Reference to related applications

This application claims priority from European patent application Nr. 22155260.7 filed on 4 February 2022, the whole content of this application being incorporated herein by reference for all purposes.

Technical Field

[0001] The present invention provides compounds that include a fluorinated allyl xanthate group. The invention also relates to a process for making these compounds and the use of said compounds as chain transfer agents or as monomers. The invention relates also to polymers, including copolymers, comprising recurring units deriving from compounds comprising fluorinated allyl xanthate groups.

Background Art

[0002] In certain instances, it is beneficial to provide polymers comprising functional groups.

[0003] Polymers with attached functional groups may be prepared directly by polymerization of functional monomers. Oligomers and polymers prepared by a controlled polymerization processes may have functionality at specific locations along the chain and a specific amount of functionality. For example, functional monomers may be placed periodically along the polymer chain, the initiator may have attached functionality, or the group providing for controlled polymerization may be removed and replaced with a desired functional group. However, there are several functional monomers that may not be directly copolymerized by polymerization process. Further, the monomers with desired functionality may not copolymerize in the desired manner using the selected polymerization process.

[0004] For instance, introducing monomers bearing -CF2SO3H and -CF2SH side chain functional groups is very difficult.

[0005] The access to polymers having super short side chains bearing -CF2SO3H groups in the field of fuel cell and electrolysis applications would lead to systems, such as membranes, higher electrochemical performance and better mechanical properties than the current technology. [0006] The Applicant has found a novel monomer, more specifically a monomer comprising a fluorinated allyl xanthate, which can be used for the synthesis of polymers bearing pendant functionalities that can suitably be converted into several functional groups in post-polymerization processes. The resulting polymers can be tailored to different applications.

[0007] Alkyl xanthates are compounds having the general formula ROC(=S)SR'. They are widely used in the field of engineering such as flotation and as chain transfer agents in controlled radical polymerizations, and they are generally prepared by substitution reaction of xanthates with chloro alkyl compounds.

Summary of invention

[0008] Thus a first object of the present invention is a compound (AX) complying with formula (I): wherein Ra is a (per)fluorinated allyl group and Rb is a straight or branched alkyl group. Rb is a C1-C12 straight or branched alkyl group, typically a Ci-Cs straight or branched alkyl group, preferably a C1-C6 straight or branched alkyl group.

[0009] The invention also relates to processes for preparing the compound (AX) according to the first object.

[0010] Another object of the present invention is a polymer (P) comprising recurring units deriving from compound (AX) of formula (I) as above defined.

[0011] The invention also relates to a process for manufacturing a polymer (P) comprising recurring units deriving from compound (AX) of formula (I) as above defined.

[0012] Polymer (P) may suitably be subjected to chemical transformations in order to convert the -S(=S)ORb groups present in the recurring units deriving from compound (AX) into different functional groups, thus providing further functionalized polymers. [0013] In a further object, the present invention is directed to the use of a compound (AX) of formula (I) as above defined as chain transfer agent in controlled radical polymerizations.

Description of embodiments

[0014] In the present application:

- any description, even though described in relation to a specific embodiment, is applicable to and interchangeable with other embodiments of the present disclosure;

- where an element or component is said to be included in and/or selected from a list of recited elements or components, it should be understood that in related embodiments explicitly contemplated here, the element or component can also be any one of the individual recited elements or components, or can also be selected from a group consisting of any two or more of the explicitly listed elements or components; any element or component recited in a list of elements or components may be omitted from such list;

- any recitation herein of numerical ranges by endpoints includes all numbers subsumed within the recited ranges as well as the endpoints of the range and equivalents;

- the use of parentheses before and after symbols or numbers identifying formulae or parts of formulae has the mere purpose of better distinguishing that symbol or number with respect to the rest of the text; thus, said parentheses could also be omitted.

[0015] For the purpose of the invention, the term “(per)fluorinated allyl group” is intended to denote any partially or fully fluorinated allyl group, i.e. wherein all or only a part of the hydrogen atoms of the hydrocarbon allyl structure have been replaced by fluorine atoms attached to unsaturated and/or to saturated carbons. When the allyl group is fully fluorinated, the term perfluorinated is used.

