considered as a linear polymer of isoprene (2-methyl-1, 3-butadiene) and is also called as cis - 1, 4 - polyisoprene. See fig.1.

● The cis-polyisoprene molecule consists of various chains held together by weak van der Waals interactions and has a coiled structure.

● Therefore, it can be stretched like a spring and exhibits elastic properties.

color{green}("Vulcanisation of Rubber ") : Natural rubber becomes soft at high temperature (>335 K) and brittle at low temperatures (<283 K) and shows high water absorption capacity.

● It is soluble in non-polar solvents and is non-resistant to attack by oxidising agents.

● To improve upon these physical properties, a process of vulcanisation is carried out.

● This process consists of heating a mixture of raw rubber with sulphur and an appropriate additive at a temperature range between 373 K to 415 K.

● On vulcanisation, sulphur forms cross links at the reactive sites of double bonds and thus the rubber gets stiffened.

=> In the manufacture of tyre rubber, 5% of sulphur is used as a cross-linking agent.

=> The probable structures of vulcanised rubber molecules are depicted in fig.2.

### Synthetic rubbers :

=> Synthetic rubber is any vulcanisable rubber like polymer, which is capable of getting stretched to twice its length.

=> But, it returns to its original shape and size as soon as the external stretching force is released.

=> Therefore, synthetic rubbers are either homopolymers of 1, 3 - butadiene derivatives or copolymers of 1, 3 - butadiene or its derivatives with another unsaturated monomer.

### Preparation of Synthetic Rubbers :

Some of the synthetic rubbers are prepared as follow :

### Neoprene :

=> Neoprene or polychloroprene is formed by the free radical polymerisation of chloroprene.

color{red}("undersettext(Chloroprene)(n CH_2 = overset( overset(Cl)(|))C- CH= CH_2) oversettext(Polymerisation)→ undersettext(Neoprene)([-CH_2 - overset( overset(Cl)(|))C = CH - CH_2-]_n))

=> It has superior resistance to vegetable and mineral oils.

color{green}("Uses ") : It is used for manufacturing conveyor belts, gaskets and hoses.

### Buna-N :

Buna –N is obtained by the copolymerisation of 1, 3 – butadiene and acrylonitrile in the presence of a peroxide catalyst.

color{red}(undersettext(1 , 3 - Butadiene)(nCH_2 = CH-CH=CH_2)+undersettext(Arcylonitrile)(nCH_2 = overset( overset(CN)(|))CH) oversettext(Copolymerisation)→ undersettext(Buna - N) ([-CH_2-CH = CH - CH_2 - CH_2 - overset ( overset(CN)(|))CH-]_n))

● It is resistant to the action of petrol, lubricating oil and organic solvents.

color{green}("Uses ") : It is used in making oil seals, tank lining, etc.