WO2017016943A1 - Method for recovering sulfonic esters or sulfonyl halides from salts of sulfonic acids - Google Patents

Abstract

A method for the obtainment of an ester or a halide of a fluorinated sulfonic acid from an salt of a sulfonic acid is herein disclosed. The method is particularly useful for recovering waste sulfonate salts from reaction mixtures obtained by reaction of sulfonic esters of alcohols having a pKa lower than 15 with nucleophile compounds.

Description

Method for recovering sulfonic esters or sulfonyl halides from salts of sulfonic acids Cross-reference to related application

This application claims priority from European patent application No. 15178203.4, the whole content of this application being incorporated herein by reference for all purposes.

Technical Field

The present invention relates to a method for the recovery of sulfonic esters or sulfonyl halides, in particular esters or halides of (per)fluoroalkyl sulfonic acids or of (per)fluoro(poly)oxyalkyl sulfonic acids, from salts of sulfonic acids.

Background Art

Sulfonyl halides are used in organic chemistry for converting certain alcohols, like fluorinated alcohols and phenols, which have a pK_a lower than 15, into the corresponding sulfonic esters, thereby making such alcohols reactive to nucleophilic substitution. For example, TONELLI, et al, Linear perfluoropolyether difunctional oligomers: chemistry, properties and applications, Journal of Fluorine Chemistry, 1999, 95, 51 - 57, teach that (per)fluoropolyether (PFPEs) alcohols of the formula R_fCH₂OH (wherein R_f is a perfluoropolyether chain) and having a pK_a of about 12.4 can be conveniently activated to nucleophilic substitution by reaction with perfluorobutanesulfonyl fluoride (CF₃CF₂CF₂CF₂SO₂F) to provide the corresponding sulfonic esters R_fCH₂OSO₂CF₂CF₂CF₂CF₃ (nonaflate esters), which can be reacted with nucleophile compounds in the presence of a base. At the end of the reaction, beside the desired substitution product, the reaction mixture contains a salt of the sulfonic acid corresponding to the sulfonic halide used. For example, if CF₃CF₂CF₂CF₂SO₂F is used to activate an alcohol and sodium t-butoxide is used as base in the nucleophilic substitution reaction, sodium perfluorobutanesulfonate (CF₃CF₂CF₂CF₂SO₃ ^-Na⁺) would be obtained as by-product. The sulfonic acid salt (sulfonate salt) is usually separated from the mixture and discarded, which has negative economic impact, especially on an industrial scale; it would therefore be useful to provide a method avoiding the discharge of the sulfonate salt.

CONTE, et al, A new method for recovering waste alkaline perfluoro-n-butanesulfonate, Journal of Fluorine Chemistry, 1991, 53, 277-283, report on a process for the recovery of perfluorobutane sulfonic acid from waste CF₃(CF₂)₃SO₃-Na⁺ obtained as product in reactions wherein CF₃(CF₂)₃SO₂-OCH₂CF₃ is used as alkylating agent. The authors teach that CF₃(CF₂)₃SO₃ ^-Na⁺ can be converted into the corresponding sulfonic acid CF₃(CF₂)₃SO₃H and that this acid can either be transformed into CF₃(CF₂)₃SO₂-OCH₂CF₃ or into CF₃(CF₂)₃SO₂Cl or CF₃(CF₂)₃SO₂F that is subsequently esterified with CF₃CH₂OH. Conte et al further teach to prepare CF₃(CF₂)₃SO₂Cl by treatment of CF₃(CF₂)₃SO₃H with PCl₅ and to convert CF₃(CF₂)₃SO₂Cl into CF₃(CF₂)₃SO₂F by ion exchange. This method provides CF₃(CF₂)₃SO₂F in fairly good yields (58-74%); however, it employs PCl₅ as key reagent, which is toxic. Moreover, PCl₅ is converted to POCl₃, which cannot be re-used in a further sulfonate salt conversion reaction because it is not sufficiently reactive.

US 2732398 to Minnesota Mining & Manufacturing Company relates to certain fluorocarbon sulfonic acids and to a process for their manufacture by means of an electrochemical process which leads to "... the saturated perfluoro sulfonyl compounds, from which the sodium and potassium salts can be made ... and ... converted ... to the perfluoro sulfonic acids" (col. 5, lines 22 - 26). Example 1 (col. 8, lines 50 - 65) discloses in particular the conversion of CF₃SO₃H into CF₃SO₃Cl by treatment with PCl₅. Therefore, this method suffers from the same disadvantages as those affecting the method disclosed in the above-discussed article of Conte et al.

SCOTT, et al, A general method for the preparation of perfluoroalkanesulfonyl chlorides, Journal of Fluorine Chemistry, 2005, 126, 1196 - 1201, teach to prepare a perfluoroalkylsulfonyl chloride through a two-step one-pot synthesis comprising the reaction of a iodoperfluoroalkane with sodium dithionite to provide a sodium perfluoroalkyl sulfinate salt which is subsequently oxidized with N-chlorosuccinimide. This document does not provide any hint or suggestion to the conversion of an alkaline perfluoroalkyl sulfonyl salt into the corresponding halide.

The need is thus still felt for an industrially applicable method for the conversion of salts of sulfonic acids into sulfonyl derivatives that can be conveniently used as sulfonating agent, said method avoiding the use of toxic reagents and providing high yields.

Summary of invention

The Applicant has surprisingly found out that salts of fluorinated sulfonic acids can be easily converted into corresponding phenolic esters by treatment with a phenol that bears at least one electron-withdrawing group and that the phenolic esters can in turn be converted into sulfonyl halides by treatment with an anhydrous alkali- or alkali-earth metal halide. Either the phenolic esters or the sulfonyl halides can be used as sulfonating agents, in particular as sulfonating agents for fluorinated and aromatic alcohols having a pK_a lower than 15.

Accordingly, the present invention relates to a method [method (M-1)] for the obtainment of an ester or a halide of a fluorinated sulfonic acid from an salt of a sulfonic acid, said process comprising the following steps:

• a) reacting a salt of a fluorinated sulfonic acid [salt (S)] with a strong mineral acid [acid (A_M)] to provide the corresponding fluorinated sulfonic acid [acid (A_S)];
• b) reacting the fluorinated sulfonic acid (A_S) from step a) with a phenol substituted with at least one electron-withdrawing group to provide the corresponding ester of sulfonic acid (A_S) [sulfonic ester (E_S)]; and, optionally
• c) reacting ester (E_S) with an anhydrous alkali- or alkali-earth metal halide to provide the corresponding fluorinated sulfonyl halide [sulfonyl halide (A_SH)].

