The persistent radical effect (PRE) in chemistry describes and explains the selective product formation found in certain free-radical cross-reactions.
In 1981, Geiger and Huber found that the photolysis of dimethylnitrosamine into dimethylaminyl radical and nitrous oxide was also completely reversible.
[8][9] The term 'persistent radical effect' was coined in 1992 by Daikh and Finke in their work related to the thermolysis of a cyanocobalamin model compound.
The dormant species are activated (with a rate constant kact) either spontaneously/thermally, in the presence of light, or with an appropriate catalyst (as in ATRP) to reform the growing centers.
However, persistent radicals (X), as stated above, cannot terminate with each other but only (reversibly) cross-couple with the growing species (kdeact).