KR920002489B1 - Polymer inhibiting composition and process for inhibiting the polymerization of vinyl aromatic compounds - Google Patents

Abstract

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Description

Anti-polymerization composition and anti-polymerization method of vinyl aromatic compound

1 is a schematic diagram showing an embodiment of the process of the present invention in a distillation series consisting of three columns.

2 is a schematic representation of an embodiment of the process of the present invention using a method of directly injecting a feed of a vinyl aromatic compound into a recycle column.

The present invention relates to a polymerization preventing composition and a method for preventing the polymerization of a vinyl aromatic compound which can be easily polymerized.

It is well known that vinyl aromatic compounds such as styrene monomers, lower alkylated styrenes such as α-methylstyrene and the like readily polymerize, and moreover, their polymerization rates increase as the temperature rises. If the vinylaromatic compound produced by conventional industrial methods contains impurities, it must undergo a separation and purification process to suitably apply to most additional industrial applications. This separation and purification is generally carried out by distillation.

In order to prevent polymerization in the storage of vinyl aromatic compounds, various known polymerization inhibitors have been generally used under refrigeration conditions. For example, conventional inhibitors useful for preventing the polymerization of vinyl aromatic compounds under storage conditions include 4-tert-butylcatechol (TBC) and hydroquinone.

On the other hand, sulfur has been widely used as a polymerization inhibitor in the distillation of various vinyl aromatic compounds. However, while sulfur is provided as a relatively effective inhibitor, the use of sulfur in the distillation process has a very serious drawback, that is, a waste which is highly contaminated by sulfur, forming a reboiler mood in the distillation column. This waste also causes serious problems of pollution and waste disposal.

Although several compounds are effective for preventing the polymerization of vinyl aromatic compounds under different conditions such as storage, only a few of them have been found to be practically useful for preventing the polymerization of vinyl aromatic compounds under distillation conditions. One of the compounds effective in preventing polymerization is 2,6-dinitro-p-cresol (DNPC), which is described in Watson, US Pat. No. 4,105,506. It is also known that known polymerization inhibitors can be mixed in order to obtain a greater protection effect than using each inhibitor alone. The synergistic effect of mixing two known inhibitors is described in Watson (US Pat. No. 4,061,545), where phenothiazine and tert-butyl catechol (TBC) are used as polymerization inhibitors in the presence of oxygen. ) Are used together. The synergistic effect of N-nitrosodiphenylamine mixed with DNPC in preventing the polymerization of vinyl toluene under vacuum conditions is described in Watson, US Pat. No. 4,341,600. However, it has been found that increasing the distillation temperature reduces the effectiveness of the inhibitor.

Higher temperatures in the distillation apparatus are preferred to increase the yield during distillation of the vinyl aromatic compound and to perform distillation with higher energy efficiency. At these high temperatures, however, the rate of polymerization increases, producing an unacceptable level of polymer in the distillation apparatus. Therefore, there is a great need for an anti-polymerization agent that effectively prevents polymerization of vinyl aromatic compounds during distillation at high temperatures.

It is an object of the present invention to provide a method for preventing the polymerization of a polymer preventing composition and a vinyl aromatic compound.

It is also an object of the present invention to provide both a method for preventing polymerization of vinyl aromatic compounds at high temperatures and a polymer preventing composition which obtains high purity unsaturated vinyl aromatic compounds in high yield and produces small amounts of undesired by-products.

It is a further object of the present invention to provide both a method for preventing the polymerization of vinyl aromatic compounds and a polymer preventing composition which can operate the distillation apparatus at a higher temperature and increase the production rate without reducing the efficiency.

In order to achieve the above and other objects, an effective amount of 2,6-dinitro is used for the prevention of polymerization of vinyl aromatic compounds which are easily polymerized when treated in the presence of oxygen at a high temperature as in the distillation process according to the present invention. A composition is provided comprising -p-cresol and an effective amount of phenylenediamine represented by the general formula below.

Wherein R ₁ and R ₂ are alkyl, aryl or hydrogen.

