Some radicals of particular importance

The allylic radical plays an extraordinary role in polymerizations. Qualitatively its reactivity can be estimated from the series of increasing particle stability [56] 

The resonance energy of the allylic radical is 48.8 kJ mol-1 [57, 58]. It is easily formed and its reactivity is low. It reacts reluctantly with monomers. This is why propene, 2-methylpropene, 1-butene (and higher members of the homologous series) do not polymerize by the radical mechanism 

Compounds which readily yield the allyl radical are potent transfer agents. The reaction of the generated radical with monomer is slow and the probability of its decay by some other reaction is large polymerization dies out  

The chain rarely grows on two or more active centres. Multifunctional active centres are sometimes intentionally formed [59] but more often they result from transfer to polymer. Their practical importance is not great so far. Not much is known about how reactivity is mutually affected in multifunctional radicals. We assume that centres mutually isolated by several methylene groups (several monomer units) behave as individual centres. 

Rarely, the unpaired electron of the active centre can belong to an atom other than carbon. 

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