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Monday, May 22, 2017

Types of Polymerization Reactions

1. Addition polymerization (or) chain growth polymerization:
The molecules of the same monomer or different monomers add together on a large scale to form a polymer.
The monomers used are unsaturated compounds.
Eg: Alkenes, alkadienes and their derivatives.
Can take place through the formation of either free radicals or ionic species.

Free radical mechanism: A variety of alkenes or dienes and their derivatives are polymerized in the presence of a free radical generating initiator (catalyst) like benzoyl peroxide, acetyl peroxide, tert-butyl peroxide etc.,
Eg: Polymerisation of ethene to polythene.

The free radical mechanism involves following steps.

Chain initiation step:
Ċ₆H₅ - CH₂ = CH₂ → 2C₆H₅ - CH₂ - ĊH₂
Chain propagation step:

C₆H₅ - CH₂ - ĊH₂ + CH₂ = CH₂ → C₆H₅ - CH₂ - CH₂ - CH₂ - ĊH₂ → C₆H₅ (- CH₂ - CH₂-)n – CH₂ - ĊH₂
Chain terminating step: For termination of the long chain, these free radicals can combine in different ways to form polythene.
Eg: C₆H₅ - (- CH₂ - CH₂ -)n - CH₂ - ĊH₂ + C₆H₅ - (- CH₂ - CH₂ -)n - CH₂  - ĊH₂ → C₆H₅ - (- CH₂ - CH₂ -)n - CH₂ - CH₂ - CH₂ - CH₂ - (- CH₂ - CH₂ -)n - C₆H₅

Preparation of some important addition polymers:

a) Polythene: These are of two types.

i) Low density polythene (LDP):-
By polymerization of ethene
Temperature: 350 K to 570 K in the presence of traces of dioxygen or peroxide initiator.
LDP obtained through the free radical addition and H atom abstractions has highly branched structure.
It is chemically inert and tough but flexible and a poor conductor of electricity.

ii) High density polythene (HDP):-
Addition polymerization of ethene in a hydrocarbon solvent in the presence of Ziegler - Natta Catalyst.

These are two sets of Zieglar - Natta catalyst/ co - catalyst systems. Either way, we have four chlorine atoms.
Temperature: 333 K to 343 K
Pressure of 6 - 7 atm.
HDP molecules consists linear molecules, its high density is due to close packing. It is also chemically inert and more tough and hard.
It is used for manufacturing of buckets, bottles, pipes, etc.,

b) Polytetrafluoro ethene (Teflon):
By heating tetrafluoroethene with a free radical or per sulphate catalyst at high pressure.
It is chemically inert and resistant to attack by corrosive reagents.
It is used in making oil seals and gaskets.

c) Polyacrylonitrile:
The addition polymerization of acrylonitrile in the presence of a peroxide catalyst leads to the formation of polyacryionitrile.
It is used as a substitute for wool in making commercial fibres as Orlon or acrilan.

2. Condensation polymerization or step Growth polymerization:
Involves a repetitive condensation reaction between two bifunctional monomers.
Results in the loss of simple molecules as water, alcohol, etc.
The product of each step is again a bi functional species and the sequence of condensation goes on.
So this process is also called as step growth polymerization.

Some important condensation polymerization reactions characterized by their linking units.

a. Polyamides:
Possess amide linkages.
Prepared by condensation polymerization of diamines with dicarboxylic acids and also of amino acids and their lactums.

Nylon 6, 6: Prepared by the condensation polymerization of hexamethylenediamine with adipic acid under high pressure and at high temperature.
Uses: It is used in making sheets, textile industry.

Nylon 6: Heating caprolactum with water at a high temperature.
Uses: It is used in the manufacture of tyre cords, fabrics and ropes.

b. Polyesters: Poly condensation products of dicarboxylic acids and diols.
Eg: Dacron or terylene.
It is manufactured by heating a mixture of ethylene glycol and terephthalic acid at 420 to 460 k in the presence of Zinc acetate antimony trioxide catalyst.

c. Phenol - formaldehyde polymer:
These are obtained by condensation reaction of phenol with formaldehyde in the presence of either acid or base catalyst.
The reaction starts with the initial formation of o – and/ or p – hydroxylmethylphenol derivatives, which further react with phenol to form compounds having rings joined to each other through – CH₂ groups.
The initial product could be a linear product- Novolac
Novolac is used in paints.

Bakelite: Novolac on heating with formaldehyde undergoes cross linking to form an infusible solid mass called Bakelite.
Uses: It is used in combs, phonograph records, etc.
Structure:

d. Melamine formaldehyde polymer: Melamine formaldehyde polymer is formed by the condensation polymerization of melamine and formaldehyde.
Uses: It is used in the manufacture of unbreakable crockery.

3. Copolymerization:
A mixture of more than one monomeric species is allowed to polymerize and form a copolymer.
The copolymer can be made not only by chain growth polymerization but by step growth polymerization also.
Example: Mixture of 1, 3 - butadiene and styrene can form a copolymer.
Copolymers have properties quite different from homopolymers.

For example, butaiene - styrene copolymer, is quite tough and is a good substitute of natural rubber. It is used in manufacture of auto tyres, footwear components, cable insulation etc.

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