WO2000036052A1 - Inhibiting polymerization of vinyl aromatic monomers using synergistic mixtures containing nitroxide stabilizers - Google Patents

Abstract

The premature polymerization of vinyl aromatic compounds, such as styrene, in the presence of saturated precursors during distillation or purification processes is very effectively inhibited by the presence of at least one stable nitroxide compound together with at least one phenolic, aromatic amine, hindered amine, or phosphite costabilizer.

Description

Inhibiting Polymerization of Vinyl Aromatic Monomers Using Synerqistic Mixtures

Containing Nitroxide Stabilizers

The instant invention pertains to a composition and a method for effectively inhibiting premature polymerization of readily polymerizable vinyl aromatic compounds during the monomer manufacturing and purification processes.

It is well known that vinyl aromatic compounds, such as styrene, a-methylstyrene, and other substituted vinyl benzenes, have a strong tendency to polymerize when subjected to elevated temperatures. Vinyl aromatic monomers are produced by several different industrial methods. The most common method is to dehydrogenate saturated alkyl- substituted aromatic compounds. The vinyl aromatic monomers produced in this way contain byproducts and impurities, such as ethylbenzene and substituted ethylbenzenes, and therefore must be subjected to separation and purification processes in order to be suitable for further industrial applications. Such separation and purification is generally accomplished by distillation.

A typical distillation system for vinyl aromatic compounds is described in detail in U.S. Pat. Nos. 4,252,615 and 4,341 ,600, the relevant parts of which are incorporated herein by reference. A typical system requires a series of distillation columns consisting of one or more pre-finishing columns for purifying crude product and a finishing column for further purifying nearly pure product. When the vinyl aromatic monomers are produced by dehydrogenating alkyl-substituted aromatics, the pre-finishing columns contain high percentages of the intermediate alkyl-substituted aromatic compounds.

In a common example of such a process, the feed streams introduced into the first column of a distillation system for vinyl aromatic monomers that are produced by dehydrogenating alkyl-substituted aromatics consist of on average about 40 % alkyl-substituted aromatics. Low-boiling aromatic hydrocarbons such as benzene and toluene are removed in this first column.

In the next pre-finishing column of this example, in which the alkyl-substituted aromatic compound is distilled away from the vinyl aromatic monomer, variable amounts of the alkyl- substituted aromatic compound are present. The percentage of alkyl-substituted aromatic compound in this column varies from very high at the top of the column to very low at the bottom. The reboiler(s) of this pre-finishing column contain nearly pure vinyl aromatic monomer and only parts per million levels of alkyl-substituted aromatic compounds.

Another common method for the preparation of vinyl aromatic monomers is by dehydrating an hydroxyalkyl-substituted aromatic compound. For example sec-phenethyl alcohol is dehydrated to produce styrene. Distillation trains of styrene produced in this manner may contain high levels of the intermediate hydroxyalkyl-substituted aromatic compound; in this case sec-phenethyl alcohol.

Despite the fact that vinyl aromatic monomers are generally purified by vacuum distillation, air is also present to some degree. Therefore oxygen is present to some degree. The starting materials of the synthetic processes to produce the vinyl aromatic monomers may be saturated with ambient air. Additionally distillation systems are commonly not perfectly sealed to the environment which means that there are often leaks that allow some air to enter the systems under vacuum distillation. The amount of oxygen present in a typical distillation system based on vinyl aromatic monomer may vary from about 0.05 parts per million (ppm) to about 1000 ppm. The most common distillation systems will have between about 0.1 ppm to about 150 ppm of oxygen present by weight of vinyl aromatic monomer. These distillation processes employ elevated temperatures which results in the premature polymerization of the vinyl aromatic monomers. This formation of polymer results in reduced process efficiency and loss of product. Occasionally, complete polymerization of the vinyl aromatic compound occurs in the distillation system causing considerable economic loss.

To minimize premature polymerization of vinyl aromatic monomers polymerization inhibitors are often employed. The vinyl aromatic compound to be distilled is generally contacted with the polymerization inhibitor before being subjected to distillation conditions. However, polymer formation in these distillation systems still remains a significant problem. Industry continues to search for better and more cost-effective solutions to this problem.

Unexpectedly, the combination of a stable nitroxide with at least one phenol, aromatic amine, hindered amine, phosphite, or with mixtures thereof, is found to be synergistic towards preventing premature polymerization in vinyl aromatic monomer industrial plant streams where alkyl-substituted aromatic compounds are present. A variety of inhibitors has been used to prevent premature polymerization of vinyl aromatic monomers. Sulfur has been widely used in the past. More recently, however, other compounds have been developed to replace sulfur as a polymerization inhibitor.

Japanese Hei 63-316745 teaches the use of a combination of a nitrosophenol and a nitrophenol to prevent polymerization of aromatic vinyl compounds.

European patent application 240,297 A1 teaches the use of a substituted hydroxylamine and a dinitrophenol to inhibit the polymerization of a vinyl aromatic compound at elevated temperatures in a distillation process.

U.S. Pat. No. 3,390,198 teaches the use of mono- and di-alkyl catechols as polymerization inhibitors for styrene.

U.S. Pat. No. 3,733,326 discloses the polymerization inhibition of vinyl monomers with hindered amines and hindered hydroxylamines.

U.S. Pat. Nos. 3,988,212 and 4,341 ,600 disclose the use of N-nitrosodiphenylamine combined with dinitro-cresol derivatives for inhibiting the polymerization of vinyl aromatic compounds under vacuum distillation conditions.

U.S. Pat. Nos. 4,086,147, 4,105,506, 4,252,615, 4,272,344 and 4,469,558 disclose the use of 2,6-dinitro-p-cresol as a polymerization inhibitor for vinyl aromatic monomers.

U.S. Pat. No. 4,132,602 discloses the use of a halo-6-nitro-p-cresol and U.S. Pat. No. 4,132,603 discloses the use of a 4-halo-3,5-dinitrotoluene as polymerization inhibitors during distillation of vinyl aromatic compounds.

U.S. Pat. No. 4,376,678 teaches the use of dinitrophenols for preventing polymerization of vinyl aromatic monomers during distillation. U.S. Pat. No. 4,434,307 disclose the stabilization of vinyl aromatic compounds with an effective blend of N,N-diarylhydroxylamines with mono- or ditertiary alkylcatechols and/or mono- or ditertiary alkylhydroquinones.

U.S. Pat. No. 4,466,904 teaches the use of phenothiazine, 4-tert-butylcatechol and 2,6- dinitro-p-cresol as a polymerization inhibitor system for vinyl aromatic compounds.

U.S. Pat. No. 4,468,343 discloses a composition and a process for utilizing 2,6-dinitro-p- cresol and either a phenylenediamine or 4-tert-butylcatechol in the presence of oxygen to prevent the polymerization of vinyl aromatic compounds during heating.

