Mechanism of ''living'' radical polymerization mediated by stable nitroxyl radicals

It is shown that in order for the ''living'' radical polymerization mediated by stable nitroxyl radical (SNR) to proceed successfully, a constant supply of initiating radicals (by, e.g., thermal initiation) to make up for the loss of polymer radicals due to irreversible bimolecular termination is essential in addition to the frequent reversible combination of polymer radical with SNR. The stationary rate of SNR-mediated polymerization is totally independent of the SNR concentration, being equal to the polymerization rate of the SNR-free system. Experimental evidence is presented showing that in the ''living'' polymerization of styrene, both of these requirements are met in fact.