Method for preparing a fluorinated polymer comprising ion exchange groups or precursor groups thereof

Abstract

The present invention relates to a method for preparing a fluorinated polymer comprising ion exchange groups or precursor groups thereof, said method comprising polymerizing a polymerizable mixture comprising: (i) tetrafluoroethylene, (ii) an ethylenically unsaturated functional monomer having at least one SO2X group, where X is halogen or -OZ group, Z being a hydrogen atom, an ammonium group or an alkali metal atom; and (iii) a redox initiating system comprising at least one reducing agent having one or more sulfinic or sulfinate groups and at least one organic hydroperoxide as oxidizing agent. The present invention also relates to a fluorinated polymer obtained with said method, a membrane comprising said fluorinated polymer, a membrane-electrode assembly comprising said membrane and an electrochemical device comprising said membrane-electrode assembly.

Description

1 / 28 SSPI 2024_023METHOD FOR PREPARING A FLUORINATED POLYMER COMPRISING ION EXCHANGE GROUPS OR PRECURSOR GROUPS THEREOFTechnical Field

[0001] The present invention relates to a method for preparing a fluorinated polymer comprising ion exchange groups or precursor groups thereof. The present invention also relates to the fluorinated polymer obtained with said method, a membrane comprising said fluorinated polymer, a membraneelectrode assembly comprising said membrane and an electrochemical device comprising said membrane-electrode assembly.Priority

[0002] This application claims priority filed on 2024-08-08 in EUROPE with N. 24193710.1 , the whole content of this application being incorporated herein by reference for all purposes.Background Art

[0003] Fuel cells, and in particular hydrogen-oxygen fuel cells, are currently drawing increasing attention as power generating systems possessing substantially no detrimental effect against the environment. In particular, polymer fuel cells have been identified as the most promising approach for implementing fuel cell technology as they enable obtaining higher power density.

[0004] The basic element of a polymer fuel cell is the so-called “membrane electrode assembly” (MEA). The MEA comprises a polymeric membrane which consists of a proton conducting polymer and whose opposing faces are in contact with electrically conductive and catalytically active layers (also called electrode layers). The electrode layers catalyse the oxidation of the fuel (e.g. H2) and the reduction of the oxidizing agent (e.g. O2), and contribute to assure the necessary electrical conductivity within the fuel cell. Such layers are generally composed of the same proton conducting polymer as the membrane containing dispersed therein an active catalyst, generally a noble metal (e.g. Pt).

[0005] Fluorinated polymers having a plurality of pendant ion exchange groups, especially perfluorinated polymers obtained by polymerizing2 / 28 SSPI 2024_023 tetrafluoroethylene and one or more fluorinated functional monomers having ion exchange groups or precursor groups thereof, have been widely used as materials for both the polymeric membrane and the electrode layers of fuel cells as well as electrolyzers. These fluorinated polymers, however, generally comprise unstable end groups, such as carboxyl- and carbonyltype end groups (e.g. — COOH, — COO, — COF and — CONH2) and non- carboxyl / carbonyl-type end groups (e.g. — CF=CF2 and — CF2H). These unstable groups, which are located at the chain ends of the fluorinated polymer, are susceptible to peroxide radical attacks and thus jeopardize the stability of the fluorinated polymer. During long-term fuel cells operation, in fact, the fluorinated polymer of the membrane and / or of the electrode layers gradually decomposes starting from these end groups leading to decrease of the mechanical strength of the membrane and / or of the MEA, possible generation of pinholes, breaking, abrasion and the like, so that the power generation voltage is progressively reduced and fuel cell life-time is limited.

[0006] Several methods have been proposed in the past aiming at improving the durability of these fluorinated polymers having a plurality of ion exchange groups by reduction of the amount of unstable end groups.

[0007] One of the most commonly used methods is the direct fluorination of the polymer that converts the unstable end groups into perfluorinated end groups. The fluorinated polymer, which is generally prepared by free-radical initiated polymerization of one or more fluorinated ethylen ically unsaturated monomers, is subjected to the fluorination treatment generally as a finely divided powder in the — SO2F form, that is the form of the polymer having the pendant side-chains terminating with — SO2F groups. Subsequently, the fluorinated polymer may undergo hydrolysis to convert the — SO2F groups into the proton conductive sulfonic acid groups (-SO3H) or metal sulfonates (-SO3- Me+) groups.