[0016] R _a is preferably a perfluorinated allyl group.

[0017] The compound (AX) of the present invention preferably complies with formula (II) wherein Rb is as defined above:

(II).

[0018] Non-limitative examples of Rb include, notably: ethyl, isopropyl, n-butyl, isobutyl, n-pentyl and isopentyl groups.

[0019] Particularly preferred is a compound (AX) of formula (III):

(III), hereinafter referred to as “FAX”.

[0020] Compound (AX) of the present invention can be prepared by a process comprising the following steps a) and b): a) providing a xanthate salt of formula (IV): wherein Rb is as above defined and M ⁺ is a monovalent cation; b) reacting the xanthate salt provided in step a) with a (per)fluoro allyl fluorosulfate of formula (V):

R _a -OSO ₂ X (V) wherein R _a is a (per)fluorinated allyl group and X is a halogen atom.

[0021] In formula (IV) M ⁺ is preferably selected from alkali metal cations, more preferably M ⁺ is selected from Na ⁺ , K ⁺ , Cs ⁺ and Li ⁺ , even more preferably M ⁺ is K ⁺ .

[0022] Among the (per)fluoro allyl fluorosulfates, perfluoro allyl fluorosulfate of formula CF2=CFCF2OSO2F (hereinafter referred to as “FAFS”) is particularly preferred.

[0023] In step b), the reaction is carried out preferably at room temperature.

[0024] The reaction in step b) is typically carried out in the presence of a solvent. Suitable solvents for the reaction in step b) are polar aprotic solvents, notably glycol ethers, ethers, nitriles. Preferably the solvent is acetonitrile.

[0025] The reaction time in step b) is suitably comprised between 1 and 5 hours. [0026] At the end of step b), the solid FSO3M by-product is filtered off from the reaction mixture and compound (AX) is recovered in the form of powder after evaporation of the solvent.

[0027] Compound (AX) of the present invention can be used in the preparation of polymers. Advantageously, the recurring units deriving from compound (AX) can serve as a precursor to other protective and/or reactive functionalities such as -CF2SO3H and -CF2SH.

[0028] Hence a further object of the invention is a polymer (P) comprising recurring units deriving from compound (AX).

[0029] Polymer (P) may be a homopolymer. That is, polymer (P) may consist of recurring units deriving from compound (AX).

[0030] Alternatively, polymer (P) may be a copolymer, comprising recurring units deriving from compound (AX) and recurring units deriving from one or more ethylenically unsaturated monomers.

[0031] Polymer (P) of the present invention is preferably a copolymer.

[0032] More preferably, polymer (P) is a copolymer comprising recurring units deriving from compound (AX) as above defined and recurring units deriving from at least one fluoromonomer [fluoromonomer (FM)]. The expression “fluoromonomer” is used herein according to its usual meaning, that is to say for designating an ethylenically unsaturated monomer comprising at least one fluorine atom.

[0033] In a preferred embodiment of the invention, polymer (P) comprises 85 to 5 % by moles of recurring units deriving from compound (AX), with respect to the total moles of recurring units of polymer (P), and 15 to 95 % by moles of recurring units deriving from the at least one fluoromonomer (FM), with respect to the total moles of recurring units of polymer (P).

[0034] Polymer (P) may comprise at least 5 %, at least 10 %, at least 15%, at least 20% at least 25 %, at least 35 %, at least 45%, at least 50 %, even at least 60 %, at least 70 % by moles of compound (AX) with respect to the total moles of recurring units of polymer (P). Polymer (P) may comprise less than 80 %, less than 75%, less than 65% and even less than 50%, less than 45%, less than 30 % by moles of compound (AX) with respect to the total moles of recurring units of polymer (P). The remainder of recurring units in polymer (P) derives from one or more fluoromonomer (FM).

