The photoproduction of a free radical or ion capable of initiating a chain reaction such as a polymerization.
See: photoinduced polymerization
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2261 (https://doi.org/10.1351/pac199668122223)