[0008] The fluorination treatment requires highly reactive fluorine gas to be contacted with the powdered polymer at elevated temperatures for several hours. The degree of fluorination is highly dependent on the polymer particle size, the temperature, the fluorine content of the fluorination gas, and the contact time. These parameters are difficult to control reproducibly, therefore, although it may be an effective method to reduce the amount of3 / 28 SSPI 2024_023 unstable end-groups, fluorination is a time-consuming technique and increase the manufacturing costs of the fluorinated polymers.

[0009] An advantageous way to prepare fluorinated polymers having pendant ion exchange groups or precursor groups thereof is emulsion or suspension polymerization in the presence of high-molecular weight fluorinated polymeric dispersants comprising functional groups (e.g. -SOsXa and - COOXa groups, wherein Xa is H, an ammonium group or a monovalent metal), such as those disclosed in WO 2018 / 167190A1 and WO 2023 / 165912. These polyfunctional polymeric dispersants act as effective dispersing agents of the growing polymer chains and can thus be used in replacement of environmentally hazardous non-polymeric (low molecular weight) surfactants. The polymerization employing such polyfunctional polymeric dispersants therefore allows the preparation of fluorinated polymers having pendant ion exchange groups or precursor groups thereof with reduced environmental impact. Moreover, the fluorinated polymeric dispersants have a molecular weight comparable to that of the product polymer, which makes the removal of the polyfunctional polymeric dispersants from the polymer at the end of the polymerization process unnecessary.

[0010] The Applicant has observed, however, that fluorination of polymers prepared using these polyfunctional polymeric dispersants is particularly difficult to control reproducibly and may leads to fluorinated polymers that, when used as ion exchange membranes in electrolytic devices, do not possess adequate durability against peroxide radicals. Therefore, the unavailability of suitable fluorination treatments may represent a limitation to the full exploitation of this environmental-friendly polymerization technique.

[0011] Since the composition of the polymer chain end groups is linked to that of the radical initiators used to initiate the polymerization, in the state of the art it has been proposed to produce stabilized fluorinated polymers using selected initiators or initiating systems capable of producing stable end groups on the final fluorinated polymer.

[0012] US 5285002 discloses a method for the preparation of a fluorine-containing polymer comprising polymerizing, under free-radical conditions, an aqueous4 / 28 SSPI 2024_023 emulsion or suspension of a polymerizable mixture comprising a fluoroaliphatic-radical containing sulfinate and an oxidizing agent capable of oxidizing said sulfinate to a sulfonyl radical. The oxidizing agent is selected among sodium, potassium, and ammonium persulfates, perphosphates, perborates, and percarbonates. The reaction between the sulfonate and the oxidizing agent is believed to eliminate SO2, forming a fluorinated radical that initiates the polymerization of the ethylenically unsaturated monomers leading to a final fluorinated polymer in which the majority of the end-groups is fluoroaliphatic.

[0013] WO 97 / 02300 discloses a process in which fluorine containing olefins are polymerized using an initiation system which is a combination of a fluoroaliphatic sulfinate or sulfinic acid and chlorate, bromate or hypochlorite ions. The resulting polymer is described as containing fewer deleterious end groups and is more stable and / or easier to process.

[0014] WO 2006 / 119224 discloses fluorinated ionomers with reduced amounts of carbonyl end groups and a method of preparation thereof. The fluoropolymer comprises a plurality of pendant groups terminating in — CF2SO3X, — CF2SO2F, or combinations thereof, where X is selected from a group consisting of H+and a monovalent cation, and at least one — CF2Y end group, where Y is selected from a group consisting of a chlorine atom, a bromine atom, an iodine atom, a nitrile group, and an — SO3X group. The preparation method comprises free-radically polymerizing fluorinated monomers in the presence of a salt (e.g. halogen or cyanide salt), a pseudohalogen (e.g. pseudohalogen nitrile-containing compounds), or a combination of the salt and the pseudohalogen, to make the fluoropolymer. The free-radical initiator is preferably a redox initiation system, such as sodium disulfite and ammonium persulfate.