In a preferred aspect, the invention relates to a method [method (M-2)] which comprises using method (M-1) to obtain sulfonic esters (E_S) or sulfonyl halides (A_SH) from waste sulfonic acids salts (S), i.e. salts (S) separated from reaction mixtures containing salts (S) as by-products.

In a further preferred aspect, the invention relates to a method [method (M-3)] for the manufacture of a sulfonic ester of a fluorinated or aromatic alcohol, in particular of a (per)fluoropolyether (PFPE) alcohol, said method comprising obtaining an ester (E_S) or a sulfonyl halide (A_SH) as defined above according to method (M-1) or (M-2) and then reacting said ester or halide with a fluorinated or aromatic alcohol.

General definitions, symbols and abbreviations

For the avoidance of doubt, throughout the present application, the expression “fluorinated sulfonic acid” denotes a (per)fluoroalkyl sulfonic acid or a (per)fluoro(poly)oxyalkyl sulfonic acid.

A (per)fluoroalkyl sulfonic acid is a fully or partially fluorinated alkyl sulfonic acid, while a (per)fluoro(poly)oxyalkyl sulfonic acid is a fully or partially fluorinated alkyl sulfonic acid comprising one or more ethereal oxygen atoms in the alkyl chain.

The acronym “PFPE” stands for “(per)fluoropolyether", i.e. fully or partially fluorinated polyether. When this acronym is used as substantive in the plural form, it is referred to as “PFPEs”.

The term “(per)haloalkyl” denotes a straight or branched alkyl group wherein one or more hydrogen atoms have been replaced with halogen atoms.

Unless otherwise indicated, the term “halogen” includes fluorine, chlorine, bromine or iodine.

The use of parentheses “(…)” before and after names, symbols or numbers identifying formulae or parts of formulae like, for example “formula (S)”, “chain (R_f)” has the mere purpose of better distinguishing those names, symbols or numbers from the rest of the text; thus, said parentheses could also be omitted.

The adjective “aromatic” denotes any cyclic moiety having a number of π electrons equal to 4n+2, wherein n is 0 or any positive integer.

The expression "average functionality (F)" denotes the average number of functional groups per polymer molecule and can be calculated according to methods known in the art. For example, the average functionality (F) of PFPE alcohols can be calculated following the method reported in EP 1810987 A to Solvay Solexis S.p.A .

When ranges are indicated, range ends are included.

The expression “as defined above” is intended to comprise all generic and specific or preferred definitions referred to by that expression in preceding parts of the description, unless indicated otherwise.

Detailed description of the invention Method (M-1) – step a)

The method of the present invention can be applied both to inorganic and organic salts of sulfonic acids. In other words, the method can be used to obtain sulfonic esters (E_s) and sulfonyl halides (A_SH) from inorganic and organic salts of sulfonic acids.

For the purpose of the present description, inorganic salts are alkali- or alkali-earth metal (i. e. a metal belonging to Group I or Group II of the periodic table) salts of sulfonic acids, preferably alkali metal salts, while organic salts are ammonium and phosphonium salts. Examples of ammonium salts are those formed by reaction of a sulfonic acid with a tertiary amine, while examples of phosphonium salts are those formed by reaction of a sulfonic acid with a trialkylphoshine. Preferred examples of ammonium salts are those formed by reaction of a sulfonic acid with a tertiary amine selected from trimethylamine, triethylamine and tributylamine. Preferred examples of phosphonium salts are those formed by reaction of a sulfonic acid with trimethylphosphine, triethylphosphine and tributylphosphine.

According to a preferred embodiment, the alkali salt of fluorinated sulfonic acid [salt (S)] used in step a) of method (M-1) is an alkali metal salt complying with formula:
(S) R-SO₃-Me⁺
wherein:
- R is a (per)fluoroalkyl or a (per)fluoro(poly)oxyalkyl chain comprising a -CF₂- group directly bound to the sulphur atom of the -SO₃ ^- group and
- Me⁺ is an alkali metal cation, preferably a sodium or potassium cation.

In a first aspect, the salt (S) is an alkali metal salt of a (per)fluoroalkyl sulfonic acid [salt (S-1)].

Typically, salt (S-1) complies with formula:
(S-1) R¹-SO₃ ^-Me⁺,
wherein:

• R¹ is a C₁-C₁₀ (per)fluoroalkyl chain, i.e. a straight or branched fully or partially fluorinated C₁-C₁₀ alkyl chain, preferably a straight or branched C₁-C₄ (per)fluoroalkyl chain, with the proviso that chain R¹ contains a -CF₂- group directly bound to the sulphur atom of the -SO₃ ^- group. More preferably, chain R¹ is a perfluoroalkyl chain, still more preferably a straight perfluoroalkyl chain, even more preferably a straight C₁-C₄ perfluoroalkyl chain; and
• Me⁺ is an alkali metal cation, preferably a sodium or potassium cation.

Preferred examples of salts (S-1) are sodium and potassium trifluoromethanesulfonate (triflate), sodium and potassium pentafluoroethane sulfonate, sodium and potassium heptafluoropropanesulfonate and sodium and potassium nonafluorobutanesulfonate (nonaflate), sodium and potassium nonaflate being preferred.

In a second aspect, the salt (S) is an alkali metal salt of a (per)fluoro(poly)oxyalkyl sulfonic acid [salt (S-2)].

Typically, salt (S-2) complies with formula:
(S-2) R²-SO₃ ^-Me⁺
wherein:
- R² is a straight or branched C₁-C₁₀ (per)fluoro(poly)oxyalkyl chain, i.e. a straight or branched fully or partially fluorinated C₁-C₁₀ alkyl chain comprising one or more ethereal oxygen atoms, preferably a straight or branched C₁-C₄ (per)fluoro(poly)oxyalkyl chain, with the proviso that chain R² contains a -CF₂- group directly bound to the sulphur atom of the -SO₃ ^- group. More preferably, chain R² is a perfluoro(poly)oxyalkyl chain, even more preferably a straight perfluoro(poly)oxyalkyl chain, still more preferably a straight C₁-C₄ perfluoro(poly)oxyalkyl chain; and
- Me⁺ is an alkali metal cation, preferably a sodium or potassium cation.

A preferred example of salt (S-2) is C₂F₅OCF₂CF₂SO₃ ^- K⁺ (potassium perfluoro(2-ethoxyethane)sulfonate).

Preferably, salts (S) is a salt (S-1).

The strong mineral acid [acid (A_M)] used in step a) is an inorganic acid having a pK_a lower than 0, preferably lower than -3. Preferred examples of acids (A_M) are fluorosulfonic and sulphuric acid; still more preferably, acid (A_M) is sulphuric acid. It is preferred that acid (A_M) is anhydrous.