In another aspect of the invention, the prevention composition consists of an effective amount of 2,6-dinitro-p-cresol and 4-tert-butyl catechol.

Vinyl aromatic compounds of the present invention include styrene, substituted styrene, divinylbenzene, vinyltoluene, vinylnaphthalene and polyvinylbenzene. This includes all structural isomers thereof.

Also in carrying out the above other objects, the present invention provides an effective amount of 2,6-dinitro-p-cresol and each of the aforementioned phenylenediamine derivatives or 4-tert-butylcatechol and oxygen, Provided is a method for preventing polymerization of an easily polymerized vinyl aromatic compound, which is characterized by treating the vinyl aromatic compound under heating conditions such as distillation.

According to the process of the present invention, the amount of polymerization produced in the distillation apparatus at a high temperature of 150 ° C. is greatly reduced compared to the conventionally used method. Moreover, the distillation apparatus can be operated at high temperature and high pressure than when using conventional inhibitors, thus allowing higher distillation yields.

Other objects, features and advantages of the invention will be apparent from the following detailed description and claims.

The present invention uses 2,6-dinitro-p-cresol (hereinafter referred to as DNPC) and phenylenediamine derivatives in the presence of oxygen as a polymerization aid inhibitor composition upon heating of a vinyl aromatic compound. Phenylenediamine of the present invention is represented by the following general formula.

Wherein R ₁ is an alkyl group, an aryl group or hydrogen, and R ₂ is an alkyl group, an aryl group or hydrogen.

It is preferred that the alkyl groups of R ₁ and R ₂ each contain 1 to 12 carbon atoms.

P-phenylenediamine, N, N'-dimethylphenylenediamine, N, N'-diethylphenylenediamine, N, N'-bis (1,4-dimethylpentyl) -p- in preferred phenylenediamine derivatives Phenylenediamine, and N-4-methyl-2-phenyl-N'-phenyl-p-phenylenediamine.

Particular preference is given to N, N'-bis (1,4-dimethylpentyl) -p-phenylenediamine and N-4-methyl-2-pentyl-N'-phenyl-p-phenylenediamine, although N, N ' Most preferred is -bis (1,4-dimethylpentyl) -p-phenylenediamine.

The distillation technique of the present invention is suitable for use in any form that substantially separates the compound from the mixture when the vinyl aromatic compound that can be readily polymerized is treated above room temperature. The polymerization inhibitor can prevent the polymerization of unwanted vinyl aromatic monomers at elevated temperatures such as in a distillation apparatus. Increasing the temperature in the device has the advantage of increasing the distillation rate, but also increases the polymerization rate, which is offset by introducing the inhibitor of the present invention. As the most useful application, the composition is applied to a distillation mixture of vinyl aromatic compounds selected from the group consisting of styrene, substituted styrenes such as α-methylstyrene, divinylbenzene, vinyltoluene, vinylnaphthalene and polyvinylbenzene. This group includes all structural isomers of the aforementioned compounds. Preferred applications of the invention relate to distillation of crude styrene.

The amount of the polymerization inhibitor added varies widely depending on the distillation conditions. In general, stability depends on the amount of inhibitor added. According to the present invention, based on the feed of the vinyl aromatic compound fed to the BT column 10 or the recycle column 90, generally at a phenylenediamine concentration of about 50 ppm to 200 ppm and a DNPC concentration of about 100 ppm to 2000 ppm It has been found primarily to produce appropriate results depending on the temperature of the distillation mixture and the desired degree of protection. However, the phenylenediamine inhibitor of the present invention is used at a concentration of about 50 ppm to about 1000 ppm, and DNPC is preferably used at a concentration of about 250 ppm to about 1000 ppm. The preferred ppm ratio of phenylenediamine to DNPC is 2: 3. There is no specific order of mixing the compounds of the present invention. One particular embodiment is to combine and inject these compounds at ambient temperature and pressure outside the distillation series. The distillation technique of the present invention is suitable for virtually any type of separation in which distillative separation of easily polymerizable vinyl aromatic compounds from the mixture, in which the vinyl aromatic compounds are processed above room temperature.