[0015] Despite the methods available in the state of the art, there is still a need of further methods for preparing fluorinated polymers that have stability toward radical attacks which is comparable to or higher than that of the fluorinated polymers stabilized through fluorination treatments.Summary of invention

[0016] It has now been found that the above technical problem, as well as other technical problems that will become more apparent hereinafter, can be5 / 28 SSPI 2024_023 solved by fluorinated polymers that are obtained by carrying out the polymerization of a polymerizable mixture comprising tetrafluoroethylene and at least one ethylenically unsaturated functional monomer in the presence of a redox initiating system, wherein the redox initiating system comprises at least one compound having one or more sulfinic or sulfinate groups acting as reducing agent and at least one organic hydroperoxide as oxidizing agent.

[0017] The polymerization is a free-radical-initiated polymerization carried out in an aqueous medium (e.g. aqueous emulsion or suspension polymerization). When the polymerization is initiated by the redox initiating systems according to the present invention, fluorinated polymers are obtained that are stable against peroxide radical attacks and have durability properties comparable to those of the same polymers that are prepared using the redox initiating systems of the prior art or conventional thermal initiators (e.g. ammonium persulfate) followed by fluorination treatment.

[0018] Besides avoiding performing complex and expensive post-fluorination treatments, the method of the present invention enables to perform the polymerization reaction with adequate polymerization rates at relatively lower temperatures compared to methods employing thermal initiators, thus reducing the energy consumption of the synthesis process.

[0019] Notably, it has been observed that despite a number of redox couples may achieve an adequate polymerization rate and / or the formation of low amounts of unstable -COOH groups, only the redox couples of the present invention lead to final fluorinated polymers with improved stability toward radical attacks and adequate durability of the polymeric membranes comprising thereof (as measured by Open Circuit Voltage (OCV) decay test). It has also been observed that certain other redox couples are not capable of initiating the polymerization, even at relatively high temperatures.

[0020] According to a first aspect, therefore, the present invention relates to a method for preparing a fluorinated polymer comprising ion exchange groups, or precursor groups thereof, said method comprising polymerizing a polymerizable mixture comprising:- tetrafluoroethylene,6 / 28 SSPI 2024_023- an ethylenically unsaturated functional monomer having at least one — SO2X group, where X is a halogen atom or — OZ group, Z being a hydrogen atom, an ammonium group or an alkali metal atom; and- a redox initiating system comprising at least one reducing agent having one or more sulfinic or sulfinate groups and at least one organic hydroperoxide as oxidizing agent.

[0021] According to a second aspect, the present invention relates to a fluorinated polymer comprising ion exchange groups, or precursor groups thereof, as obtainable by the method according to the first aspect.

[0022] According to a third aspect, the present invention relates to a membrane comprising a fluorinated polymer comprising ion exchange groups, or precursor groups thereof, according to the second aspect, wherein the fluorinated polymer preferably comprises ion exchange groups of formula — SO3Z, where Z is a hydrogen atom, an ammonium group or an alkali metal atom, more preferably a hydrogen atom.

[0023] According to a fourth aspect, the present invention relates to a membraneelectrode assembly comprising a membrane comprising ion exchange groups, or precursor groups thereof, according to the third aspect.

[0024] According to a fifth aspect, the present invention relates to an electrochemical device, such as a fuel cell or an electrolyzer, comprising a membrane-electrode assembly according to the fourth aspect.

[0025] As used herein, the compositions of the present invention may “comprise”, “consists of” or “consists essentially of” the essential and optional components disclosed in the description and annexed claims. The expression “consists essentially of” means that the composition or the component may include additional ingredients insofar they do not materially affect the essential characteristics of the composition or component.Description of invention

[0026] The method of the present invention allows to synthesize a fluorinated polymer comprising a fluorinated backbone chain with a plurality of pendant side chains, which are covalently bound to the backbone chain and terminate in ion exchange groups or precursor groups thereof.

[0027] The backbone chain of the fluoropolymer may be partially or fully fluorinated. Suitable fluorine concentrations in the backbone chain include7 / 28 SSPI 2024_023 about 40% or more by weight, based on the total weight of the backbone chain. In one embodiment of the present invention, the backbone chain of the fluoropolymer is perfluorinated.

[0028] As used herein, the term "ion exchange group" has its general meaning as intended in organic chemistry and it encompasses atoms or combination of atoms bonded to the carbon skeleton of the ethylenically unsaturated functional monomer, which confer to said ethylenically unsaturated functional monomer the ability to catch and release (i.e. exchange) ions in an ion exchange process.

