Condensation Polymerisation of Polyesters

Polyesters are a type of plastic made up of simpler units or monomers held together via ester bonds.

Let us first consider the condensation reaction of carboxylic acid and alcohol to form ester.

230403 esterification

During esterification, the oxygen in alcohol acts as a nucleophile and attacks the acid carbon that acts as an electrophile.

The products formed are ester and water.

For one acid and one alcohol, we can only get one ester and one water.

If we have a di-acid and di-ol, something interesting can happen.

The di-acid can form 2 ester bonds with 2 different di-ols on either side.

Similarly the di-ol can also form 2 ester bonds with 2 different di-acids on either side.

This will form a continuous chain of alternating di-acid and di-ol, linked together via ester bonds.

Hence this polymer is called a polyester, made up of monomers di-acid and di-ol.

The balanced equation for this polymerisation is as shown:

230403 polymerisation

Notice n moles of di-acid and n moles of di-ol will give 2n-1 moles of water instead of 2n moles.

This difference in 1 water is due to an -OH group at the beginning and a -H group at the end of the polyester chain.

Topic: Carboxylic Acid and Derivatives, Organic Chemistry, A Level Chemistry, Singapore

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