WO1998045385A1 - Composition inhibiting radical polymerisation of ethylenically unsaturated monomers and method for inhibiting radical polymerisation of said monomers - Google Patents

Abstract

The invention concerns a composition for inhibiting the radical polymerisation of ethylenically unsaturated monomers characterised in that it comprises at least a nitroxide derivative of formula (I') in which: the point on the nitroxyl function oxygen atom represents a non-paired electron; C represents with the nitroxyl function a heterocycle with five vertices, saturated or unsaturated and comprising in position 3 relative to the nitroxyl function a heteroatom selected among nitrogen, sulphur and oxygen, provided that when the heteroatom is a nitrogen atom it carries as the case may be a hydrogen atom or a (C1-C22) alkyl group; n represents a whole number between 1 and 7; R represents a hydrocarbon radical which can be an acyclic aliphatic or a saturated or unsaturated cycloaliphatic or an aromatic radical or an optionally substituted arylalkyl radical; or two R groups carried by a common carbon of the heterocycle constitute with the latter an optionally substituted cycloalkyl radical.

Description

 "Composition inhibiting the radical polymerization of ethylenically unsaturated monomers and method for inhibiting the radical polymerization of said monomers.

The invention relates to a composition which can be used as an inhibitor of the radical polymerization of ethylenically unsaturated monomers and to a process intended to prevent the radical polymerization of such unsaturated monomers. 5 The ethylenically unsaturated monomers are inclined to spontaneously polymerize under the action of heat. However, premature polymerization must be avoided during the manufacture, purification and storage of said monomers. During manufacture or purification, early polymerization is detrimental since it causes a drop in production yields and fouling of the installations often making temporary stoppage of production necessary for maintenance reasons. Due to the exothermicity of the polymerization reaction, explosions and fires are also to be feared.

The distillation of certain ethylenically unsaturated monomers is particularly problematic when it requires the use of high temperatures: this is particularly the case for the distillation of vinyl aromatic derivatives such as styrene, α-methylstyrene and other vinylbenzenes.

In order to prevent the polymerization of ethylenically saturated monomers, it is known in the art to add one or more polymerization inhibitors or retarders, either preventively during manufacture, or even directly to said monomers before their use.

Thus, for the inhibition of the polymerization of styrene at high temperature, US 4,086,147 and US 4,252,615 recommend the use of nitroaromatic derivatives; US 3,988,212 indicates the possibility of using N-πitrosodiphenylamine in combination with dinitrocresol; US 5,510,547 teaches the association of phenylenediamine and hydroxyiamiπe; and SU 1 027 150, JP-1-165 534 and SU 1 139 722 indicate the possibility of using nitroxide derivatives as inhibitors of radical polymerization.

The best known of the nitroxide derivatives described in the literature is 2,2,6,6-tetramethylpιpéπdine-1 -oxyl. This compound is however very toxic and sparingly soluble in many monomers, so much so that its use on an industrial scale has not even been envisaged. In general, the inhibitors of ^the state of the art are toxic, unstable and / or have a too low efficiency.

Thus, despite the numerous variants proposed in the literature, the development of an effective, stable and economical inhibitor would be of great use in the art.

The object of the invention is to provide a composition intended to prevent the premature polymerization of ethylenically unsaturated monomers.

More specifically, the composition of the invention comprises: a nitroxide derivative of formula (I '):

^^ (R) n

I

O ^m

(!') in which :

- the dot on the oxygen atom of the nitroxyl function designates an unpaired electron,

- C represents with the nitroxyl function a heterocycle with five vertices, saturated or unsaturated and comprising in position 3 with respect to the nitroxyl function a heteroatom chosen from nitrogen, sulfur and oxygen, it being understood that when the heteroatom is a nitrogen atom, it optionally carries a hydrogen atom or a (dC ₂₂ ) alkyl group;

- n represents an integer between 1 and 7 and

- R represents a hydrocarbon radical which can be an acyclic aliphatic or cycloaliphatic saturated or unsaturated or aromatic radical or an aryialkyl radical optionally substituted; or else two R groups carried by the same carbon of the heterocycle constitute with the latter an optionally substituted cycloalkyl radical.

Within the meaning of the invention, the hydrocarbon chain symbolized by C is a substituted hydrocarbyl as indicated above. Except for the heteroatom and the nitrogen of the nitroxyl function, it only incorporates carbon atoms and of hydrogen at the level of its skeleton. Thus, no ketone function is present in the skeleton of said heterocycle.

As regards the substituents of said heterocycle represented by R, they can themselves be substituted by a hydroxyl function, a (Cι- ₂₂ ) alkyl or (Cι. ₂₂ ) alkoxy group.

More particularly, the composition claimed comprises:

(i) one or more nitroxides of formula (I):

dans laquelle A représente 0, S ou N; les pointillés désignent la présence éventuelle d'une seule double liaison entre les sommets 2 et 3, 3 et 4 ou 4 et 5 ou bien la présence de deux doubles liaisons entre les sommets 2 et 3 et 4 et 5; étant entendu que lorsque A représente 0 ou S, une simple liaison est présente entre les sommets 2 et 3 et 4 et 5; le point sur l'atome d'oxygène de la fonction nitroxyle désigne un électron non apparié;

wherein A represents 0, S or N; the dotted lines indicate the possible presence of a single double bond between the vertices 2 and 3, 3 and 4 or 4 and 5 or else the presence of two double bonds between the vertices 2 and 3 and 4 and 5; it being understood that when A represents 0 or S, a single bond is present between the vertices 2 and 3 and 4 and 5; the dot on the oxygen atom of the nitroxyl function designates an unpaired electron;

