Radical-scavenging activity of the reaction products of isoeugenol with thiol, thiophenol, mercaptothiazoline or mercaptomethylimidazole using the induction period method.

The reaction products in the presence of Lewis acid of isoeugenol (1) with ethanethiol, thiophenol, 2-mercaptothiazoline or 2-mercapto-1-methylimidazole (ISO-S1-ISO-S-4) were obtained. The radical-scavenging activity of these compounds was investigated using the induction period method for polymerization of methyl methacrylate (MMA) initiated by thermal decomposition of 2,2'-azobisisobutyronitrile (AIBN) and benzoyl peroxide (BPO) and monitored by differential scanning calorimetry (DSC). For BPO, the stoichiometric factor (number of free radicals trapped by one mole of antioxidant moiety, n) declined in the order isoeugenol (1.8) > ISO-S-1 (1.6) > ISO-S-2 (1.2) > ISOS- 3 (0.9) > ISO-S-4 (0.3), whereas for AIBN, their n values were about 1, except for ISOS- 3 (0.6). The ratio of the rate constant of inhibition to that of propagation (k(inh)/k(p)) for BPO declined in the order ISO-S-4 (56) > ISO-S-3 (15) > ISO-S-2 (11) >ISO-S-1 (9) > isoeugenol (8). Similarly, for AIBN the k(inh)/k(p) of the reaction products (33-57) was greater than that of isoeugenol (31). The reaction products of isoeugenol with a SH group showed greater inhibition rate constants (kinh) than the parent compound isoeugenol.