Blends of quinone alkide and nitroxyl compounds and polymerization inhibitors

A method for inhibiting the premature polymerization of ethylenically unsaturated monomers comprising adding to said monomers an effective amount of:

• A) at least one nitroxyl compound of the structure:
• wherein R1 and R4 are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl and R2 and R3are (1) independently selected from the group consisting of alkyl and heteroatom-substituted alkyl or (2) taken together, form a ring structure with the nitrogen, and wherein the ring is selected from the group consisting of piperidine, pyrrolidine, pyridine and pyrrole;
• and X1 and X2 (1) are independently selected from the group consisting of halogen, phosphorus (in any of it oxidation states), cyano, COOR7, —S—COR7, —OCOR7, —S—R7 (wherein R7 is alkyl or aryl), amido, carbonyl, alkenyl, or alkyl of 1 to 15 carbon atoms, or (2) taken together, form a ring structure with the nitrogen, and
• B) at least one quinone alkide compound of the formula
• wherein:
• R31 and R32 are independently alkyl of 4 to 18 carbon atoms, cycloalkyl of 5 to 12 carbon atoms or phenylalkyl of 7 to 15 carbon atoms, and
• R33 is —CN, —COOH, —COOR34, —COR35, —OCOR36, —CONR37R38 or —PO(OR39)2 where R34 is alkyl of 1 to 18 carbon atoms, cycloalkyl of 5 to 12 carbon atoms, phenyl or benzyl,
• R35 is alkyl of 1 to 18 carbon atoms, aryl of 6 to 10 carbon atoms or said aryl substituted by 1 or 2 alkyl of 1 to 4 carbon atoms or by hydroxyl,
• R36 is alkyl of 1 to 18 carbon atoms, aryl of 6 to 10 carbon atoms or said aryl substituted by 1 or 2 alkyl of 1 to 4 carbon atoms or by hydroxyl,
• R37 and R38 are independently hydrogen, alkyl of 1 to 18 carbon atoms or said alkyl substituted by alkylamino of 1 to 4 carbon atoms, by dialkylamino of 2 to 8 carbon atoms or by hydroxyl;
• benzyl, aryl of 6 to 10 carbon atoms or said aryl substituted by alkyl of 1 to 4 carbon atoms, by alkylamino of 1 to 4 carbon atoms, by dialkylamino of 2 to 8 carbon atoms, by phenylamino or by hydroxyl,
• R39 is hydrogen or alkyl of 1 to 18 carbon atoms, and
• R40 is hydrogen, alkyl, aryl, hydroxyl, alkoxy, —CN, —COOH, —COOR34, —COR35, —OCOR36, —CONR37R38 or —PO(OR39)2.