U.S. Pat. No. 4,654,451 teaches that a mixture of alkyl-substituted p-nitrosophenols and p- nitrosophenol is useful for stabilizing vinyl aromatic compounds against undesired polymerization.

U.S. Pat. No. 4,967,027 teaches a method of inhibiting polymerization of styrene under distillation conditions with the use of a combination of nitrosophenols and p-tert- butylcatechol.

The use of nitroxides towards preventing unwanted polymerization in vinyl aromatic monomers is also reported. Soviet Patent Nos. 1 ,027,150, 1 ,139,722 and 1 ,558,888 disclose the stabilization of styrene by using nitroxide radicals.

Japanese Hei 1 -165534 discloses the use of nitroxide derivatives as polymerization inhibitors for styrene.

U.S. Pat. No. 3,163,677 is aimed at hydroxylamines and nitroxides and processes for their preparation. An example is given where styrene is stabilized against polymerization with a nitroxide.

U.S. Pat. Nos. 5,545,782 and 5,545,786 disclose that nitroxides in the presence of oxygen are effective for the stabilization of vinyl aromatic compounds. The use of costabilizers with nitroxides for preventing premature polymerization of vinyl aromatic monomers is also reported.

Q. Wang, et al., in The Journal of Lanzhou University, 23(3), 1987, pp. 138-140, reports the effective inhibiting effect of di-tert-butylnitroxide with hydroquinone, tert-butylcatechol, benzoquinone derivatives or 4-hydroxy-1 -oxyl^^.e.δ-tetramethylpiperidine in styrene.

U.S. Pat. No. 5,254,760 teaches that the use of blends of nitroxides with nitroaromatic retarders outperforms the individual components in preventing premature polymerization of vinyl aromatic monomers.

U.S. Pat. No. 5,616,774 discloses the synergistic mixture of nitroxide inhibitors with quinone methide retarders in preventing premature polymerization of vinyl aromatic monomers.

WO 98/02400 discloses the use of a mixture of an aromatic amine and an organic acid together with a nitroxide as an inhibitor for aromatic and aliphatic vinyl compounds. Some of the aromatic amines of the examples also contain a phenolic portion of the molecule. Synergism is shown for such stabilizer mixtures in styrene.

DE 19510184 A1 and related WO 96/29311 discloses the stabilization of styrene with 4- acylaminopiperidinyl nitroxides. Costabilizers may be used which include aromatic nitro and nitroso compounds or substituted phenols. No synergy is disclosed.

The use of costabilizers with nitroxides is reported in other non-aromatic, monomers as well.

An unexamined application, Japanese Hei 5-320217, reports that blends of nitroxides and one or more heteroatom substituted aromatic compounds were effective in (meth)acrylic acid stabilization.

U.S. Patent No. 5,71 1 ,767 discloses the use of nitroxides to prevent oxidative degradation and gum or deposit formation in gasoline. A costabilizer may be employed which is an aromatic amine or a phenolic antioxidant or a mixture of an aromatic amine and phenolic antioxidant. Synergism for these systems is shown. Copending application Serial No. 09/200,368 discloses the joint use of nitroxides and phenylenediamines towards preventing unwanted polymerization in reactive light olefins. DE 19609312 A1 and related WO 97/32833 disclose the use of nitroxides as inhibitors for monomers in which the vinyl group is attached to a heteroatom. The compositions may additionally contain one or more costabilizers of the group of phenothiazines, quinones, hydroquinones and their ethers, hydroxylamines or phenylenediamines. No synergy is disclosed however.

The issue of nitroxide activity towards preventing premature polymerization of vinyl aromatic monomers in the presence of high concentrations of alkyl-substituted aromatic compounds has not been addressed by the prior art. That is to say the use of nitroxides towards preventing premature polymerization in the intermediate columns of a typical distillation system used for purifying vinyl aromatic monomers has not been addressed or suggested.

In each of the prior art references involving vinyl aromatic monomer stabilization, no mention is made of the effect of the alkyl-substituted aromatic intermediates present in the process stream. It is shown here that the activity of nitroxides towards preventing premature polymerization of vinyl aromatic monomers is hampered in the presence of high concentrations of alkyl-substituted aromatic compounds. It is also shown here that the combination of a stable nitroxide compound with at least one phenol, aromatic amine, hindered amine, phosphite, or with mixtures thereof is synergistic towards preventing premature polymerization of vinyl aromatic monomers in the presence of alkyl-substituted aromatic compounds. This is quite unknown in the prior art. The superior performance of these particular stabilizer combinations towards preventing polymerization of vinyl aromatic monomers in industrial distillation processes under these conditions is not disclosed or suggested in the prior art.

One object of this invention is to provide a novel composition stabilized against premature polymerization during distillation or purification comprising a mixture of a vinyl aromatic compound and an alkyl-substituted or hydroxyalkyl-substituted aromatic compound and an effective inhibiting amount of a combination of a stable nitroxide compound with at least one phenol, aromatic amine, hindered amine or phosphite, or with mixtures thereof. Another object of this invention is to provide a novel method for inhibiting the premature polymerization of a vinyl aromatic compound during distillation or purification in the presence of an alkyl-substituted or hydroxyalkyl-substituted aromatic compound which comprises incorporating therein an effective inhibiting amount of a combination of a stable nitroxide compound with at least one phenol, aromatic amine, hindered amine or phosphite, or with mixtures thereof.

The instant invention pertains to the combination of a stable nitroxide with at least one phenol, aromatic amine, hindered amine or phosphite, or with mixtures thereof to inhibit premature polymerization during distillation or purification of vinyl aromatic compounds in the presence of alkyl-substituted or hydroxyalkyl-substituted aromatic compounds. These combinations are synergistic for this purpose.

Specifically, the novel compositions of this invention, stabilized against premature polymerization during distillation or purification, comprise

(a) a vinyl aromatic compound, and

(b) an alkyl-substituted or hydroxyalkyl-substituted aromatic compound, and

(c) an effective stabilizing amount of a mixture of i.) at least one stable nitroxide, and ii.) at least one phenol, aromatic amine, hindered amine or phosphite, or mixture thereof.

The vinyl aromatic compounds of component (a) have at least one carbon-carbon double bond capable of undergoing free radical induced polymerization. Typical examples of such monomers are styrene, oc-methylstyrene, vinyltoluene, divinylbenzene, styrenesulfonic acid and structural isomers, derivatives of said compounds, mixtures of said compounds, and the like.

Preferably the vinyl aromatic monomer is styrene, α-methylstyrene, vinyltoluene, or divinylbenzene; most preferably styrene.