Oxygen must be supplied to the composition system of the present invention in order for the phenylenediamine adjuvant to function properly. Oxygen is added separately into the system to achieve a higher oxygen concentration in the required area. Oxygen used in the present invention may be in the form of oxygen or a gas containing oxygen. If an oxygen containing gas is used, the remaining components of the gas must be inert to the vinyl aromatic compound under distillation conditions. The most useful and inexpensive oxygen source is preferred in the present invention as air. The amount of oxygen varies widely, but in general it is preferred to use the amount of oxygen in the air.

1 illustrates a conventional styrene class family consisting of a benzene-toluene fractional column 10, ethylbenzene or recycle column 12, industrially referred to as a B-T column, and a styrene or terminating column 14. The operating principle of the distillation process of the present invention is very suitable for use with minimal modification to other distillation apparatus used for the purification of other vinyl aromatic compounds. As in FIG. 1, crude styrene is introduced into the middle portion of the B-T column 10 via the supply line 16. The B-T column 10 may be of any suitable type known to those skilled in the art and may include a suitable number of vapor-liquid contact devices such as bubble cap trays, porous trays, and the like. However, column 10 generally includes less than 40 distillation trays. Column 10 is also equipped with a suitable reboiler 18 for supplying heat thereto. The temperature of the reboiler 18 is generally about 190 ° F. to about 250 ° F.

While most of the polymer is produced in ethylbenzene or recycle column 12, a small or large amount of polymer in the prepolymer produced upon distillation is produced in B-T column 10. Therefore, a polymerization inhibitor is needed in this column. To prevent polymerization in the BT column 10, 2,6-dinitro-p-cresol (hereinafter referred to as DNPC) is introduced into the BT column 10 as a separate stream via line 20, or The crude styrene feed flowing through line 16 may be mixed. Phenylene diamine inhibitors may also be introduced into line B-T column 10 via line 20 or incorporated into the crude styrene feed flowing through line 16 to facilitate the process. The phenylenediamine inhibitor has little or no protection in the BT column 10 due to lack of oxygen, but is conveyed to the recycle column 12 via line 24 with the distillation balance of the BT column, where it is the main inhibitor. Works. When DNPC and / or phenylenediamine polymerization inhibitor is added as a separate stream to the B-T column 10, it is preferably dissolved in a volatile aromatic hydrocarbon diluent such as ethylbenzene. The inhibitor supply line 20 is usually located in the middle of the B-T column 10 so that the inhibitor distribution closely matches the distribution of the vinyl aromatic compound that can be easily integrated in the column 10.

Under the distillation conditions imposed on column 10, the upper stream comprising benzene and toluene is removed via line 22. These low boiling aromatic hydrocarbons are condensed continuously and stored for later use. The distillation residue in the B-T column containing styrene, ethylbenzene, inhibitor and taar is recycled or feedstock for the ethylbenzene column 12. This distillation residue is introduced into the middle of the ethyl benzene column 12 via line 24 and pump 26. Recirculation column 12 may be in a suitable form known to those skilled in the art and may include 40 to 100 trays. However, the recycle column is preferably of parallel path design. The recycle column may also include as many as 72 trays to properly separate styrene and ethyl benzene having similar boiling points. The B-T column distillation residue is preferably introduced into the middle of the recycle column 12.

A certain amount of inhibitor in the ethylbenzene column 12 introduces line 28 into the middle of column 12 with DNPC, which is introduced into the BT column distillation balance fraction through line 30 or into the BT column via line 20. Protected by a phenylenediamine composition. Due to DNPC protection it is necessary to supply air only to the recycle column 12 of the remaining distillation series. Higher temperatures up to 150 ° C. are allowed in the column for more energy-efficient distillation, so a more effective polymerization inhibitor is required in the recycle column 12. Low pressure steam can be recovered from a cooler (not shown) above the recycle column 12 by obtaining a temperature of at least 118 ° C., preferably 130 ° C. or higher.

