Monomer

Summary: A small molecule that can be joined together with many others to form a polymer. Tags: igcse chemistry definition organic-chem Created: 2026-07-14 Last Updated: 2026-07-14

A monomer is a relatively small, reactive molecule that can join together with many identical or similar units through covalent bonds to form a long-chain polymer. In addition polymerisation, the monomer must contain a carbon-carbon double bond (C=C); the double bond opens up and monomers add to one another without the loss of any atoms — ethene (C₂H₄) polymerises to form poly(ethene), and propene (C₃H₆) yields poly(propene), both IGCSE staples. In condensation polymerisation, each monomer carries two reactive functional groups (e.g., a diol and a dicarboxylic acid), and each joining step eliminates a small molecule such as water. The structure of the polymer can be deduced directly from the structure of its monomer: the repeating unit has the same atoms as the monomer but with the C=C double bond converted to a single bond and the “spare” bonds extending to either side. In exam questions, students must be able to draw the polymer repeating unit given the monomer structure (or vice versa), and they should note that the polymer name is placed in brackets — for example, poly(ethene) rather than polyethene.

Key term — IGCSE Chemistry (0620/0971) glossary.