Condensation polymers, polyesters and recycling polymers

Polyesters are made by condensation polymerisation: two different monomers join and give off water each time. Getting rid of polymers safely is a big challenge.

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Condensation polymerisation

Each monomer has two functional groups, one at each end.
A diol (two -OH groups) reacts with a dicarboxylic acid (two -COOH groups).
Each time they join, an ester link forms and a small molecule, water, is given off.
Example: ethanediol (HO-CH2-CH2-OH) + hexanedioic acid (HOOC-(CH2)4-COOH) → a polyester + water.
The equation is usually written: n diol + n dicarboxylic acid → polyester + 2n H2O.

Comparing with addition polymers

Addition polymerCondensation polymer
MonomersOne type, with a C=C double bondUsually two types, each with two functional groups
Other productNoneWater (a small molecule)
ExamplePoly(ethene)Polyester

Disposal and recycling

Most polymers are not biodegradable, so they last for a very long time in landfill and the sea.
Burning releases energy but can make toxic gases and adds carbon dioxide.
Recycling saves crude oil and reduces landfill, but polymers must be collected and sorted by type, which costs money and energy, and they can only be recycled a limited number of times.
Biopolyesters are biodegradable: microorganisms can break them down.
Worked example

Using the equation n diol + n dicarboxylic acid → polyester + 2n H2O, how many water molecules are shown when n = 50?

  1. Each repeat unit (one diol and one acid) has two ester links.
  2. Each ester link forms with loss of one water molecule.
  3. 2n = 2 × 50 = 100

Answer: 100 molecules of water

Key idea

Polyesters form by condensation polymerisation of a diol and a dicarboxylic acid, losing water at each ester link. Addition polymers come from alkenes and give no other product. Most polymers do not biodegrade; recycling needs sorting; biopolyesters are biodegradable.

Key words
Acid
A substance that releases hydrogen ions, H^{+}, in water (a proton donor). Its solution has a pH below 7.
Ester
An organic compound made from a carboxylic acid and an alcohol, with a fruity smell, for example ethyl ethanoate.
Monomer
A small molecule that joins with many others to form a polymer.
Crude oil
A finite mixture of hydrocarbons formed over millions of years from the remains of ancient sea organisms.

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  1. Using the equation n diol + n dicarboxylic acid → polyester + 2n H2O, how many water molecules are shown when n = 50?

    Show the answer

    Answer: 100 molecules

    2n = 2 × 50 = 100 water molecules.

  2. Which two monomers could react together to make a polyester?

    1. Ethanediol and benzene-1,4-dicarboxylic acid
    2. Two alkenes
    3. Ethene and propene
    4. Ethanol and ethane
    Show the answer

    Answer: Ethanediol and benzene-1,4-dicarboxylic acid

    A diol and a dicarboxylic acid link by ester groups.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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