[0035] Fluoromonomer (FM) is selected generally from the group consisting of:

- C2-C8 perfluoroolefins, such as tetrafluoroethylene, hexafluoropropene;

- C2-C8 hydrogen-containing fluoroolefins, such as vinyl fluoride, 1 ,2- difluoroethylene, vinylidene fluoride, trifluoroethylene, pentafluoropropylene, and hexafluoroisobutylene;

- (per)fluoroalkylethylenes complying with formula CH2=CH-Rfo, in which Rro is a Ci-Ce (per)fluoroalkyl or a Ci-Ce (per)fluorooxyalkyl having one or more ether groups ;

- chloro- and/or bromo- and/or iodo-C2-Ce fluoroolefins, like chlorotrifluoroethylene;

- fluoroalkylvinylethers complying with formula CF2=CFORfi in which Rn is a Ci-Ce fluoro- or perfluoroalkyl, e.g. -CF3, -C2F5, -C3F7 ;

- hydrofluoroalkylvinylethers complying with formula CH2=CFORfi in which Rfi is a Ci-Ce fluoro- or perfluoroalkyl, e.g. -CF3, -C2F5, -C3F7 ;

- fluoro-oxyalkylvinylethers complying with formula CF2=CFOXo, in which Xo is a C1-C12 oxyalkyl, or a C1-C12 (per)fluorooxyalkyl having one or more ether groups, like perfluoro-2-propoxy-propyl;

- fluoroalkyl-methoxy-vinylethers complying with formula CF2=CFOCF2ORf2 in which Rf2 is a Ci-Ce fluoro- or perfluoroalkyl, e.g. - CF3, -C2F5, -C3F7 or a Ci-Ce (per)fluorooxyalkyl having one or more ether groups, like -C2F5-O-CF3;

- functional fluoro-alkylvinylethers complying with formula CF2=CFOYo, in which Yo is a C1-C12 alkyl or (per)fluoroalkyl, or a C1-C12 oxyalkyl or a Ci- 012 (per)fluorooxyalkyl, said Yo group comprising a carboxylic or sulfonic acid group, in its acid, acid halide or salt form;

- fluorodioxoles, of formula: wherein each of Rf3, Rf4, Rfs, Rf6, equal or different each other, is independently a fluorine atom, a Ci-Ce fluoro- or per(halo)fluoroalkyl, optionally comprising one or more oxygen atom, e.g. -CF3, -C2F5, -C3F7, - OCF3, -OCF2CF2OCF3.

[0036] Polymer (P) may comprise recurring units deriving from at least one additional monomer different from fluoromonomer (FM), that is to say a monomer free from fluorine, otherwise generally referred to as a hydrogenated monomer [monomer (HM)]. Examples of hydrogenated monomers (HM) are notably C2-C8 non-fluorinated olefins, in particular C2- Cs non-fluorinated alpha-olefins, including ethylene, propylene, 1 -butene; diene monomers; styrene monomers. Monomer (HM) is preferably selected from with C2-C8 alpha-olefins.

[0037] In one preferred embodiment of the present invention, polymer (P) is a copolymer comprising recurring units deriving from compound (AX) as above defined, recurring units deriving from at least one C2-C8 perfluoroolefin and recurring units deriving from at least one functional fluoro-alkylvinylether of formula CF2=CFOYo as above defined.

[0038] The C2-C8 perfluoroolefin is preferably tetrafluoroethylene. The functional fluoro-alkylvinylether of formula CF2=CFOYo is preferably selected from the fluoro-alkylvinylethers in which Yo is a C1-C12 (per)fluorooxyalkyl group comprising a sulfonic acid group, in its acid, acid halide or salt form.

[0039] The functional fluoro-alkylvinylether of formula CF2=CFOYo is preferably selected from the group consisting of:

(j) sulfonyl halide fluorovinylethers of formula: CF2=CF-O-(CF2)mSO2X’, wherein m’ is an integer between 1 and 10, preferably between 1 and 6, more preferably between 2 and 4, even more preferably m’ equals 2; and X’ is chosen among halogens (Cl, F, Br, I), preferably F, or is -O'M’ ⁺ , wherein M’ ⁺ is a cation selected among H ⁺ , NH4 ⁺ , K ⁺ , Li ⁺ , Na ⁺ , or mixtures thereof, preferably M’ ⁺ is H ⁺ ; and

(jj) sulfonyl fluoride fluoroalkoxyvinylethers of formula: CF2=CF-(OCF2CF(RFi))w-O-CF2(CF(RF2))ySO ₂ X’ wherein X’ is chosen among halogens (Cl, F, Br, I), preferably F, or is -O’ M’ ⁺ , wherein M’ ⁺ is a cation selected among H ⁺ , NH4 ⁺ , K ⁺ , Li ⁺ , Na ⁺ , or mixtures thereof, preferably M’ ⁺ is H ⁺ ; and wherein w is an integer between 0 and 2, RFI and RF2, equal or different from each other, are independently F, Cl or a C1-C10 fluoroalkyl group, optionally substituted with one or more ether oxygens, y is an integer between 0 and 6; preferably w is 1 , RFI is - CF ₃ , y is 1 and RF2 is F.