The alkyl-substituted aromatic compounds of component (b) have at least one alkyl group attached to an aromatic ring. Typical examples of such compounds are ethylbenzene, isopropylbenzene, ethyltoluene, ethylstyrene, ethylbenzenesulfonic acid and structural isomers, derivatives of said compounds, mixtures of said compounds, and the like. An example of an hydroxyalkyl-substituted aromatic compound of component (b) is sec- phenethyl alcohol.

The compositions and methods of the present invention are effective where the weight ratio of the vinyl aromatic compound (a) to the alkyl-substituted aromatic compound (b) varies from about 95:5 to about 5:95. They are particularly effective where the weight ratio of the vinyl aromatic compound (a) to the alkyl-substituted or hydroxyalkyl-substituted aromatic compound (b) varies from about 2:1 to about 1 :9.

Preferably the alkyl-substituted aromatic compound is ethylbenzene, isopropylbenzene, ethyltoluene, or ethyl styrene; most preferably ethylbenzene. Preferably the hydroxyalkyl- substituted aromatic compound is sec-phenethyl alcohol.

The stable nitroxide compounds of component i.) useful in this invention have the generic structure

where each R is alkyl and T is a group required to complete a 5- or 6-membered ring.

Two or more nitroxide groups may be present in the same molecule by being linked through the T moiety as exemplified below where E is a linking group.

Typical nitroxides of component i.) include bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate, 4-hydroxy-1 -oxyl-2,2,6,6-tetramethylpiperidine, 4-ethoxy-1 -oxyl-2,2,6,6- tetramethylpiperidine, 4-propoxy-1 -oxyl-2,2,6,6-tetramethylpiperidine, 4-acetamido-1 -oxyl- 2,2,6,6-tetramethylpiperidine, 1 -oxyl-2,2,6,6-tetramethylpiperidine, 1 -oxyl-2,2,6,6- tetramethylpiperidin-4-one, 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl acetate, 1 -oxyl-2,2,6,6- tetramethylpiperidin-4-yl 2-ethylhexanoate, 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl stearate, 1 -oxyl-2,2,6,6-tetramethylpiperidin-4-yl benzoate, 1 -oxyl-2,2,6,6-tetramethylpiperidin-4-yl 4-t- butyl-benzoate, bis(1-oxyI-2,2,6,6-tetramethylpiperidin-4-yl) succinate, bis(1-oxyl-2,2,6,6- tetramethylpiperidin-4-yl) adipate, bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) n- butylmalonate, bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) phthalate, bis(1-oxyl-2,2,6,6- tetramethyipiperidin-4-yl) isophthalate, bis(1 -oxyl-2,2,6,6-tetramethylpiperidin-4-yl) terephthalate, bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) hexahydroterephthalate, N,N'- bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipamide, N-(1-oxyl-2,2,6,6-tetramethylpiperidin- 4-yl)caprolactam, N-(1 -oxyl^^.e^-tetramethylpiperidin^-y dodecylsuccinimide, 2,4,6-tris- [N-butyl-N-(1 -oxyl-2,2,6,6-tetramethylpiperidin-4-yl)]-s-triazine, 4,4'-ethylenebis(1 -oxyl- 2,2,6,6-tetramethylpiperazin-3-one), 2-oxyl-1 ,1 ,3,3-tetramethyl-2-isobenzazole, 1-oxyl- 2,2,5,5-tetramethylpyrrolidine, and N,N-bis-(1 ,1 ,3,3-tetramethylbutyl)nitroxide.

Preferred nitroxides of component i.) are bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate, 4-hydroxy-1 -oxyl^^.e.δ-tetramethylpiperidine, 4-ethoxy-1 -oxyl-2,2,6,6- tetramethylpiperidine, 4-propoxy-1 -oxyl-2,2,6,6-tetramethylpiperidine, 4-acetamido-1 -oxyl- 2,2,6,6-tetramethylpiperidine, 1-oxyl-2,2,6,6-tetramethylpiperidine, and 1 -oxyl-2,2,6,6- tetramethylpiperidin-4-one.

Most preferred nitroxides of component i.) are bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate and 4-hydroxy-1 -oxyl-2,2,6,6-tetramethylpiperidine. The phenols of component ii.) include

alkylated monophenols, for example 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butylphenol, 2-tert-butyl-4,6-dimethylphenol, 2,6-di-tert-butyl-4-ethylphenol, 2,6-di-tert-butyl-4-n- butylphenol, 2,6-di-tert-butyl-4-isobutylphenol, 2,6-dicyclopentyl-4-methylphenol, 2-( a- methylcyclohexyl)-4,6-dimethylphenol, 2,6-dioctadecyl-4-methylphenol, 2,4,6- tricyclohexylphenol, 2,6-di-tert-butyl-4-methoxymethylphenol, nonylphenols which are linear or branched in the side chains, for example, 2,6-di-nonyl-4-methylphenol, 2,4-dimethyl-6-(1- methylundec-1 -yI)phenol, 2,4-dimethyl-6-(1 -methylheptadec-1 -yl)phenol, 2,4-dimethyl-6-(1 - methyltridec-1-yl)phenol and mixtures thereof,

alkylthiomethylphenols, for example 2,4-dioctylthiomethyl-6-tert-butylphenol, 2,4- dioctylthiomethyl-6-methylphenol, 2,4-dioctylthiomethyl-6-ethylphenol, 2,6-di- dodecylthiomethyl-4-nonylphenol,

hindered hydroquinones, for example 2,6-di-tert-butyl-4-methoxyphenol, 2,5-di-tert- butylhydroquinone, 2,5-di-tert-amylhydroquinone, 2,6-diphenyl-4-octadecyloxyphenol, 2,6-di- tert-butylhydroquinone, 2,5-di-tert-butyl-4-hydroxyanisole, 3,5-di-tert-butyl-4-hydroxyanisole, 3,5-di-tert-butyl-4-hydroxyphenyl stearate, bis-(3,5-di-tert-butyl-4-hydroxyphenyl) adipate,

tocopherols, for example a-tocopherol, b-tocopherol, g-tocopherol, d-tocopherol and mixtures thereof (vitamin E),

hydroxylated thiodiphenyl ethers, for example 2,2'-thiobis(6-tert-butyl-4-methylphenol), 2,2'- thiobis(4-octylphenol), 4,4'-thiobis(6-tert-butyl-3-methylphenol), 4,4'-thiobis(6-tert-butyl-2- methylphenol), 4,4'-thiobis-(3,6-di-sec-amylphenol), 4,4'-bis(2,6-dimethyl-4- hydroxyphenyl)disulfide,

