JEE Chemistry Polymers Notes Set 02

What is a Polymer

A polymer is a very large molecule built by joining many small molecules called monomers. The number of repeating units is the degree of polymerisation, and polymers have high molecular masses.

Classification by Structure

Linear polymers have long straight chains, for example PVC and high density polythene. Branched polymers have side chains attached to the main chain, for example low density polythene. Cross linked polymers have covalent bonds between chains and form a network, for example Bakelite and melamine formaldehyde resin.

Classification by Mode of Polymerisation

Addition (chain growth) polymers form by repeated addition of monomers with nothing lost. Homopolymers use one monomer: \( n\,\ce{CH2=CH2}\to[-\ce{CH2-CH2}-]_n \) polythene; PVC from vinyl chloride; polyacrylonitrile (Orlon) from acrylonitrile.
Copolymers use two monomers: Buna-S from 1,3-butadiene and styrene, and Buna-N from 1,3-butadiene and acrylonitrile.

Condensation Polymers

Condensation (step growth) polymers form when monomers join with loss of a small molecule such as water, an alcohol or HCl.
Nylon-6,6: hexamethylenediamine and adipic acid. Nylon-6,10: hexamethylenediamine and sebacic acid. Terylene (Dacron): ethylene glycol and terephthalic acid. Nylon-6: caprolactam heated with water, giving a chain of 6-aminohexanoic acid units.

Classification by Intermolecular Forces

Elastomers have the weakest forces between chains and can be stretched: Buna-S, Buna-N, neoprene. Fibres have strong forces and high tensile strength: nylon-6,6, polyesters. Thermoplastics are linear or slightly branched and soften on heating and harden on cooling again: polythene, polystyrene, polyvinyls. Thermosetting plastics are cross linked, set permanently and cannot be reused: Bakelite, urea formaldehyde resin.
Order of intermolecular forces: fibres > thermosetting > thermoplastics > elastomers.

Some Important Polymers

Low density polythene is made by free radical addition with a peroxide initiator at high pressure; it is chemically inert and a poor conductor, used for insulation and squeeze bottles. High density polythene is made with the Ziegler-Natta catalyst (titanium tetrachloride and triethylaluminium); it is harder and tougher and is used for buckets and bottles.
Bakelite comes from phenol and formaldehyde and is used for switches. Natural rubber is cis-1,4-polyisoprene. Neoprene is the polymer of chloroprene (2-chloro-1,3-butadiene). Teflon is the polymer of tetrafluoroethylene and is used for non-stick coatings.

Vulcanisation of Rubber

Raw rubber is soft and sticky at high temperature. Heating it with sulphur forms sulphur cross links at the double bonds, which makes it harder, stronger and more elastic over a wide temperature range.

Monomers and Polymers at a Glance

Ethene gives polythene. Propene gives polypropylene. Vinyl chloride gives PVC. Styrene gives polystyrene. Acrylonitrile gives PAN (Orlon). Vinyl acetate gives PVA. Tetrafluoroethylene gives Teflon. Isoprene gives natural rubber. Chloroprene gives neoprene. Methyl methacrylate gives PMMA. Ethylene glycol with phthalic acid gives Glyptal. Melamine with formaldehyde gives melamine formaldehyde resin. Urea with formaldehyde gives urea formaldehyde resin.