Ri and R ₂ independently of one another represent a hydrogen atom, a group (Ci-C _∑∑ Jalkylθ optionally substituted by hydroxy or (C ₁ -C ₂ 2) alkyl, a group (C ₆ -Cι ₄ ) aryl or a (C ₆ -Cι ₄ ) aryl- (Cι-C ₂₂ ) alkyl group, the aryl nucleus of the aryl and aryialkyl groups possibly being substituted by one or more (Cι-C ₂₂ ) alkyl groups, (Cι- C ₂₂ ) alkoxy or hydroxy; or else Ri and R ₂ represent, together with the carbon atom to which they are linked, a (C ₃ -C _β ) cycloalkyl radical optionally substituted by one or more (Cι-C ₂ ) alkyl groups, it being understood that when a double bond is present between the vertices 3 and 4 or, 4 and 5, at least one of Ri and R ₂ represents nothing; R ₃ and R ₄ independently of one another represent a group (Cι-Cι ₂ ) alkyl, (C ₆ -Cι ₄ ) aryl, or else R ₃ and R ₄ together represent with the carbon atom to which they are linked a (C ₃ -C ₈ ) cycloalkyl group optionally substituted by one or more (Cι-C ₂₂ ) alkyl groups, it being understood that when a double bond is present between the vertices 4 and 5, at least one of R ₃ and R ₄ represent nothing;

ou bien R₅ et R₆ représentent ensemble avec l'atome de carbone auquel ils sont liés un groupe (C₃-C₈)cycloalkyleR ₅ and Rβ independently of one another represent an (Cι-Cι ₂ ) alkyl group,

or R ₅ and R ₆ together represent with the carbon atom to which they are linked a (C ₃ -C ₈ ) cycloalkyl group

IO optionally substituted by one or more (Cι-C ₂₂ ) alkyl groups; it being understood that when a double bond is present between the vertices 2 and 3, at least one of R ₅ and Rβ represents nothing;

R ₇ represents a hydrogen atom or a (Cι-C ₂₂ ) alkyl group; it being understood that R ₇ represents nothing when A represents 0

15 or S or when a double bond is present between vertices 2 and 3 or 3 and 4;

(ii) optionally in combination with one or more vehicles compatible with said monomer and with each of the constituents of the composition. 0 The ethylenically unsaturated monomers more specifically comprise olefinic hydrocarbons such as styrene, α-methylstyrene, divmylbenzene, vinyltoluene and styrenesulfonic acids but also dienes of the type of isoprene and butadiene; halogenated monomers of the type of vinyl chloride, chloroprenβ, vinylidene chloride, fluoride

Vinylidene and vinyl fluoride; unsaturated acids such as acrylic acid, methacrylic acid and crotonic acid; unsaturated esters of the type of vinyl acetate, methyl methacrylate, ethyl acrylate, methyl acrylate, 2-hydroxyethyl acrylate or methacrylate, ethylene bismethacrylate, triacrylate tπméthylolpropane, ^resins) acrylated epoxy and polyethylene glycol diacrylate, the msaturés amides such as ^acrylamide, N, N-diméthylacrylamιde the méthylèπebisacrylamide and N- vinylpyrrolidone, πitriles msaturés the type of the acryloπitrile, the ethers unsaturated such as vinyl ether, methyl ylpyridines, diethyl vmyiphosphonate and sodium styrenesulfonate II should be understood that this list is by no means exhaustive, the invention generally relating to the stabilization of any type of ethylenically unsaturated monomer.

Nevertheless, the compositions of the invention are particularly effective in the case of the stabilization of vinylaromatic monomers of the styrene, α-methylstyrene, vinyltoluene, divmylbenzene and styrenesulfonic acids type. The term “alkyl group” means a linear or branched saturated hydrocarbon radical such as methyl, ethyl, propyl, isopropyl, butyl and sec-butyl.

The cycloalkyl group denotes a saturated mono- or bicyclic hydrocarbon radical. Examples are cyclopropyl, cyclohexyl, cyclopentyl and cycloheptyl.

The term aryl also denotes a mono- or polycyclic aromatic group such as phenyl, naphthyl or anthryl.

The respective sizes of these groupings are specified in parentheses in each case. Thus, an alkyl group (Cι-Cι ₂ ) has from 1 to 12 carbon atoms whereas a group (Cι-C ₂₂ ) has from 1 to 22 carbon atoms. Similarly, (C ₃ -C _β ) cycloalkyl denotes a cycloalkyl radical of 3 to 8 carbon atoms and (C ₆ -Ci4) aryl denotes aryl groups comprising from 6 to 14 carbon atoms. The dotted lines in formula (I) of the nitroxides which can be used according to the invention designate the possible presence of one or two double bonds.

When the compound of formula (I) comprises a single double bond, this can be located between the vertices 2 and 3 or 3 and 4 or 4 and 5.

When the compound of formula (I) comprises two double bonds, these are respectively located between the vertices 2 and 3 and 4 and 5 6

In the general formula above, one of R, and R ₂ , one of R ₃ and R ₄ or / and one of Rs and R _s may not exist. Likewise, R ₇ may represent nothing.

Insofar as in formula (I) each carbon atom is tetravalent, it is easily understood that when a double bond is present between the vertices 2 and 3, one of R ₅ and R ₆ represents nothing.

Similarly, when a double bond is present between the vertices 4 and 5, one of Ri and R ₂ represents nothing and one of R ₃ and R ₄ represents nothing. The situation is similar when a double bond is present between the vertices 3 and 4, in which case one of Ri and R ₂ represents nothing and R ₇ represents nothing.

Since oxygen and sulfur are divalent atoms in formula (I) above, it goes without saying that when A represents O or S, R ₇ represents nothing and a single bond is respectively present between the vertices 2 and 3 and 3 and 4.

In other words, the compounds of formula (I) or (I ') include the compounds of formula (IA), (IB), (IC), (ID) and (IE) below:

(IA) (IB) (IC)

(IA) (IB) (IC)

(ID) (E) 98/5

7

In the above formulas and in formula (I), the point attached to the oxygen atom of the nitroxyl -NO ^* function denotes an unpaired electron. Recall that the nitroxyl function is the characteristic of nitroxide derivatives.