[0040] Still more preferably, the at least one functional fluoro-alkylvinylether is a sulfonated perfluorovinylether of formula (FM1 ): wherein m’ is an integer between 1 and 10, preferably between 1 and 6, more preferably between 2 and 4, and X’ is chosen among halogens (Cl, F, Br, I), preferably F, -O’M’ ⁺ , wherein M’ ⁺ is a cation selected among H ⁺ , NH4 ⁺ , K ⁺ , Li ⁺ , Na ⁺ , or mixtures thereof, preferably M’ ⁺ is H ⁺ .

[0041] Advantageously, the sulfonated perfluorovinylether of formula (FM1) is selected in the group consisting of the compounds of formulae (FM1-A), (FM1-B) and (FM1-C):

(FM1-A) (FM1-B) (FM1-C) wherein X’ has the same meaning as above defined.

[0042] The sulfonated perfluorovinylether is preferably perfuoro-5-sulfonylfluoride-3-oxa-1 -pentene (hereinafter referred to as

VEFS”) of formula (FM1-D):

(FM1-D) which can be in its -SO2F form as shown above or it can be in its -SO3X form, with X being H or alkaline metal or NH ⁴⁺ .

[0043] Advantageously, polymer (P) is a copolymer of formula (VI):

wherein n, m and p are, independently from each other, integers greater than 0, together denoting the molar fraction of each monomer in the polymer (P). Typically n is between 0.50 and 0.75, m is between 0.10 and 0.30 and p is between 0.05 and 0.40.

[0044] In the above notation for copolymers, the n, m and p units may appear in any order: the formula (VI) is only intended to define the relative proportion of monomer units, and not the exact order (which is random) in the copolymer. Similarly, the orientation of the recurring units in a tail-to-tail pattern in formula (VI) is only indicative and not intended to limit the structure of the polymer. The recurring units in the polymer (P) of formula (VI) may be in a head-to-head, tail-to-tail or head-to-tail arrangement.

[0045] In a preferred embodiment of the invention, polymer (P) comprises:

- 5 to 25 %, 10 to 20 % by moles of recurring units deriving from compound (AX),

- 45 to 85 %, 60 to 70 % by moles of recurring units deriving from tetrafluoroethylene, and

- 10 to 30 %, 15 to 25% by moles of recurring units deriving from the functional fluoro-alkylvinylether of formula (FM1-D), the amount by moles being referred to the total moles of recurring units of polymer (P).

[0046] Polymer (P) may be prepared by a process that comprises the polymerization of a monomer mixture (MM) comprising:

(i) at least one compound (AX) as above defined;

(ii) at least one fluoromonomer (FM) as above defined; in the presence of at least free radical initiator and optionally at least one surfactant. The polymerization may be performed in an aqueous emulsion. [0047] The monomer mixture (MM) may optionally comprise: (iii) at least one monomer different from fluoromonomer (FM), that is to say a monomer (HM) as above defined.

[0048] Monomer mixtures (MM) comprising a compound (AX), one or more than one fluoromonomer (FM) and optionally monomer (HM) are generally employed in the preparation of polymer (P) of the present invention.

[0049] The polymerization initiators used in the process of the present invention are organic or inorganic. As organic initiators, diisopropyl peroxydicarbonate (IPP) or di-tert-butyl peroxide (DTBP) can for example be mentioned. Radical inorganic initiators, such for example ammonium and/or potassium and/or sodium persulfate, optionally in combination with ferrous, cupreous or silver salts, are preferably used. The initiator feeding procedures can be in a continuous way or by a single addition at the start of the polymerization.

