Photoinitators can create reactive species by different pathways including photodissociation and electron transfer. As an example of dissociation, hydrogen peroxide can undergo homolytic cleavage, with the O-O bond cleaving to form two hydroxyl radicals.
H2O2 → 2 ·OH
Certain azo compounds (such as azobisisobutyronitrile), can also photolytically cleave, forming two alkyl radicals and nitrogen gas:
RCH2-N=N-H2CR → 2 RCH2 + N2
These free radicals can now promote other reactions.