Oxygen is introduced into the reboiler 32 through each of air purge lines 36 and 38. Oxygen also passes through line 42 and through sump or boot 40 when the air pressure / volume is sufficient to reach reboiler 32 via line 43. Can be introduced. Equal amounts of oxygen are dissolved in the liquid phase of column 12 in order to make phenylenediamine effective. However, the amount of oxygen introduced should not exceed the amount that would cause an explosion in the column. Oxygen is dispersed through column 12 where it acts together with phenylenediamine to prevent polymerization. The amount of oxygen required depends on the number and spacing of the oxygen inlets around the column 12 and the degree of efficient mixing of oxygen and liquid hydrocarbons in the column 12. Thus, in practical operation, as long as the production of the polymer is reduced, the flow of oxygen will increase within a range that does not produce an explosive mixture. In general, DNPC acts as an auxiliary inhibitor where air is not fully dispersed into column 12, where the lack of air reduces the effect of phenylenediamine. Thus, DNPC has an anti-polymerization effect continuously in the air-deficient portion of the recycle column 12, thus providing a higher overall anti-polymerization effect than when phenylenediamine is used alone or in combination with other oxygen-activated inhibitors. do.

It is a groundbreaking fact that DNPC can be used with phenylenediamine and also acts as a polymerization inhibitor, with or without air. Therefore, DNPC, together with phenylenediamine in the recirculation column, in which air is effectively dispersed, may act as a polymerization inhibitor. However, it has been found that DNPC is consumed more rapidly when DNPC is used alone as an inhibitor in the presence of air. This may be due to the fact that more polymer free radicals are produced in the presence of air. Therefore, it is necessary to add a higher amount of DNPC inhibitor in order to maintain effective DNPC / oxygen polymerization prevention for a long time. It is described in US Pat. No. 4,272,344 that the tar containing DNPC / phenylenediamine can be recycled to the recycle column 12 to obtain additional protection of DNPC and phenylenediamine.

The phenylenediamine inhibitor can also be introduced into the B / T column 10 via line 20 as a DNPC / phenylenediamine mixture with a DNPC inhibitor. There is no preferred mixing order of these. Proper results can be obtained by mixing in any order under ambient temperature and pressure. The DNPC / phenylenediamine inhibitor fraction is conveyed to the recycle column 12 with the B-T column distillation balance.

The distillation balance of the recycle column 12 comprising the styrene inhibitor and the tar is recovered from the reboiler portion of the recycle column 12 via line 60. This recycle base balance is then supplied via line 64 to the middle of the styrene or termination column 14 by pump 62. Optionally such base material may be introduced to the bottom of styrene column 14 via line 66.

Termination column 14 may be of any suitable form known to those skilled in the art. Typical columns contain, for example, about 24 distillation trays. The reboiler 68 is connected to the sump 76 via a line 78 and a pump 80. Reboiler 68 is generally operated at a temperature of about 823 ° C to about 121 ° C. In general, the protection of the inhibitor is properly achieved in the column by the DNPC and phenylenediamine inhibitors present in the base feed. The taar fraction from column 14 may be recycled to at least line 88 to ethylbenzene column 12 to further replenish DNPC in the system.

High purity styrene product from the top of the column is recovered from styrene column 14 via line 74. The styrene column 14 distillation balance, consisting of polystyrene, undistilled styrene, heavy by-products and DNPC / phenylenediamine adjuvant, is recovered from the reboiler recycle line 78 and flash port through line 85 for subsequent processing. It is moved to 84. Flash port 84 removes residual styrene from the distillation balance of styrene column 14 and recycles it to column 14 via line 86. The flare generated in the flash port 84 is continuously withdrawn from the system via line 83 or recycled to recycle column 12 or B-T column 10 via line 88.

