VINYL AROMATIC COMPOSITONS STABILIZED AGAINST POLYMERIZATION

Abstract: Vinyl aromatic compounds are stabilized against polymerization by the addition of an effective amount of a polymerization inhibition composition comprising (a) a phenothiazine compound; and (b) an aryl-substituted phenylenediamine compound. In other aspects, this invention is directed to a vinyl aromatic composition stabilized against polymerization by such polymerication inhibitor composition, as well as to a method of stabilizing a vinyl aromatic composition against polymerization which method comprises adding an effective amount of such polymerization inhibitor composition.

Description: This invention is directed to a vinyl aromatic composition stabilised against polymerisation by a polymerisation inhibitor composition comprising (i) a phenothiazine compound; and (ii) an aryl-substituted phenylenediamine compound, as well as to a method of stabilising a vinyl aromatic composition against polymerisation which method comprises adding an effective amount of such polymerisation inhibitor composition. Commercial processes for the manufacture of vinyl aromatic compounds such as monomeric styrene, divinyl benzene and lower alkylated styrenes (such as alphamethylstyrene and vinyltoluene) typically produce products contaminated with various impurities, such as benzene, toluene and the like. These impurities must be removed in order for the monomer product to be suitable for most applications. Such purification of vinyl aromatic compounds is generally accomplished by distillation. However, it is well known that vinyl aromatic compounds polymerize readily and that the rate of polymerization increases rapidly as the temperature increases. In order to prevent polymerization of the vinyl aromatic monomer under distillation conditions various polymerization inhibitors have been employed. In general, the compounds which are commercially employed as such polymerization inhibitors are of the dinitrophenolic class. Thus, for example, Drake et al, in United States Patent 2,526,567, show the stabilization of nuclear chlorostyrenes employing 2,6-dinitrophenols. Similarly, United States Patent 4,105,506, to Watson, discloses the use of 2,6-dinitro-p-cresol as a polymerization inhibitor for vinyl aromatic compounds. In addition, it has been disclosed by Butler et al, in United States Patent 4,466,905, that, in the presence of oxygen, the presence of phenylenediamines in the distillation column with 2,6-dinitro-p-cresol will further reduce the amount of polymerization which occurs. While dinitrophenols are effective polymerization inhibitors, there are several disadvantages associated with their use, either alone or in blends. For example, dinitrophenols are solids that, if subjected to temperatures above their melting points, are unstable and may explode (see United States Patent 4,457,806). Moreover, dinitrophenols are highly toxic, many having an LD₅₀ (rat) of less than 30 mg/kg (Sax, Hazardous Properties of Industrial Chemicals). Recently, it has been disclosed by Kolich, in United States Patent 4,633,026, that halogenated vinyl aromatic compounds (such as bromostyrene) may be inhibited from polymerising by the addition of an amine polymerisation inhibitor selected from the group consisting of certain alkyl-substituted phenylenediamine compounds and phenothiazine compounds in the presence of air. Japanese patent application 40-133336 discloses use of a blend of phenothiazine and phenylenediamine as an antioxidant for rubber. While such prior art inhibitors may inhibit the polymerisation of vinyl aromatic compounds to some degree, it would be desirable to possess polymerisation inhibitors which would more effectively delay the onset of polymerisation and/or which would avoid the use of highly toxic compounds such as dinitrophenols. Preferred embodiments of this invention provide a vinyl aromatic composition which is stabilised against polymerisation. Preferred embodiments of this invention provide an improved method for inhibiting the polymerisation of vinyl aromatic compounds. According to a first aspect of the present invention a vinyl aromatic composition stabilised against polymerisation, said composition comprising: (a) vinyl aromatic compound; and (b) a polymerisation inhibitory effective amount of a polymerisation inhibitor composition comprising: (i) a phenothiazine compound having the structure: wherein R is hydrogen or C₁-C₁₂ alkyl; and R¹ and R² are each independently selected from the group consisting of hydrogen, C₆-C₁₀ aryl, C₇-C₁₁ aralkyl, C₇-C₁₆ alkaryl and C₁-C₁₂ alkyl; and (ii) a phenylenediamine compound having the structure: wherein R³ is C₆-C₁₀ aryl or C₇-C₁₆ alkaryl; and R⁴, R⁵ and R⁶ are each