the condensation polymerisation of the monomers urea and formaldehyde. H C = O formaldehyde H N NH urea -H O HO NH NH -H O H C = O HO NH NH -H O H C = O H C H C O = C urea formaldehyde polymer . . Co-polymers: A polymer containing two or more different kinds of monomer units is called a co- polymer.
For example, SBR rubber(Buna-S) contains styrene and butadiene monomer units. Co-polymers have properties quite different from the homopolymers. . .
Natural and Synthetic rubbers: Rubber is a naturally occurring polymer. It is obtained from the latex that excludes from cuts in the bark of rubber tree (Ficus elastic). The monomer unit of natural rubber is cis isoprene ( -methyl buta- , -diene). Thousands of isoprene units are linearly linked together in natural rubber.
Natural rubber is not so strong or elastic. The properties of natural rubber can be modified by the process called vulcanization. XII U15 Chemistry in XII U15 Chemistry in - - - - n cispolyisoprene Vulcanization: Cross linking of Rubber In the year , Charles Good year accidently dropped a mixture of natural rubber and sulphur onto a hot stove. He was surprised to find that the rubber had become strong and elastic.
This discovery led to the process that Good year called vulcanization. Natural rubber is mixed with - % sulphur and heated at - ˚C causes cross linking of the cis- , -polyisoprene chains through disulphide (-S-S-) bonds. The physical properties of rubber can be altered by controlling the amount of sulphur that is used for vulcanization. In sulphur rubber, made with about to % sulphur is soft and stretchy.
When to % sulphur is used the resultant rubber is somewhat harder but flexible. Synthetic rubber: Polymerisation of certain organic compounds such as buta- , -diene or its derivatives gives rubber like polymer with desirable properties like stretching to a greater extent etc., such polymers are called synthetic rubbers. Preparation of Neoprene: The free radical polymeristion of the monomer, -chloro buta- , -diene(chloroprene) gives neoprene. CH