[0029] The fluorinated polymer comprises at least a plurality of sulfonic-type ion exchange groups, i.e. groups of formula -SO2X, wherein X is a halogen atom or — OZ group, wherein Z is a hydrogen atom (H), an ammonium group (NH4), a monovalent metal ion or mixtures thereof. Preferably, the halogen atom is selected from Cl, F, Br, I and mixture thereof, more preferably from F and Cl; the monovalent metal ion is preferably selected from K, Li, Na and mixture thereof.

[0030] When in the -SO2X group X is an — OZ group as detailed above, the pendant group provides ionic conductivity to the fluorinated polymer. When in the -SO2X group X is a halogen atom (e.g. -SO2F), the pendant group is non-ionic and therefore the polymer is scarcely ion conductive.

[0031] Herein, the non-ionic-SO2X group where X is a halogen atom is also named “precursor group” as it can be converted into an ionic conductive ion exchange -SO3Z group by modification and / or post-treatment of the polymer, e.g. by means of hydrolysis and acidification processes known in the art.

[0032] The fluorinated polymer of the present invention is prepared by free-radical polymerization, preferably emulsion or suspension polymerization in aqueous medium, of a polymerizable mixture containing tetrafluoroethylene (TFE), at least one ethylenically unsaturated functional monomer having at least one — SO2X group where X has the meaning indicate above, a redox initiating system and further optional ingredients, such as emulsifiers (e.g. surfactants) and suspending agents.

[0033] Preferably, the ethylenically unsaturated functional monomer is a monomer of general formula F2C=CF-R1-SO2F, wherein R1is a branched or8 / 28 SSPI 2024_023 unbranched perfluoroalkyl, perfluoroalkoxy, or perfluoroether group, which contains one to fifteen carbon atoms and zero to four oxygen atoms.

[0034] More preferably, the ethylenically unsaturated functional monomer is selected from:- sulfonated perfluoroolefins of formula (M1)wherein n is an integer between 0 and 6 and X’ is a halogen atom or — OM group, wherein M is H, NH4, alkali metal or mixtures thereof; preferably X’ is fluorine; preferred sulfonated perfluoroolefins are those of formulae (M1- A) and (M1-B):wherein X’ has the same meaning as above defined;- sulfonated perfluorovinylethers of formula (M2):wherein m is an integer between 1 and 10 and X’ is a halogen atom or — OM group, wherein M is H, NH4, alkali metal or mixtures thereof; preferablyX’ is fluorine; preferred sulfonated perfluorovinylethers are those of the formulae (M2-A), (M2-B) and (M2-C):9 / 28 SSPI 2024_023 wherein X’ has the same meaning as above defined; most preferably, the sulfonated perfluorovinylether is perfluoro-5-sulphonylfluoride-3-oxa-1 pentene (also known as “SFVE”) of formula (M2-D):which can be in its -SO2F or in any of the -SO2X’ forms, as above detailed;- sulfonated perfluoroalkoxyvinylethers of formula (M3):wherein w is an integer between 0 and 2; RFi and RF2, equal or different from each other and at each occurrence, are independently F, Cl or a C1- C10 perfluoroalkyl group, optionally substituted with one or more ethereal oxygen atoms; y is an integer between 0 and 6 and X’ is a halogen atom or — OM group, wherein M is H, NH4, alkali metal or mixtures thereof; preferably X’ is fluorine; preferred sulfonated perfluoroalkoxyvinylethers of formula (M3) here above, are those wherein w is 1 , RF1 is -CF3, y is 1 and RF2 is F and X’ is F (formula (M3-A), also called “PSEPVE” (perfluoro 2 (2 fluorosulfonylethoxy)propylvinyl ether)):which can be in its -SO2F or in any of the -SO2X’ forms, as above detailed- sulfonated aromatic (per)fluoroolefins of formula (M4):10 / 28 SSPI 2024_023 wherein Ar is a C3-C15 aromatic or heteroaromatic moiety, preferably perfluorinated, and X’ is a halogen atom or — OM group, wherein M is H, NH4, alkali metal or mixtures thereof; preferably X’ is fluorine; and- mixtures thereof.

[0035] In the above formulae M1 to M4, the halogen atom is preferably selected from F, Cl, Br and I, more preferably from F and Cl.