alkylidenebisphenols, for example 2,2'-methylenebis(6-tert-butyI-4-methylphenol), 2,2'- methylenebis(6-tert-butyl-4-ethylphenol), 2,2'-methylenebis[4-methyl-6-( a-methylcyclohexyl)- phenol], 2,2'-methylenebis(4-methyl-6-cyclohexylphenol), 2,2'-methylenebis(6-nonyl-4- methylphenol), 2,2'-methylenebis(4,6-di-tert-butylphenol), 2,2'-ethylidenebis(4,6-di-tert- butylphenol), 2,2'-ethylidenebis(6-tert-butyl-4-isobutylphenol), 2,2'-methylenebis[6-( a- methylbenzyl)-4-nonylphenol], 2,2'-methylenebis[6-( a,a-dimethylbenzyl)-4-nonylphenol], 4,4'-methylenebis(2,6-di-tert-butylphenol), 4,4'-methylenebis(6-tert-butyl-2-methylphenol), 1 ,1 -bis(5-tert-butyl-4-hydroxy-2-methylphenyl)butane, 2,6-bis(3-tert-butyl-5-methyl-2- hydroxybenzyl)-4-methylphenol, 1 J ,3-tris(5-tert-butyl-4-hydroxy-2-methylphenyl)butane, 1 ,1- bis(5-tert-butyl-4-hydroxy-2-methyl-phenyI)-3-n-dodecylmercaptobutane, ethylene glycol bis[3,3-bis(3'-tert-butyl-4'-hydroxyphenyl)butyrate], bis(3-tert-butyl-4-hydroxy-5-methyl- phenyl)dicyclopentadiene, bis[2-(3'-tert-butyl-2'-hydroxy-5'-methylbenzyl)-6-tert-butyl-4- methylphenyl] terephthalate, 1 J-bis-(3,5-dimethyl-2-hydroxyphenyl)butane, 2,2-bis-(3,5-di- tert-butyl-4-hydroxyphenyl)propane, 2,2-bis-(5-tert-butyl-4-hydroxy-2-methylphenyl)-4-n- dodecylmercaptobutane, 1 ,1 ,5,5-tetra-(5-tert-butyl-4-hydroxy-2-methylphenyl)pentane,

O-, N- and S-benzyl compounds, for example 3,5,3',5'-tetra-tert-butyl-4,4'-dihydroxydi-benzyl ether, octadecyl-4-hydroxy-3,5-dimethylbenzylmercaptoacetate, tridecyl-4-hydroxy-3,5-di- tert-butylbenzylmercaptoacetateJris(3,5-di-tert-butyl-4-hydroxybenzyl)amine, bis(4-tert-butyl- 3-hydroxy-2,6-dimethyIbenzyl)dithioterephthalate, bis(3,5-di-tert-butyl-4-hydroxy - benzyl)sulfide, isooctyl 3,5-di-tert-butyl-4-hydroxybenzylmercaptoacetate,

hydroxybenzylated malonates, for example dioctadecyl 2,2-bis-(3,5-di-tert-butyl-4-hy- droxybenzyl)-malonate, di-octadecyl 2-(3-tert-butyl-4-hydroxy-5-methylbenzyl)-malonate, di- dodecylmercaptoethyl-2,2-bis-(3,5-di-tert-butyl-4-hydroxybenzyl)malonate, bis[4-(1 ,1 ,3,3- tetramethylbutyl)phenyl] 2,2-bis(3,5-di-tert-butyl-4-hydroxybenzyl)malonate,

aromatic hydroxybenzyl compounds, for example 1 ,3,5-tris-(3,5-di-tert-butyl-4-hydroxy- benzyl)-2,4,6-trimethylbenzene, 1 ,4-bis(3,5-di-tert-butyl-4-hydroxybenzyl)-2,3,5,6-tetrame- thylbenzene, 2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)phenol_I

triazine compounds, for example 2,4-bis(octylmercapto)-6-(3,5-di-tert-butyl-4-hydroxy- anilino)-1 ,3,5-triazine, 2-octylmercapto-4,6-bis(3,5-di-tert-butyl-4-hydroxyanilino)-1 ,3,5- triazine, 2-octylmercapto-4,6-bis(3,5-di-tert-butyl-4-hydroxyphenoxy)-1 ,3,5-triazine, 2,4,6-tris- (3,5-di-tert-butyl-4-hydroxyphenoxy)-1 ,2,3-triazine, 1 ,3,5-tris-(3,5-di-tert-butyl-4-hydroxyben- zyl)isocyanurate, 1 ,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)isocyanurate, 2,4,6-tris- (3,5-di-tert-butyl-4-hydroxyphenylethyl)-1 ,3,5-triazine, 1 ,3,5-tris(3,5-di-tert-butyl-4-hydroxy- phenylpropionyl)-hexahydro-1 ,3,5-triazine, 1 ,3,5-tris(3,5-dicyclohexyl-4- hydroxybenzyl)isocyanurate, benzylphosphonates, for example dimethyl 2,5-di-tert-butyl-4-hydroxybenzylphosphonate, diethyl-3,5-di-tert-butyl-4-hydroxybenzylphosphonate, dioctadecyl 3,5-di-tert-butyl-4-hy- droxybenzylphosphonate, dioctadecyl 5-tert-butyl-4-hydroxy-3-methylbenzylphosphonate, the calcium salt of the monoethyl ester of 3,5-di-tert-butyl-4-hydroxybenzylphosphonic acid,

acylaminophenols, for example 4-hydroxylauranilide, 4-hydroxystearanilide, octyl N-(3,5-di- tert-butyl-4-hydroxyphenyl)carbamate,

esters of b-(3,5-di-tert-butyl-4-hydroxyphenyl) propionic acid with mono- or polyhydric alcohols, e.g. with methanol, ethanol, n-octanol, isooctanol (a mixture of octanols), octadecanol, 1 ,6-hexanediol, 1 ,9-nonanediol, ethylene glycol, 1 ,2-propanediol, neopentyl glycol, thiodiethylene glycol, diethylene glycol, triethylene glycol, pentaerythritol, tris(hydroxyethyl) isocyanurate, N,N'-bis(hydroxyethyl)oxamide, 3-thiaundecanol, 3- thiapentadecanol, trimethylhexanediol, trimethylolpropane, 4-hydroxymethyl-1 -phospha- 2,6,7-trioxabicyclo[2.2.2]octane,

esters of b-(5-tert-butyl-4-hydroxy-3-methylphenyl) propionic acid with mono- or polyhydric alcohols, e.g. with methanol, ethanol, n-octanol, isooctanol (a mixture of octanols), octadecanol, 1 ,6-hexanediol, 1 ,9-nonanediol, ethylene glycol, 1 ,2-propanediol, neopentyl glycol, thiodiethylene glycol, diethylene glycol, triethylene glycol, pentaerythritol, tris(hydroxyethyl) isocyanurate, N,N'-bis(hydroxyethyl)oxamide, 3-thiaundecanol, 3- thiapentadecanol, trimethylhexanediol, trimethylolpropane, 4-hydroxymethyl-1-phospha- 2,6,7-trioxabicyclo[2.2.2]octane,

