monomer

A monomer is a small molecule that can chemically bind to other similar or different molecules to form a larger macromolecule called a polymer. The term comes from the Greek mono- meaning single and meros meaning part. In a polymer, repeated units derived from monomers create a long chain; the repeating unit in the polymer may be identical to the monomer or result from changes that occur during polymerization.

Polymers are typically produced by two broad classes of reactions. Addition (or chain-growth) polymerization involves unsaturated monomers with a reactive functional group, such as a carbon–carbon double bond, joining to a growing chain without byproducts. Condensation (or step-growth) polymerization involves monomers with at least two functional groups that react to form bonds while releasing small molecules like water or methanol. The choice of monomer functionality, such as di-, tri-, or polyfunctional groups, influences chain length, branching, and the formation of cross-links that can yield thermoplastic or thermosetting materials.

Common monomers include ethylene, propylene, styrene, vinyl chloride, and acrylates, which yield polymers such as polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polyacrylates. Natural monomers—amino acids, nucleotides, and simple sugars—give rise to biopolymers like proteins, nucleic acids, and polysaccharides.

Key concepts include the degree of polymerization (the number of repeating units), the distinction between the monomer and the polymer’s repeating unit, and the possibility of forming copolymers from two or more different monomers. Monomer selection, functionality, and polymerization conditions determine a material’s properties and applications.