independently selected from the group consisting of hyrogen, C₁-C₁₂ alkyl, C₃-C₁₂ cycloalkyl, C₇-C₁₁ aralkyl, and C₇-C₁₆ alkaryl. In another aspect, this invention provides a method for inhibiting the polymerisation of a vinyl aromatic compound, which method comprises adding a polymerisation inhibiting effective amount of a polymerisation inhibitor composition comprising: (i) a phenothiazine compound having the structure: wherein R is hydrogen or C₁-C₁₂ alkyl; and R¹ and R² are each independently selected from the group consisting of hydrogen, C₆-C₁₀ aryl, C₇-C₁₁ aralkyl, C₇-C₁₆ alkaryl and C₁-C₁₂ alkyl; and (ii) a phenylenediamine compound having the structure: wherein R³ is C₆-C₁₀ aryl or C₇-C₁₆ alkaryl; and R⁴, R⁵ and R⁶ are each independently selected from the group consisting of hydrogen, C₁-C₁₂ alkyl, C₃-C₁₂ cycloalkyl, C₇-C₁₁ aralkyl, and C₇-C₁₆ alkaryl. The vinyl aromatic,polymerisation inhibitory compositions comprise (i) a phenothiazine compound; and (ii) an aryl-substituted phenylenediamine compound. The phenothiazine compounds which may be employed are of the structural formula: wherein R is hydrogen or C₁-C₁₂ alkyl; and R¹ and R² are each independently selected from the group consisting of hydrogen, C₆-C₁₀ aryl, C₇-C₁₁ aralkyl, C₇-C₁₆ alkaryl and C₁-C₁₂ alkyl. Particularly suitable phenothiazine compounds which can be employed herein include phenothiazine, 2-methylphenothiazine, 2-octylphenothiazine, 2-nonylphenothiazine, 2,8-dimethylphenothiazine, 3,7-dimethylphenothiazine, 3,7-diethylphenothiazine, 3,7-dibutylphenothiazine, 3,7-dioctylphenothiazine, 2,8-dioctylphenothiazine, 3,7-dinonylphenothiazine, 2,8-dinonylphenothiazine, 2(alpha, alpha-dimethylbenzyl)phenothiazine, 3,7-bis(alpha, alpha-dimethylbenzyl)phenothiazine, 2,8-bis(alpha, alpha-dimethylbenzyl)phenothiazine, N-methyl-2,8-dioctylphenothiazine and N-methyl-3,7-dioctylphenothiazine. Moreover, mixtures of two or more phenothiazine compounds may also be employed. The phenylenediamine compounds which may be employed possess the structural formula: wherein R³ is C₆-C₁₀ aryl or C₇-C₁₆ aralkyl; and R⁴, R⁵ and R⁶ are each independently selected from the group consisting of hydrogen, C₁-C₁₂ alkyl, C₃-C₁₂ cycloalkyl, C₇-C₁₁ aralkyl and C₇-C₁₆ alkaryl. Preferred compounds include those wherein the amine groups are in the para position. Particularly preferred compounds are para-phenylenediamines wherein R⁴ and R⁵ are hydrogen; R³ is phenyl; and R⁶ is C₃-C₈ alkyl or C₃-C₈ cycloalkyl. Illustrative preferred phenylenediamine compounds which may be employed include N-phenyl-N′-isopropyl-p-phenylenediamine, N-phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine, N-phenyl-N′-(1,4-dimethylpentyl)-p-phenylenediamine and N-phenyl-N′-cyclohexyl-p-phenylenediamine. Moreover, mixtures of phenylenediamine compounds may be employed. The phenylenediamine compounds may be of the oxygenated species described in US 4774374. The phenothiazine and phenylenediamine compounds are generally employd in weight ratios of between 10: 1 and 1: 10. Preferably, weight ratios of between 4: 1 and 1: 4 are employed, with ratios of between 2: 1 and 1: 2 being particularly preferred. The polymerisation inhibitor compositions may further comprise an aromatic hydrocarbon solvent. Illustrative of such solvents are benzene, toluene, xylene, ethylbenzene and other alkyl-benzenes as well as vinyl aromatic compounds themselves such as styrene, alphamethylstyrene. Typically, when solvents are employed they hydrogenated precursors of the vinyl aromatic to be stabilised are the preferred solvents. Thus, for the stabilisation of the styrene, ethylbenzene is the preferred solvent. Similarly for the stabilisation of alpha-methylstyrene, isopropylbenzene is the preferred solvent. The vinyl aromatic composition comprises (a) a vinyl aromatic compound; and (b) a polymerisation inhibitory effective amount of the polymerisation inhibitor composition described above. Illustrative of the vinyl aromatic compounds which may be stabilised against polymerisation by the process of this invention are styrene, alpha-methylstyrene, vinyltoluene and divinylbenzene, as well as halogenated species thereof. The stabilised vinyl aromatic composition may be in the form of a reaction mixture additionally comprising the starting materials of the vinyl aromatic compound to be stabilised as well as byproducts of the production process. Thus, in the case of styrene, the reaction mixture will typically include starting materials such as benzene, ethylbenzene and ethylene, as well as byproducts such as diethylbenzene, vinyl toluene. The primary use of the polymerisation inhibitor composition is to prevent the polymerisation of vinyl aromatics during purification/distillation