esters of b-(3,5-dicyclohexyl-4-hydroxyphenyl) propionic acid with mono- or polyhydric alcohols, e.g. with methanol, ethanol, octanol, isooctanol (a mixture of octanols), octadecanol, 1 ,6-hexanediol, 1 ,9-nonanediol, ethylene glycol, 1 ,2-propanediol, neopentyl glycol, thiodiethylene glycol, diethylene glycol, triethylene glycol, pentaerythritol, tris(hydroxyethyl)isocyanurate, N,N'-bis(hydroxyethyl)ox-amide, 3-thiaundecanol, 3- thiapentadecanol, trimethylhexanediol, trimethylolpropane, 4-hydroxymethyl-1 -phospha- 2,6,7-trioxabicyclo[2.2.2]octane, esters of 3,5-di-tert-butyl-4-hydroxyphenyl acetic acid with mono- or polyhydric alcohols, e.g. with methanol, ethanol, octanol, isooctanol (a mixture of octanols), octadecanol, 1 ,6- hexanediol, 1 ,9-nonanediol, ethylene glycol, 1 ,2-propanediol, neopentyl glycol, thiodiethylene glycol, diethylene glycol, triethylene glycol, pentaerythritol, tris(hydroxyethyl)isocyanurate, N,N'-bis(hydroxyethyl)oxamide, 3-thiaundecanol, 3-thiapentadecanol, trimethylhexanediol, trimethylolpropane, 4-hydroxymethyl-1 -phospha-2,6,7-trioxabicyclo[2.2.2]octane, and

amides of b-(3,5-di-tert-butyl-4-hydroxyphenyl) propionic acid e.g. N,N'-bis(3,5-di-tert-butyl-4- hydroxyphenylpropionyl)hexamethylenediamide, N,N'-bis(3,5-di-tert-butyl-4- hydroxyphenylpropionyI)trimethylenediamide, N,N'-bis(3,5-di-tert-butyl-4- hydroxyphenylpropionyI)hydrazide, N,N'-bis[2-(3-[3,5-di-tert-butyl-4- hydroxyphenyl]propionyloxy)ethyl]oxamide (Naugard^® XL-1 supplied by Uniroyal).

The aromatic amines of component ii.) include for example 4-(p-toluene- sulfamoyl)diphenylamine, diphenylamine, N-allyldiphenylamine, 4-isopropoxydiphenylamine, N-phenyl-1 -naphthylamine, N-(4-tert-octylphenyl)-1 -naphthylamine, N-phenyl-2- naphthylamine, octylated diphenylamine, for example 4,4'-di-tert-octyldiphenylamine, 4-n- butylaminophenol, 4-butyrylaminophenol, 4-nonanoylaminophenol, 4- dodecanoylaminophenol, 4-octadecanoylaminophenol, bis(4-methoxyphenyl)amine, 2,6-di- tert-butyl-4-dimethylaminomethylphenol, 2,4'-diaminodiphenylmethane, 4,4'- diaminodiphenylmethane, N,N,^N'-tetramethyl-4,4'-diaminodiphenylmethane, 1 ,2-bis[(2- methylphenyl)amino]ethane, 1 ,2-bis(phenylamino)propane, (o-tolyl)biguanide, bis[4-(1 ',3'- dimethylbutyl)phenyl]amine, tert-octylated N-phenyl-1 -naphthylamine, a mixture of mono- and dialkylated tert-butyl/tert-octyl-diphenylamines, a mixture of mono- and dialkylated nonyldiphenylamines, a mixture of mono- and dialkylated dodecyldiphenylamines, a mixture of mono- and dialkylated isopropyl isohexyldiphenylamines, a mixture of mono- and dialkylated tert-butyldiphenylamines, 2,3-dihydro-3,3-dimethyl-4H-1 ,4-benzothiazine, phenothiazine, a mixture of mono- and dialkylated tert-butyl/tert-octylphenothiazines, a mixture of mono- and dialkylated tert-octyl-phenothiazines, N-allylphenothiazin, and N,N,N',N'-tetraphenyl-1 ,4-diaminobut-2-ene.

