Inhibition of acrylic acid and acrylate autoxidation

Acrylic acid (AA) is a versatile monomer whose high reactivity can present a challenge for transport and storage due to its highly exergonic oligomerization, which can lead to runaway polymerization and explosion. To prevent premature polymerization of acrylic acid, hydroquinone monomethyl ether (MeHQ) and phenothiazine (PTZ) are commonly used as inhibitors/stabilizers. Despite their widespread use, the limited radical-trapping stoichiometry of MeHQ and oxidative consumption of PTZ at process temperatures are clear limitations. Herein, we apply a recently devised spectrophotometric approach employing the autoxidizable STY-BODIPY dye to monitor reaction progress in autoxidations of acrylic acid, <i>n</i>-butyl acrylate and the non-polymerizable 2-ethylhexanol, and the impact of a panel of radical-trapping antioxidants (RTAs, including MeHQ and PTZ) upon them. We find that the radical-trapping stoichiometry is highly substrate-dependent, with nitroxides and aromatic amines that can be converted to nitroxides <i>in situ</i> exhibiting superstoichiometric activities in substrates where hydroperoxyl radicals are formed or in the presence of acid. <i>N</i>-Alkyl derivatives of phenoxazine, the most potent RTA uncovered to date, are found to be particularly excellent inhibitors of AA autoxidation. It is proposed that gradual acid-mediated dealkylation to phenoxazine minimizes accumulation of the phenoxazine-derived nitroxide, which can otherwise undergo acid-catalyzed disproportionation and diminish radical-trapping capacity. These results suggest that <i>N</i>-alkylated phenoxazine derivatives should be explored further as stabilizers of AA.