SK36398A3 - Method of inhibiting polymerization - Google Patents

Abstract

A process for the prevention of premature polymerisation of styrene monomer during its production by the dehydrogenation of ethylbenzene into styrene. The process comprises: (a) passing a supply current of ethylbenzene over a dehydrogenation catalyst in a reactor, to form a current of styrene; (b) removing an exhaust effluent containing by-products consisting of H2, ethylbenzene vapour, styrene vapour, benzene, toluene, CO, CO2 and water vapour from the reactor; (c) injecting a styrene polymerisation inhibitor; and (d) compressing the exhaust effluent for the ultimate treatment stages, the gases containing no more than a trace of polystyrene. Also claimed is the system and sub-systems for the production of styrene monomer. Preferably the polymerisation inhibitor is a radical chain terminator type, of the class of sterically hindered amines. It is chosen from the following derivatives of 1-oxyl-2,2,6,6,-tetramethylpiperidine-4-yl :- -the acetate, 2-ethylhexanoate, stearate, benzoate and 4-tert-butylbenzoate; and from the bis(1-oxyl-2,2,6,6-tetramethylpiperidine-4-yl derivatives: succinate, adipate, sebacate, n-butylmalonate, phthalate, isophthalate, terephthalate and hexahydroterephthalate; and also -N, N<1>-bis(1-oxyl-2,2,6,6-tetramethylpiperidine-4-yl)-adipamide, N-(1-oxyl-2,2,6,6-tetramethyl-piperidine-4-yl)caprolactam, N-(1-oxyl-2,2,6,6-tetramethylpiperidine-4-yl)dodecylsuccinimide, 2,4,6-trisN-butyl-N-(1-oxyl-2,2,6,6-tetramethylpiperidine-4-yl)-s-tria zine, and 4,4<1>-ethylenebis(1-oxyl-2,2,6,6-tetramethylpiperazine-3-one). The more preferred inhibitor is the sebacate of bis(1-oxyl-2,2,6,6-tetramethylpiperidine-4-yl).

Description

The method of inhibition comprises passing the ethylbenzene feed through a hydrogenation catalyst in a dehydrogenation reactor to form a styrene stream; a gas stream circuit containing by-products including hydrogen, ethylbenzene in the gas phase, styrene, benzene and toluene vapors, carbon monoxide, carbon dioxide and water vapor from the reactor; injecting a styrene polymerization inhibitor into the gas stream and then compressing it without generating more than trace amounts of polystyrene in the styrene-forming process, said inhibitor being a shielded amine capable of terminating radical reactions.

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Process for inhibiting polymerization

Technical field

The present invention relates to the inhibition of polymerization in a process stream comprising a polymerizable monomer component, and more particularly to the polymerization of monovinyl aromatic monomers, such as styrene, contained in a gas withdrawn from a dehydrogenation unit.

In the production of a monovinyl aromatic monomer such as styrene from a chemical feedstock such as ethylbenzene, the monomer is prepared by dehydrogenating the feedstock in a dehydrogenation unit. For example, ethylbenzene (EB) is treated to styrene by passing EB through a dehydrogenation unit, which removes hydrogen atoms from the EB molecules to form styrene molecules. The gaseous by-products of this chemical reaction, containing mostly hydrogen, are withdrawn from the dehydrogenation unit under vacuum as dehydrogenation gas for further processing.

The by-product gaseous product from the dehydrogenation unit normally contains hydrogen, carbon dioxide, carbon monoxide, water vapor and vapor phase hydrocarbons. In one alternative, the gas may be distilled to remove heavy fractions (ethylbenzene and styrene) that are recycled to the EB dehydrogenation unit and to the styrene product line with optional combustion of the light fractions including hydrogen as a fuel. Similarly, the entire gaseous product removed from the dehydrogenation can also be combusted as a fuel gas.

Alternatively, the dehydrogenation gas may be fed to a phenylacetylene reduction system comprising one or more reactors through which a styrene monomer containing an undesired phenylacetylene admixture is passed through a suitable catalyst to reduce the phenylacetylene component to styrene by reaction with the hydrogen contained in the dehydrogen gas.

In each of the above cases, the dehydrogenation gas must be removed from the dehydrogenation unit, compressed and fed to another reactor, distillation unit or burner. Regardless of the end use, the dehydrogenation gas, which is under reduced pressure, must be compressed to about 6.5 x 10 ³ Pa to be transported to the next stage of processing.