The hindered amines of component ii.) include for example 2,2,6,6-tetramethylpiperidin-4-ol, 2,2,6,6-tetramethylpiperidin-4-one, 4-acetamido-2,2,6,6-tetramethylpiperidine, bis(2,2,6,6- tetramethyl-4-piperidyl) sebacate, bis(2,2,6,6-tetramethyl-4-piperidyl) succinate, bis(1 ,2,2,6,6-pentamethyl-4-piperidyl) sebacate, bis(1-octyloxy-2,2,6,6-tetramethyl-4- piperidyl) sebacate, bis(1 ,2,2,6,6-pentamethyl-4-piperidyl) n-butyl-3,5-di-tert-butyl-4- hydroxybenzylmalonate, the condensate of 1-(2-hydroxyethyl)-2,2,6,6-tetramethyl-4- hydroxypiperidine and succinic acid, linear or cyclic condensates of N,N'-bis(2,2,6,6- tetramethyl-4-piperidyl)hexamethylenediamine and 4-tert-octylamino-2,6-dichloro-1 ,3,5- triazine, tris(2,2,6,6-tetramethyl-4-piperidyl)nitrilotriacetate, tetrakis(2,2,6,6-tetramethyl-4- piperidyl)-1 ,2,3,4-butane-tetracarboxylate, 1 ,1'-(1 ,2-ethanediyl)-bis(3,3,5,5- tetramethylpiperazinone), 4-benzoyl-2,2,6,6-tetramethylpiperidine, 4-stearyloxy-2,2,6,6-te- tramethylpiperidine, bis(1 ,2,2,6,6-pentamethylpiperidyl)-2-n-butyl-2-(2-hydroxy-3,5-di-tert- butylbenzyl)malonate, 3-n-octyl-7,7,9,9-tetramethyl-1 ,3,8-triazaspiro[4.5]decan-2,4-dione, bis(1-octyloxy-2,2,6,6-tetramethylpiperidyl) succinate, linear or cyclic condensates of N,N'- bis-(2,2,6,6-tetramethyl-4-piperidyl)hexamethylenediamine and 4-morpholino-2,6-dichloro- 1 ,3,5-triazine, the condensate of 2-chloro-4,6-bis(4-n-butylamino-2,2,6,6-tetramethylpiperidyl )-1 ,3,5-triazine and 1 ,2-bis(3-aminopropylamino)ethane, the condensate of 2-chloro-4,6-di- (4-n-butylamino-1 ,2,2,6,6-pentamethylpiperidyl)-1 ,3,5-triazine and 1 ,2-bis-(3- aminopropylamino)ethane, 8-acetyl-3-dodecyI-7,7,9,9-tetramethyl-1 ,3,8- triazaspiro[4.5]decane-2,4-dione, 3-dodecyl-1-(2,2,6,6-tetramethyl-4-piperidyl)pyrrolidin-2,5- dione, 3-dodecyl-1-(1 ,2,2,6,6-pentamethyl-4-piperidyl)pyrrolidine-2,5-dione, a mixture of 4- hexadecyloxy- and 4-stearyloxy-2,2,6,6-tetramethylpiperidine, a condensation product of N,N'-bis(2,2,6,6-tetramethyl-4-piperidyl)hexamethylenediamine and 4-cyclohexylamino-2,6- dichloro-1 ,3,5-triazine, a condensation product of 1 ,2-bis(3-aminopropylamino)ethane and 2,4,6-trichloro-1 ,3,5-triazine as well as 4-butylamino-2,2,6,6-tetramethylpiperidine (CAS Reg. No. [136504-96-6]); N-(2,2,6,6-tetramethyl-4-piperidyl)-n-dodecylsuccinimid, N-(1 , 2,2,6,6- pentamethyl-4-piperidyl)-n-dodecylsuccinimid, 2-undecyl-7,7,9,9-tetramethyl-1-oxa-3,8- diaza-4-oxo-spiro[4,5]decane, a reaction product of 7,7,9,9-tetramethyl-2-cycloundecyl-1- oxa-3,8-diaza-4-oxospiro [4,5]decane and epichlorohydrin, 1 ,1-bis(1 ,2,2,6, 6-pentamethyl-4- piperidyloxycarbonyl)-2-(4-methoxyphenyl)ethene, N,N'-bis-formyl-N,N'-bis(2,2,6,6- tetramethyl-4-piperidyl)hexamethylenediamine, diester of 4-methoxy-methylene-malonic acid with 1 ,2,2,6,6-pentamethyl-4-hydroxypiperidine, poly[methylpropyl-3-oxy-4-(2,2,6,6- tetramethyl-4-piperidyl)]siloxane, and the reaction product of maleic acid anhydride-a-olefin- copolymer with 2,2,6,6-tetramethyl-4-aminopiperidine or 1 ,2,2,6,6-pentamethyl-4- aminopiperidine. The phosphites of component ii.) include for example triphenyl phosphite, diphenyl alkyl phosphites, phenyl dialkyl phosphites, tris(nonylphenyl) phosphite, trilauryl phosphite, trioctadecyl phosphite, distearyl pentaerythritol diphosphite, tris(2,4-di-tert-butylphenyl) phosphite, diisodecyl pentaerythritol diphosphite, bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite, bis(2,6-di-tert-butyl-4-methylphenyl)-pentaerythritol diphosphite, diisodecyloxypentaerythritol diphosphite, bis(2,4-di-tert-butyl-6-methylphenyl)pentaerythritol diphosphite, bis(2,4,6-tris(tert-butylphenyl)pentaerythritol diphosphite, tristearyl sorbitol triphosphite, tetrakis(2,4-di-tert-butylphenyl) 4,4'-biphenylene diphosphonite, 6-isooctyloxy- 2,4,8,10-tetra-tert-butyl-dibenz[d,f]-[1 ,3,2]-dioxaphosphepin, 6-fluoro-2,4,8,10-tetra-tert-butyl- 12-methyl-12H-dibenz[d,g]-1 ,3,2-dioxaphosphocin, bis(2,4-di-tert-butyl-6-methylphenyl) methyl phosphite, bis(2,4-di-tert-butyl-6-methylphenyl) ethyl phosphite, 2,2',2"- nitrilo[triethyltris(3,3',5,5'-tetra-tert-butyl-1 ,1 '-biphenyl-2,2'-diyl)phosphite], and 2- ethylhexyl(3,3',5,5'-tetra-tert-butyl-1 ,r-biphenyl-2,2'-diyl^hosphite.

Preferred is a composition in which component ii.) is at least one phenol, aromatic amine, hindered amine or phosphite selected from the group consisting of 2,6-di-tert-butyl-4- methylphenol (BHT), 2,6-di-tert-butylphenol, isooctyl 3,5-di-tert-butyl-4- hydroxyhydrocinnamate (IRGANOX^® L135), vitamin E, a mixture of mono- and dialkylated tert-butyl/tert-octyl-diphenylamines (IRGANOX^® L57), a mixture of mono- and dialkylated nonyldiphenylamines (IRGANOX^® L67), 2,2,6,6-tetramethylpiperidin-4-ol, 2,2,6,6- tetramethylpiperidin-4-one, bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate, bis(1 ,2,2,6,6- pentamethyl-4-piperidyl) sebacate, bis(1-octyloxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate, tris-(2,4-di-tert-butylphenyl)phosphite (IRGAFOS^® 168), bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite, tris(nonylphenyl) phosphite, and bis(2,4-di-tert-butyl-6- methylphenyl) ethyl phosphite.

More preferred is a composition in which component ii.) is at least one phenol, aromatic amine, hindered amine or phosphite selected from the group consisting of isooctyl 3,5-di- tert-butyl-4-hydroxyhydrocinnamate (IRGANOX^® L135), vitamin E, a mixture of mono- and dialkylated tert-butyl/tert-octyl-diphenylamines (IRGANOX^® L57), 2,2,6,6- tetramethylpiperidin-4-ol, and tris-(2,4-di-tert-butylphenyl)phosphite (IRGAFOS^® 168).

The effective stabilizing amounts of components i.) and ii.) are each about 0.1 to about 10,000 ppm by weight based on the weight of the monomer of component (a). Preferably, the amounts of components i.) and ii.) are each about 1 to about 2000 ppm by weight based on the monomer of component (a). Most preferably, the amount of components i.) and ii.) are each about 1 to about 1000 ppm by weight based on the monomer of component (a). The relative amounts of components i.) to ii.) employed are generally in the range of about 95:5 to about 5:95 by weight. In preferred embodiments, the ratios of i.) to ii.) generally fall in the range of about 75:25 to about 25:75 by weight.

In another preferred embodiment oxygen is present in the range of from about 0.05 parts per million to about 1000 parts per million of component (a), particularly preferred in the range of from about 0J parts per million to about 150 parts per million of component (a).

