`=>` This type of polymerisation generally involves a repetitive condensation reaction between two bi-functional monomers.

`=>` These polycondensation reactions may result in the loss of some simple molecules as water, alcohol, etc., and lead to the formation of high molecular mass condensation polymers.

`=>` In these reactions, the product of each step is again a bi-functional species and the sequence of condensation goes on.

`=>` Each step produces a distinct functionalised species and is independent of each other, this process is also called as step growth polymerisation.

`color{red}("Example ")` : The formation of terylene or dacron by the interaction of ethylene glycol and terephthalic acid.

Some important condensation polymerisation reactions characterised by their linking units are described below :

`=>` These polymers possessing amide linkages are important examples of synthetic fibres and are termed as nylons.

`=>` The general method of preparation consists of the condensation polymerisation of diamines with dicarboxylic acids and also of amino acids and their lactams.

(a) `color{green}("Preparation of Nylons ")` :

(i) `color{green}("Nylon 6, 6 ")` : It is prepared by the condensation polymerisation of hexamethylenediamine with adipic acid under high pressure and at high temperature.

`color{red}(nHOOC (CH_2)_4COOH + n H_2 N (CH_2)_6 NH_2 undersettext(High pressure) overset(553 K)→ undersettext(Nylon 6 , 6) ( -[- overset(overset(H)(|))N - (CH_2)_6 - overset( overset(H)(|))N - overset( overset(O)(||))C (CH_2)_4 - overset(overset(O)(||))C-]_n))`

`color{green}("Uses ")` : Nylon 6, 6 is used in making sheets, bristles for brushes and in textile industry.


(ii) `color{green}("Nylon 6 ")` : It is obtained by heating caprolactum with water at a high temperature. See fig.

`color{green}("Uses ")` : Nylon 6 is used for the manufacture of tyre cords, fabrics and ropes.

`=>` These are the polycondensation products of dicarboxylic acids and diols.

`color{green}("Dacron ")` : Dacron or terylene is the best known example of polyesters.

● 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 as per the reaction given earlier.

● Dacron fibre (terylene) is crease resistant.

`color{green}("Uses ")` : It is used in blending with cotton and wool fibres and also as glass reinforcing materials in safety helmets, etc.

`=>` Phenol-formaldehyde polymers are the oldest synthetic polymers.

`=>` These are obtained by the condensation reaction of phenol with formaldehyde in the presence of either an acid or a base catalyst.

`=>` The reaction starts with the initial formation of `color{red}(o)`-and/or `color{red}(p)`-hydroxymethylphenol derivatives, which further react with phenol to form compounds having rings joined to each other through `color{red}(–CH_2)` groups.

`=>` The initial product could be a linear product – Novolac used in paints. See fig.1.

`=>` Novolac on heating with formaldehyde undergoes cross linking to form an infusible solid mass called bakelite. See fig.2.

`color{green}("Uses ")` : It is used for making combs, phonograph records, electrical switches and handles of various utensils.

`=>` Melamine formaldehyde polymer is formed by the condensation polymerisation of melamine and formaldehyde.

`color{green}("Uses ")` : It is used in the manufacture of unbreakable crockery.