Monomer – IGCSE Chemistry Definition
IGCSE Chemistry definition of monomer: a small molecule that joins with many others to form a polymer. Covers examples and how to identify monomers for 0620.
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Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Understanding monomers is essential for answering polymerisation questions on Paper 4. The examiner frequently asks candidates to draw the monomer from a given polymer (or vice versa), so you must be confident converting between the two representations.
The 0620 definition
A monomer is a small molecule that can join with many other identical or similar molecules to form a polymer through polymerisation.
The word comes from Greek: mono = one, mer = part.
Monomers for addition polymerisation
The monomers must be unsaturated — they must contain a C=C double bond (alkenes).
| Monomer | Formula | Polymer formed |
|---|---|---|
| Ethene | CH₂=CH₂ | Poly(ethene) |
| Propene | CH₂=CHCH₃ | Poly(propene) |
| Chloroethene | CH₂=CHCl | Poly(chloroethene) / PVC |
| Styrene | CH₂=CHC₆H₅ | Polystyrene |
Monomers for condensation polymerisation
The monomers need two functional groups (one at each end) so they can link to monomers on both sides.
| Monomer type | Example | Links to form |
|---|---|---|
| Diol + dicarboxylic acid | Ethane-1,2-diol + terephthalic acid | Polyester (PET) |
| Diamine + dicarboxylic acid | 1,6-diaminohexane + hexanedioic acid | Nylon |
| Amino acids (both −NH₂ and −COOH) | Glycine, alanine | Proteins |
Converting between monomer and polymer
Monomer → polymer (addition)
- Open the C=C double bond to C−C
- Draw extending bonds from each side
- Enclose in brackets with subscript n
Polymer → monomer (addition)
- Identify the repeating unit within the brackets
- Remove the extending bonds
- Restore the C=C double bond
Worked exam question
The repeating unit of a polymer is −(CH₂−CHCl)ₙ−. (a) Draw the structural formula of the monomer. [1] (b) Name the type of polymerisation. [1]
Mark scheme
(a) CH₂=CHCl (chloroethene) — double bond restored between the two carbons [1]
(b) Addition polymerisation [1]
Common exam mistakes
- Forgetting to restore the C=C double bond when working backwards from polymer to monomer — without the double bond, the monomer is wrong.
- Confusing monomer (one small molecule) with polymer (the long chain) — “mono” = one.
- Assuming all monomers must be alkenes — condensation polymer monomers do not contain C=C bonds.
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