Mechanistic studies on the decomposition of water soluble azo-radical-initiators

Abstract

Spin trapping with nitrones coupled with the use of electron paramagnetic resonance (EPR) is one of the most effective techniques to observe reactive radical oxygen species (RROS), even though the distinction between, for example, alkoxyl and alkylperoxyl radicals is not an easy task. In the light of this very problem, this work summarizes our results using three nitrones as spin traps and three water soluble amidino-azo-initiators as sources for alkylperoxyl and/or alkoxyl radicals. The principal conclusion of this study is that only the corresponding alkoxyl radicals were found to add to the nitrones, and no evidence was found for the alkylperoxyl radical-spin adducts. The spin traps 5-(diethoxyphosphoryl)-5-methyl-4,5-dihydro-3H-pyrrole N-oxide (DEPMPO) as well as methyl-N-durylnitrone (MDN) rendered a discriminative spin adduct assignment much simpler as compared to 5,5-dimethyl-4,5-dihydro-3H-pyrrole N-oxide (DMPO), where liquid chromatography–electrospray mass spectrometry (LC–ESMS) was employed to substantiate the EPR results. For the kinetic decomposition behavior of the azo initiators it was found that at pH 6.2 2,2′-azobis(2-methylpropionamidine) dihydrochloride (AAPH) undergoes hydrolysis to the corresponding amides while the cyclic initiators 2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA-44) and 2,2′-azobis{2-[2-(4-methyl)imidazolin-2-yl]propane} dihydrochloride (Me-VA-44) are resistant towards hydrolysis over the employed timescale of 30 h.