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.