Among the nitroxides of formula I, some are particularly preferred.

Particularly preferred are the nitroxides of formula (IA)

(IA) in which: - the dotted lines denote the possible presence of a single double bond between the vertices 3 and 4,

- A represents N;

- R ₃ , R4, Rs and R _β are independently a hydrogen atom or a (Cι-Cι ₂ ) alkyl group;

- R ₇ represents (Cι-C ₂₂ ) alkyl; - Ri and R2 being as defined in claim 2, it being understood that when a double bond is present between the vertices 3 and 4, at least one of Ri and R ₂ represents nothing and R ₇ represents nothing.

Among these compounds, mention may be made more particularly of those in which at least four of the substituents R ₃ , R ₄ , R _{5 |} R _β and R ₇ represent a (Cι-Cι ₂ ) alkyl group.

Preferably, among this subgroup of compounds, those for which R ₃ , R ₄ , Rs, Rβ and R ₇ represent methyl are very particularly preferred. Mention may in particular be made, as representative of the compounds which can be used in the claimed composition:

- 2,2,3,4,5,5-hexamethylimidazolidine-l-oxyle, - 2,2,4,5,5-pentamethyl-3-imidazoiine-1-oxyle,

- 2,2,5,5-tetramethyl-4-phenyl-3-ιmidazolιne-1 -oxyle,

- 2,4,5,5-tetramethyl-2-octyl-3-imidazoline-1-oxyle,

- 2,2,3,5,5-pentamethyl-4-phenylimidazolidine-1-oxyle. Thus, the compositions of the invention comprising at least one nitroxide of formula (IA) constitute a particularly preferred embodiment of the invention.

The compositions of the invention form either true solutions, that is to say that they consist of perfectly miscible ingredients, or emulsions or else suspensions. According to a preferred embodiment, however, the compositions are in the form of true solutions.

The presence in the composition of one or more vehicles is optional. However, it may prove necessary when the solubility of the nitroxide constituents of the composition in the monomer to be stabilized is low, or even insufficient. In this case, in fact, it is preferable to incorporate into the composition one or more vehicles compatible on the one hand with the monomer to be stabilized, and on the other hand with each of the other constituents of the composition. By "compatible" is meant according to the invention a vehicle which is chemically inert with respect to the various ingredients of the composition and of the monomer. The nature of the vehicle therefore depends on the various constituents present as well as on the very nature of the monomer.

When the monomer is a vinyl aromatic derivative, particularly suitable vehicles are benzene, toluene, xylene, ethylbenzene, styrene, acetophenonβ, methylphenylcarbinol or mixtures of these solvents. In this case, it is preferred to use ethylbenzene. It will nevertheless be noted that these solvents are suitable in the case of a large number of ethylenically unsaturated monomers.

Depending on the nature of the monomer to be stabilized, those skilled in the art will easily determine the best combination of nitroxides and solvents leading to the best results. For example, in the case of nitroxides which are particularly insoluble in the monomer considered, it is possible either to incorporate the appropriate solvent into the composition, or to change the nitroxide constituent or 9 add a nitroxide having, at the substituents R ,, R ₂ and R ₇ , lipophilic groups, for example (C ₆ -C ₂₂ ) alkyl, (C ₃ -C ₈ ) cycioalkyl groups substituted by one or more (C ₆ -C ₂₂ ) alkyl, (C ₆ -Cι ₄ ) aryl or (C _β -Cι ₄ ) aryl- (Cι- C ₂₂ ) alkyl. Preferably, the compositions of the invention also contain one or more nitroaromatic derivatives. Such compounds are known in the art as a polymerization retarder. It is also known to combine them with polymerization inhibitors. It should be noted that the polymerization inhibitor prevents polymerization until a certain time beyond which the polymerization reaction starts normally. This time is the induction time. The longer the induction time, the more effective the inhibition. The role of the self-timer is different. It does not prevent polymerization but slows down the kinetics of polymerization. A more or less significant synergy is observed by association of an inhibitor and a retarder. Thus, the compositions of the invention containing at least one polymerization retarder and at least one polymerization inhibitor form a preferred subject of the invention.

The nitroaromatic derivative is advantageously chosen from 1, 3-dinitrobenzene, 1, 4-dinitrobenzene, 2,6-dinitro-4-methylphéπol, 2-nιtro-4-méthyipénol, 2,4-dinitro-1-naphthol, 2, 4,6-trinotrophenol (picπque acid), 2,4-dinitro-6-methylphenol, 2,4-dinitrochlorobenzene, 2,4-dinitrophenol, 2,4-dιnιtro-6- sec-butylphéπol, 4-cyano-2- nitrophenol, and 3-iodo-4-cyano-5-nitrophenol.

We prefer to use 2,6-dinitro-4-methyiphenol, 2,4-dιnιtro-6-methylphenol, 2,4-dinitrophenol and 2,4-dinitro-6-sec-butylρhenol, 2,4-dinιtro-6 - sec-butylphenol being particularly advantageous.

The nitroaromatic derivatives are added to the composition so that the ratio of the total mass of the nitroxide type constituents of formula I to the total mass of the nitroaromatic type constituents is between 90/10 and 10/90, preferably between 80 / 20 and 20/80, better still between 60/40 and 40/60. This weight ratio is calculated by making the ratio of the total amount (by weight) of the nitroxides of formula I present in the composition to the total amount (by weight) of the nitroaromatic derivatives of the composition.

The compositions of the invention may contain other polymerization inhibitors such as hydroquinones; catechols of the tert-butyl-catechol type; alkylated phenols; nitrosophenols and mtrosophenylhydroxylamines or any other inhibitor known in the art, provided that these are compatible with the other ingredients of the composition under the operating conditions to which the monomer to be stabilized is subjected. Certain nitroxides of formula (I ') and (I) are commercial. The others are easily prepared by a person skilled in the art from commercial products.