The polymerization inhibitors can be introduced into the monomer to be protected by any conventional method. It may be added just upstream of the point of desired application by any suitable means. In addition, this mixture may be injected separately into the distillation train along with the incoming feed of monomer or through separate entry points providing efficient distribution of the activated inhibitor mixture. Each component of the synergistic blend may be added separately into the monomer stream. Since the inhibitors are gradually depleted during operation, it is generally necessary to maintain the appropriate amount of the inhibitors in the distillation system by adding additional inhibitor during the course of the distillation process. Such additions may be carried out either on a continuous basis or by intermittently charging fresh stabilizer into the distillation system if the concentration of the inhibitor is to be maintained above the minimum required level.

A further subject of the invention is a method for preventing the premature polymerization of vinyl aromatic compounds during distillation or purification comprises

adding to a vinyl aromatic compound in the presence of an alkyl-substituted or hydroxyalkyl- substituted aromatic compound an effective polymerization inhibiting amount of a mixture of

i.) at least one stable nitroxide compound, and

ii.) at least one phenol, aromatic amine, hindered amine or phosphite, or mixtures thereof.

Still a further subject of the invention is the use of a mixture of i.) at least one stable nitroxide, and ii.) at least one phenol, aromatic amine, hindered amine or phosphite, or mixture thereof for stabilizing a mixture of

(a) a vinyl aromatic compound, and

(b) an alkyl-substituted or hydroxyalkyl-substituted aromatic compound against premature polymerization during distillation or purification.

The polymerization inhibiting compositions and methods of this invention are well suited for protecting the reboiler sections of a distillation column.

Other inhibitors or retarders may also be present in the compositions and methods of this invention, such as nitrophenols, nitrosophenols, phenylenediamines, hydroquinone and its ethers, or catechols.

The definitions and preferences given above for the composition also apply for the method.

The following examples are meant for illustrative purposes only.

Example 1 Inhibition of Styrene Monomer Containing Ethylbenzene Commercial grade styrene is freed of t-butyl catechol storage stabilizer by distillation under reduced pressure. Commercial grade ethylbenzene is freed of any existing hydroperoxide by slow filtration through alumina under a nitrogen purge. A 300 mL three-necked flask equipped with condenser, rubber septum, and magnetic stirrer bar is charged with 55 mL of purified styrene containing the appropriate stabilizer(s). To this is added 55 mL of purified ethylbenzene (or chlorobenzene as a control where indicated). An oxygen-free atmosphere is established by five consecutive evacuations and backfilling with nitrogen while stirring the solution. While the vessel is kept under a slight pressure of nitrogen, 2 mL of air is injected into the solution to simulate trace levels of air present in industrial plant streams. The vessel is then immersed into a thermostatically controlled oil bath at 120°C. Samples are withdrawn at 15 minute intervals and the amount of polystyrene formed is then determined by refractive index measurements. Induction periods are determined by fitting a line to the failure portion of the generated graph, and extrapolating back to 0 % polymer. The amounts of inhibitors used are reported in parts per million (ppm) by weight based on styrene monomer. The inhibitors are:

Bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate (PROSTAB™ 5415)

Isooctyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate (IRGANOX^® L135)

A mixture of mono- and dialkylated tert-butyl/tert-octyl-diphenylamines (IRGANOX^® L57)

2,2,6,6-Tetramethylpiperidin-4-o! (Hindered amine)

Tris-(2,4-di-tert-butylphenyl) phosphite (IRGAFOS^® 168)

IRGAFOS^® and IRGANOX^® are registered trademarks and PROSTAB™ is a trademark of

Ciba Specialty Chemicals Corporation.

Table 1

Induction Period Test Solvent (minutes)

1 chlorobenzene 75

2 ethylbenzene 37

PROSTAB™ 5415 is present at 100 ppm in tests 1 and 2 based on styrene. Table 1 shows that nitroxide inhibitors are less effective at preventing premature thermal polymerization of vinyl aromatic monomers when in the presence of alkyl-substituted aromatic compounds.

Table 2

Test Costabilizer Induction Period (100 ppm) (minutes)

2 none 37

3 IRGANOX^® L57 60

4 Vitamin E 82

5 IRGANOX^® L135 82

6 Hindered amine 75

7 IRGAFOS^® 168 60

-.TM

PROSTAB 5415 is present at 100 ppm in tests 2-7 based on styrene. Table 2 shows that the use of a phenol, aromatic amine, hindered amine or a phosphite is effective towards improving the activity of nitroxides towards preventing premature polymerization in styrene in the presence of ethylbenzene at elevated temperatures.

Table 3

Test PROSTAB™ 5415 IRGANOX^® L57 IRGANOX^® L135 Induction Period (ppm) (ppm) (ppm) (minutes)

2 100 37

3 100 100 60

5 100 100 82

8 — 150 18

9 100 150 86

10 ~ 50 6

11 100 50 69 Table 3 shows that the use of a combination of the costabilizers of the present invention with a nitroxide is synergistic towards stabilizing styrene in the presence of an alkyl-substituted aromatic compound against premature thermal polymerization.

Example 2 Inhibition of Styrene Monomer Containing Ethylbenzene The styrene inhibition experiments are run as in Example 1 except that only 0.5 mL of air are injected into the solution instead of 2 mL. The combined use of PROSTAB™ 5415 with at least one phenol, aromatic amine, hindered amine or phosphite of the present invention provides a synergistic method for inhibiting premature polymerization of styrene under these conditions.

Example 3 Inhibition of Styrene Monomer Containing Ethylbenzene The styrene inhibition experiments are run as in Example 1 using 4-hydroxy-1-oxyl-2,2,6,6- tetramethylpiperidine and 1-oxyl-2,2,6,6-tetramethylpiperidin-4-one in place of bis(1 -oxyl- 2,2,6, 6-tetramethylpiperidin-4-yl) sebacate. The combined use of these nitroxides with at least one phenol, aromatic amine, hindered amine or phosphite of the present invention provides a synergistic method for inhibiting premature polymerization of styrene in the presence of ethylbenzene. The same effects are observed when only 0.5 mL of air are injected into the solution.

Example 4

Inhibition of Vinyl Aromatic Monomers Containing the Corresponding Alkyl-Substituted Aromatic Compound

The polymerization inhibition experiments are run as in Example 1 replacing the styrene/ethylbenzene combination with the combinations of a- methylstyrene/isopropylbenzene, vinyltoluene/ethyltoluene, divinylbenzene/ethylstyrene, and styrenesulfonic acid/ethylbenzenesulfonic acid. The combined use of a nitroxide with at least one phenol, aromatic amine, hindered amine or phosphite of the present invention provides a synergistic method for inhibiting premature polymerization of vinyl aromatic monomers in the presence of the corresponding alkyl-substituted aromatic compounds. The same effects are observed when only 0.5 mL of air are injected into the solution.