[0050] A surfactant may optionally be used. The surfactant may be a fluorinated or a non-fluorinated surfactant.

[0051] Among fluorinated surfactants mention may be made of functional (per)fluoropolyether compounds comprising at least one (per)fluoropolyoxyalkylene chain and at least one functional end-group selected from carboxylic acid, phosphonic acid and sulfonic acid groups as well as cyclic fluorocompounds such as those described in WO 2010/00392.

[0052] In a typical polymerization process, formation of a mixture containing water, an optional surfactant and the monomers (i), (ii) and optionally (iii), at the polymerization temperature, is placed in a reaction vessel; the polymerization reaction is started by addition of the free radical initiator. In an emulsion polymerization process a surfactant is generally present in the mixture, thereby forming an emulsion.

[0053] The polymerization reaction is generally carried out at temperatures in the range 25°-130° C. The polymerization is typically performed at atmospheric pressure or under pressure, for example from 2 bar up to 60 bar. [0054] Preferably the polymerization reaction is generally carried out at temperatures in the range 40°-70° C, preferably 50°-60° C, under pressure up to 20 bar, preferably higher than 5 bars.

[0055] A polymerization latex or suspension comprising the polymer dispersed in an aqueous liquid phase is obtained at the end of the process. The polymer (P) can be recovered from said polymerization latex using well- known techniques, such as a freeze-thawing coagulation method or by or addition of electrolytes such as aluminum sulfate or nitric acid.

[0056] Additional treatments such as purification of the latex or suspension, washing from contaminants, drying and the like can be performed on such coagulum before isolating polymer (P) in the form of powder.

[0057] Advantageously, a polymer (P) of the present invention is preferably obtained by a process comprising the polymerization of a monomer mixture (MM) comprising a compound (AX) of formula (III) as above defined, tetrafluoroethylene and a fluoromonomer (FM) of formula (FM1-D). A polymer (P) of formula (VI) as defined above is thus obtained.

[0058] Polymer (P) according to the present invention may be subjected to a chemical transformation.

[0059] Advantageously, the -S(=S)ORb groups present in the recurring units deriving from compound (AX) may be converted into different functional groups.

[0060] In a further aspect, the present invention thus provides a process for the chemical transformation of recurring units deriving from compound (AX) of polymer (P) into different functional groups.

[0061] According to a first embodiment of the present invention, the process comprises the chemical hydrolysis of the xanthate moiety of the recurring units deriving from compound (AX), namely the moiety of formula to the group -SH.

[0062] Chemical hydrolysis of the xanthate moiety to sulphydryl group -SH may be suitably carried out in the presence of acid or basic aqueous solutions. Hence, the process comprises reacting polymer (P) with an acid or a base in an aqueous solution.

[0063] Suitable acid aqueous solutions are notably solutions comprising HCI, HBr or H3 O4.

[0064] Suitable basic aqueous solutions are notably solutions comprising NaOH or KOH.

[0065] The reaction is generally carried out at a temperature in the range 25°- 100° C.

[0066] Conversion of the -S(=S)ORb groups into -SH groups may be observed by analytical techniques such as infrared spectroscopy.

[0067] According to a further embodiment of the present invention, the process of chemical transformation to be applied on polymer (P) is represented by conversion of the xanthate moiety of the recurring units deriving from compound (AX), namely the moiety of formula to the group -SO3H by chemical oxidation.

[0068] Chemical oxidation of the xanthate moiety to sulfonic acid group -SO3H may be suitably carried out in the presence of an oxidant such as hydrogen peroxide. Hence, the process comprises the reaction of polymer (P) with an oxidant, preferably hydrogen peroxide.

[0069] The reaction is generally carried out at temperatures in the range 25°-100° C.

[0070] Conversion of the -S(=S)ORb groups into -SO3H groups may be observed by analytical techniques such as infrared spectroscopy.

