7-substituted quinone methides as inhibitors for unsaturated monomers

A composition for inhibiting the premature polymerization of an ethylenically unsaturated monomer which comprises

• (a) an ethylenically unsaturated monomer or mixture of monomers, and
• (b) an effective inhibiting mount of a synergistic mixture of
• (i) at least one compound of formula I ##STR5## wherein R.sub.1 and R.sub.2 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
• R.sub.3 is --CN, --COOH, --COOR.sub.4, --COR.sub.5, --OCOR.sub.6, --CONR.sub.7 R.sub.8 or --PO(OR.sub.9).sub.2 where
• R.sub.4 is alkyl of 1 to 18 carbon atoms, cycloalkyl of 5 to 12 carbon atoms, phenyl or benzyl,
• R.sub.5 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,
• R.sub.6 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,
• R.sub.7 and R.sub.s 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, or
• --NR.sub.7 R.sub.8 is morpholino, piperidino or pyrrolidino, and
• R.sub.9 is hydrogen or alkyl of 1 to 18 carbon atoms, and
• (ii) at least one 7-aryl quinone methide of formula II ##STR6## where E.sub.1 and E.sub.2 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
• E.sub.3 is 2-, 3- or 4-pyridyl, 2- 3-thienyl, 2- 3-pyrryl, 2- 3-furyl, aryl of 6 to 10 carbon atoms, or said aryl substituted by one to three alkyl of 1 to 8 carbon atoms, alkoxy of 1 to 8 carbon atoms, alkylthio of 1 to 8 carbon atoms, alkylamino of 1 to 8 carbon atoms, dialkylamino of 2 to 8 carbon atoms, alkoxycarbonyl of 2 to 8 carbon atoms, hydroxy, nitro, amino, cyano, carboxy, aminocarbonyl, chloro or mixtures of said substituents.