METHOD FOR STABILIZING UNSATURATED CARBOXYLIC ACID OR ESTER THEREOF

PURPOSE:To contrive stabilization of an unsaturated carboxylic acid or ester thereof having wide applications as a raw material for synthetic resins, by using p-phenylenediamines as a readily handleable polymerization inhibitor having great polymerization inhibitory effects. CONSTITUTION:An unsaturated carboxylic acid, such as methacrylic acid, or an ester thereof is stabilized by the coexistence of at least one of p- phenylenediamines [e.g., N-phenyl-N'-(n-hexyl)-p-phenylenediamine] expressed by the formula (R1 is 6C alkyl; R2 is phenyl) in an amount of 5-2000ppm, especially 50-1000ppm based on the unsaturated carboxylic acid or ester thereof. Not only stabilization can be carried out with the inhibitor in a relatively small amount by the above-mentioned method but also effects thereof are not inhibited even in the coexistence of water, SO2, etc.