Condensation Polymerisation
Summary: Condensation polymerisation occurs when monomers with two functional groups join together, eliminating a small molecule (usually water or HCl) as a byproduct. Examples include nylon, PET/terylene, and proteins. Tags: igcse chemistry organic-chem polymers Created: 2026-07-14 Last Updated: 2026-07-23
Definition
Condensation polymerisation is the reaction in which monomers with two reactive functional groups join together, with the elimination of a small molecule (usually water, H₂O, or hydrogen chloride, HCl) each time a bond forms.
Unlike addition polymerisation, the monomers do NOT need C=C double bonds — any molecule with two functional groups can potentially form a condensation polymer.
How It Works
- Each monomer has two functional groups (one at each end)
- Functional groups on different monomers react together
- Each time two monomers join, a small molecule is eliminated
- The polymer grows from both ends
- A long chain polymer is formed containing functional groups in the backbone (ester linkages, amide linkages, etc.)
Types of Condensation Polymers
Polyesters
Formed when:
- Dicarboxylic acid (HOOC—R—COOH) reacts with
- Diol (HO—R’—OH)
Each ester linkage eliminates one H₂O molecule:
HOOC—R—COOH + HO—R’—OH → —[OC—R—COO—R’—O]— + H₂O
Example: Terylene / PET — ethane-1,2-diol + benzene-1,4-dicarboxylic acid
Polyamides
Formed when:
- Dicarboxylic acid (HOOC—R—COOH) reacts with
- Diamine (H₂N—R’—NH₂)
Each amide linkage eliminates one H₂O molecule:
HOOC—R—COOH + H₂N—R’—NH₂ → —[OC—R—CONH—R’—NH]— + H₂O
Example: Nylon — hexane-1,6-dioic acid + 1,6-diaminohexane
Natural Polyamides (Proteins)
Proteins are natural condensation polymers formed from amino acids. Each amino acid has both:
- —NH₂ (amine group)
- —COOH (carboxylic acid group)
Amino acids join via peptide bonds (amide linkages), eliminating water each time.
Comparison: Addition vs Condensation
| Feature | Addition Polymerisation | Condensation Polymerisation |
|---|---|---|
| Monomer requirement | C=C double bond (unsaturated) | Two functional groups per monomer |
| Byproduct? | None | Small molecule eliminated (H₂O, HCl) |
| Atoms in polymer | All atoms from monomer | Some atoms lost as byproduct |
| Polymer backbone | C—C bonds only | May contain O, N, ester (−COO−), amide (−CONH−) |
| Empirical formula | Same as monomer | Different from monomer |
| Examples | Poly(ethene), PVC, PTFE | Nylon, Terylene/PET, proteins |
| Biodegradability | Not biodegradable | Some are biodegradable (proteins) |
Drawing Condensation Polymers at IGCSE
The examiner expects:
- The functional group linkage shown clearly (ester −COO− or amide −CONH−)
- Extension bonds extending through brackets
- The eliminated small molecule (H₂O) shown alongside
- Repeating unit in brackets with ‘n’ subscript
Example — Nylon repeating unit:
O O H H
‖ ‖ | |
—[—C—(CH₂)₄—C—N—(CH₂)₆—N—]—ₙ
Key Facts
- Monomers must have two functional groups each
- A small molecule (H₂O or HCl) is eliminated with each bond formed
- Polymer contains ester (−COO−) or amide (−CONH−) linkages in the backbone
- Examples: Nylon (polyamide), Terylene/PET (polyester), proteins (natural polyamides)
- Mass of polymer < mass of monomers (byproduct lost)
- The repeating unit’s empirical formula is DIFFERENT from the monomer’s