Persistent radical effect

The persistent radical effect in chemistry describes and explains the selective product formation found in certain free-radical cross-reactions. In this type of reaction different radicals compete in secondary reactions. The so-called persistent (long-lived) radicals do not self-terminate and only react in cross-couplings. In this way the cross-coupling products in the product distribution are more prominent

The effect was first described in 1936 by Bachmann & Wiselogle. They heated pentaphenylethane and observed that the main reaction product was the starting product (87%) with only 2% of tetraphenylethane formed. They concluded that the dissociation of pentaphenylethane into triphenylmethyl and diphenylmethyl readicals was reversible and that persistent triphenylmethyl did not self terminate and transient diphenylmethyl did to a certain extent. In 1964 Perkins performed a similar reaction with phenylazotriphenylmethane in benzene. Again the dimerization product of the persistent radical (phenylcyclohexydienyl) was absent as reaction product. In 1981 Geiger and Huber found that the photolysis of dimethylnitrosamine into the dimethylaminyl radical and nitrous oxide was also completely reversible. A similar effect was observed by Kräutler in 1984 for methylcobalamin. The term persistent radical effect was coined in 1992 by Daikh and Finke in work related to the thermolysis of a cyanocobalamin model compound. The persistent radical effect is relevant to living free-radical polymerization.

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