NEET Chemistry Notes Polymers – Polymerisation
Polymerisation
Polymerisation
The process or technique through which monomer units combine to give a polymer is known as polymerisation. The polymerisation reaction cannot be controlled easily.
General Methods of Polymerisation
These are the following methods through which monomers combine to give polymers:
Addition Polymerisation
It is the process in which molecules of same or different monomers combine together to give a polymer without the elimination of small molecules. Since, there is an increase in the length of chain (i.e. chain grows) by this mode of polymerisation, it is also known as chain growth polymerisation. This mode of polymerisation is observed in molecules having multiple bonds, mainly in CH2 = CHy type molecules (where, y may be —H, —X, —CO2R, —CN etc.)
The addition or chain growth polymerisation can proceed by the following two mechanisms
- By the formation of free radicals, i.e. free radical polymerisation and
- By the formation of ionic species, i.e. ionic polymerisation.
Condensation Polymerisation
- In this mode of polymerisation, there is combination of molecules of same or different monomers in a controlled stepwise manner with the elimination of small molecules such as water, ammonia, alcohol, etc., as by product.
- These monomem have functional groups such as alcohol, amine or carboxylic acid groups instead of double bonds.
Since, there is a repetitive condensation reaction between bifunctional monomers, the process is also known as step growth polymerisation.
Examples of Addition Homopolymers
- Polymer Polytetrafluoroethene (teflon)
Monomer Tetrafluoroethene (CF2=CF2)
Uses For making oil seals and gaskets, non-stick surface coated utensils. - Polymer Polythene (polyethylene)
Monomer Ethene (CH2= CH2)
Polythene is of two types:
- Low density polythene is obtained by the polymerisation of ethene at 1000 to 2000 atm pressure and 350 to 570 K temperature in the presence of peroxide initiator (catalyst).
- High density polythene is obtained by the polymerisation of ethene at 6-7 atm pressure, 333-343 K temperature in the presence of Ziegler-Natta catalyst.
- Polymer Polyacrylonitrile (PAN)
Monomer Acrylonitrile (CH2=CHCN)
Uses Substitute for wool in making commercial fibres as orlon or acrilan. - Polymer Polypropene Monomer Propene
Uses In the manufacture of ropes, toys, pipes, fibres etc. - Polymer Polystyrene
Monomer Styrene, (CH2=CH—C6H5)
Uses As insulator, wrapping material, manufacture of toys, radio and television cabinets. – - Polymer Polyvinyl chloride (PVC)
Monomer Vinyl chloride (CH2 =CH—Cl)
Uses In the manufacture of raincoats, handbags, vinyl flooring, water pipes.
Polymer Natural rubber or polyisoprene (linear polymer)
Monomer Isoprene (2-methyl-l, 3-butadiene)
Natural rubber can be stretched like a spring and exhibits elastic properties. - Polymer Neoprene (synthetic rubber)
Monomer Chloroprene (2-chloro-l, 3-butadiene)
Uses In the manufacture of conveyor belts, gaskets and hoses.
Examples of Addition Copolymers
(i) Polymer Butadiene-styrene (Buna-S)
Monomers 1,3-butadiene (CH2=CH— CH=CH2) and styrene (C6H5—CH=CH2).
Uses Buna-S is quite tough and is a good substitute for natural rubber. It is used for the manufacture of auto tyres, floor tiles, footwear components, cable insulation etc.
(ii) Polymer Buna-N
Monomers 1,3-butadiene (CH2 = CH—CH = CH2) and acrylonitrile (CH2= CHCN).
Uses It is resistant to the action of petrol, lubricating oil and organic solvents. It is used in making oil seals, tank lining etc.
Examples of Condensation Copolymers
- Polymer Nylon-6, 6
Monomers Adipic acid [HOOC(CH2)4COOH] and hexamethylenediamine |H2N(CH2)6NH2]
Uses Nylon-6, 6 is used in making sheets, bristles for brushes and in textile industry. - Polymer Terylene or dacron Monomers Terephthalic acid
and ethylene glycol
Uses Blending with cotton and wool fibres and also as glass reinforcing materials in safety helmets, etc. - Polymer Phenol-formaldehyde polymer (bakelite) Monomers Phenol (C6H5OH) and formaldehyde (HCHO).
Uses It is used for making combs, phonograph records, electrical switches, and handles of various utensils. - Polymer Melamine-formaldehyde polymer Monomer Melamine and formaldehyde (HCHO).
Uses For making unbreakable cups and laminated sheets,
- Polymer Glyptal (alkyd resin)
Uses In the manufacture of paints and lacquers.
Examples of Condensation Homopolymers
Polymer Nylon-6 (perlon).
Monomer Caprolactum
Uses Nylon-6 is used for the manufacture of tyre cords, fabrics and ropes.
Copolymerisation
When two or more different monomer units are allowed to polymerise, a copolymer is formed which contains multiple units of each monomer used, in the same polymeric chain. The process by which copolymers are formed is referred to as copolymerisation
Some Important Polymers
Rubber
On the basis of their occurrance, these are of following two types:
- Natural Rubber
It is a natural polymer prepared from rubber latex (colloidal dispersion of rubber in water) obtained from the bark of rubber tree. It is poly cis-isoprene. It is very soft.
- Synthetic Rubber
These rubbers are formed by different monomers as shown below:
(a) Buna-S or styrene-butadiene rubber (SBR) is addition copolymer of 1, 3-butadiene and styrene
Thiokol or polysulphide rubber Thiokol is a type of synthetic rubber. It is a copolymer of ethyiene chloride and sodium tetrasulphide (Na2S4).