In general, the compounds of formula (I ') and (I) in which

A is O or N are obtained by implementing a process comprising the oxidation of a compound of formula (II):

dans laquelle Ri, R_2l R₃, R4, Rs et R_β sont tels que définis ci-dessus et P est un groupe protecteur de l'azote, lorsque A représente N, ou bien P ne représente rien, lorsque A représente O. En tant qu'agent oxydant, on préfère utiliser un hydroperoxyde tel que l' hydroperoxyde de tert-butylβ ou un peracide tel que l'acide m- chloroperbenzoique, l'acide monopβrphtalique ou le sel de magnésium de monoperoxyphtalate (MMPP).

in which Ri, R _2l R ₃ , R4, Rs and R _β are as defined above and P is a nitrogen protecting group, when A represents N, or else P does not represent anything, when A represents O. As the oxidizing agent, it is preferred to use a hydroperoxide such as tert-butyl β hydroperoxide or a peracid such as m-chloroperbenzoic acid, monopβrphthalic acid or the magnesium salt of monoperoxyphthalate (MMPP).

The oxidation reaction is preferably carried out in the presence of 1 to 3 molar equivalents of oxidizing agent relative to the total amount of cyclic amme of formula (II), in a solvent, for example a polar aprotic solvent such as a halogenated hydrocarbon of the dichloromeihane type The reaction temperature can be adjusted between -10 and + 30 ° C. Advantageously, the oxidation takes place at room temperature in dichloromethane.

The protective group P is a protective group of amino function, non-oxidizable under the oxidation conditions involved.

Those skilled in the art will easily select this group as a function of the reactivity of the amino function which it is desired to oxidize. For this, he could draw inspiration from Protective Groups in Organic Chemistry, 2nd Edition, T.W. Green and P.G.M. Wuts, Wiley, 1991. Preferably, the group P is a formyl function.

When A is protected by P, the oxidation of the compound of formula (II) does not lead directly to a compound of formula (I). It is the deprotection of the nitrogen atom carrying the P function which will make it possible to isolate a compound of formula (I) in which R ₇ represents a hydrogen atom. The deprotection conditions naturally depend on the nature of the protective group and are easily determined by a person skilled in the art. When P is a formyl group, this can be hydrolyzed by the action of a base such as potassium hydroxide. Optimal conditions for this hydrolysis reaction are a temperature of 30 to 60 ° C, preferably 40 to 50 ^e C and a methaπol / water medium, for example consisting of 2 to 5 parts by volume of methanol per part in volume of water.

The compounds of formula (I) in which R ₇ represents (d-C ₂₂ ) alkyl are easily obtained by alkylation of the corresponding compounds of formula (I) in which R ₇ represents hydrogen. This alkylation reaction can be carried out in a conventional manner by the action of an alkylating agent such as a dialkyl sulfate in the presence of a base such as potassium hydroxide and at the reflux of a polar protic solvent such as methanol. Other alkylating agents can be used such as an alkyl halide.

The compounds of formula II are easily accessible either because ^they are commercial, or because they are readily synthesized from commercial products by known methods. For the synthesis of these different compounds of formula II, inspiration may be drawn from the following publications: JFW Keana, J Am Chem Soc 100 (3), 1978, 934-937; JF W. Keana, J. Org. Chem, 53, 1988, 2365-2367.

Nitroaromatic derivatives are also commercially available products.

The compositions of the invention are easily prepared by mixing the various constituents. Heating of the reaction mixture with stirring can be envisaged in certain cases to dissolve the nero-aromatic derivative (s) and / or the constituent (s) of nitroxide type. The invention also relates to a process for preventing the radical polymerization of an ethylenically unsaturated monomer. This method comprises, for example, adding to said monomer, an effective amount of a composition of the invention as defined above. More generally, the process of the invention comprises the addition to an ethylenically unsaturated monomer, of an effective amount of one or more nitroxides of formula (I ¹ ):

- (R) n

(I ^* ) in which:

- the dot on the oxygen atom of the nitroxyl function designates an unpaired electron,

- C represents with the nitroxyl function a heterocycle with five vertices, saturated or unsaturated and comprising in position 3 with respect to the nitroxyl function a heteroatom chosen from nitrogen, sulfur and oxygen, it being understood that when the heteroatom is a nitrogen atom, where appropriate it carries a hydrogen atom or a (Cι-C ₂₂ ) alkyl group;

- π represents an integer between 1 and 7 and - R represents a hydrocarbon radical which can be an acyclic or cycloaliphatic saturated or iπsaturated or aromatic aliphatic radical or an optionally substituted aryialkyl radical; or else two R groups carried by the same carbon of the heterocycle constitute with the latter an optionally substituted cycloalkyl radical.

More particularly, the process of the invention comprises the addition of at least one nitroxide of formula (I):

dans laquelle

in which

A represents O, S or N; the dotted lines indicate the possible presence of a single double bond between the vertices 2 and 3, 3 and 4 or 4 and 5 or else the presence of two double bonds between the vertices 2 and 3 and 4 and 5; it being understood that when A represents O or S, a single bond is present between the vertices 2 and 3 and 4 and 5; the dot on the oxygen atom of the nitroxyl function designates an unpaired electron;

R and R ₂ independently of one another represent a hydrogen atom, a (Cι-C ₂ 2) alkyl group optionally substituted by hydroxy or