2 illustrates the application of the distillation process of the present invention to another typical distillation series. Styrene is preferably introduced via line 91 into the middle of the recycle column 90 in the form of a parallel distillation passage. Line 92 feeds phenylenediamine inhibitor to recycle column 90. Heat is supplied to the base of column 90 by reboiler 94. Oxygen is introduced into the reboiler 94 through air purge lines 96 and 98. Oxygen is introduced directly into reboiler 94 through line 101 and sump or boot 100 when the oxygen pressure and volume is sufficient to reach reboiler 94 through line 97. Benzene and toluene in the B-T column 122 are recovered through line 123 as top fraction and condensed for subsequent use. The ethylbenzene distillation balance is recovered via line 124 and recycled for later use. Reboiler 126 supplies the heat necessary for distillation to B-T column 122.

The distillation balance of the recycle column comprising polystyrene, undistilled styrene, heavy by-products, phenylenediamine and DNPC is recovered from recycle column 90 via line 128. The impure styrene fraction is then fed to the top of the steren column 130 via pump 132 and line 133. Optionally, impure styrene can be introduced through line 134 to the bottom of styrene column 130. The reboiler circuit, which consists of the reboiler 148 and the pump 150, is connected to the styrene or terminating column 130 to supply the necessary heat therein. DNPC is introduced into recycle column 90 via line 92, preferably with phenylenediamine. The purified styrene distillate is recovered via line 144.

Reboiler 148 is connected to termination column sump 136 via recirculation line 145 and pump 150. The termination column distillate balance is withdrawn from reboiler recycle line 145 for further processing in flash pot 146. Flash pot 146 is recycled to termination column 130 through line 149. The tar generated during the distillation process is recovered via line 152 or recycled to line distillation via line 154.

Another aspect of the present invention is the introduction of an effective amount of 4-tert-butyl catechol into the distillation series through recycle column 12 or 90 to act as an adjuvant with DNPC instead of phenylenediamine. DNPC and 4-tert-butylcatechol (hereinafter referred to as TBC) have been found to form an effective adjuvant system in the presence of air at temperatures below 140 ° C. An effective amount of TBC based on the feed of vinyl aromatic compound to the B-T column 10 or the recycle column 90 is from about 50 ppm to about 2000 ppm. Preferred amounts of TBC are about 200 ppm to about 1000 ppm. The ppm ratio of TBC to DNPC is preferably 2: 3.

The use of the methods and compositions of the present invention permits high temperatures in the recycle column due to the introduction of effective amounts of DNPC / phenylenediamine and / or DNPC / TBC inhibitor compositions, thus allowing the distillation apparatus to be produced at high production rates as opposed to conventional prior art processes. It can work. In addition, DNPC inhibitors may be used to effectively prevent polymerization in the remaining fractional columns that are low temperature and deficient in air. Thus, hot distillation temperatures and high pressures can be used without producing unacceptable amounts of polymer. In this way, the distillation rate can be increased without increasing the amount of polymerization previously experienced in conventional distillation processes.

In addition, by optimizing the distribution of DNPC / phenylenediamine or DNPC / TBC inhibitors in the recycle column and by optimizing the distribution of DNPC inhibitors in the remaining fraction columns of the distillation series, higher temperatures can be obtained in the recycle column to achieve more efficient Energy recovery is possible.

In order to make the present invention more clear, the following examples are given, but this is not a limitation of the present invention.

Example 1

Prepare two 100 ml reaction flasks. The first flask (1) was filled with 25 g of styrene, in which 100 ppm DNPC and 50 ppm Flexone 4L (Flexone 4L, a trademark of Universal Chemical Co., Ltd.) were incorporated in the Uniroyal Material Safety Sheet used herein as a reference. , N'-bis (1,4-dimethylpentyl) -p-phenylenediamine found to have the formula) is added. The second flask (2) is filled with 25 g of styrene, to which 200 ppm DNPC is added. Two flasks are mounted with a magnetic stirrer and septum closure and heated to 138 ± 2 ° C. in a stirred oil bath. The flask 1 is purged just below the liquid surface with about 3 ml / min of air during the distillation period. The flask 2 is placed under a nitrogen blanket. After 2 hours the change in refractive index of the sample is measured to test the degree of styrene polymerization as an inspection standard. Also as an inspection standard, the monomers are sometimes removed and the remaining polymer is weighed. The final polymer yields of flasks (1) and (2) are 14.94% and 18.24%, respectively, suggesting an excellent protective effect of the phenylenediamine / DNPC adjuvant system at high temperatures than with DNPC alone.

