Inhibition of acrylic acid polymerization by phenothiazine and p-methoxyphenol

The major features of polymerization induction periods for acrylic acid inhibited with phenothiazine and p-methoxyphenol have been characterized at 100°C, including duration of the induction periods, and rates of inhibitor disappearance, molecular oxygen absorption, and peroxide formation. Surprisingly, thermally produced radicals react more rapidly with phenothiazine than with oxygen since there is no detectable oxygen absorption or peroxide for mation during phenothiazine-induced induction periods. Thus, phenothiazine has been used to estimate the thermal rate of radical formation. Phenothiazine's effectiveness as an inhibitor is not directly affected by oxygen, although it does undergo oxygen-promoted, noninhibition-related side reactions. p-Methoxyphenol, on the other hand, depends entirely on the presence of oxygen to function as an inhibitor. Compared with equivalent concentrations of p-methoxyphenol, induction periods obtained with phenothiazine are very long, and the rate of inhibitor disappearance is slower by at least an order of magnitude. The characteristics of p-methoxyphenol inhibition reflect a greater radical flux deriving from the significant rates of oligomeric peroxide formation and decomposition which we measured during p-methoxyphenol-induced induction periods at 100°C. MEHQ is an effective inhibitor at ambient temperatures in part because of the greater stability of the peroxides at these lower temperatures.