[0071] Hence a further object of the invention is a polymer (P ^ox ) of formula (VII):

I

SO ₂ X'

(VII) wherein: w is either 0 or 1 , Q is an integer between 1 and 4, preferably 1 or 2; X’ is selected among halogens (Cl, F, Br, I), preferably F or is -O’M’ ⁺ , wherein M’ ⁺ is a cation selected among H ⁺ , NH4 ⁺ , K ⁺ , Li ⁺ , Na ⁺ , or mixtures thereof, preferably M’ ⁺ is H ⁺ ; n and p are, independently from each other, integers greater than 0; m is equal to greater than 0; m, n and p denoting the molar fraction of each monomer in the polymer. Typically, n is between 0.50 and 0.75, m is between 0 and 0.30 and p is between 0.05 and 0.50. In formula (VII), n may be between 0.60 and 0.70, m between 0.10 and 0.30 and p between 0.10 and 0.20.

[0072] In a particularly preferred embodiment of the present invention, a polymer (P) of formula (VI) as above defined is post treated by chemical oxidation with hydrogen peroxide to provide a post treated polymer (P ^ox ) of formula (VIII): - wherein n, m and p are independently from each other integers greater than 0, denoting the molar fraction of each monomer in the polymer. Typically n is between 0.50 and 0.65, m is between 0.10 and 0.30 and p is between 0.05 and 0.40. In formula (VIII) n may be between 0.60 and 0.70, m between 0.10 and 0.30 and p between 0.10 and 0.20.

[0073] In the above notation for copolymers, the n, m and p units may appear in any order: formule (VII) and (VIII) are only intended to define the relative proportion of monomer units, and not the exact order (which is random) in the copolymer. Similarly, the orientation of the recurring units in a tail-to- tail pattern in formulae (VII) and (VIII) is only indicative and not intended to limit the structure of the polymer. The recurring units in the polymer of formula (VII) or (VIII) may be in a head-to-head, tail-to-tail or head-to-tail arrangement.

[0074] Polymers (P ^ox ) such as those of formula (VII) or (VIII) obtained from the inventive process are ion conducting polymers or precursors thereof. They are particularly suitable to be used in electrochemical applications. Polymers of formula (VII) or (VIII) may be used in the preparation of membranes for fuel cells, of membranes for electrochemical applications, such for example chloro-soda cells, lithium batteries. They may additionally be used as membranes in electrodialysis applications and in reactors in which membranes made of the polymer act as a superacid catalyst.

[0075] Sulfonic perfluorinated ion conducting polymers, owing to the combination of chemical stability in harsh environments and good proton conductivity in a wide range of humidity conditions, are today considered the material benchmark for fuel cells (mainly low temperature fuel cells for transportation) and electrolyzers (producing the so-called green hydrogen from renewables). Commercially available perfluorinated ion conducting polymers are copolymers of tetrafluoroethylene and vinyl ethers of different length bearing -SOsH groups. The ion conducting polymer having the shortest side chain (two -CF2- groups) currently available on the market is Aquivion® ion conducting polymer from Solvay whereas the ion conducting polymer with the longest side chain is Nation® from Chemours. The access to ion conducting polymer with shorter side chains would have great impact in increasing electrochemical performance and mechanical strength.

[0076] This invention thus gives access to perfluorinated ion conducting polymer having the side chain containing only one -CF2- group. Polymers (P) in which the -S(=S)ORb group has been oxidized to the -SO3H group, including polymers of formula (VII) or (VIII), are advantageously endowed with higher proton conductivity, higher crystallinity and higher mechanical strength than perfluorinated ion conducting polymers available on the market.

[0077] A further object of the invention is therefore an article comprising polymer (P) or polymer (P ^ox ). The article could be in the form of a membrane, such as an ion conducting membrane. The ion conducting membrane comprising polymer (P) or, advantageously, polymer (P ^ox ) might be used in electrolysis applications or in fuel cell applications.

[0078] In another aspect of the present invention, the use of a compound (AX) of formula (I) as above defined as chain transfer agent in controlled radical polymerizations is provided.

[0079] Among controlled radical polymerization techniques, reversible additionfragmentation chain transfer (RAFT) and macromolecular design via interexchange of xanthate compounds (MADIX) can be mentioned.

[0080] RAFT/MADIX agents are capable to act as a reversible chain transfer agent in free-radical polymerizations, thereby inducing reversible-addition fragmentation transfer reactions to create an equilibrium between propagating radicals (i.e. the growing polymer chain) and so-called dormant species (containing the chain transfer agent fragment) that can become active again.

