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.