Attempts to squeeze the EB dehydrogenation gas cause problems due to the presence of styrene monomer in the gas, since styrene is very reactive and polymerizes rapidly. Due to the heat released by the compression of this gas in the compressor, the styrene monomer would rapidly polymerize on the components and surfaces of the compressor, thereby causing its operational malfunctions and poor efficiency. When the compressor is down, the polymer will freeze. Re-commissioning is complex and costly.

A common solution to prevent polymer deposits in a compressor to dehydrogenation gas is to continuously rinse with ethylbenzene injected into the compressor along with dehydrogenation gas. It does not have a significant beneficial effect on the polymerization of the monomer, but allows the compressor to run continuously by physically dissolving the polymer formed on the compressor components. However, the resulting polymer solution in ethylbenzene must then be treated to remove ethylbenzene from the polymerized monomer, which is usually a low-molecular-weight, low-molecular-weight material of low commercial value. A disadvantage of this method is that the treatment of the polymer solution in ethylbenzene is expensive. This polymer increases the amount of tar in the business waste and means the loss of valuable raw material.

It is known that aromatic vinyl compounds such as styrene, α-methylstyrene and other substituted vinylbenzenes tend to polymerize at elevated temperatures. Since aromatic vinyl compounds produced by conventional industrial processes contain by-products and impurities, these compounds need to be subjected to technological separation and purification steps prior to further industrial use. This separation and purification is usually accomplished by distillation.

BACKGROUND OF THE INVENTION

Various compounds are used as polymerization inhibitors to prevent premature polymerization of aromatic vinyl polymers when purified by distillation. In the past, sulfur has been widely used to inhibit the polymerization of aromatic vinyl compounds. More recently, however, many chemical compounds have been found or developed to replace sulfur in inhibiting polymerization. These compounds have been successful in the industrial application of distillation processes to varying degrees.

In a typical method of distilling aromatic vinyl compounds using a polymerization inhibitor, the mixture containing the aromatic vinyl compounds to be distilled is typically contacted with the polymerization inhibitor before being subjected to distillation conditions in the still. A persistent serious problem is that the amount of polymer formed in the distillation system and in the high purity product obtained therefrom is substantially higher than desired. Even worse, sometimes the entire aromatic vinyl compound polymerizes in the distillation system, causing considerable economic losses. U.S. Patent Applications 4,252,615 and 4,341,600 disclose typical distillation apparatus, the disclosure of which is herein incorporated by reference.

U.S. Pat. No. 3,733,326 discloses the inhibition of polymerization of vinyl monomers by free radical precursors. US Patent 1,027,150 describes the stabilization of styrene using a nitroxyl radical. Soviet patent 1,139,722 describes the use of a bis-nitroxyl radical as an inhibitor of the thermal polymerization of styrene. Japanese patent Hei-1-165534 discloses the use of 1-piperidyloxy derivatives as styrene polymerization inhibitors. US Patent 1,588,888 describes the inhibition of styrene polymerization by a nitroxyl radical.

U.S. Patent 4,087,147 discloses a method of using 2-nitro-β-cresol as a polymerization inhibitor. U.S. Patents 4,105,506 and 4,252,516 disclose a method of using 2,6-dinitro-β-cresol as a polymerization inhibitor. U.S. Patents 4,132,602 and 4,132,603 disclose the use of a halogenated aromatic nitro compound as a polymerization inhibitor for use in the distillation of aromatic vinyl compounds. However, these aromatic nitro compounds have relatively low activities and must therefore be used at high concentrations, especially at higher distillation temperatures. In view of their relatively high toxicity to humans, these aromatic nitro compounds cannot be considered acceptable agents for inhibiting polymerization.