[0081] The Applicant has surprisingly found that compound (AX) can be suitably used as RAFT/MADIX agent in emulsion polymerization of fluorinated monomers in order to control microstructure of the polymer.

[0082] Thus, in a further aspect, the present invention provides a method for emulsion polymerization of at least one fluoromonomer, said method comprising:

(i) providing at least one aqueous emulsion comprising a monomer mixture comprising at least one fluoromonomer [monomer (F)] and at least one compound (AX) of formula (I): wherein R _a is a (per)fluorinated allyl radical group, Rb is a straight or branched alkyl radical group and optionally at least one surfactant; (ii) initiating the polymerization of said monomer mixture in said aqueous emulsion adding at least one radical initiator;

(iii) continuing the polymerization by adding additional amounts of said at least one monomer (F) and/or said compound (AX), until converting the targeted amount of said monomer mixture, and

(iv) terminating the polymerization and recovering a latex of fluoropolymer [polymer (F)].

[0083] The expression “fluoromonomer” is used herein according to its usual meaning, that is to say for designating an ethylenically unsaturated monomer comprising at least one fluorine atom.

[0084] Fluoromonomer (F) can notably be a fluoromonomer (FM) as above defined.

[0085] The method of the invention is suitable for the manufacture of a large variety of fluoropolymers, including notably non-melt processable tetrafluoroethylene polymers (including PTFE homopolymers and its copolymers comprising low amounts of perfluorinated comonomers), thermoplastic fluoropolymers (e.g. vinylidene fluoride homopolymers and its plastomeric copolymers, copolymers of ethylene with chlorotrifluoroethylene, thermoplastic copolymers of tetrafluoroethylene and perfluoroalkyl vinylethers, thermoplastic copolymers of tetrafluoroethylene and hexafluoropropylene), and fluoroelastomers.

[0086] The invention will be now described with reference to the following examples, whose purpose is merely illustrative and not intended to limit the scope of the invention.

Experimental section

[0087] EXAMPLE 1 : FAX synthesis

[0088] A three-necked round bottom flask equipped with a thermometer, a condenser and a dropping funnel was charged with 109.20 g of CF2=CFCF2OSO2F (FAFS) under nitrogen. Then, 70.57 g of Potassium Ethylxanthogenate dissolved in 1482 ml of Acetonitrile was added dropwise in 25 minutes, at room temperature. After two hours of stirring the reaction was complete and a white solid (FSOsK) precipitated. The white solid was filtered and the resulting clear solution was washed three times with distilled water (1 :1 volume vs organic phase). The organic phase was separated and distilled under vacuum obtaining 82.73 g of pure CF2=CFCF2S(C=S)OCH ₂ CH3 (FAX).

[0089] ¹⁹ F NMR in Acetone (HFMX reference): -82 ppm (m; 2F; -SCF ₂ CF=CF2); - 93.5 ppm (m; 1 F; cis -SCF2CF=CF2); -105 ppm (m; 1 F; trans - SCF ₂ CF=CF ₂ ); -184 ppm (m; 1 F; -SCF ₂ CF=CF2). ¹ H NMR in Acetone (TMS reference): +4.8 ppm (q; 2H; -OCH2CH3); +1.5 ppm (m; 3H; - OCH2CH3).

[0090] EXAMPLE 2: Polymerization of TFE+VEFS+FAX

[0091] Deionized water (1.8L), VEFS (212 g) and solution in water of Fluorolink 7800 (540 g, 5 wt%) were loaded in a 5L reactor and then it was pressurized with 7.5 bar of TFE and the system was heated to 50°C under stirring. The reaction took place after the feeding of a solution of potassium persulfate (cone. 10.5 g/L, 200 mL). Every 10% TFE conversion, 45 g of VEFS and 50 g of a solution of VEFS (90 wt%) and FAX (10 wt%) were fed. When 4.95 g of FAX and 257 g of VEFS were added, the reaction was stopped, cooled down and the pressure was reduced by removing TFE. The polymer latex thus obtained was freeze-thawed and the polymer was recovered as a yellowish powder.

