In chemistry, radical initiators are substances that can produce radical species under mild conditions and promote radical reactions. These substances generally possess weak bonds—bonds that have small bond dissociation energies. Radical initiators are utilized in industrial processes such as polymer synthesis. Typical examples are halogen molecules, azo compounds, and peroxides.

• Like all diatomic molecules, halogens can generate two free radicals resulting from the homolysis of the bond, but halogens undergo the homolytic fission relatively easily. Chlorine, for example, gives two chlorine radicals (Cl•) by irradiation with ultraviolet light. This process is used for chlorination of alkanes.

• Azo compounds (R-N=N-R') can be the precursor of two carbon-centered radicals (R• and R'•) and nitrogen gas upon heating and/or by irradiation.

• Peroxides each have a peroxide bond (-O-O-), which is readily cleaved to give two oxygen-centered radicals. The oxyl radicals are rather unstable and believed to be transformed into relatively stable carbon-centered radicals. For example, di-t(tertiary)-butylperoxide (^tBuOO^tBu) gives two t-butanoyl radicals (^tBuO•) and the radicals become methyl radicals (CH₃•) with the loss of acetone. Benzoylperoxide ((PhCOO)₂) generates benzoyloxyl radicals (PhCOO•), each of which loses carbon dioxide to be converted into a phenyl radical (Ph•).

Radical initiators, especially azo compounds and peroxides, are inherently unstable. They must be kept in a cool place or refrigerated. Care should be taken with the handling of the compounds or an explosion may occur.

See also: Initiator