In addition, U.S. Patents 3,988,212 and 4,341,600 disclose the use of N-nitrosodiphenylamine in conjunction with dinitrocresol derivatives to inhibit the polymerization of aromatic vinyl compounds under vacuum distillation conditions. U.S. Pat. No. 4,466,904 describes the use of phenothiazine, 4-tert-butylcatechol and 2,6-dinitro-β-cresol as a polymerization inhibition system in the heating of aromatic vinyl compounds in the presence of oxygen. U.S. Patent 4,468,343 discloses a composition and method of use

2,6-dinitro-β-cresol and either phenylenediamine or 4-tert-butylcatechol in the presence of oxygen to prevent polymerization of aromatic vinyl compounds upon heating. European Patent Application 240.297 A1 describes the use of substituted hydroxylamine and dinitrophenol to inhibit the polymerization of an aromatic vinyl compound in distillation at elevated temperatures. However, the efficiency of these systems depends on the presence of oxygen. This results in inconsistent inhibition due to inconsistent air distribution in the distillation column, increasing the risk of explosion.

More recently, U.S. Patent 5,254,760 has disclosed an inhibitor composition designed to prevent the polymerization of an aromatic vinyl monomer in the distillation and purification of these monomers. Such inhibitors may or may not be useful in process gas effluents due to the presence of aqueous condensates and the solubility of these inhibitors in aqueous condensates. The contamination of the aqueous phase is undesirable in terms of waste water treatment and the need to protect the catalyst from poisoning due to the recycling of the aqueous phase into the reactors.

All of the aforementioned patents and references are hereby incorporated by reference herein in their entirety.

SUMMARY OF THE INVENTION

The present invention overcomes these drawbacks by proposing a system and method in which an EB purge is not required to remove polymer deposits from the dehydrogenation gas compressor components, as this method prevents monomer polymerization in the compressor.

The method and system therein of a polymerization bioreactor which prevented the polymerization unit from preventing the formation of the polymer described and claimed uses an inhisa injected into the gas from an EB dehycompressor for that gas to be inside the compressor.

In particular, the present invention provides a method for preventing premature polymerization of a styrene monomer in its preparation by dehydrogenating ethylbenzene to styrene, the method comprising:

passing the ethylbenzene feed through the dehydrogenation catalyst in a dehydrogenation reactor to form a styrene stream;

extracting a gaseous stream containing by-products including hydrogen, ethylbenzene in the gas phase, styrene, benzene and toluene vapors, carbon monoxide, carbon dioxide and water vapor from the reactor;

injecting a styrene polymerization inhibitor into the gas stream; and compressing the gaseous stream for further processing without generating more than trace amounts of polystyrene in the styrene production system.

Figure 1 is a flow diagram of a system installed in a styrene manufacturing method.

In Figure 1, which is a flow diagram of the styrene production system, feed of ethylbenzene EBF is fed to a dehydrogenation unit for ethylbenzene EBD. In the EBD reactor, hydrogen is removed from ethylbenzene and a styrene monomer stream is removed from the reactor. The by-product gaseous product is withdrawn from the EBD reactor via the VG line, compressed in the compressor for the VGC product gas, wherein the inhibitor reservoir I is connected to the VG line via the supply line for the inhibitor IS. The styrene inhibitor is injected through the IS line into the VG line upstream of the VGC compressor. The compressed mixture of inhibitor and hydrogen gas removed from the dehydrogenation is then passed through valve V and directed either to the waste heat boiler B and combusted as fuel, or returned via valve V through the RC recycle line and injected back into the raw styrene from the EBD reactor. Alternatively, a phenylacetylene PAR separation system can be used in conjunction with the present invention by means of a second PV valve to recycle all or part of the dehydrogenation compressed gas to the phenylacetylene PAR reduction reactor. Similarly, the styrene monomer feed from EBD is passed through a phenylacetylene PAV valve to a PAR reactor. The inhibited dehydrogenation gas stream flowing through the PV valve to the PAR reactor is combined with the styrene monomer stream, and the hydrogen in this gas reduces the phenylacetylene fraction contained in the styrene monomer to styrene, which is then discharged through the PM line for the purified monomer.

In an alternative embodiment not shown here, the PV valve may be replaced by a purification system such as a distillation unit allowing the separation of heavier liquid components such as ethylbenzene and styrene flowing through the valve V to the RC recycle line to recycle these heavier liquid components back to the ethylbenzene feed. The remainder of the dehydrogenation gas without these heavier liquid fractions is then recycled to a PAR reactor to reduce phenylacetylene to further purify the styrene monomer stream from the EBD reactor.

The inhibitor used may be any suitable polymerization inhibitor capable of being injected into the process gas stream. A particularly preferred inhibitor has been found bis- (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate, which is a shielded amine type inhibitor capable of terminating radical reactions.

