Stabilization of polyamides IV. Thermo-oxidation of hexano-6-lactam in the presence of alkali metal salts

The effect of alkali metal acetates, halides and carbonates on the kinetics and mechanism of the branched chain oxidation of hexano-6-lactam was studied. The salts affect the rate of the initiation and termination reactions of this chain process depending on the cation and anion of the salt, on its initial concentration and on the reaction temperature. The initiation effect of the salts, mainly KCl and KBr, lies in the initiation of the homolysis of the product of the propagation reaction, 6-hydroperoxyhexano-6-lactam, i.e. of the reaction that is the main source of radicals and simultaneously the branched reaction of the chain oxidation of N-alkylamides. The inhibition effect of the salts lies in the initiation of nonhomolytic reactions of the hydroperoxide, one of them being most probably the reaction of the hydroperoxide with 5-formylpentanamide, the intermediate in subsequent reactions of 6-hydroperoxyhexano-6-lactam homolysis, leading to adipamic acid. Under certain conditions, salts can initiate a bimolecular termination reaction of peroxy radicals of 6-hydroperoxyhexano-6-lactam leading to one molecule of adipimide and one molecule of peroxyadipamic acid, while the termination reaction of peroxy radicals of the chain oxidation of N-alkylamides leads to two molecules of the respective imide and one molecule of hydroperoxide.