Claims

What is Claimed is:

1. A composition stabilized against premature polymerization during distillation or purification comprising

(a) a vinyl aromatic compound, and

(b) an alkyl-substituted or hydroxyalkyl-substituted aromatic compound, and

(c) an effective stabilizing amount of a mixture of i.) at least one stable nitroxide, and ii.) at least one phenol, aromatic amine, hindered amine or phosphite, or mixture thereof.

2. A composition according to claim 1 in which component (a) is a vinyl aromatic compound selected from styrene, α-methylstyrene, vinyltoluene, divinylbenzene, and styrenesulfonic acid.

3. A composition according to claim 1 in which component (a) is styrene.

4. A composition according to claim 1 in which component (b) is an alkyl-substituted aromatic compound selected from ethylbenzene, isopropylbenzene, ethyltoluene, ethylstyrene, and ethylbenzenesulfonic acid.

5. A composition according to claim 1 in which component (b) is ethylbenzene.

6. A composition according to claim 1 in which component (b) is sec-phenethyl alcohol.

7. A composition according to claim 1 in which component i.) is at least one nitroxide selected from the group consisting of bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate, 4-hydroxy-1 -oxyl-2,2,6,6-tetramethylpiperidine, 4-ethoxy-1 -oxyl-2,2,6,6-tetramethylpiperidine, 4-propoxy-1 -oxyl-2,2,6,6-tetramethylpiperidine, 4-acetamido-1 -oxyl-2,2,6,6- tetramethylpiperidine, 1 -oxyl-2,2,6,6-tetramethylpiperidine, 1 -oxyl-2,2,6,6- tetramethylpiperidin-4-one, 1 -oxyl-2,2,6,6-tetramethylpiperidin-4-yl acetate, 1 -oxyl-2,2,6,6- tetramethylpiperidin-4-yl 2-ethylhexanoate, 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl stearate,

1 -oxyl-2,2,6,6-tetramethylpiperidin-4-yl benzoate, 1 -oxyl-2,2,6,6-tetramethylpiperidin-4-yl 4-t- butyl-benzoate, bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) succinate, bis(1-oxyl-2,2,6,6- tetramethylpiperidin-4-yl) adipate, bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) n- butylmalonate, bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) phthalate, bis(1-oxyl-2,2,6,6- tetramethylpiperidin-4-yl) isophthalate, bis(1 -oxyl-2,2,6,6-tetramethylpiperidin-4-yl) terephthalate, bis(1 -oxyl-2,2,6,6-tetramethylpiperidin-4-yl) hexahydroterephthalate, N,N'- bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipamide, N-(1-oxyl-2,2,6,6-tetramethylpiperidin- 4-yl)caprolactam, N-(1 -oxyl-2,2,6,6-tetramethylpiperidin-4-yl)dodecylsuccinimide, 2,4,6-tris- [N-butyl-N-(1 -oxyl-2,2,6,6-tetramethylpiperidin-4-yl)]-s-triazine, 4,4'-ethylenebis(1 -oxyl- 2,2,6,6-tetramethylpiperazin-3-one), 2-oxyl-1 ,1 ,3,3-tetramethyl-2-isobenzazole, 1-oxyl- 2,2,5,5-tetramethylpyrrolidine, and N,N-bis-(1 ,1 ,3,3-tetramethylbutyl)nitroxide.

8. A composition according to claim 1 in which component i.) is at least one nitroxide selected from the group consisting of bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate, 4-hydroxy-1 -oxyl-2,2,6,6-tetramethyIpiperidine, 4-ethoxy-1 -oxyl-2,2,6,6-tetramethylpiperidine, 4-propoxy-1 -oxyl-2,2,6,6-tetramethylpiperidine, 4-acetamido-1 -oxyl-2,2,6,6- tetramethylpiperidine, 1-oxyl-2,2,6,6-tetramethylpiperidine, and 1-oxyl-2,2,6,6- tetramethylpiperidin-4-one.

9. A composition according to claim 1 in which component ii.) is at least one phenol, aromatic amine, hindered amine or phosphite selected from the group consisting of 2,6-di- tert-butyl-4-methylphenol (BHT), 2,6-di-tert-butylphenol, isooctyl 3,5-di-tert-butyl-4- hydroxyhydrocinnamate (IRGANOX^® L135), vitamin E, a mixture of mono- and dialkylated tert-butyl/tert-octyl-diphenylamines (IRGANOX^® L57), a mixture of mono- and dialkylated nonyldiphenylamines (IRGANOX^® L67), 2,2,6,6-tetramethylpiperidin-4-ol, 2,2,6,6- tetramethylpiperidin-4-one, bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate, bis(1 ,2,2,6,6- pentamethyl-4-piperidyl) sebacate, bis(1-octyloxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate, tris-(2,4-di-tert-butylphenyl)phosphite (IRGAFOS^® 168), bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite, tris(nonylphenyl) phosphite, and bis(2,4-di-tert-butyl-6- methylphenyl) ethyl phosphite.

10. A composition according to claim 1 in which component ii.) is at least one phenol, aromatic amine, hindered amine or phosphite selected from the group consisting of isooctyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate (IRGANOX^® L135), vitamin E, a mixture of mono- and dialkylated tert-butyl/tert-octyl-diphenylamines (IRGANOX^® L57), 2,2,6,6- tetramethylpiperidin-4-ol, and tris-(2,4-di-tert-butylphenyl)phosphite (IRGAFOS^® 168).

1 1. A composition according to claim 1 in which components i.) and ii.) are each present in the range of from about 0.1 part per million to about 10,000 parts per million of component (a).

12. A composition according to claim 1 in which the weight ratio of components i.) to ii.) is in the range of about 95:5 to about 5:95.

13. A composition according to claim 1 in which the weight ratio of the vinyl aromatic compound (a) to the alkyl-substituted aromatic compound (b) is in the range of from about 95:5 to about 5:95.

14. A composition according to claim 1 in which oxygen is present in the range of from about 0.05 parts per million to about 1000 parts per million of component (a).

15. A method for preventing the premature polymerization of vinyl aromatic compounds during distillation or purification comprising

adding to said vinyl aromatic compound in the presence of an alkyl-substituted or hydroxyalkyl-substituted aromatic compound an effective polymerization inhibiting amount of a mixture of i.) at least one stable nitroxide compound, and ii.) at least one phenol, aromatic amine, hindered amine or phosphite, or mixture thereof.

16. Use of a mixture of i.) at least one stable nitroxide, and ii.) at least one phenol, aromatic amine, hindered amine or phosphite, or mixture thereof for stabilizing a mixture of

(a) a vinyl aromatic compound, and

(b) an alkyl-substituted or hydroxyalkyl-substituted aromatic compound against premature polymerization during distillation or purification.