Polymers

A polymer (macromolecule) is composed of small structural repeating units called monomers. The process of formation of polymers from respective monomers is called polymerisation. Polymers can be classified on the basis of source (Natural polymers, Semi-synthetic polymers and Synthetic polymers), structure (Linear polymers, Branched chain polymers and Cross linked or network polymers), mode of polymerisation (addition polymers and condensation polymers) and molecular forces (elastomers, fibres, thermoplastic polymers and thermosetting). Polymers are formed by either addition (or chain growth polymerisation) or by condensation or step growth polymerisation. Polythene is an important polymer. There are two types of polythene: Low density polythene and High density polythene. Polytetrafluoroethene (Teflon) is another important polymer which is obtained by heating tetrafluoroethene with a free radical or persulphate catalyst at high pressure. It is used for making non–stick coating on utensils because teflon coatings undergo decomposition at temperatures above 300°C. Polyacrylonitrile (Orlon) is obtained from vinyl cyanide (acrylonitrile) in presence of a peroxide catalyst . It is used as a substitute for wool. Some important condensation polymers are Phenol-formaldehyde polymer (Bakelite), Melamine–formaldehyde polymer, Nylon 6,6 (obtained from adipic acid and hexamethylenediamine), Nylon 6 (obtained from caprolactum). Natural rubber is a linear polymer of isoprene (2-methyl-1, 3-butadiene) also called as cis - 1, 4 – polyisoprene. The process of heating natural rubber with sulphur to improve its properties is called vulcanisation of rubber. Neoprene, Buna-N etc. are synthetic rubbers. There are two ways of expressing molecular mass of polymers: Number average molecular mass and Weight average molecular mass. Biodegradable polymer can be degraded by microorganisms within a suitable period. They do not cause any serious affect on the environment. Poly β-hydroxybutyrate–co-β-hydroxyvalerate (PHBV) and Nylon 2–Nylon 6 are biodegradable polymers.

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