An excess of acid (A_M) with respect to salt (S) is typically used in step a). Preferably, acid (A_M) is used in an equivalent ratio ranging from 2 to 5 with respect to salt (S).

Step a) is generally carried out by adding salt (S) to acid (A_M) to obtain a mixture that is heated at a temperature of at least 10°C, preferably ranging from 10°C to 100°C, more preferably from 20°C to 50°C, for a time typically ranging from 1 to 4 hours. A person skilled in the art will be able to determine the reaction temperature and time according to known methods on a case-by-case basis according to the selected salt (S) and acid (A_M). In any case, the completion of the reaction can be followed withdrawing a sample and submitting the sample to ¹⁹F-NMR analysis. The reaction is complete when shifts from -114.3 ppm (–CF₂SO₃M signal) to -112 (-CF₂SO₃H signal) and from -121 ppm (-CF₂-CF₂SO₃M signal) to -122 (-CF₂CF₂SO₃H signal) are observed.

The resulting fluorinated sulfonic acid (S_A) is recovered from the reaction mixture according to methods known in the art, typically distillation which can be determined by the person skilled in the art on a case-by-case basis.

If a salt (S-1) is used as starting material, step a) leads to a (per)fluoroalkyl sulfonic acid (S_A-1) complying with formula:
(S_A-1) R¹-SO₃H
wherein R¹ is as defined above.

If a salt (S-2) is used as starting material, step a) leads to a (per)fluoro(poly)oxyalkyl sulfonic acid (S_A-2) complying with formula:
(S_A-2) R²-SO₃H
wherein R² is as defined above.

Method (M-1) – step b)

In step b), the sulfonic acid (S_A) obtained from step a) is reacted with a phenol [phenol (W_p) substituted with at least one electron-withdrawing group, herein after identified with the symbol (W_p). “At least one” means that the phenol can contain from 1 to 5 electron-withdrawing groups. For the purposes of the present invention, the at least one electron-withdrawing group is typically selected from one or more of nitro, cyano, perfluoro alkyl, typically C₁-C₃ perfluoroalkyl, and fluorine. More preferably phenol (W_p) comprises at least one nitro- or cyano- group. Still more preferably, the phenol is 4-nitrophenol or 4-cyanophenol.

Phenol (W_p) can thus be represented with the formula here below:

wherein E_w is an electron-withdrawing group as defined above and n is an integer ranging from 1 to 5.

Typically, an excess of phenol (W_p) with respect to sulfonic acid (S_A) is used. Preferably, phenol (W_p) is used in a molar ratio of 1.3 with respect to sulfonic acid (S_A). More preferably, the molar ratio between phenol (W_p) and sulfonic acid (S_A) ranges from 1.1 to 1.5.

Sulfonic acid (A_S) and phenol (W_p) are placed into contact in a reactor at room temperature, and then heated to a temperature typically ranging from 100°C to 200°C and for a time ranging from 1 to 5 hours. A person skilled in the art will be able to determine the reaction temperature and time according to known methods on a case-by-case basis according to sulfonic acid (A_S) and phenol (W_p). In any case, the completion of the reaction can be followed withdrawing a sample and by submitting the samples to ¹⁹F-NMR. The reaction is complete when shifts from -112 ppm (-CFSO₃H signal) to -114.5 ppm (-CF ₂ -SO₃-Ar signal) and -122 (–CF ₂ CF₂SO₃H signal) to -121 ppm (-CF ₂ CF₂SO₃-Ar signal) are observed.

The resulting sulfonic ester (E_S) is recovered from the reaction mixture according to methods known in the art, typically distillation, which can be chosen by the person skilled in the art on a case-by-case basis.

If sulfonic acid (A_S) is an acid (S_A-1) as defined above, the resulting sulfonic ester (E_S) is an ester (E_S-1) complying with formula:
(E_S-1)

wherein R¹, E_w and n are as defined above.

Sulfonic ester (E_S-1) in which R¹ is -CF₃, E_w is –NO₂ and n is 1 is a preferred embodiment of the present invention. Indeed, as explained in detail below, it can be most advantageously used as sulfonating agent of alcohols having a pKa lower than 15.

If sulfonic acid (A_S) is an acid (S_A-2) as defined above, the resulting ester (E_S) is an ester (E_S-2) complying with formula:
(E_S-2)

wherein R², E_w and n are as defined above.

Method (M-1) – step c)

In step c), the alkali- or alkali-earth metal halide [herein after “metal halide (MX_n’)”] is a halide of an alkali- or alkali-earth metal [metal (M)], i. e. a metal belonging to Group I or Group II of the periodic table. Advantageously, metal (M) is selected from Li, Na, K, Mg and Ca; more preferably, the metal is Li or Na.

Advantageously, in halide (MX_n’), X is a halogen selected from fluorine or chlorine and n’ is 1 or 2; preferably, X is fluorine.

According to a preferred aspect of the invention, metal halide (MX_n’) is KF.

An excess of metal halide (MX_n’) with respect to sulfonic ester (E_S) is typically used in step c).

Advantageously, metal halide (MX_n’) is used in a molar ratio of at least 1.2:1 with respect to ester (E_S); preferably, the molar ratio between metal halide (MX_n’) and sulfonic ester (E_S) ranges from 1.5 to 2.5.

Sulfonic ester (E_S) and metal halide (MX_n’) are contacted into a reactor in the presence of a polar organic solvent, to provide a mixture that is heated to the solvent’s reflux temperature for a time typically ranging from 50°C to 100°C and for a time typically ranging from 1 to 5 hours.

The polar organic solvent is advantageously selected from dimethylsulfoxide, sulfolane, dimetylacetamide and acetonitrile; according to a preferred embodiment, the solvent is acetonitrile.

The reaction time can be determined by the person skilled in the art on a case-by-case basis according to sulfonic ester (E_S) and metal halide (MX_n’). In any case, the completion of the reaction can be followed by withdrawing a sample and submitting the sample to ¹⁹F-NMR. The reaction is complete when appearance of a signal at +48 ppm (-SO₂ F) is observed.

The resulting sulfonyl halide (A_SH) is recovered from the reaction mixture according to methods known in the art, typically distillation, which can be chosen by the person skilled in the art on a case-by-case basis.

If sulfonic ester (E_S) is an ester (E_S-1) as defined above, the sulfonyl halide (A_SH) is a sulfonyl halide (A_SH-1) of formula:
(A_SH-1) R¹-SO₂X
wherein R¹ and X are as defined above.