Example 2

A 100 ml reaction flask is filled with 25 g of styrene, to which 100 ppm DNPC and 90 ppm TBC are added. The flask is mounted with a magnetic stirrer and a diaphragm stopper and heated to 118 ± 2 ° C. in a stirred oil bath. Purge the flask directly under the liquid surface with 1-2 ml / min of air while heating. The results are shown in the table below.

Comparative Example 2A

Fill a 100 ml flask with 25 g of styrene and add 100 ppm DNPC. It carried out according to the method of Example 2, and the result is as follows.

Comparative Example 2B

Fill 100 ml flasks with 25 g of styrene and add 90 ppm TBC. It carried out according to the method of Example 2, and the result is as follows.

The results of Example 2 and Comparative Examples 2A and 2B show an increased effect of the DNPC / TBC adjuvant composition at lower temperatures than when using DNPC or TBC alone.

Example 3

A 1: 1 mixture of ethylbenzene and styrene was distilled off using a 12 kPa pilot plant fractionation column packed with Norton Intalox Packing. As in a conventional recycle column, the column has a continuous feed and distillate recovery. 300 ppm DNPC (based on styrene content) and 200 ppm flexon 4L (based on styrene content) are introduced into the column. Air is introduced into the column at a rate of 1.2 l / min. The temperature of the reboiler is maintained at 118 ° C. The distillation balance is recovered at 30 to 60 minute intervals to maintain a constant reboiler capacity. Record feed rate, recovery rate, column temperature distribution, reflux ratio and top pressure at 1 hour intervals. Collect distilled balance and distillate samples at 2 hour intervals. The aldehyde and peroxide content in the distillate is measured at 6-8 hour intervals. The percent polymer in the distillation balance is obtained by vacuum evaporation drying a portion of the distillation balance, placing the dried product in a flask, heating the flask to remove monomers and then measuring the weight of the remaining amount constituting the polymer.

The results are as follows.

Polymer yield: 0 to 0.21%

Aldehyde: 181-427ppm

Peroxide: 13-25 ppm

As a further test, the polymer yield is 0.24 to 0.36% when the oxygen feed rate is reduced from 0.50 to 0.25 l / min and then to 0.10 l / min.

Example 4

It is carried out according to the method of Example 3 using 300 ppm DNPC and 200 ppm TBC. The reboiler temperature is maintained at 118 ° C.

The results are as follows.

Polymer yield: 0.80-1.22%

Aldehyde: 220 to 420ppm

Peroxide: 25 to 141 ppm

As a further test, the polymer yield slowly increases from 1.29 to 2.09% when the oxygen flow rate is reduced from 0.50 to 0.25 l / min and then to 0.10 l / min.

Comparative Example 3/4

The procedure of Example 3 was conducted except that 500 ppm DNPC was used alone as an inhibitor without introducing air into the column. The reboiler is maintained at 118 ° C. Polymer yield is 3.97-4.25%.

Example 5

The procedure of Example 3 is carried out using 300 ppm DNPC and 200 ppm flexon 4L (the same concentration used at 118 ° C. in Example 3). The temperature of the reboiler is maintained at 132 ° C.

The results are as follows.

Polymer yield: 0.80-1.22%

Aldehyde: 280 to 346ppm

Peroxide: 48 to 80 ppm

As a further test, the polymer yield increases slightly from 1.24 to 1.47% when the oxygen flow rate is reduced from 0.50 to 0.25 l / min and then to 0.101 / min.

Example 6

600 ppm DNPC and 400 ppm TBC were used according to the method of Example 3 (this is twice the concentration used at 118 ° C. of Example 4). The temperature of the reboiler is maintained at 132 ° C.

The results are as follows.