(Cι-C2 ₂ ) alkyl, a group (C _β -Cι ₄ ) aryl or a group (C ₆ -Cι ₄ ) aryl- (Cι-C ₂₂ ) alkyl, the aryl nucleus of the aryl and aryialkyl groups being able to be substituted by one or more (Cι-C ₂₂ ) alkyl, (Cι-C ₂₂ ) alkoxy or hydroxy groups; or Ri and R ₂ together with the carbon atom to which they are linked represent a (C ₃ -C _β ) cycloalkyl radical optionally substituted by one or more (Cι-C ₂₂ ) alkyl groups, it being understood that when a double link is present between vertices 3 and 4 or 4 and 5, at least one of Ri and R ₂ represents nothing; R ₃ and R ₄ independently of one another represent a group (C, -C, ₂ ) alkyl, (C ₆ -Cι ₄ ) aryl, or else R ₃ and R ₄ together represent with the carbon atom to which they are linked a (C ₃ -C ₈ ) cycioalkyl group optionally substituted by one or more (CrC ₂₂ ) alkyl groups; it being understood that when a double bond is present between the vertices 4 and 5, at least one of R ₃ and R ₄ represents nothing;

ou bien R_s et R₆ représentent ensemble avec l'atome de carbone auquel ils sont liés un groupe (C₃-C₈)cycloalkyle éventuellement substitué par un ou plusieurs groupes (Cι-C₂₂)alkyle; étant entendu que lorsqu'une double liaison est présente entre les sommets 2 et 3, l'un au moins de R₅ et R_β ne représente rien;R ₅ and R ₆ independently of one another represent an (Cι-Ci ₂ ) alkyl group,

or else R _s and R ₆ represent together with the carbon atom to which they are linked a (C ₃ -C ₈ ) cycloalkyl group optionally substituted by one or more (Cι-C ₂₂ ) alkyl groups; it being understood that when a double bond is present between the vertices 2 and 3, at least one of R ₅ and R _β represents nothing;

R ₇ represents a hydrogen atom or a (dC ₂₂ ) alkyl group; it being understood that R ₇ represents nothing when A represents O or S or else when a double bond is present between the vertices 2 and 3 or 3 and 4.

It goes without saying that this process is preferred when it involves the addition to said monomer of at least one preferred nitroxide according to the invention, that is to say the addition of at least one nitroxide of formula (IA ), better still at least one nitroxide of formula (IA) in which at least four of the substituents R ₃ , R, Rs, Rβ and R ₇ represent a group (Cι-Cι ₂ ) alkyl and more preferably at least a nitroxide of formula (IA) in which R ₃ , R ₄ , Rβ, Rβ and R ₇ represent methyl.

A preferred way of stabilizing the monomer is to add to it a nitroxide derivative chosen from:

2,2,3,4,5,5-hexamethylimidazolidine-1-oxyl,

- 2,2,4,5,5-pentamethyl-3-imidazoline-1-oxyle,

2,2,5,5-tetramethyl-4-phenyl-3-imidazoline-1-oxyl,

- 2,4,5,5-tetramethyl-2-octyl-3-imidazoline-1-oxyl, - 'a 2,2,3,5,5-pentamethyl-4-phenylimιdazolidine-1 -oxyle, and more preferably , 2,2,3,4,5,5-hexamethylιmιdazolιdιne- 1-oxyle. 15

The amount of nitroxide to be added to achieve effective inhibition of polymerization varies widely. It is a function of the monomer to be stabilized and of the operating conditions to which this monomer is subjected. It is clear that at high temperatures, the amount of inhibitor will be greater. The process of the invention is in fact applicable for the stabilization of the monomer, during manufacture and purification. However, it is not uncommon for the purification to be carried out by distillation of the monomer, the temperature at the reboiler can exceed 150 ° C.

Thus, the ideal amount of inhibitor should be assessed on a case-by-case basis.

Anyway, for information, a total amount of nitroxide derivative of formula (I) of between 1 and 2000 ppm, preferably between 5 and 1000 ppm is generally sufficient, this amount being expressed relative to the total weight of the monomer to stabilize. It should be understood that the total amount of nitroxide derivative of formula (I) denotes the sum of the contents of each nitroxide of formula (I).

As indicated above, it is desirable to add to the monomer an effective amount of one or more nitroaromatic derivatives as a polymerization retarder. As preferred retarder, mention may again be made of 2,6-dinitro-4-methyiphenol, 2,4-dinιtro-6-methylphenol, and 2,4-dinitrophenol, but above all 2,4-dinitro-6- sec-butylphenol.

The proportion of the nitroaromatic type retarder (s) is preferably such that the ratio of the total mass of nitroxide type constituents of formula I to the total mass of the nitroaromatic type constituents is between 90/10 and 10/90, better still between 80/20 and 20/80.

The radical polymerization inhibitor and the nitroaromatic derivative can be added to the monomer in a completely conventional manner. The point of introduction, in the case of a distillation, is very variable: it is conceivable to add each of these compounds at ^the heat exchanger, pipes, pumps, reboiler, compressor or more usually containers. It should be understood that the addition can be carried out continuously or repeated over time in one or different particular sites

According to the invention, it is possible to envisage the simultaneous or separate addition of the inhibitor of the nitroxide type and of the derivative of the nitroaromatic type. In order to increase the efficiency of the inhibition, it is recommended to incorporate into the monomer to stabilize oxygen or air, in an effective amount, at the same time as the inhibitor of formula (I) is added thereto. This addition of air makes it possible to reinforce the activity of inhibition of the nitroxide compound.

The process of the invention is particularly advantageous in terms of effectiveness of the inhibition but also because of the stability of the nitroxides of formula (I).

The following examples are given by way of illustration and relate to preferred embodiments of the invention.