If sulfonyl ester (E_S) is an ester (E_S-2) as defined above, the sulfonyl halide (A_SH) is a sulfonyl halide (A_SH-2) of formula:
(A_SH-2) R²-SO₂X
wherein R¹ and X are as defined above.

Method (M-2)

As mentioned above, method (M-1) can be advantageously used to obtain sulfonic esters (E_S) or sulfonyl halides (A_SH) from sulfonic acids salts (S) separated from reaction mixtures comprising said sulfonic salts (S) as by-products.

Accordingly, in a preferred embodiment, the present invention further relates to a method (M-2) which comprises a step a₀) comprising isolating a sulfonic acid salt (S) from a reaction mixture and then submitting salt (S) to steps a) – c) as defined above.

Accordingly, method (M-2) comprises the following steps:
a₀) isolating a sulfonic acid salt (S) from a reaction mixture;
a) reacting a sulfonic acid [salt (S)] with a strong mineral acid [acid (A_M)] to provide the corresponding fluorinated sulfonic acid [acid (A_S)];
b) reacting the fluorinated sulfonic acid (A_S) from step a) with a phenol substituted with at least one electron-withdrawing group to provide the corresponding ester of sulfonic acid (A_S) [sulfonic ester (E_S)]; and, optionally
c) reacting ester (E_S) with an anhydrous alkali- or alkali-earth metal halide to provide the corresponding fluorinated sulfonyl halide [sulfonyl halide (A_SH)].

Advantageously, the mixture is one obtained from the reaction of a fluorinated sulfonic ester of a fluorinated alcohol [ester (E’_S)] of formula :
A_f-O-SO₂R
with a reagent NuH, according to the following equation (1):

wherein:

• R is as defined above;
• A_f is a (per)fluoroalkyl or (per)fluoro(poly)oxyalkyl chain, optionally comprising one or more -OSO₂R groups, with the condition that A_f is selected in such a way that its electron-withdrawing power is lower than that of the -OSO₂R groups, which means that a corresponding alcohol A_f-OH, wherein A_f is as defined above, must have a pK_a at most of 13, and preferably ranging from 7.5 to 13; and
• NuH is a a nucleophile reagent.

Usually, in order for A_f to have an electron-withdrawing power lower than that of the -OSO₂R group, as defined above, it is selected in such a way as the carbon atom directly bound to the -OSO₂R group does not bear fluorine atoms. In other words, it is a fluoroalkyl or a fluoro(poly)oxyalkyl chain comprising a -CH₂- group, a secondary carbon atom (-CH-) or a tertiary carbon atom directly bound to the -OSO₂R group.

Non limiting examples of NuH reagents include aliphatic and aromatic alcohols, amines and thiols, but also carbon-anions.

Typically, R is a group R¹ or R² as defined above, preferably a chain R¹. Preferred examples of groups R¹ are trifluoromethyl, pentafluoroethyl, heptafluoroethyl and nonafluorobutyl, trifluoromethyl and nonafluorobutyl being more preferred.

In a first preferred aspect, sulfonic ester (E’_s) is a sulfonic ester of a (per)fluoroalkyl alcohol, wherein A_f is a (per)fluoroalkyl chain optionally comprising one or more -OSO₂R groups. Advantageously, according to this aspect, ester (E’_S) is an ester (E’_S-1) complying with formula:
(E’_S-1) T-R¹ _fa-OSO₂R
wherein:
- T is (per)fluoromethyl or -OSO₂R;
- R is as defined above and
- R¹ _fa is a straight or branched fluoroalkylene chain comprising from 1 to 10 carbon atoms, preferably from 1 to 4 carbon atoms, with the proviso that it contains a -CH₂- group or a tertiary C-atom directly bound to the -OSO₂R group(s).

In a second preferred aspect, sulfonic ester (E’_s) is a sulfonic ester of a PFPE alcohol, wherein A_f is a straight or branched fluoro(poly)oxyalkylene chain, optionally comprising one or more -OSO₂R groups. Advantageously, according to this aspect, ester (E’_S) is an ester (E’_S-2) complying with formula:
(E’_S-2) T’-O-R² _fa-OSO₂R
wherein:
- T’ is a (per)haloalkyl or a -SO₂R group;
- R is as defined above and
- R² _fa is a fluoro(poly)oxyalkylene chain comprising recurring units (R°) selected from:
(i) -CFX_fO-, wherein X is F or CF₃;
(ii) -CFX_fCFX_fO-, wherein X_f, equal or different at each occurrence, is F or CF₃, with the proviso that at least one of X_f is –F;
(iii) -CF₂CF₂CW_fO-, wherein each of W_f, equal or different from each other, is F, Cl, H,
(iv) -CF₂CF₂CF₂CF₂O-;
(v) -(CF₂)_j-CFZ_f-O- wherein j is an integer from 0 to 3 and Z_f is a group of general formula -OR*_afT, wherein R*_af is a fluoropolyoxyalkene chain comprising a number of repeating units from 0 to 10, said recurring units being chosen among the followings : -CFX_fO- , -CF₂CFX_fO-, -CF₂CF₂CF₂O-, -CF₂CF₂CF₂CF₂O-, with each of each of X_f being independently F or CF₃ and T’ is a (per)haloalkyl group
with the proviso that chain R² _fa contains a -CH₂- group or a tertiary C-atom directly bound to the -OSO₂R group(s).

In one preferred embodiment, esters (E’_S-2) are monofunctional, i.e. they have an average functionality (F) lower than 1.2, while in another preferred embodiment they are bifunctional, i.e. they have an average functionality (F) higher than 1.90.

According to a preferred embodiment, ester (E’_S-2) complies with formula (E’_S-2’):
(E’_S-2’) T_F-O-R² _Fa-CFX_F-CH₂-O-(CHY_FCHY_FO)_nFSO₂R
wherein:
- T_F is -CFX_F-CH₂-O-(CHY_FCHY_FO)_nFSO₂R or a (per)haloalkyl group, typically a C₁-C₃ haloalkyl group, preferably selected from -CF₃, -CF₂Cl, -CF₂CF₂Cl, -C₃F₆Cl, -CF₂Br, -CF₂CF₃ and -CF₂H, -CF₂CF₂H;
- R² _Fa is a PFPE chain comprising recurring units (i) – (v) as defined above;
- X_F is F or CF₃, preferably F;
- Y_F is, independently of each other, hydrogen or lower alkyl, typically C₁-C₄ alkyl, preferably methyl;
- n_F is 0 or a positive number preferably ranging from 1 to 10 and
- R is as defined above.

