Addition Polymerisation – IGCSE Chemistry Definition
IGCSE Chemistry definition of addition polymerisation: unsaturated monomers joining without loss of atoms. Covers mechanism, examples, and exam tips for 0620.
Published by IGCSEChemistry.com.my
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.
Addition polymerisation is one of the most diagram-heavy topics in IGCSE organic chemistry. Paper 4 regularly asks candidates to draw the repeating unit of a polymer from its monomer, or identify the monomer from the polymer. Understanding how the double bond opens is the key to both directions.
The 0620 definition
Addition polymerisation is the process in which many small unsaturated molecules (monomers, typically alkenes) join together to form a long-chain molecule (polymer) with no other product formed.
How it works
- The monomer must contain a C=C double bond (it must be unsaturated)
- The double bond opens up (one bond breaks)
- Each monomer links to the next through the freed bonds
- No atoms are lost — every atom from the monomers appears in the polymer
General equation: nCH₂=CH₂ → −(CH₂−CH₂)ₙ−
Key examples
| Monomer | Polymer | Use |
|---|---|---|
| Ethene, CH₂=CH₂ | Poly(ethene) / polythene | Plastic bags, bottles |
| Propene, CH₂=CHCH₃ | Poly(propene) | Rope, carpet, containers |
| Chloroethene, CH₂=CHCl | Poly(chloroethene) / PVC | Pipes, window frames |
| Tetrafluoroethene, CF₂=CF₂ | PTFE (Teflon) | Non-stick coatings |
Drawing the repeating unit
From monomer to polymer:
- Remove the double bond (change C=C to C−C)
- Extend single bonds from each carbon that was in the double bond
- Enclose in brackets with subscript n
From polymer to monomer:
- Take the repeating unit
- Remove the extending bonds
- Restore the C=C double bond between the two carbons
Worked exam question
Propene (CH₂=CHCH₃) undergoes addition polymerisation. (a) Define the term addition polymerisation. [2] (b) Draw the repeating unit of poly(propene). [1]
Mark scheme
(a) Many unsaturated monomers / alkene molecules [1] join together to form a polymer with no other product / no small molecule lost [1]
(b) The repeating unit shows −(CH₂−CHCH₃)ₙ− with a single C−C bond (double bond opened) and extending bonds from the first and second carbon [1]
Common exam mistakes
- Leaving the double bond in the polymer — the C=C must open to C−C when the monomer polymerises.
- Forgetting the extending bonds outside the brackets — these show the unit continues.
- Confusing addition with condensation polymerisation — addition produces no by-product; condensation produces water (or HCl).
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