Preferred shielded amine nitroxyl compounds thus useful according to this patent contain at least one radical corresponding to formula (I) per molecule.

Particularly useful are those shielded amine nitroxyl compounds which are derived from the corresponding shielded amines described in patent application EP-A-592.363.

Among these, 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl acetate deserves special attention;

-oxyl -2,2,6,6-tetramethylpiperidin-4-yl-2-ethylhexanoate;

l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl stearate;

l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl benzoate;

1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl-4- tert -butylbenzoate;

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) sebacate;

bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) n-butyl malonate;

bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) phthalate;

bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) isophthalate;

bis (l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) terephthalate;

bis (l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) hexahydrotereftalát; 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- (l-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 and

4,4'-ethylenebis (1-oxyl-2,2,6,6-tetramethylpiperazin-3-one);

Most preferably, the compound is bis (1-oxyl-2,2,6,6, tetramethylpiperidin-4-yl) sebacate;

2,4,6-tris [N-butyl-N- (l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)] - s-triazine; or

4,4'-ethylenebis (1-oxyl-2,2,6,6-tetramethylpiperazin-3-one).

The inhibitor is preferably injected in an amount of 0.1 ppm to 1000 ppm, more preferably in an amount of 1ppm to 500 ppm and most preferably in an amount of 5 ppm to 100 ppm by weight of the monomer.

This inhibitor is injected into the dehydrogenation gas stream through the IS line upstream of the gas compressor so that it can successfully prevent styrene polymerization on the compressor components. The inventors conclude that other similar injectable styrene polymerization inhibitors will also serve to prevent styrene polymerization on the internal components of the compressor. Although this method has not yet been tried in a true compressor for ethylbenzene hydrogenation gases, the inventors believe that, due to the nature of the inhibitor and the known characteristics of the styrene monomer in the dehydrogenation gas applications, the described inhibitor will sufficiently prevent polymerization in the compressor no polymer.

Another object of the present invention is a system for the dehydrogenation of ethylbenzene to styrene, comprising:

a catalytic dehydrogenation reactor modified to dehydrogenate ethylbenzene to styrene;

an ethylbenzene stream connected to the reactor;

a dehydrogenation gas removal subsystem coupled to said reactor and capable of withdrawing from said reactor by-products of dehydrogenation of ethylbenzene;

a compressor for these gaseous products coupled to said gas product removal system which is capable of receiving; and a polymerization inhibitor subsystem between said dehydrogenation gas removal subsystem and said compressor capable of injecting the polymerization inhibitor into the gaseous product from said reactor.

The definitions and preferences set forth above for this process also apply to the ethylbenzene dehydrogenation system.

Although a specific preferred embodiment of the present invention is described above, it is not intended to limit the present invention to the particular embodiments described herein, as they are to be construed as illustrative rather than limiting, and it will be apparent to those skilled in the art that the invention is not so limited. For example, while the embodiments described herein relate to the gas fraction from dehydrogenation of ethylbenzene to styrene, it is understood that the patent also covers other gas systems containing polymerizable monomers. Therefore, the present invention is claimed as a patent covering all changes and modifications herein to illustrate the specific example described herein, which do not depart from the spirit and purpose of the invention. Embodiments of the invention covered by the specific claims are defined as follows.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

A styrene unit constructed similar to the flow diagram of Figure 1, operating under typical conditions without adding an inhibitor to the stream fed to the dehydrogenation gas compressor. After three months, polystyrene accumulates on the internal components of the compressor. The presence of polymer on the turbine blades results in imbalance and vibration. The deposition of the polymer reduces the efficiency of the compressor. Shutting down the compressor for cleaning is necessary after about five months if the unit is not to be permanently damaged.

Example 2

The same unit is operated as described in the example

1. 20 ppm of a water-insoluble polymerization inhibitor of bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate is injected into the stream fed to the dehydrogenation gas compressor.

After four months of operation, the compressor operates continuously without increasing vibration and loss of efficiency. Traces of polystyrene can be found in the water condensation tank downstream of the EBD reactor. No trace of inhibitor is detected in the aqueous phase of the condensation tank.