EXAMPLE 1

Process for the preparation of 2,2,3,4,5,5-hexamethylimi- dazolidiπe-1-oxyle

a) 2-methyl-2,3-diaminobutane

This compound is prepared using the method described in US 4,681,948 starting from isoprene. The reaction scheme followed is recalled below (scheme 1): 17

1) Na / πθuOH 2) HCI

DIAGRAM 1

b) 2,2,3,5,5-pentamethylimidazoiidine

Is successively introduced into a 250 ml reactor placed under a nitrogen atmosphere: toluene (100 ml), acetone (33.8 g; 770 mmol, 10 eq.), Anhydrous potassium carbonate (10 g) , p-toluenesulfonic acid monohydrate (40 mg) and 2-methyl-2,3-dιamιnobutane (7.86 g; 77 mmol; 1 eq.). It is brought to reflux for 24 hours with good stirring. The solvents are removed under reduced pressure and then the expected imidazolidine is distilled under reduced pressure (-60 ^e C / 12 mm Hg). 7.5 g of the title imidazolidine are obtained in the form of a colorless oil which tends to crystallize at room temperature.

c) 1 -formyl-2,2,4,4,5-pentamethyl-lmidazolldiπe

Formic-acetic anhydride is prepared with a mixture of acetic anhydride (6 g; 59 mmol; 1, 2 eq.) And formic acid (2.7 g; 59 mmol; 1, 2 eq.), stirred for 2 hours at 55 ° C.

This solution is then cooled and then added slowly to a solution of 2,2,3,5,5-pentamethylιmιdazolιdιne (7g; 49 mmol; 1 éq) in CH CI ₂ (30 ml) maintained at 0 ° C. It is allowed to return to 25 ° C and then stirring is continued for 12 hours. The reaction medium is washed with a saturated aqueous solution of Na ₂ C0 ₃ (50 ml). The organic phase is concentrated on a rotary evaporator. A pale yellow oil is obtained. After a few hours at + 4 ° C., this oil crystallizes, the solid is recrystallized from a hexane / ethyl acetate mixture (1/1). The expected imidazolidine is obtained in the form of white crystals (5.1 g).

d) 2,2,3,5,5-pentamethyl-4-formyl-imidazolidinθ-1-oxyle A solution is prepared containing the product obtained in step c)

(5g; 29 mmol; 1 eq.), Mo (CO) ₆ (50 mg) in 1, 2-dichloroethane (30 ml). This solution is brought to reflux, then a 4M solution of tert-butyl hydroperoxide (14.5 ml; 58 mmol; 2 eq.) In 1, 2-dichloroethane is added dropwise. Maintained at reflux and with stirring for 4 hours. The reaction medium is then washed with a 5% aqueous solution of sodium sulfite (100 ml). The aqueous phase is extracted with dichloromethaπe (2 x 50 ml) then the combined organic phases are dried over MgS0 ₄ and concentrated under reduced pressure. The residue is crystallized from hexane. The expected nitroxide is obtained in the form of yellow-orange crystals (4.5 g).

e) 2,2,3,5,5-pentaméthyiimidazolidine-l-oxyle

The nitroxide obtained in step d) (4 g; 21 mmol; 1 eq.) Is dissolved in a solution made from KOH (10 g), methanol (30 ml) and water (8 ml). This solution is brought to 45 ° C. and stirred for 21 hours. The solvents are removed on a rotary evaporator then the residue is taken up in CH ₂ CI ₂ (30 ml) and filtered through a column of celite. The filtrate is dried over K ₂ C0 ₃ and concentrated on a rotary evaporator. The expected product is thus obtained in the form of a brown solid (3.1 g).

f) 2,2,3,4,5,5-hexamethyllmidazolldlne-l-oxyle

The nitroxide obtained in step e) (3.0 g; 19 mmol; 1 eq.) Is dissolved in toluene (30 ml). Brought to reflux followed by _j ny gout drop a solution of dimethyl sulfate (2.4 g; 19 mmol; 1 eq) in toluene (30 ml). Maintained for 1 hour under reflux and with stirring. The reaction medium is washed with a saturated aqueous solution of sodium carbonate (2 × 50 ml) The organic phase is dried over K ₂ C0 ₃ and then concentrated on a rotary evaporator. The residue is crystallized from an AcOEt / hexane mixture. The expected nitroxide is thus obtained after filtration in the form of orange crystals (2.7 g; mp 43-45 ^β C).

EXAMPLE 2

In order to evaluate the inhibition properties of the reference products, vis-à-vis the radical polymerization of vinyl monomers, inhibition tests with styrene were undertaken.

Before each test, the styrene used (marketed by the Merck Company) is previously destabilized by passage over an activated alumina column (obtained from the Procatalyze Company) in order to completely eliminate the tertbutylcatechol initially present at a rate of 15-20 ppm .

The resulting styrene (10 ml) is placed in a test tube and the appropriate amount of inhibitor is then added. Argon is introduced into the reactor sky by bubbling (5 min.) As well as into the liquid phase by bubbling (5 min.). The tube is closed and placed in an oil bath thermostatically controlled at 100 ° C for 2 hours.

The level of polymer formed after 2 hours is determined by the methanol precipitation method. To this end, a cooled sample of 10 ml of styrene is transferred to a glass flask containing approximately 50 ml of methanol, in order to precipitate the polystyrene formed, insoluble in methanol. This medium is then filtered on a millipore filter and the residue (corresponding to the polymer formed) is dried in an oven at 40 ° C before being weighed. The test results are summarized in Table I TABLE I

HMIO = 2,2,3,4,5,5-hexamethylimidazolidine-1-oxyle DNBP = 2,4-dinitro-6-sec-butyl-phenol

DNP = 2,4-dinitrophenol.

The results show that 2,2,3,4,5,5-hexamethyimidazolidine-1-oxyl is an excellent inhibitor of the polymerization of styrene and that it can be used either alone or in combination with dinitrophenolic compounds.

EXAMPLE 3

By reproducing the protocol described in Example 2, the inhibition properties of other products in accordance with the invention were evaluated with regard to the radical polymerization of styrene.

