Inhibition of acrylic acid polymerization by phenothiazine and p‐methoxyphenol. II. Catalytic inhibition by phenothiazine

Inhibition of acrylic acid by p‐methoxyphenol fits a conventional stoichiometric mechanism but phenothiazine inhibits acrylic acid via a completely different, catalytic cycle which does not depend on the presence of oxygen. We propose that this mechanism may involve a pair of single electron transfer reactions between free radicals, phenothiazine N‐radicalcation, and phenothiazine itself, the latter being cyclically regenerated. Arrhenius equations were derived for the rates of disappearance of inhibitor and oxygen in acrylic acid stabilized with phenothiazine and with p‐methoxyphenol and also with phenothiazine in the absence of oxygen. The practical implication of high oxygen to p‐methoxyphenol consumption ratios is quite important: if commercial acrylic acid (usually stabilized with p‐methoxyphenol) is inadvertently heated during storage, the limiting substance determining the onset of polymerization will be the dissolved oxygen and not p‐methoxyphenol unless oxygen (air) is being supplied to and dissolved in the liquid at a rate sufficient to overcome the rate of its consumption.