[0036] In the above formulae M1 to M4, the alkali metal is preferably selected from K, Li, Na and mixture thereof

[0037] Preferred ethylenically unsaturated functional monomers are notably sulfonated perfluorovinylethers of formula (M2) as above detailed and sulfonated perfluoroalkoxyvinylethers of formula (M3) as above detailed, and mixtures thereof.

[0038] Optionally, the fluorinated polymer comprises recurring units deriving from one or more ethylenically unsaturated monomers different from TFE and the ethylenically unsaturated functional monomer as detailed above. These optional ethylenically unsaturated monomers are hereinafter referred to as “comonomers”.

[0039] The polymerizable mixture, therefore, may optionally comprise one or more comonomers, different from one another, which can be either hydrogenated (i.e. free of fluorine atoms) or fluorinated (i.e. containing at least one fluorine atom).

[0040] Non-limitative examples of suitable hydrogenated comonomers are notably ethylene, propylene, vinyl monomers such as vinyl acetate, acrylic monomers such as methyl methacrylate, acrylic acid, methacrylic acid and hydroxylethyl acrylate, as well as styrene monomers, such as styrene and p-methylstyrene.

[0041] Non limitative examples of suitable fluorinated comonomers are notably:C2-C8 perfluoroolefins, such as tetrafluoroethylene, hexafluoropropylene, perfluoroisobutylene;- C2-C8 hydrogen-containing fluoroolefins, such as trifluoroethylene, vinylidene fluoride, vinyl fluoride, pentafluoropropylene, and hexafluoroisobutylene;- C2-C8 chloro- and / or bromo- and / or iodo-containing fluoroolefins, such as chlorotrifluoroethylene and bromotrifluoroethylene;11 / 28 SSPI 2024_023- fluoroalkylvinyl ethers of formula CF2=CFORfi, wherein Rn is a Ci-Ce fluoroalkyl, e.g. -CF3, -C2F5, -C3F7;- fluorooxyalkylvinyl ethers of formula CF2=CFOXo, wherein Xo is a Ci-012 fluorooxyalkyl group comprising one or more than one ethereal oxygen atom, including notably fluoromethoxyalkylvinyl ethers of formula CF2=CFOCF2ORf2, with Rf2 being a C1-C3 fluoro(oxy)alkyl group, such as - CF2CF3, -CF2CF2-O-CF3 and -CF3;- fluorodioxoles having the following general formulawherein each of Rf3, Rf4, Rfs, Rf6, equal to or different from each other, is independently a fluorine atom or a Ci-Ce (halo)fluoroalkyl, optionally comprising one or more oxygen atom, e.g. -CF3, -C2F5, -C3F7, -OCF3, - OCF2CF2OCF3.

[0042] Preferably the fluorinated polymer comprises recurring units derived from TFE and recurring units derived from PSEPVE and / or SFVE, in their -SO2F or -SO2X form, wherein X is a halogen atom or — OM group, wherein M is H, NH4, alkali metal (K, Li, Na) or mixtures thereof; preferably in their -SO2F form.

[0043] Advantageously, the fluorinated polymer comprises, consists essentially of, or consists of:- from 50 to 95 mol%, preferably from 55 to 93 mol% of recurring units deriving from TFE;- from 5 to 50 mol%, preferably from 7 to 45 mol% of recurring units deriving from PSEPVE and / or SFVE;- from 0 to 25 mol%, preferably from 0 to 20 mol% of recurring units deriving from one or more comonomers.

[0044] Advantageously, the fluorinated polymer comprises, consists essentially of, or consists of:12 / 28 SSPI 2024_023- from 50 to 95 mol%, preferably from 55 to 93 mol% of recurring units deriving from TFE;- from 5 to 50 mol%, preferably from 7 to 45 mol% of recurring units deriving from a functional monomer of formula (M2), preferably of formula (M2-D);

[0045] - from 0 to 25 mol%, preferably from 0 to 20 mol% of recurring units deriving from one or more comonomers.

[0046] According to the present invention, the polymerization is initiated by means of a redox initiating system comprising at least one reducing agent having one or more sulfinic or sulfinate groups and at least one organic hydroperoxide acting as oxidizing agent.

