JEE Chemistry Polymers Notes Set 04

Polymers and Classification by Monomer Type

A polymer is a very large molecule built by joining many small repeating units (monomers) in a regular way; the process is called polymerisation. Example: n \( \ce{CH2=CH2} \) gives polythene.
A homopolymer is made from one kind of monomer (polythene, PVC, polystyrene, Teflon, nylon-6, neoprene, cellulose, starch). A copolymer is made from two or more kinds of monomer (nylon-6,6, Terylene, Bakelite, Glyptal, Buna-S, Buna-N).

Classification by Source and by Structure

Natural polymers occur in plants and animals (proteins, cellulose, starch, natural rubber). Synthetic polymers are made in the laboratory (polythene, nylon-6,6, Dacron, PAN). Semi-synthetic polymers are natural polymers modified chemically (cellulose acetate or rayon, cellulose nitrate).
Linear polymers have long straight chains (HDPE, PVC); HDPE is made with the Ziegler-Natta catalyst \( \ce{(C2H5)3Al + TiCl4} \) at 6-7 atm and 333-345 K. Branched polymers have side chains (LDPE), made at 1000-2000 atm and 350-570 K with traces of oxygen or a peroxide. Cross-linked (network) polymers form when bi- and tri-functional monomers are linked by strong covalent bonds (Bakelite, melamine-formaldehyde).

Addition and Condensation Polymerisation

Addition polymers form by repeated addition of unsaturated monomers (polythene, polystyrene, PVC, Teflon, Buna-S, Buna-N, neoprene) without loss of any atom.
Condensation polymers form by repeated reaction between bi- or poly-functional monomers with loss of a small molecule such as water, alcohol or HCl. Examples: nylon-6,6 (hexamethylenediamine and adipic acid), nylon-6 (caprolactam, ring opening after hydrolysis), Terylene or Dacron (ethylene glycol and terephthalic acid), Glyptal (ethylene glycol and phthalic acid), Bakelite (phenol and formaldehyde).

Classification by Intermolecular Forces

Elastomers are rubber-like solids whose chains are held by the weakest forces (Buna-S, Buna-N, neoprene, natural rubber). Fibres have the strongest forces, from hydrogen bonding (polyamides such as nylon) or dipole-dipole attraction (polyesters, polyacrylonitrile), and have high tensile strength. Thermoplastics have intermediate forces and soften repeatedly on heating (polythene, polystyrene, PVC). Thermosetting polymers are cross-linked, set permanently on heating and cannot be remoulded (Bakelite, urea-formaldehyde resin).
Order of forces: elastomer < thermoplastic < fibre.

Mechanisms of Polymerisation

Chain growth (addition) polymerisation proceeds by a free radical, cationic or anionic mechanism. In the free radical mechanism an initiator such as benzoyl peroxide gives radicals that add to the alkene, generating a new radical at each step (initiation, propagation, termination).
Step growth polymerisation occurs by stepwise reaction between functional groups of bifunctional monomers, so the chain grows through dimers, trimers and so on; condensation polymers are formed this way.

Important Polymers and Their Monomers

Polythene: ethene. PVC: vinyl chloride. Polystyrene: styrene. PAN or Orlon: acrylonitrile. Teflon: tetrafluoroethene. Neoprene: chloroprene (2-chlorobuta-1,3-diene). Natural rubber: cis-1,4-polyisoprene. Buna-S: butadiene and styrene. Buna-N: butadiene and acrylonitrile.
Nylon-6,6: hexamethylenediamine and adipic acid. Nylon-6: caprolactam. Terylene: ethylene glycol and terephthalic acid. Glyptal: ethylene glycol and phthalic acid (paints and lacquers). Bakelite: phenol and formaldehyde. Melamine polymer: melamine and formaldehyde. PHBV: 3-hydroxybutanoic and 3-hydroxypentanoic acids, a biodegradable copolyester. Nylon-2-nylon-6: glycine and 6-aminohexanoic acid, biodegradable.

Vulcanisation

Raw natural rubber is soft and sticky and has poor strength. Heating it with 3-5 percent sulphur introduces disulphide cross-links between the polyisoprene chains at allylic positions, making it stronger, more elastic and more resistant to wear and heat. About 1-3 percent sulphur is used for rubber bands and about 5 percent for tyres.