The results are presented in Table II below. BOARD

Claims

1. Composition intended to prevent the radical polymerization of ethylenically unsaturated monomers, characterized in that it comprises at least one nitroxide derivative of formula (T): ^ ^• (R) n

d ') in which: - the dot on the oxygen atom of the nitroxyl function designates an unpaired electron, - C represents with the nitroxyl function a heterocycle with five vertices, saturated or unsaturated and comprising in position 3 with respect to the nitroxyl function a chosen heteroatom among nitrogen, sulfur and oxygen, it being understood that when the heteroatom is a nitrogen atom it optionally carries a hydrogen atom or a (Cι-C ₂₂ ) alkyl group; - n represents an integer between 1 and 7 and - R represents a hydrocarbon radical which can be an acyclic aliphatic or cycloaliphatic saturated or unsaturated or aromatic radical or an aryialkyl radical optionally substituted; or else two R groups carried by the same carbon of the heterocycle constitute with the latter an optionally substituted cycloalkyl radical. 2. Composition according to claim 1, characterized in that it comprises (i) one or more nitroxides of formula (I):

in which - A represents O, S or N; - the dotted lines denote the possible presence of a single double bond between the vertices 2 and 3, 3 and 4 or 4 and 5 or else the presence of two double bonds between the vertices 2 and 3 and 4 and 5; it being understood that when A represents O or S, a single bond is present between the vertices 2 and 3 and 4 and 5; - the dot on the oxygen atom of the nitroxyl function designates an unpaired electron; - R, and R ₂ independently of one another represent a hydrogen atom, a (Cι-C ₂₂ ) alkyl group optionally substituted by hydroxy or

a group

or a (C ₆ -Cι ₄ ) aryl- (Cι-C ₂₂ ) alkyl group, the aryl nucleus of the aryl and aryialkyl groups possibly being substituted by one or more (Cι-C ₂₂ ) alkyl groups, (Cι-C ₂₂ ) alkoxy or hydroxy; - or Ri and R ₂ together with the carbon atom to which they are linked represent a (C ₃ -C ₈ ) cycloalkyl radical optionally substituted by one or more (Cι-C2 ₂ ) alkyl groups, it being understood that when a double bond is present between vertices 3 and 4 or 4 and 5, at least one of Ri and R ₂ represents nothing; - R ₃ and R ₄ independently of one another represent a (Cι-Cι ₂ ) alkyl group (Cβ-Cujaryl, or else R ₃ and R ₄ together represent with the carbon atom to which they are linked a (C ₃ -C ₈ ) cycloalkyl group optionally substituted by one or more (Cι-C ₂₂ ) alkyl groups; it being understood that when a double bond is present between the vertices 4 and 5, at least one of R ₃ and R ₄ represents nothing; R ₅ and R _β represent independently of one another a group (CrCι ₂ ) alkyl,

or else R ₅ and R ₆ together with the carbon atom to which they are bonded represent a (C ₃ -C ₈ ) cycloalkyl group optionally substituted by one or more (Cι-C ₂₂ ) alkyl groups; it being understood that when a double bond is present between the vertices 2 and 3, at least one of R _s and R _β represents nothing; R ₇ represents a hydrogen atom or a (CrC ₂₂ ) alkyl group; it being understood that R ₇ represents nothing when A represents O or S or else when a double bond is present between the vertices 2 and 3 or 3 and 4; (n) optionally in combination with one or more vehicles compatible with said monomer and with each of the constituents of the composition. 3. Composition according to claim 1 or 2, characterized in that it comprises at least one nitroxide of formula (IA).

(IA) in which: - the dotted lines indicate the possible presence of a single double bond between the vertices 3 and 4, - A represents N; - R ₃ , R ", Rs and Rβ are independently a hydrogen atom or a (Cι-Ci ₂ ) alkyl group; - R ₇ represents (Cι-C ₂₂ ) alkyl; Ri and R ₂ being as defined in claim 2, it being understood that when a double bond is present between the vertices 3 and 4, at least one of Ri and R ₂ represents nothing and R ₇ represents nothing. 4. Composition according to claim 2 or 3, characterized in that at least four of the substituents R ₃ , R ₄ , R ₅ , R ₆ and R ₇ represent a group (Cι-C ₁₂ ) alkyl. 5. Composition according to claim 2, 3, or 4 characterized in that the substituents R ₃ , R ₄ , R ₅ , R _β and R ₇ represent a methyl group. 6. Composition according to any one of claims 1 to 4, characterized in that it comprises at least one derivative chosen from: - 2,2,3,4,5,5-hexamethylιmιdazolιdιπe-l -oxyle, - 2,2,4,5,5-pentamethyl-3-ιmιdazolιπe-1 -oxyle, - 2,2,5,5-tetramethyl-4-phenyl-3-ιmιdazolιne-1-oxyle, - 2,4,5,5-tetramethyl-2-octyl-3-ιmιdazolιne-1-oxyie, - 2,2,3,5,5-pentamethyl-4-phenylιmιdazolιdιne-1-oxyle 7. Composition according to any one of the preceding claims, characterized in that it comprises, as vehicle, one or more solvents chosen from benzene, toluene, xylene, ethylbenzene, styrene, acetophenone and methylphenylcarbinol 8. Composition according to any one of the preceding claims, further comprising at least one nitroaromatic derivative. 9. Composition according to claim 8, characterized in that the nitroaromatic derivative is chosen from 2,6-dinιtro-4-methylphenol, 2,4-dinιtro-6-methylphenol, 2,4-dinitrophenol, 2, 4-dinιtro-6-sec-butylphenol and 2-nιtro-4-methylphenol, the nitroaromatic derivative preferably being 2,4-dιnιtro-6-sec-butylphenol. 10. Composition according to any one of claims 8 or 9, characterized in that the ratio of the total mass of the nitroxide constituents to the total mass of the constituents of nitroaromatic derivative type is between 90/10 and 10/90, preferably between 80/20 and 20/80. 11 A method for preventing the radical polymerization of an ethylenically unsaturated monomer, comprising adding to said monomer an effective amount of one or more nitroxides of formula (I '). ^ - (R) n

