Mechanism of stable radical generation in aromatic polyamides on exposure to nitrogen dioxide

Exploring the Chemistry of Electron-Accepting Molecules in the Cavities of the Basic Microporous P4VP Polymer by in situ FTIR Spectroscopy

“…[45][46][47][48] It readily attacks double bonds in most of the polymers (such as polybutadienes, polyisoprenes, and their copolymers), ester groups in poly(methyl)methacrylate, and also amide fragments in polyamides and polyurethanes, thus determining their degradation. 49 It is assumed that the process involves an electron transfer from the substrate to the nitrosonium cation, to give NO and a cationradical product. When polymers are considered, a further decomposition of cation-radicals can occur, with the consequent release of protons and the formation of macro-radicals, which undergo recombination with NO and are transformed into the corresponding nitroso compounds.…”

Oxidative radical generation via nitrogen dioxide dimer conversions induced by amide groups of macromolecules

“…$ A_\parallel^{\rm N} $ = 4.1 mT, g ∥ = 2.0024 and $ A_\bot^{\rm N} $ = 2.6 mT, g ⊥ = 2.005 (radical II)9 and with $ A_\parallel^{\rm N} $ = 1.94 mT, g ∥ = 2.003 (radical III) 3. Using this procedure, the maximum concentrations of radicals I, II, and III were separately determined in composites with the various contents of AP and PVP after exposure to NO 2 within 24 h. It should be noted that the parameters of spectra of iminoxyl radicals II are identical in AP and PVP, therefore the same designation is accepted for these radicals.…”

“…Nevertheless, effective formation of stable radicals is observed also in polymers not containing labile hydrogen atoms or double bonds. For example, aromatic polyamidoimides, nylon, polyvinylpyrrolidone (PVP),3 and aromatic polyamides (AP)9 exhibit high activity in respect to nitrogen dioxide. These facts allow considering other probable mechanisms of radical initiation.…”

“…These facts allow considering other probable mechanisms of radical initiation. The fact is that major radical products of the nitrogen dioxide interaction with AP and PVP are iminoxyl and acylalkylaminoxyl radicals that are produced from oximes and acylnitroso compounds 3, 9. The occurrence of these precursors of stable radicals in turn associates with presence of nitric oxide.…”

“…As NN has strong oxidative properties,10 the generation of radicals can take place by an electron transfer from donor functional groups with the formation of intermediate radical cations: The recombination of radicals with nitric oxide gives nitroso compounds that undergo isomerization into oximes11 to produce iminoxyl radicals in the reaction with NO 2 : The nitroso compounds are effective spin traps and a source of stable aminoxyl radicals: Thus, the mechanism involving reactions (1–3) formally could explain an appearance of stable radicals in the polymers not containing specific chemical bonds reacting with NO 2 mono radicals 9. However, there are certain obstacles connected with energetic properties of NO 2 dimers for realizing such mechanism; the energy of syn‐ and anti‐forms of NN exceeds that of PD, respectively, 29.8 and 18.4 kJ mol −1 8; that is the equilibrium should be shifted to PD in gas phase.…”