Preferably, chain R² _Fa complies with formula (R² _Fa-I):
(R² _Fa-I)
-(CFX¹O)_g1(CFX²CFX³O)_g2(CF₂CF₂CF₂O)_g3(CF₂CF₂CF₂CF₂O)_g4-
wherein:
- X¹ is independently selected from -F and -CF₃;
- X², X³, equal or different from each other and at each occurrence, are independently -F, -CF₃, with the proviso that at least one of X is -F;
- g1, g2 , g3, and g4, equal or different from each other, are independently integers ≥0, such that g1+g2+g3+g4 is in the range from 2 to 300, preferably from 2 to 100; should at least two of g1, g2, g3 and g4 be different from zero, the different recurring units are generally statistically distributed along the chain.

More preferably, chain (R² _Fa) is selected from chains of formulae (R² _Fa -IIA) - (R² _Fa-IIE):
(R² _Fa-IIA) -(CF₂CF₂O)_a1(CF₂O)_a2-
wherein:
- a1 and a2 are independently integers ≥ 0 such that the number average molecular weight is between 400 and 10,000, preferably between 400 and 5,000; both a1 and a2 are preferably different from zero, with the ratio a1/a2 being preferably comprised between 0.1 and 10;
(R² _Fa-IIB) -(CF₂CF₂O)_b1(CF₂O)_b2(CF(CF₃)O)_b3(CF₂CF(CF₃)O)_b4-
wherein:
- b1, b2, b3, b4, are independently integers ≥ 0 such that the number average molecular weight is between 400 and 10,000, preferably between 400 and 5,000; preferably b1 is 0, b2, b3, b4 are > 0, with the ratio b4/(b2+b3) being ≥1;
(R² _Fa-IIC) -(CF₂CF₂O)_c1(CF₂O)_c2(CF₂(CF₂)_cwCF₂O)_c3-
wherein:
- cw = 1 or 2;
c1, c2, and c3 are independently integers ≥ 0 such that the number average molecular weight is between 400 and 10,000, preferably between 400 and 5,000; preferably c1, c2 and c3 are all > 0, with the ratio c3/(c1+c2) being generally lower than 0.2;
(R² _Fa-IID) -(CF₂CF(CF₃)O)_d-
wherein:
- d is an integer >0 such that the number average molecular weight is between 400 and 10,000, preferably between 400 and 5,000;
(R² _Fa-IIE) -(CF₂CF₂C(Hal)₂O)_e1-(CF₂CF₂CH₂O)_e2-(CF₂CF₂CH(Hal)O)_e3-
wherein:
- Hal, equal or different at each occurrence, is a halogen selected from fluorine and chlorine atoms, preferably a fluorine atom;
- e1, e2, and e3, equal to or different from each other, are independently integers ≥ 0 such that the (e1+e2+e3) sum is comprised between 2 and 300.

Still more preferably, chain (R² _Fa) complies with formula (R² _Fa-III) here below:
(R² _Fa -III) -(CF₂CF₂O)_a1(CF₂O)_a2-
wherein:
- a1, and a2 are integers > 0 such that the number average molecular weight is between 400 and 4,000, with the ratio a2/a1 being generally comprised between 0.2 and 5.

Most conveniently, method (M-2) is applied to PFPE esters (E’_S-2), more preferably to bifunctional esters of formula (E’_S-2’) as defined above in which:
- R² _Fa complies with formula (R² _Fa -III);
- X_F is fluorine;
- Y_F is H;
- n_F is 0 or a positive number ranging from 1 to 10 and
- R is as defined above.

Sulfonic esters (E’_s) can be obtained according to methods known in the art by reaction of a fluorinated alcohol [alcohol (A_OH)] with a sulfonyl halide (A_SH).

In particular, esters (E’_s-2) can be obtained by reaction of a PFPE alcohol [alcohol (A_OH-1)] comprising at least one –CH₂OH group according to methods known in the art, for example following the teaching of TONELLI, et al, Linear perfluoropolyether difunctional oligomers: chemistry, properties and applications, Journal of Fluorine Chemistry, 1999, 95, 51 - 70 Tonelli, et al. Linear perfluoropolyether difunctional oligomers: chemistry, properties and applications. Journal of Fluorine Chemistry. 1999, vol.95, p.51-70. , in particular as reported on page 64, par. 3.2.15. Esters (E’_s-2’) can be obtained according to such method using a PFPE alcohol [alcohol (A_OH-2)] of formula:
T’_F-O-R² _Fa-CFX_F-CH₂-O-(CHY_FCHY_FO)_nFH
wherein:
T’ is the same as T_F or a -CH₂-O-(CHY_FCHY_FO)_nFH group;
and R² _Fa, X_F, Y_F and n_F are as defined above,
said alcohol (A_OH-2) having a functionality higher than 1.90.

Perfluoropolyether alcohols are known in the art and can be manufactured according to known methods. In particular, PFPE alcohols (A_OH-2) in which R² _Fa complies with formula (R² _Fa -III) and nF = 0 are commercially available from Solvay Specialty Polymers Italy S.p.A. with trade name Fomblin^®. Corresponding PFPE alcohols wherein nF is equal to or higher than 1 can be obtained by reaction of a Fomblin^® PFPE with an epoxide in the presence of a base, according to known methods.

Method (M-3)

As stated above, either ester (E_S) or sulfonyl halide (A_SH) obtained according to method (M-1) or to method (M-2) can be used as sulfonating agents for alcohols having a pK_a lower than 15 to provide the corresponding sulfonic esters. Such alcohols comprise both aromatic alcohols and fluorinated alcohols A_f-OH as defined above.

Accordingly, in a further preferred aspect, the present invention relates to a method [method (M-3)] for obtaining a sulfonic ester of a PFPE alcohol which comprises the following steps:
a) reacting a salt of a fluorinated sulfonic acid [salt (S)] with a strong mineral acid [acid (A_M)] to provide the corresponding fluorinated sulfonic acid [acid (A_S)];
b) reacting the fluorinated sulfonic acid (A_S) from step a) with a phenol substituted with at least one electron-withdrawing group to provide the corresponding ester of sulfonic acid (A_S) [sulfonic ester (E_S)];
c) optionally reacting ester (E_S) with an anhydrous alkali- or alkali-earth metal halide to provide the corresponding fluorinated sulfonyl halide [sulfonyl halide (A_SH)];
d) reacting ester (E_S) or sulfonyl halide (A_SH) with an alcohol having a pK_a lower than 15 to provide the corresponding sulfonic ester [sulfonic ester (E*_S)].

