Monomers

Monomers are small molecules that can join together to form larger, more complex structures called polymers. A polymer is a chain of repeating units derived from monomers; the specific monomer and the way units are linked determine the polymer’s properties. When a polymer is built from a single type of monomer, it is called a homopolymer; when it contains two or more different monomers, it is a copolymer.

Monomers are defined by their reactivity and functional groups. The number of reactive sites, or functionality, influences how many other monomer molecules a given molecule can bond to and affects chain length and cross-linking. Common monomer examples include ethylene, propylene, styrene, and vinyl chloride, which form polymers such as polyethylene, polypropylene, polystyrene, and polyvinyl chloride. Monomers from biology, such as amino acids, nucleotides, and monosaccharides, serve as building blocks for proteins, nucleic acids, and polysaccharides, respectively.

Polymerization occurs by two broad mechanisms. Addition (chain-growth) polymerization links unsaturated monomers without releasing small molecules, as in polyethylene and polystyrene. Condensation (step-growth) polymerization joins monomers with the loss of a small molecule like water or methanol, producing polyesters and polyamides such as nylons. In natural systems, enzymes and specific conditions guide polymer formation, yielding biopolymers with defined structures.

Key terms include monomer, the basic building block; repeat unit, the portion of the polymer’s backbone corresponding to one monomer; and cross-links, which connect chains to form a network and influence material properties.