to remove unreacted starting materials and distillable byproducts. Typically, this involves the sequential distillation of the vinyl aromatic reaction product through a plurality of distillation columns. In the first of such columns, a relatively large amount of starting material and byproducts will be present, while in the last column essentially pure vinyl aromatic compound (plus polymerisation inhibitors and heavy, nondistillable byproducts) will be present. The method of this invention involves adding to a vinyl aromatic compound an effective amount of the polymerization inhibitor composition of this invention. When the polymerization inhibitor composition of this invention is employed during the purification/distillation of vinyl aromatic compounds, it is preferred that oxygen, whether in the form of air or otherwise, be present. It is also noted that the polymerization inhibitor composition of this invention will be effective for uses other than during distillation _ e.g., during the shipment or storage of vinyl aromatic compounds. The vinyl aromatic composition and method of this invention comprise or involve the addition of an effective amount of polymerization inhibitor composition. As employed herein, the term "effective amount" refers to that amount of inhibitor composition which is needed to prevent the formation of more than 1 weight percent of vinyl aromatic polymer in less than 3 hours at temperatures of between 90° and 150°C. Although the amount of polymerization inhibitor required will vary somewhat (based upon such factors as the particular vinyl aromatic compound stabilized; the particular phenylenediamine and phenothiazine species employed; and the like) such an effective amount may-be readily determined by routine experimentation. In general, such an effective amount will be between 50 and 1,500 parts per million by weight of vinyl aromatic compound. The polymerization inhibitor composition of this invention will provide stability against vinyl aromatic polymerization at temperatures typically employed for the purification of vinyl aromatic compound (e.g., of between 90° and 140°C) for periods well in excess of those typically employed for such purification. This stability is achieved without the use of undesirably toxic dinitrophenolic compounds which are generally employed in commercial operations today. Examples The following Examples illustrate the present invention. Examples 1 and 2 and Comparative Experiment A To a fifty milliliter flask (Example 1) charged with forty grams of styrene were added 50 ppm of 4-isopropyl-aminodiphenylamine and 50 ppm of phenothiazine. The flask was fitted with a magnetic stirrer and septum closure and heated in an oil bath to 118°C (plus or minus 2°C). The flask was purged with approximately 5 cc/min air run beneath the liquid surface during the period of the test. During the test period, samples were removed from the flask every one-half hour and tested for degree of polymerization by measuring the changes in refractive index. The time until onset of polymerization, defined as the point at which 1 weight percent of the styrene had polymerized, was determined to be 4 hours. Employing this procedure, a second aryl-substituted phenylenediamine, N-(1,4-dimethylpentyl)-N′-phenyl-p-phenylenediamine, was similarly tested in combination with phenothiazine (Example 2). In order to compare these results with prior art vinyl aromatic polymerization inhibitors, a third flask (Comparative Experiment A) was prepared comprising 50 ppm of phenothiazine plus 50 ppm of a non-aryl substituted phenylenediamine compound, N,N′(p-phenylene)bis(2-amino-5-methylhexane), in 40 grams of styrene. This flask was tested in accordance with the procedure described for Example 1. It is noted that United States Patent 4,633,026 to Kolich indicates that vinyl aromatic compounds may be stabilized against polymerization with amines selected from the group consisting of phenothiazines and non-aryl-substituted phenylenediamines. The results of such testing are summarized in Table I below. The above data indicate the unexpected degree of polymerization inhibition afforded by the inhibitor compositions of this invention. Examples 3-5 Employing a process essentially identical to that described in Example 1 above, several akylated phenothiazines were tested (at a concentration of 50 parts per million (ppm) by weight, based upon the weight of styrene employed) in combination with 4-isopropylamino diphenylamine (at 50 ppm) for their effectivenesss as polymerization inhibitors. The results of such testing are summarized in Table 2. The above results further indicate the desirable polymerization inhibitory activity of the compositions of this invention.