If only steps a), b) and d) are carried out in method (M-3), and if the alcohol having a pK_a lower than 15 is an alcohol of formula A_f-OH wherein A_f is as defined above, sulfonic ester (E*_S) complies with formula (E*’s):
A_f-O-SO₂R*
wherein A_f is as defined above and R* is a phenol group substituted with at least one electron-withdrawing group (E_w) as defined above.

If the alcohol having a pK_a lower than 15 is an alcohol of formula A_f-OH wherein A_f is as defined above and all steps a) – d) are carried out in method (M-3), sulfonic ester (E*_S) complies with formula (E’_S) as defined above.

According to a preferred aspect, sulfonic salt (S) is obtained from the reaction of a sulfonic ester (E’_S) with a nucleophile reagent, according to equation (1) depicted above.

Thus, the present invention further comprises a method [method (M-3’)] which comprises the following steps:
a’₀) reacting a sulfonic ester (E’_S) as defined above with a nucleophile compound, to provide a mixture comprising a sulfonic salt (S) as defined above;
a’’₀) isolating sulfonic salt (S) from the mixture;
a) reacting sulfonate salt (S) with a strong mineral acid [acid (A_M)] to provide the corresponding fluorinated sulfonic acid [acid (A_S)];
b) reacting the fluorinated sulfonic acid (A_S) from step a) with a phenol [phenol (W_p)] substituted with at least one electron-withdrawing group to provide the corresponding ester of sulfonic acid (A_S) [sulfonic ester (E_S)];
c) optionally reacting ester (E_S) with an anhydrous alkali- or alkali-earth metal halide to provide the corresponding fluorinated sulfonyl halide [sulfonyl halide (A_SH)];
d) reacting ester (E_S) or sulfonyl halide (A_SH) with a fluorinated alcohol to provide the corresponding sulfonic ester (E*_S).

According to a preferred embodiment of the invention, sulfonic ester (E_s) is used as sulfonating agent; thus, is order to obtain this ester, optional step c) is not carried out in process (M-1) and in preferred processes (M-2) and (M-3).

The omission of step c) is particularly useful in cases where sulfonate salt (S) is a trifluoromethanesulfonate salt. Indeed, carrying out step c) would lead to the obtainment of trifluoromethanesulfonyl chloride, which is a gas difficult to handle, so much so that trifluoromethanesulfonyl anhydride (CF₃SO₂)₂O is typically used as sulfonating agent, which can be most conveniently stored and handled; however, (CF₃SO₂)₂O is considerably expensive.

The omission of step c) is particularly convenient on an industrial scale, because ester (E_S) can be reacted with an alcohol having a pK_a lower than 15, preferably with a fluorinated alcohol, to provide an ester (E*’s), which can be reacted with a nucleophile compound to provide a mixture comprising a sulfonate salt (S); salt (S) can then be isolated and submitted to steps a), b) and d) according to preferred method (M-3’).

Therefore, the present invention further relates to a process [process (M-3*’)] comprising the following steps:
a’₀) reacting an ester (E’_S) as defined above with a nucleophile compound as defined above, to provide a mixture comprising a sulfonate salt (S);
a’’₀) isolating sulfonate salt (S) from the mixture;
a) reacting sulfonate salt (S) with a strong mineral acid [acid (A_M)] to provide the corresponding fluorinated sulfonic acid [acid (A_S)];
b) reacting the fluorinated sulfonic acid (A_S) from step a) with a phenol substituted with at least one electron-withdrawing group to provide the corresponding ester of sulfonic acid (A_S) [sulfonic ester (E_S)];
d) reacting ester (E_S) with an alcohol having a pK_a lower than 15 to provide the corresponding sulfonic ester (E’*_S); and optionally
e) reacting sulfonic ester (E’*_S) with a nucleophile compound, to provide a mixture comprising a sulfonic salt (S) and recycling sulfonic ester (E’*_S) to step a’₀) and repeating steps a’’₀) – d) or a’’₀) - e).

According to a particularly preferred embodiment, in process (M-3*’), ester (E’_S) is a PFPE ester complying with formula (E’_S-2) as defined above. Preferably, it is a bifunctional ester of formula (E’_S-2’) as defined above in which:
- R² _Fa complies with formula (R² _Fa -III);
- X_F is fluorine;
- Y_F is H;
- n_F is 0 or a positive number ranging from 1 to 0 and
- R is as defined above.

Advantageously, method (M-3*’) is applied to esters (E’_S) in which R is trifluoromethyl.

The invention will be illustrated in greater detail in the following Experimental section by means of non-limiting examples.

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

Experimental section Material and methods

All reagents were used as received by the manufacturer without further purification. The solvent (acetonitrile) was previously distilled and maintained over molecular sieves.

Example 1

Step a) Conversion of perfluorobutane sulfonate sodium salt into perfluorobutane sulfonic acid

Anhydrous sulfuric acid (37 g, 360 mmoles) was placed in a glass round-bottomed flask equipped with a magnetic stir bar, a reflux condenser, a thermometer and a solid dispenser. Perfluorobutane sulfonate sodium salt (C₄F₉SO₃Na, 35 g, 109 mmols) was placed in the dispenser and added to the acid in 10 minutes. No exothermic reaction was observed. The mixture was stirred at 120°C for 30 minutes at 1,000 rpms. The progress of the reaction was followed by ¹⁹F-NMR. Once complete conversion of acid was observed (the –CF ₂ -SO₃H peak shifted from -114.5 ppm to -112 ppm and the –CF ₂ CF₂SO₃H peak shifted from -121 ppm to -122 ppm; total shift = 3.5 ppms), the internal temperature was raised to 140°C and vacuum (0.8 mbar residual pressure) was applied to the reaction mixture to distil perfluorobutane sulfonic acid as a clear liquid.

Yield: 28.1 g (94 mmols); 94 mol%.

Step b) Conversion of perfluorobutane sulfonic acid into perfluorobutane para-nitro phenyl sulfonate

Perfluorobutane sulfonic acid (25 g, 83 mmols) from step a), was placed in a glass reactor identical to the one used for step a) and para-nitrophenol was added (30 g, 216 mmoles) at room temperature in about 20 minutes. The reaction temperature was raised to 150°C with 900 rpm stirring for 3.5 hrs. During the course of the reaction the reaction mixture turned deep red. The progress of the reaction was followed by ¹⁹F-NMR. Once the conversion of the acid to the corresponding para-nitro phenyl ester was completed (the –CF ₂ -SO₃-Ar peak shifted to -114.3 ppm and the –CF ₂ CF₂SO₃-Ar peak shifted to -121 ppm; a total shift = 3 ppm), the reaction mixture was distilled at 160°C and 0.9 residual mbar pressure. The distillate was a pale yellow liquid that became an off-white, crystalline solid at room temperature.

