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Nitroso compounds, such as 2-methyl-2-nitrosopropane, nitrosobenzene, and 4-nitrosophenol, were tested as volatile inhibitors for vapor-phase inhibition of acrylic acid polymerization during its reflux at 113 °C under reduced pressure. The experimental parameters were set to mimic the conditions employed for the industrial distillation of acrylic acid. Nitrosobenzene was found to be the most efficient vapor-phase inhibitor for the distillation of acrylic acid. The inhibition time was also found to be dependent on the pressure used for the experiments. Decreasing the pressure from 110 to 85 mbar resulted in an almost 2-fold increase in the inhibition time. The combination of nitrosobenzene with a liquid-phase inhibitor, namely, 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl, significantly improved the efficiency of vapor-phase inhibition by decreasing the consumption of nitrosobenzene in the liquid phase.

Phenothiazine and other compounds with similar structures, namely, phenoxazine, promazine, promazine hydrochloride, N,N′-dimethylphenazine, carbazole, N-ethylcarbazole, N-benzylphenothiazine, and N-(1-phenylethyl)phenothiazine were tested as liquid-phase inhibitors for acrylic acid. N-Alkylated phenothiazine (PTZ) derivatives, such as Nbenzylphenothiazine and especially N-(1-phenylethyl)phenothiazine, showed improved efficiency for liquid-phase inhibition, in comparison with PTZ. It was also shown that N-alkylated PTZ derivatives could be used in combination with nitroso compounds that can be employed as vapor-phase inhibitors. The combination of N-(1-phenylethyl)phenothiazine and nitrosobenzene showed superior liquid-phase-inhibition efficiency, with time to gelation of >122 h, in comparison with 64 h obtained with PTZ alone. The structure of liquid-phase inhibitors had no influence on the extent of Michael addition reactions during heating of acrylic acid.

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