[0047] The reducing agent has one or more sulfinic or sulfinate groups covalently bound to a carbon atom. Preferably, the reducing agent is not fluorinated.

[0048] Advantageously, the reducing agent comprises at least one compound of formula (I)wherein:- M is a hydrogen atom, an ammonium ion, a monovalent metal ion;- Ri is OH or N(R4)(Rs) where each of R4 and Rs, identical or different from one another, are hydrogen atom or linear or branched alkyl group having from 1 to 6 carbon atoms;- R2 is hydrogen atom linear or branched alkyl group having from 1 to 6 carbon atoms, 5- or 6-membered cycloalkyl group, 5- or 6-membered aryl group;- R3is a group -COOM, -SO3M, -C(=O)R4, -C(=O)N(R4)(Rs), -C(=O)OR4, wherein M, R4 and Rs are as defined above.

[0049] In a preferred embodiment, in formula (I):- M is an alkali metal ion or an equivalent of an alkaline earth metal ion or zinc ion;- R1 is a hydroxyl or amino group, preferably hydroxyl;13 / 28 SSPI 2024_023- R2 is H or alkyl, preferably H; and- R3 is COOM or COOR4, where M is H, an alkali metal ion or an equivalent of an alkaline earth metal ion, and R4 is Ci-Ce-alkyl.

[0050] A preferred compound of formula (I) is 2-hydroxy-2-sulfinatoacetic acid disodium salt.

[0051] The reducing agent may be a reducing agent composition comprising at least one compound of formula (I) and optionally at least one compound of formula (II) and / or one compound of formula (III) described below. Preferably, the total weight of the compound of formula (I) in the reducing agent composition is up to 50 wt.% of the weight of the reducing agent composition.

[0052] The compound of formula (II) is represented by the following formula:wherein:- M is a hydrogen atom, an ammonium ion, a monovalent metal ion;- R1 is OH, or N(R4)(Rs) where each of R4 and Rs, identical or different from one another, are hydrogen atom or linear or branched alkyl group having from 1 to 6 carbon atoms;- R2 is hydrogen atom, linear or branched alkyl group having from 1 to 6 carbon atoms, 5- or 6-membered cycloalkyl group, 5- or 6-membered aryl group;- R3is -COOM, -SO3M, -C(=O)R4, -C(=O)N(R4)(R5), -C(=O)OR4, wherein M, R4 and Rs are as defined above.

[0053] In a preferred embodiment, in formula (II):- M is an alkali metal ion or an equivalent of an alkaline earth metal ion or zinc ion;- R1 is a hydroxyl or amino group, preferably hydroxyl;- R2 is H or alkyl, preferably H; and- R3 is COOM or COOR4, where M is H, an alkali metal ion or an equivalent of an alkaline earth metal ion, and R4 is Ci-Ce-alkyl.14 / 28 SSPI 2024_023

[0054] A preferred compound of formula (II) is 2-hydroxy-2-sulfonatoacetic acid disodium salt.

[0055] Preferably, the total weight of the compound of formula (II) in the reducing agent composition is up to 50 wt.% of the weight of the reducing agent composition.

[0056] The compound of formula (III) is represented by the following formula:

[0057] wherein M is a hydrogen atom, an ammonium ion, a monovalent metal ion, preferably is Na2SOs.

[0058] Preferably, the total weight of the compound of formula (III) in the reducing agent composition is up to 10 wt.% of the weight of the reducing agent composition.

[0059] The reducing agent composition may comprise water, preferably in a concentration up to 30 wt.% of the weight of the reducing agent composition.

[0060] The reducing agent according to the formulae (I) to (III) can be prepared according to techniques generally known in the art and are commercially available. For example, suitable reducing agents according to formulae (I) to (III) and compositions thereof are commercially available from BRUGGEMANN-GROUP under the trade name Bruggolite®. In particular, the mixture commercialized as Bruggolite® E28, which contains a mixture of a 2-hydroxy-2-sulfinatoacetic acid salt and a 2-hydroxy-2-sulfonatoacetic acid salt, has been found to be particularly useful within the frame of the present invention.

[0061] Further information on the preparation of the reducing agents according to formulae (I) to (III) and related compositions are available in US 2002 / 0042353.