Yield: 33 g (92 mol%); 76 mmoles.

Step c) Conversion of perfluorobutane para-nitro phenyl sulfonate into perfluorobutane sulfonyl fluoride

Perfluorobutane para-nitrophenyl sulfonate from step b) (15 g, 34 mmoles) was dissolved in anhydrous CH₃CN (60 ml) in a glass reactor equipped with a reflux condenser, a heating oil-bath, an internal thermometer, a dropping funnel and a magnetic stirring bar. Anhydrous KF (4 g, 69 mmol) was added. and mixture was heated to the solvent’s reflux temperature (i.e. about 80°C). The water condenser maintained all products in the reactor, until complete conversion. The progress of the reaction was followed by ¹⁹F-NMR. During the course of the conversion the colourless solution turned yellow. Once the ¹⁹F-NMR analysis showed complete conversion (the –CF₂SO₂F peak shifted to -108 ppm and the –CF₂CF₂SO₂F peak shifted to -122; a total shift = 7 ppm). The new fundamental peak of –SO₂ F appeared at + 48 ppm and integrated 50% of the area of each of the peaks at -108, -122 and –126 ppm), the desired perfluorobutane sulfonyl fluoride was isolated by distillation (b.p. 60°C). Some product was also successfully isolated from the acetonitrile solvent by a quick washing in ice-cold water; the product separated as a colourless liquid in the bottom layer.

Yield 10.3 g (32 mmoles), 93 mol%.

Overall yield of perfluorobutane sulfonyl fluoride: 80.7%.

Example 1A

Example 1 was repeated using, as esterifying agent, 4-cyanophenol instead of 4-nitrophenol.

The overall yield of the target perfluorobutane sulfonyl fluoride was 82%.

Example 2

Step a) Conversion of perfluoroethane sulfonate sodium salt into perfluorobutane sulfonic acid

Anhydrous sulfuric acid (50 g, 487 mmoles) was placed in a glass round-bottomed flask equipped with a magnetic stirring bar, a reflux condenser, a thermometer and a solid dispenser. Perfluoroethane sulfonate sodium salt (C₂F₅SO₃Na, 20 g, 90 mmols) was placed into the solid dispenser and added to the acid in 10 minutes. No exothermic reaction was observed. The mixture was stirred at 120°C for 30 minutes at 1,000 rpms. The progress of the reaction was followed by ¹⁹F-NMR. Once complete conversion of the acid was observed (same chemical shifts observed as in Example 1, step a), the internal temperature was raised to 120°C and vacuum (1 mbar residual pressure) was applied to the reaction mixture to distil the perfluoroethane sulfonic acid as a clear liquid.

Yield: 17 g (85 mmols); 94 mol%.

Step b) Conversion of perfluorobutane sulfonic acid into perfluoroethane para-nitrophenyl sulfonate

Perfluoroethane sulfonic acid (25 g, 125 mmoles) from step a) was placed in a glass reactor identical to that use in step a) and para-nitrophenol (30 g, 216 mmoles) was added at room temperature in about 20 minutes. The reaction temperature was raised to 150°C with 900 rpm stirring for 3.5 hrs. During the course of the reaction, the reaction mixture turned deep red. The progress of the reaction was followed by ¹⁹F-NMR. Once the conversion of the acid to the corresponding phenylester was completed (same chemical shifts as observed in Example 1, step b), the reaction mixture was distilled at 160°C and 0.9 residual mbar pressure. The distillate was a pale yellow liquid which solidified at room temperature becoming an off-white, crystalline solid.

Yield: 37.7g (117 mmoles); 94 mol%.

Step c) Conversion of perfluoroethane para-nitrophenyl sulfonate into perfluoroethanesulfonyl fluoride

Perfluoroethane para-nitrophenyl sulfonate (20 g, 62 mmols) from step b) was dissolved in anhydrous CH₃CN (70 ml) to which anhydrous KF ( 5g, 86 mmol) was added. The reaction temperature was maintained at the reflux temperature of the solvent (ca. 80°C) and the just formed perfluoroethane sulfonylfluoride was distilled off and collected in an ice-cooled flask. The progress of the reaction was followed by ¹⁹F-NMR analysis. During the course of the reaction the colourless solution became yellow. Once the ¹⁹F-NMR analysis showed complete conversion (same chemical shifts as observed in Example 1, step a), the desired perfluoroethanesulfonyl fluoride was completely removed from the reaction mass (b.p. ca. 20°C). Some product was also successfully isolated by a quick washing in ice-cold water from the residual acetonitrile solvent. The desired product separated as a colourless liquid in the bottom layer.

Yield :12.8 g (59 mmoles); 95 mol%.

Overall yield 83.9%

Example 3

Synthesis of Fomblin ^® Z DOL PFPE nonaflate starting from perfluorobutane para-nitro phenyl sulfonate

A glass reactor was charged with perfluorobutane para-nitro phenyl sulfonate (17.7 g, 40.8 meq) prepared accordingly to Example 1. The internal temperature of the reaction mass was maintained at room temperature. Fomblin^® Z DOL PFPE (20 g, 38 meq) of formula:
HOCH₂CF₂O(CF₂CF₂O)_a1(CF₂O)_a2CF₂CH₂OH
(where a1 and a2 as defined above and are selected in such a way as the MW is 1,000 and EW is 500; a1/a2 = 2)
was added drop-wise under vigorous stirring. Thereafter, the reaction mass was warmed up to room temperature, under mechanical stirring. The reaction was monitored by ¹⁹F-NMR analysis. After 1 hour at room temperature a sample was taken for NMR analysis and the observed conversion of the hydroxy groups into perfluorobutanesulfonate groups was 70%. The internal temperature was increased to 70°C until completion of the reaction. After complete conversion, the reaction mass was cooled to room temperature and the product was washed twice with ethanol (20 g each time). The organic phase was separated and ethanol was stripped at 70°C under vacuum. The resulting product (Fomblin^® Z DOL PFPE nonaflate) was isolated with a purity > 96% and a yield > 90%. The typical diagnostic ¹⁹F-NMR signals of this product resonate at -107.5 ppm, while the diagnostic peak of any perfluorosulfonate (hydrolysed nonaflate) resonates at -111.5 ppm. The resulting Fomblin^® Z DOL PFPE nonaflate had a MW of 1,630 and an EW of 820.

