INHIBITION OF ACRYLIC-ACID POLYMERIZATION BY PHENOTHIAZINE AND PARA-METHOXYPHENOL .2. 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-radical-cation, 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.