Inhibition of polymerization of unsaturated monomers

A method for inhibiting the premature polymerization of vinyl aromatic monomers selected from the group consisting of styrene, α-methylstyrene, styrene sulfonic acid, vinyltoluene, divinylbenzenes, polyvinylbenzenes, alkylated styrene and 2-vinylpyridine comprising adding to said monomers a polymerization inhibiting amount of a mixture comprising

• A) a first inhibitor comprising 4-amino-2,2,6,6-tetramethylpiperidinyloxy, wherein said amino group is a primary, secondary, or tertiary amino group;
• and
• B) at least one other inhibitor having the structural formula:
• wherein R1 and R4 are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl and R2 and R3 are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl, and the
• portion represents the atoms necessary to form a five-, six-, or seven-membered heterocyclic ring.

A composition comprising:

• (a) a vinyl aromatic monomer selected from the group consisting of styrene, α-methylstvrene, styrene sulfonic acid, vinyltoluene, divinylbenzenes, polyvinylbenzenes, alkylated styrene and 2-vinvipyridine and
• (b) an effective inhibiting amount, to prevent premature polymerization of the vinyl aromatic monomer, of a mixture of
• (i) a first inhibitor comprising 4-amino-2,2,6,6-tetramethylpiperidinyloxy, wherein said amino group is a primary, secondary, or tertiary amino group;
• and
• (ii) at least one other inhibitor having the structural formula:
• wherein:
• R1 and R4 are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl;
• R2 and R3 are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl;
• X1 and X2 (1) are independently selected alkyl groups of 1 to 15 carbon atoms or (2) taken together, form a ring structure with the nitrogen.

A composition comprising:

• (a) a vinyl aromatic monomer selected from the group consisting of styrene, α-methylstyrene, styrene sulfonic acid, vinyltoluene, divinylbenzenes, polyvinylbenzenes, alkylated styrene and 2-vinvipyridine, and
• (b) an effective inhibiting amount, to prevent premature polymerization of the vinyl aromatic monomer, of a mixture of
• (i) 1 to 99 percent by weight, based on the total weight of components (i) and (ii), of a first inhibitor comprising 4-amino-2,2,6,6-tetramethylpiperidinyloxy, wherein said amino group is a primary, secondary, or tertiary amino group, and, correspondingly,
• (ii) 99 to 1 percent by weight of at least one other inhibitor having the structural formula:
• wherein R1 and R4 are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl and R2 and R3 are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl, and the
• portion represents the atoms necessary to form a five-, six-, or seven-membered heterocyclic ring.