Method for inhibiting the polymerization of acrylic acid or its esters

A method for inhibiting the polymerization of a member selected from the group consisting of acrylic acid and acrylic acid esters during a distillation process for separating the acrylic acid or the acrylic acid esters derived from said acrylic acid obtained by the vapor phase catalytic oxidation of a member selected from the group consisting of propylene, acrolein, and the acrylic esters derived from said acrylic acid, said method comprising distilling one of said members selected from the group consisting of acrylic acid and acrylic esters in the presence of (A) 10 - 5000 ppm, based on the weight of the vapor rising in the distillation zone, of at least one compound selected from the group consisting of hydroquinone, hydroquinonemonomethyl ether, cresol, phenol, t-butyl catechol, diphenylamine, phenothiazines and methylene blue;

• (B) 5 - 2000 ppm, based on the weight of the vapor rising in the distillation zone, of at least one compound selected from the group consisting of copper dimethyldithiocarbamate, copper diethyldithiocarbamate, copper dibutyldithiocarbamate and copper salicylate;
• and (C) 0.01 - 5%, based on the volume of the vapor rising in the distillation zone, of molecular oxygen and then recovering the purified acrylic acid or acrylic esters.