Polymer yield: 1.01-1.36%

Aldehyde: 310 to 440ppm

Peroxide: 107-167ppm

As a further test, the oxygen flow rate was reduced from 0.50 to 0.25 l / min and then to 0.10 l / min, resulting in a slow increase in polymer yield from 1.57 to 2.92%.

[Example 5/6]

100 ppm of DNPC alone was used as the inhibitor, followed by the method of Example 3 (this is twice the concentration of DNPC used at 118 ° C of Comparative Example 3/4).

The results are as follows.

Polymer yield: 2.70-3.00%

Aldehyde: 220 to 260ppm

Peroxide: 49 to 96 ppm

While the invention has been described in terms of various preferred embodiments and examples, those skilled in the art may modify, substitute, omit and modify it without departing from the spirit of the invention.

Claims (17)

An effective amount of 2.6-dinitro-p-cresol and an effective amount of phenylenediamine of the following general formula, wherein the composition prevents the polymerization of the vinyl aromatic compound in the presence of oxygen.

Wherein R ₁ and R ₂ are alkyl, aryl or hydrogen. The composition of claim 1 wherein the vinyl aromatic compound is selected from the group consisting of styrene, substituted styrene, divinylbenzene, vinyltoluene, vinylnaphthalene, polyvinylbenzene and structural isomers thereof. The composition of claim 1, wherein the alkyl groups each contain 1 to 12 carbon atoms. The composition of claim 1 wherein 2,6-dinitro-p-cresol is present in an amount of 100 ppm to 2000 ppm and phenylenediamine is present in an amount of 50 ppm to 2000 ppm. A composition comprising an effective amount of 2,6-dinitro-p-cresol and an effective amount of 4-tert-butylcatechol, wherein the presence of oxygen prevents the polymerization of the vinyl aromatic compound. The composition of claim 5 wherein the vinyl aromatic compound is selected from the group consisting of styrene, substituted styrenes, divinylbenzene, vinyltoluene, vinylnaphthalene, polyvinylbenzene and structural isomers thereof. The composition of claim 5, wherein the 2,6-dinitro-p-cresol is present in an amount of 100 ppm to 2000 ppm and the 4-tert-butylcatechol is present in an amount of 50 ppm to 2000 ppm. Treating a vinyl aromatic compound with an effective amount of 2,6-dinitro-p-cresol and an effective amount of phenylenediamine of the following general formula in the presence of oxygen when heated, thereby preventing the polymerization of the vinyl aromatic compound.

Wherein R ₁ and R ₂ are alkyl, aryl or hydrogen. The method of claim 8, wherein the vinyl aromatic compound is selected from the group consisting of styrene, substituted styrene, divinylbenzene, vinyltoluene, vinylnaphthalene, polyvinylbenzene, and structural isomers thereof. The method of claim 8, wherein the alkyl groups each contain 1 to 12 carbon atoms. The method of claim 8, wherein the vinyl aromatic compound is heated to a temperature of 150 ° C. or less. The method of claim 8, wherein the effective amount of 2,6-dinitro-p-cresol is 100 ppm to 2000 ppm and the effective amount of phenylenediamine is 50 ppm to 2000 ppm. The method of claim 8, wherein the compound is heated while distilling the vinyl aromatic compound. Treating the vinyl aromatic compound with an effective amount of 2,6-dinitro-p-cresol and an effective amount of 4-tert-butylcatechol in the presence of oxygen upon heating, thereby preventing the polymerization of the vinyl aromatic compound. 15. The process of claim 14, wherein the vinyl aromatic compound is selected from the group consisting of styrene, substituted styrenes, divinylbenzene, vinyltoluene, vinylnaphthalene, polyvinylbenzene, and structural isomers thereof. The method of claim 14, wherein the vinyl aromatic compound is heated to a temperature of 140 ° C. or less. The effective amount of 2,6-dinitro-p-cresol is from 100 ppm to 2000 ppm based on the vinyl aromatic compound to be purified, and the effective amount of 4-tert-butylcatechol is a feed of the vinyl aromatic compound. 50 ppm to 2000 ppm based on the method.

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