Claims: 1. A vinyl aromatic composition stabilized against polymerization said composition comprising: (a) a vinyl aromatic compound; and (b) a polymerization inhibitory effective amount of a polymerization inhibitor composition comprised of: (i) A phenothiazine compound having the structure: wherein R is hydrogen or C₁-C₁₂ alkyl; and R¹ and R² are each independently selected from the group consisting of hydrogen, C₆-C₁₀ aryl, C₇-C₁₁ aralkyl, C₇-C₁₆ alkaryl and C₁-C₁₂ alkyl; and (ii) a phenylenediamine compound having the structure: wherein R³ is C₆-C₁₀ aryl or C₇-C₁₆ alkaryl; and R⁴, R⁵ and R⁶ are each independently selected from the group consisting of hydrogen, C₁-C₁₂ alkyl, C₃-C₁₂ cycloalkyl, C₇-C₁₁ aralkyl, and C₇-C₁₆ alkaryl. 2. A composition in accordance with claim 1 wherein said composition further comprises the starting materials and byproducts resulting from the production of said vinyl. 3. A composition in accordance with claim 1 wherein component (i) is selected from the group consisting of phenothiazine, 2-methylphenothiazine, 2-octylphenothiazine, 2-nonylphenothiazine, 2,8-dimethylphenothiazine, 3,7-dimethylphenothiazine, 3,7-diethylphenothiazine, 3,7 dibutylphenothiazine, 3,7-dioctylphenothiazine, 2,8-dioctylphenothiazine, 3,7-dinonylphenothiazine, 2,8-dinonylphenothiazine, 2(alpha, alpha-dimethylbenzyl)phenothiazine, 3,7-bis(alpha, alpha-dimethylbenzyl)phenothiazine, 2,8-bis(alpha, alpha-dimethylbenzyl)phenothiazine, N-methyl-3.7-dioctylphenothiazine and N-methyl-2,8-dioctylphenothiazine aromatic compound. 4. A composition in accordance with claim 1 wherein component (ii) is a para-phenylenediamine compound wherein R⁴ and R⁵ are hydrogen; R³ is phenyl; and R⁶ is C₃-C₈ alkyl or C₃-C₈ cycloalkyl. 5. A composition in accordance with claim 1 wherein the weight ratio of component (i) to component (ii) is between 10: 1 and 1: 10. 6. A composition in accordance with claim 5 wherein the weight ratio of component (i) to component (ii) is between 4: 1 and 1: 4. 7. A composition in accordance with claim 5 wherein the weight ratio of component (i) to component (ii) is between 2: 1 and 1: 2. 8. A method for inhibiting the polymerisation of a vinyl aromatic compound, which method comprises adding a polymerisation inhibiting effective amount of a polymerization inhibitor composition comprising: (i) a phenothiazine compound having the structure: wherein R is hydrogen or C₁-C₁₂ alkyl; and R¹ and R² are each independently selected from the group consisting of hydrogen, C₆-C₁₀ aryl, C₇-C₁₁ aralkyl, C₇-C₁₆ alkaryl and C₁-C₁₂ alkyl; and (ii) a phenylenediamine compound having the structure: wherein R³ is C₆-C₁₀ aryl or C₇-C₁₆ alkaryl; and R⁴, R⁵ and R⁶ are each independently selected from the group consisting of hydrogen, C₁-C₁₂ alkyl, C₃-C₁₂ cycloalkyl, C₇-C₁₁ aralkyl, and C₇-C₁₆ alkaryl. 9. A method in accordance with claim 8 wherein said vinyl aromatic composition is added to the reaction mixture resulting from the production of said vinyl aromatic compound. 10. A method in accordance with claim 8 wherein component (i) is selected from the group consisting of phenothiazine, 2-methylphenothiazine, 2-octylphenothiazine, 2-nonylphenothiazine, 2,8-dimethylphenothiazine, 3,7-dimethylphenothiazine, 3,7-diethylphenothiazine, 3,7-dibutylphenothiazine, 3,7-dioctylphenothiazine, 2,8-dioctylphenothiazine, 3,7-dinonylphenothiazine. 2,8-dinonylphenothiazine, 2(alpha, alpha-dimethylbenzyl)phenothiazine, 3,7-bis(alpha, alpha-dimethylbenzyl)phenothiazine, 2,8-bis(alpha, alpha-dimethylbenzyl)phenothiazine, N-methyl-3,7-dioctylphenothiazine and N-methyl-2,8-dioctylphenothiazine. 11. A method in accordance with claim 8 wherein component (ii) is a para-phenylenediamine compound wherein R⁴ and R⁵ are hydrogen; R³ is phenyl; and R⁶ is C₃-C₈ alkyl or cycloalkyl. 12. A method in accordance with claim 8 wherein the weight ratio of component (i) to component (ii) is between 10: 1 and 1: 10. 13. A method in accordance with claim 12 wherein the weight ratio of component (i) to component (ii) is between 4: 1 and 1: 4. 14. A method in accordance with claim 13 wherein the weight ratio of component (i) to component (ii) is between 2: 1 and 1: 2.

Inventors: ABRUSCATO, GERALD JOHN, STOTT, PAUL EDWIN
Applicants: UNIROYAL, INC.
Application year: 1989
Main classification: C07C7
IPC codes: C08K5, C08F112, C07C7, C08F2, C08L25