Abstract:
The polymerization of a vinylaromatic or vinylaliphatic compound at elevated temperature in the absence of air is inhibited by processing the vinylaromatic or vinylaliphatic compound in the presence of 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl or 4-acetylamino-2,2,6,6-tetramethylpiperidine-N-oxylalone or in admixture with p-nitrosophenol or 2-methyl-4-nitrosophenol.

Description:
This application is a Continuation of application Ser. No. 08/511,472, filed on Aug. 4, 1995, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a process of inhibiting the polymerization of vinylaromatic or vinylaliphatic compounds, especially styrene, during distillation. 
     2. Discussion of the Background 
     Styrene is the starting product for the preparation of polystyrene and mixed polymers. Due to the incomplete reaction and formation of by-products in the preparation of styrene, distillative separation is necessary. Since the styrene easily polymerizes thermally at the distillation temperatures, an inhibitor must be added to the process. Such inhibitors, e.g., nitrophenols, nitrosophenols, p-tert-butylpyrocatechol, or sulfur, are known and are also used on a large scale. From the publications U.S. Pat. No. 4,967,027 and European Patent No. B-0,229,515, it can be further concluded that combinations of different inhibitors can also be used. 
     Due to its thermal sensitivity, on a large scale, styrene is distilled in vacuum without exception. It is known that most inhibitors need the presence of oxygen to be effective. 
     In principle, this produces the problem of making available sufficient oxygen in a column operated in vacuum. 
     There is therefore a need for new polymerization inhibitors for vinylaromatic or vinylaliphatic compounds that are suitable for use at increased temperatures as they are used under distillation conditions and that do not have the indicated disadvantages. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of this invention is to provide a method of effectively inhibiting the polymerization of vinylaromatic and vinylaliphatic compounds during distillation. 
     Briefly, this object and other objects of the invention as hereinafter will become more readily apparent, can be attained in a method of inhibiting the polymerization of vinylaromatic and vinylaliphatic compounds at elevated temperatures under an atmosphere which excludes air by processing the vinylaromatic or vinylaliphatic compound in the presence of an inhibitor which is 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl or 4-acetylamino-2,2,6,6,-tetramethylpiperidine-N-oxyl alone or in admixture with p-nitrosophenol or 2-methyl-4-nitrosophenol. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It has been surprisingly found that the inhibitors 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (HTMPO) and 4-acetylamino-2,2,6,6-tetramethylpiperidine-N-oxyl (AA-TEMPO), alone or in combination with p-nitrosophenol or 2-methyl-4-nitrosophenol, have good inhibitory properties, even with the exclusion of oxygen, in inhibiting the polymerization of vinylaromatic or vinylaliphatic compounds at increased temperatures. 
     In German Patent No. 2,804,449, general reference is made to the effectiveness of nitroso compounds in the distillation of vinylaromatic monomer compounds. The 2,6-dinitro-p-cresol that is protected in this case has the disadvantage, as do nitrocresols generally, that these compounds are retardants. Retardants slow down polymer formation but cannot prevent it completely so that small polymer amounts are always formed. In contrast to the retardants, inhibitors prevent polymer formation completely over a certain period of time depending on their effectiveness, but have no effect thereafter. 
     HTMPO and AA-TEMPO are known as radical traps in radical-induced styrene polymerization. However, the finding of an extraordinarily good inhibitory effect in styrene distillation in vacuum was surprising. Additionally, it has been found, surprisingly, that the combinations of 2-methyl-4-nitrosophenol/HTMPO, p-nitrosophenol/HTMPO, 2-methyl-4-nitrosophenol/AA-TEMPO, and p-nitrosophenol/AA-TEMPO exhibit synergistic effects in the inhibition both with oxygen, as well as without oxygen, i.e., the effect of the mixtures is better than the effect of HTMPO or AA-TEMPO alone. An especially strong synergism exists in the combination of p-nitrosophenol/HTMPO and p-nitrosophenol/AA-TEMPO. Here, a 50% mixture has an extraordinarily good effect. 
     For the process of the invention, styrene, substituted styrene (e.g., α-methylstyrene), acrylic acid esters, methacrylic acid esters, and divinylbenzene can be used. 
     The greatest effectiveness is attained by the inhibitors or inhibitor mixtures of the invention at a temperature of 90° to 140° C., preferably 100° to 120° C. The amount of inhibitor added can be varied as a function of the distillation conditions. Usually, the degree of stabilization is proportional to the amount of inhibitor added. 
     It was determined that HTMPO and AA-TEMPO, alone or in combination with p-nitrosophenol or 2-methyl-4-nitrosophenol in amounts of 50 to 200 ppm, preferably in amounts of 100 to 150 ppm, based on the vinylaromatic or vinylaliphatic compound, produce the best results. 
     This primarily depends on the temperature of the distillation mixture and the desired degree of inhibition. 
     If HTMPO or AA-TEMPO is used with p-nitrosophenol or 2-methyl-4-nitrosophenol, the mixing ratio is 90:10 to 10:90, preferably 50:50. 
     The preparation of the inhibitors of the invention is known from the literature (e.g., DE-OS  Offenlegungsschrift=Disclosure)! 4,219,459 and Annalen 417, p. 120). ##STR1## 
    
    
     Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. 
     The effectiveness of the inhibitors or inhibitor mixtures was determined by gravimetric polymer determination according to the following procedure: 
     A 300 ml amount of freshly distilled styrene together with the inhibitor to be studied were heated to 110° C. with stirring in a 500-ml three-necked flask and the time was recorded. Samples were periodically taken and the polymer content was gravimetrically determined by precipitation with methanol. In this manner, &#34;polymer vs. time&#34; curves were obtained wherein the time until a polymer content of 3% was obtained was chosen as a measure of the effectiveness of the inhibitor or inhibitor mixture. The experiments were done under nitrogen atmosphere (nitrogen was constantly bubbled in). The results are shown in Table I. 
     
                       TABLE I______________________________________Retention times of styrene at 100° C. until apolymer content of 3% under nitrogen was achieved with the useof different inhibitors.                 Residence time to 3%Inhibitor             polymer formation______________________________________Styrene not inhibited      48 minHTMPO           50 ppm    107 minHTMPO           100 ppm   128 minp-Nitrosophenol 50 ppm    122 minp-Nitrosophenol 100 ppm   176 min2-Methyl-4-nitrosophenol           50 ppm    169 minHTMPO/          50 ppm    184 minp-Nitrosophenol 50 ppmHTMPO/          50 ppm    197 min2-Methy1-4-nitrosophenol           50 ppmAA-TEMPO        50 ppm    102 minAA-TEMPO        100 ppm   123 minAA-TEMPO/       50 ppm    184 min2-Methyl-4-nitrosophenol           50 ppmAA-TEMPO/       75 ppm    162 min2-Methyl-4-nitrosophenol           25 ppmAA-TEMPO/       50 ppm    169 minp-Nitrosophenol 50 ppm______________________________________ 
    
     In addition, HTMPO and AA-TEMPO, compared with the nitrosophenols, have the advantage of better storage stability and lower toxicity. Additionally, these materials do not tend to decompose spontaneously. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.