Polymerization inhibitors for acrylic acids and esters

A method for inhibiting the polymerization of an acrylic or methacrylic acid or ester during distillation of the acrylic or methacrylic acid or ester, which comprises conducting the distillation in the presence of an effective amount of a polymerization inhibitor composition consisting essentially of:

• (a) at least one N,N′-dinitroso phenylenediamine compound having the structure:
• wherein R is C1-C12 alkyl or C6-C10 aryl;
• R1 is C1-C12 alkyl, C6-C10 aryl, C7-C11 aralkyl, or C7-C16 alkaryl;
• and
• (b) at least one phenothiazine having the structure:
• wherein R2 is hydrogen or C1-C12 alkyl;
• and R3 and R4 are each independently selected from the group consisting of hydrogen, C6-C10 aryl, C7-C11 aralkyl, C7-C16 alkaryl and C1-C12 alkyl;
• (c) optionally a hydroquinone or hydroquinone monomethyl ether;
• and
• (d) optionally a phenylenediamine compound having the following structure
• wherein R6 is C1-C12 alkyl, C6-C10 aryl or C7-C16 alkaryl;
• and R7, R8 and R9 are independently selected from the group consisting of hydrogen, C1-C12 alkyl, C3-C12 cycloalkyl, C7-C11 aralkyl and C7-C16 alkaryl.