What small molecule is lost in condensation polymerisation?

Usually water (H₂O). In some cases, hydrogen chloride (HCl) is lost instead. The key point is that a small molecule is eliminated at each link between monomers.

Do the monomers need to be unsaturated for condensation polymerisation?

No. Unlike addition polymerisation, condensation polymerisation does not require a C=C double bond. Instead, the monomers must have functional groups at both ends (e.g. two −OH groups, two −COOH groups, or two −NH₂ groups) that can react with each other.

Condensation Polymerisation – IGCSE Chemistry Definition

IGCSE Chemistry definition of condensation polymerisation: monomers joining with loss of a small molecule. Covers polyesters, polyamides, 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.

Condensation polymerisation is a Supplement topic that examiners use to test higher-order understanding. The key distinction from addition polymerisation — the loss of a small molecule — is the most frequently tested point.

The 0620 definition

Condensation polymerisation is the process in which monomers with two functional groups join together to form a polymer, with the loss of a small molecule (usually water) at each link.

How it differs from addition polymerisation

Feature	Addition	Condensation
Monomer type	Unsaturated (C=C bond)	Two functional groups per monomer
Small molecule lost?	No	Yes (usually H₂O)
Polymer type	Poly(alkene)	Polyester or polyamide
Example	Poly(ethene)	Polyester (Terylene), nylon

Types of condensation polymer

Polyesters

Formed from a diol (two −OH groups) and a dicarboxylic acid (two −COOH groups). An ester link (−COO−) forms at each joint, and water is lost.

The linkage: −COOH + HO− → −COO− + H₂O

Example: Terylene (PET) — used in drinks bottles and clothing.

Polyamides

Formed from a diamine (two −NH₂ groups) and a dicarboxylic acid (two −COOH groups). An amide link (−CONH−) forms, and water is lost.

The linkage: −COOH + H₂N− → −CONH− + H₂O

Example: Nylon — used in clothing, rope, and engineering plastics.

Natural condensation polymers

Proteins are natural polyamides (amino acids linked by peptide bonds). Starch and cellulose are natural condensation polymers of glucose.

Worked exam question

Nylon is a condensation polymer. (a) State what is meant by condensation polymerisation. [2] (b) Name the small molecule produced during the formation of nylon. [1] (c) State one difference between condensation and addition polymerisation. [1]

Mark scheme

(a) Monomers join together / polymerise [1]; with the loss of a small molecule / water [1]

(b) Water / H₂O [1]

(c) Addition: no small molecule lost / needs unsaturated monomers; condensation: small molecule lost / monomers have two functional groups [1] (any valid comparison)

Common exam mistakes

Forgetting to mention the loss of a small molecule — this is the defining feature of condensation polymerisation and must be stated for the mark.
Saying the monomers need a double bond — that is addition polymerisation. Condensation monomers need functional groups at both ends.
Confusing polyester and polyamide — polyesters have −COO− links (from −COOH and −OH), polyamides have −CONH− links (from −COOH and −NH₂).

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