Example 3

The unit is operated as described in Example 2. In this case, 20 ppm of a water-soluble 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine polymerization inhibitor is injected into the stream of the dehydrogenation gas compressor. After twelve months of operation, the compressor works continuously without increasing vibrations and losing efficiency. No traces of polymer are detected in the aqueous phase of the condensation tank. Only a small amount of inhibitor is detected in the aqueous phase of the condensation tank.

Example 4

The unit is operating as described in Example 2.

In this case, 20 ppm of the water-soluble polymerization inhibitor 1-oxyl-2,2,6,6-tetramethylpiperidin-4-one is injected into the stream fed to the dehydrogenation gas compressor. After twelve months of operation, the compressor works continuously without increasing vibrations and losing efficiency. No polymer is detected in the aqueous phase of the condensation tank. Only a small amount of inhibitor is detected in the aqueous phase of the condensation tank.

Example 5

The unit is operated as described in Example 2. In this case, 20 ppm of a water-soluble polymerization inhibitor 1-oxyl-2,2,6,6 ^- tetramethylpiperidine, 1-oxyl are injected into the stream of the dehydrogenation gas compressor. -2,2,6,6-tetramethylpiperidin-4-ylbenzoate, 2,4,6-tris- [N-butyl-1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)] -s- triazine or 4,4'-ethylenebis (1-oxyl-2,2,6,6-tetramethylpiperazin-3-one). After four months of operation, the compressor works continuously without increasing vibrations and losing efficiency.

Industrial usability

This patent application offers a relatively extensive set of styrene polymerization inhibitors based on compounds with a shielded amine group for commercial use. Although the efficacy of all of the proposed reagents has not yet been tested in long-term industrial operation, preliminary tests with some of them have shown good results in protecting the compressor compressing the gaseous products from ethylbenzene dehydrogenation from sediment formation and subsequent failures. Also the proposed system of their application can be beneficial.

Claims (8)

A process for preventing premature polymerization of a styrene monomer in its manufacture by dehydrogenating ethylbenzene to styrene, the method comprising: passing an ethylbenzene feed over a dehydrogenation catalyst in a dehydrogenation reactor to form a styrene stream; extracting a gaseous stream containing by-products including hydrogen, ethylbenzene in the gas phase, styrene, benzene and toluene vapors, carbon monoxide, carbon dioxide and water vapor from the reactor; injecting a styrene polymerization inhibitor into the gas stream; and compressing the hydrogenation gas stream for further processing without producing larger than trace polystyrene in the styrene production system. The method of claim 1, wherein said polymerization inhibitor is a shielded amine type inhibitor capable of terminating radical reactions. The method of claim 1, wherein said polymerization inhibitor is selected from the group consisting of: l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl acetate; l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl-2-ethylhexanoate; l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl stearate; l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl benzoate; l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl-4-tert-butylbenzoate; bis (l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) succinate; bis (l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) adipate; bis (l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate; bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) n-butyl malonate; bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) phthalate; bis (l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) isophthalate; bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) terephthalate; bis (l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) hexahydrotereftalát; 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 and 4,4 'ethylenebis (1-oxyl-2,2,6,6-tetramethylpiperazin-3-one); The method of claim 3, wherein said polymerization inhibitor is bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate. 5. A system for dehydrogenating ethylbenzene to styrene, the system comprising: a catalytic dehydrogenation reactor modified to dehydrogenate ethylbenzene to styrene; an ethylbenzene stream connected to the reactor; a dehydrogenation gas removal subsystem coupled to said reactor and capable of withdrawing from said reactor by-products of dehydrogenation of ethylbenzene; a compressor for said gaseous products coupled to said gas product removal system capable of receiving; and a sub-system for the polymerization inhibitor between said dehydrogenation gas removal subsystem and said compressor capable of injecting the polymerization inhibitor into the gaseous product from said reactor. The method of claim 5, wherein said polymerization inhibitor is a shielded amine type inhibitor capable of terminating radical reactions. The system of claim 6, wherein said polymerization inhibitor is selected from the group consisting of: 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl acetate; 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl-2-ethylhexanoate; l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl stearate; l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl benzoate; l-oxyl-2,2,6,6-tetramethylpiperidin-4-yl-4-tert-butylbenzoate; 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) sebacate; bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) n-butyl malonate; 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 and 4,4 'ethylenebis (1-oxyl-2,2,6,6-tetramethylpiperazin-3-one); The system of claim 7, wherein said polymerization inhibitor is bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate. i / ι