(! ^• ) in which. - the dot on the oxygen atom of the nitroxyl function designates an unpaired electron, - C represents with the nitroxyl function a heterocycle with five vertices, saturated or iπsaturated and comprising in position 3 with respect to the nitroxyl function a heteroatom chosen from nitrogen, sulfur and oxygen, it being understood that when the heteroatom is a nitrogen atom, where appropriate it carries a hydrogen atom or a (Cι-C ₂₂ ) alkyl group; - n represents an integer between 1 and 7 and - R represents a hydrocarbon radical which can be an acyclic aliphatic or cycloaliphatic saturated or unsaturated or aromatic radical or an aryialkyl radical optionally substituted; or else two R groups carried by the same carbon of the heterocycle constitute with the latter an optionally substituted cycloalkyl radical. 12. Method according to claim 11, characterized in that at least one of the nitroxides has the formula (I):

in which - A represents O, S or N; - the dotted lines denote the possible presence of a single double bond between the vertices 2 and 3, 3 and 4 or 4 and 5 or else the presence of two double bonds between the vertices 2 and 3 and 4 and 5; it being understood that when A represents O or S, a single bond is present between the vertices 2 and 3 and 4 and 5; - the dot on the oxygen atom of the nitroxyl function designates an unpaired electron; - Ri and R ₂ independently of one another represent a hydrogen atom, a (Cι-C ₂ 2) alkyl group optionally substituted by hydroxy or (Cι-C ₂₂ ) alkyl, a group (Cβ-Cι ₄ ) aryl or a group (C ₆ -Cι ₄ ) aryl- (Cι-C ₂₂ ) alkyl, the aryl nucleus of the aryl and aryialkyl groups being able to be substituted by a or more (Cι-C_β) alkylθ, (Cι-C ₂₂ ) alkoxy or hydroxy groups; - or else R, and R ₂ together with the carbon atom to which they are linked represent a (C ₃ -C ₈ ) cycloalkyl radical optionally substituted by one or more (Cι-C ₂₂ ) alkyl groups, it being understood that when a double bond is present between the vertices 3 and 4 or 4 and 5, at least one of Ri and R ₂ represents nothing; - R ₃ and R ₄ independently of one another represent a group (Cι-Cι ₂ ) alkyl, (Cβ-C jaryl, or else R ₃ and R ₄ together represent with the carbon atom to which they are linked a (C ₃ -C ₈ ) cycloalkyl group optionally substituted by one or more (Cι-C ₂₂ ) alkyl groups; it being understood that when a double bond is present between the vertices 4 and 5, at least one of R ₃ and R ₄ represents nothing; - R ₅ and R _β independently of one another represent an (Cι-Cι ₂ ) alkyl group,

or else R ₅ and R _β represent together with the carbon atom to which they are linked a (C ₃ -C ₈ ) cycloalkyl group optionally substituted by one or more (Cι-C ₂₂ ) alkyl groups; it being understood that when a double bond is present between the vertices 2 and 3, at least one of R ₅ and R _β represents nothing; - R ₇ represents a hydrogen atom or a (Cι-C ₂₂ ) alkyl group; it being understood that R ₇ represents nothing when A represents O or S or else when a double bond is present between the vertices 2 and 3 or 3 and 4. 13. Method according to claim 12, characterized in that at least one of said nitroxides has the formula (IA)

(IA) in which: - the dotted lines indicate the possible presence of a single double bond between the vertices 3 and 4, - A represents N; - R ₃ , R ₄ , Rs and R ₆ are independently a hydrogen atom or a (Cι-Cι ₂ ) alkyl group; - R ₇ represents (Cι-C ₂₂ ) alkyl; - Ri and R ₂ being as defined in claim 12, it being understood that when a double bond is present between the vertices 3 and 4, at least one of Ri and R ₂ represents nothing and R ₇ represents nothing . 14. Method according to claim 13, characterized in that at least four of the substituents R ₃ , R ₄ , R ₅ , Rβ and R ₇ represent a group (C1-Cι) alkyl. 15. The method of claim 14, characterized in that R ₃ , R, Rs, Rβ and R ₇ represent methyl. 16. Method according to any one of claims 11 to 14, characterized in that at least one of said nitroxides is chosen from: - 2,2,3,4,5,5-hexamethylimidazolidine-1-oxyle, - 2,2,4,5,5-pentamethyl-3-imidazoline-1-oxyle, 2,2,5,5-tetramethyl-4-phenyl-3-imidazoiine-1-oxyle, - 2,4,5,5-tetramethyl-2-octyl-3-imidazoline-1-oxyle, - 2,2,3,5,5-ρentamethyl-4-phenylimidazolidine-1-oxyle. 17. Method according to any one of claims 1 1 to 16, characterized in that the total amount of added nitroxide is between 1 and 2000 ppm, preferably between 5 and 100 ppm relative to the total weight of said monomer. 18. A method according to any one of claims 1 1 to 17, characterized in that one further adds to said monomer, one or more nitroaromatic derivatives, in particular chosen from 2,6-dinitro-4-methylphenol, 2,4-dinitro-6-methylphenol, 2,4-dinitrophenol, 2,4-dinitro-6-sec-butylphenoi and 2-nitro-4-methyl phenol. 19. Method according to any one of claims 1 1 to 18, characterized in that one addition additionally to said monomer 2,4-diπitro-6-sec-butylphenol. 20. Method according to any one of claims 11 to 19, characterized in that the ratio of the total mass of the nitroxide constituents to the total mass of the constituents of nitroaromatic derivative type is between 90/10 and 10/90, preferably 80/20 and 20/80. 21. Method according to any one of claims 11 to 20, characterized in that said ethylenically unsaturated monomer is a vinyl aromatic monomer, preferably chosen from styrene, α-methylstyrene, vinyltoluene, divinylbenzene and styrenesulfonic acids . 22. Method according to any one of the preceding claims, characterized in that an effective quantity of air or oxygen suitable for improving the activity of the nitroxide (s) is incorporated into said monomer.