Illustrated Glossary of Organic Chemistry

Radical chain reaction (free radical chain reaction): A radical reaction mechanism in which one or more steps are repeated many times, until the chain (the sequence of repeated steps) is terminated, or until one of the reactants is depleted. Common examples include radical halogenation, ozone depletion by organohalides such as chlorofluorocarbons, and some forms of polymerization.

Free radical chlorination of methane (CH4) produces methyl chloride (CH3Cl) via a radical chain reaction. The mechanism includes one initiation step followed by a cycle of propagation steps. Chlorine radical produced in the last mechanism step is recycled indefinitely (second mechanism step) to produce more methyl chloride, until the chain is terminated, or until on of the reactants (methane or chlorine) is depleted.