[0092] FT-IR: 970 cm-1 (combined symmetric stretching of C-F and C-O-C); 1150 cm-1 (combined asymmetric stretching of C-O-C and stretching vibrations of C-F bonds); 1020 cm ^-1 (C=S stretching); 1220 cm ^-1 (symmetric and asymmetric stretching of CF2); 1470 cm-1 (S-F bond motion); 2365 cm- ¹ (C-F overtone/combination band); 2705 cm- ¹ (S-F overtone).

[0093] EXAMPLE 3 (comparative): Polymerization of TFE+VEFS

[0094] Deionized water (1.8 L), VEFS (212 g) and solution in water of Fluorolink 7800 (540 g, 5 wt%) were loaded in a 5L reactor and then it was pressurized with 7.5 bar of TFE and the system was heated to 50°C under stirring. The reaction took place after the feeding of a solution of potassium persulfate (cone. 10.5 g/L, 200 mL). Every 10% TFE conversion, 45 g of VEFS were fed. When 707 g of VEFS were added, the reaction was stopped, cooled down and the pressure was reduced by removing TFE. The polymer latex thus obtained was freeze-thawed and the polymer was recovered as a white powder. The powder was washed four times with demineralized water (1 L) at room temperature and under stirring and then dried in a vent oven at 80°C overnight.

[0095] FT-IR: 970 cm-1 (combined symmetric stretching of C-F and C-O-C); 1150 cm- ¹ (combined asymmetric stretching of C-O-C and stretching vibrations of C-F bonds); 1220 cm- ¹ (symmetric and asymmetric stretching of CF2); 1470 cm- ¹ (S-F bond motion); 2365 cm- ¹ (C-F overtone/combination band); 2705 cm- ¹ (S-F overtone).

[0096] EXAMPLE 4: Transformation of the xanthate group into the -SO3H group

[0097] The polymer from the EXAMPLE 2 was washed four times with demineralized water (1 L each) and with ethyl acetate (0.5 L). The washings were carried out under stirring and at room temperature. The powder was then dried in a vent oven at 80°C overnight. The polymer was stirred for 8 h at 45°C in a solution of H2O2 (15%, 200 mL) and H2SO4 (0.5 M, 2 mL) having pH of about 4. The powder thus obtained was washed with distilled water four times (1 L each) at room temperature and under stirring and finally dried in a vent oven at 80°C overnight.

[0098] FT-IR: 515 cm-1 (C-S deformation of CF2-SO3); 634 cm-1 (S-OH deformation of SO3H); 970 cm-1 (combined symmetric stretching of C-F and C-O-C); 1057 cm-1 (symmetric stretching of SO3); 1154 cm-1 (combined asymmetric stretching of C-O-C and SO3 and stretching vibrations of C-F bonds); 1220 cm-1 (symmetric and asymmetric stretching of CF2); 1300 cm-1 (symmetric and asymmetric stretching of SO3); 1470 cm-1 (S-F bond motion); 2365 cm-1 (C-F overtone/combination band); 2705 cm-1 (S-F overtone).

[0099] EXAMPLE 5 (comparative): Transformation of the -SO2F group into the -SO3H group

[00100] The polymer from the comparative EXAMPLE 3 was treated with a solution of NaOH in demineralized water (20 wt%, 1 L) at 80°C under stirring. After 8 h the powder was washed four times with demineralized water (1 L each) under stirring and at room temperature and then treated twice with a solution of HNO3 in distilled water (20 wt%, 1 L each) at room temperature and under stirring. The polymer was washed under stirring with distilled water (4 x 1 L) at room temperature and then dried in a vent oven (80°C, overnight).

[00101] FT-IR: 515 cm- ¹ (C-S deformation of CF2-SO3); 634 cm- ¹ (S-OH deformation of SO3H); 970 cm- ¹ (combined symmetric stretching of C-F and C-O-C); 1057 cm- ¹ (symmetric stretching of SO3); 1154 cm- ¹ (combined asymmetric stretching of C-O-C and SO3 and stretching vibrations of C-F bonds); 1220 cm- ¹ (symmetric and asymmetric stretching of CF2); 1300 cm- ¹ (symmetric and asymmetric stretching of SO3).