Claims (15)

1. A method for the obtainment of an ester or a halide of a fluorinated sulfonic acid from a salt of a sulfonic acid, said process comprising the following steps:

   a) reacting a salt of a fluorinated sulfonic acid [salt (S)] with a mineral acid [acid (A_M)] having a pK_a lower than 0 to provide the corresponding fluorinated sulfonic acid [acid (A_S)];

   b) reacting the fluorinated sulfonic acid (A_S) from step a) with a phenol [phenol (W_p)] substituted with at least one electron-withdrawing group selected from one or more of nitro, cyano, perfluoro alkyl and fluorine to provide the corresponding ester of sulfonic acid (A_S) [sulfonic ester (E_S)]; and, optionally

   c) reacting ester (E_S) with an anhydrous alkali- or alkali-earth metal halide to provide the corresponding fluorinated sulfonyl halide [sulfonyl halide (A_SH)].
2. The method according to claim 1, wherein salt (S) is an alkali- or alkali-metal salt.
3. The method according to claim 2, wherein salt (S) complies with formula:

   R-SO₃-Me⁺

   wherein:

   - R is a (per)fluoroalkyl or a (per)fluoro(poly)oxyalkyl chain comprising a -CF₂- group directly bound to the sulphur atom of the -SO₃ ^- group and

   - Me⁺ is an alkali metal cation.
4. The method according to claim 3, wherein salt (S) complies with formula:

   (S-1) R¹-SO₃ ^-Me⁺,

   wherein:

   - R¹ is a C₁-C₁₀ (per)fluoroalkyl chain, with the proviso that chain R¹ contains a -CF₂- group directly bound to the sulphur atom of the -SO₃ ^- group and

   - Me⁺ is an alkali metal cation.
5. The method according to claim 3, wherein salt (S) complies with formula:

   (S-2) R²-SO₃ ^-Me⁺

   wherein:

   - R² is a straight or branched C₁-C₁₀ (per)fluoro(poly)oxyalkyl chain, with the proviso that chain R² contains a -CF₂- group directly bound to the sulphur atom of the -SO₃ ^- group; and

   Me⁺ is an alkali metal cation.
6. The method according to any one of claims 1 to 5 wherein acid (A_M) is fluorosulfonic or sulphuric acid.
7. The method according to any one of claims 1 to 6 wherein phenol (W_p) complies with formula:

   

   wherein E_w is an electron-withdrawing group as defined in claim 1 and n is an integer ranging from 1 to 5.
8. The method according to claim 7 wherein phenol (W_p) is 4-nitrophenol or 4-cyanophenol.
9. The method according to any one of claims 1 to 8 wherein ester (E_s) complies with formula:

   (E_S-1)

   

   wherein:

   - R¹ is a C₁-C₁₀ (per)fluoroalkyl chain, with the proviso that chain R¹ contains a -CF₂- group directly bound to the sulphur atom of the -SO₃ ^- group;

   - E_w is an electron-withdrawing group selected from one or more of nitro, cyano, perfluoro alkyl and fluorine and n is an integer ranging from 1 to 5.
10. The method according to any one of claims 1 to 8 wherein ester (E_s) complies with formula:

    (E_s-2)

    

    wherein:

    - R² is a straight or branched C₁-C₁₀ (per)fluoro(poly)oxyalkyl chain, with the proviso that chain R² contains a -CF₂- group directly bound to the sulphur atom of the -SO₃ ^- group;

    - E_w is an electron-withdrawing group selected from one or more of nitro, cyano, perfluoro alkyl and fluorine and

    - n is an integer ranging from 1 to 5.
11. A method for the obtainment of an ester or a halide of a fluorinated sulfonic acid from a salt of a sulfonic acid which comprises a step a₀) wherein a sulfonic acid salt (S) is isolated from a reaction mixture; and submitting salt (S) to steps (a) – (c) according to any one of claims 1 to 10.
12. The method according to claim 11 wherein the reaction mixture is obtained by reaction of a fluorinated sulfonic ester (E’_S) of formula:

    A_f-O-SO₂R

    wherein:

    - R is a (per)fluoroalkyl or a (per)fluoro(poly)oxyalkyl chain comprising a -CF₂- group directly bound to the sulphur atom of the -SO₃ ^- group;

    - A_f is a (per)fluoroalkyl or (per)fluoro(poly)oxyalkyl chain selected in such a way as the carbon atom directly bound to the -OSO₂R group does not bear fluorine atoms

    with a nucleophile reagent.
13. The method according to claim 12 wherein ester (E’_S) complies with formula:

    (E’_S-1) T-R¹ _fa-OSO₂R

    wherein:

    - T is (per)fluoromethyl or -OSO₂R;

    - R is a (per)fluoroalkyl or a (per)fluoro(poly)oxyalkyl chain comprising a -CF₂- group directly bound to the sulphur atom of the -SO₃ ^- group and

    - R¹ _fa is a straight or branched fluoroalkylene chain comprising from 1 to 10 carbon atoms, with the proviso that it contains a -CH₂- group or a tertiary carbon atom directly bound to the -OSO₂R group(s).
14. The method according to claim 12 wherein ester (E’_S) complies with formula:

    (E’_S-2) T’-O-R² _fa-OSO₂R

    wherein:

    - T’ is a (per)haloalkyl or a -SO₂R group;

    - R is is a (per)fluoroalkyl or a (per)fluoro(poly)oxyalkyl chain comprising a -CF₂- group directly bound to the sulphur atom of the -SO₃ ^- group and

    - R² _fa is a fluoro(poly)oxyalkylene chain comprising recurring units (R°) selected from:

    (i) -CFX_fO-, wherein X is F or CF₃;

    (ii) -CFX_fCFX_fO-, wherein X_f, equal or different at each occurrence, is F or CF₃, with the proviso that at least one of X_f is –F;

    (iii) -CF₂CF₂CW_fO-, wherein each of W_f, equal or different from each other, is F, Cl, H,

    (iv) -CF₂CF₂CF₂CF₂O-;

    (v) -(CF₂)_j-CFZ_f-O- wherein j is an integer from 0 to 3 and Z_f is a group of general formula -OR*_afT, wherein R*_af is a fluoropolyoxyalkene chain comprising a number of repeating units from 0 to 10, said recurring units being chosen among the followings : -CFX_fO- , -CF₂CFX_fO-, -CF₂CF₂CF₂O-, -CF₂CF₂CF₂CF₂O-, with each of each of X_f being independently F or CF₃ and T’ is a (per)haloalkyl group

    with the proviso that chain R² _fa contains a -CH₂- group directly bound to the -OSO₂R group(s).
15. A sulfonic ester complying with formula:

    (E_S-1)h

    

    in which R¹ is -CF₃, E_w is –NO₂ and n is 1.