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

By using the example of poly-m-phenylene isophthalamide, the mechanism of generation of stable nitrogen-containing radicals in aromatic polyamides in the presence of nitrogen dioxide is considered. The proposed mechanism is based on the reactions of dimers of nitrogen dioxide in the form of nitrosyl nitrate. As a result of a primary reaction of electron transfer from donor functional groups of macromolecules to nitrosyl nitrate, macromolecular radical cations and nitric oxide are formed. Amide groups and phenyl rings can act as electron donors. In the subsequent reactions with participation of radical cations, nitric oxide and nitrogen dioxide oximes, nitroso compounds and nitrites are formed. Generation of stable iminoxyl radicals occurs by reactions of oximes with nitrogen dioxide. Thermolysis of the polymer nitration products gives iminoxyl and acylarylaminoxyl radicals. The structure of iminoxyl radicals and features of dynamics of their formation have been confirmed by ab initio quantum-chemical calculations.