[0062] Advantageously, reducing agents having a sulfinic or sulfinate group suitable for the present invention are fluoroaliphatic compounds of following formula (IV)Rf - SO2M’I / X (IV)15 / 28 SSPI 2024_023 wherein Rf is monovalent fluoroaliphatic radical having from 1 to 20 carbon atoms, preferably 1 to 10 carbon atoms, even more preferably 1 to 4 carbon atoms; M’ is a hydrogen atom or a cation with valence x, x being 1 or 2.

[0063] The fluoroaliphatic radical Rf can be straight chain, branched chain and, if sufficiently large, cyclic, or combinations thereof, such as alkyl cycloaliphatic radicals. Generally, Rf will have 1 to 20 carbon atoms, preferably 1 to 10, and will contain 40 to 83 wt.% fluorine, preferably 50 to 78 wt.%, based on the weight of the fluoroaliphatic compound. The preferred compounds are those in which the Rf group is fully or substantially completely fluorinated, as in the case where Rf is perfluoroalkyl, CnF2n+i, where n is 1 to 20.

[0064] Representative fluoroaliphatic sulfinate compounds useful in the practice of the present invention include the following: CFsSCkNa (sodium triflinate), C4F9SO2H, C8Fi7SO2Na, CF3C(CI)2CF2SO2K and CI(CF2)8OC2F4SO2Na. Preferred fluoroaliphatic sulfinate compound is CFsSC Na.

[0065] The reducing agents according to the above formula (IV) can be prepared as described in US 5285002.

[0066] The reducing agent is used in a total amount of from 0.01 % to 4.0 wt.%, more preferably from 0.1 % to 3.0 wt.%, based on the total monomer weight used.

[0067] The oxidizing agent comprises at least one organic hydroperoxide compound.

[0068] The organic hydroperoxide is preferably selected from: t-butyl hydroperoxide, 1 ,1 ,3,3-tetramethylbutyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, p-menthane hydroperoxide, 2,5-dimethylhexane-2,5-dihydroperoxide and mixture thereof. More, preferably the organic peroxide is t-butyl hydroperoxide.

[0069] The oxidizing agent is used in a total amount of from 0.01 % to 4.0 wt.%, more preferably from 0.1 % to 3.0 wt.%, based on the total monomer weight used.

[0070] Redox initiating systems that are particularly suitable for the practice of the present invention are the following ones:16 / 28 SSPI 2024_023

[0071] (a) system comprising: a compound of formula (I), optionally in combination with a compound of formula (II), and t-butyl hydroperoxide; wherein formula (I) and formula (II) are as defined above;(b) system comprising a triflinate salt and t-butyl hydroperoxide;.(c) system comprising a metabisulfite salt and t-butyl hydroperoxide.

[0072] The polymerization method of the present invention can be carried out according to the techniques generally known in the field of free-radical (co)polymerization of fluorine-containing olefins, see for instance K. Hintzler et al.: "Fluoropolymers, Organic", Ullmann's Encyclopedia of Industrial Chemistry, Electronic Release, https: / / doi.org / 10.1002 / 14356007.a11_393.pub2, Wiley-VCH, Weinheim (2014).

[0073] Preferably, the polymerization is aqueous emulsion or suspension polymerization. Emulsion or suspension polymerization typically involves polymerizing monomers in an aqueous medium in the presence of the redox initiating system described herein. Preferably, the polymerizable mixture comprises at least one emulsifier or suspending agent or both, which act as emulsifying agent or stabilizing agent of the monomer droplets or fluoropolymer particles in the aqueous medium.

[0074] As emulsifier or suspending agent, surfactants or other compounds known in the art capable of emulsifying and / or stabilizing fluoropolymer particles may be used.

[0075] Preferably, the emulsifier or suspending agent is a polyfunctional fluorinated polymer.

[0076] A suitable polyfunctional fluorinated polymer (hereinafter also indicated as “polymeric dispersant”) generally comprises a backbone chain comprising recurring units deriving from one or more ethylenically unsaturated monomers and a plurality of ionic groups selected from -SOsXa, -POsXa, - COOXa, wherein Xais H, ammonium group, monovalent alkali metal or a combination thereof.

[0077] Advantageously, the polymeric dispersant may be a polymer comprising, consisting essentially of or consisting of, recurring units deriving from: a. at least one ethylenically unsaturated functional monomer containing at least one -SO2X group; and17 / 28 SSPI 2024_023 b. recurring units deriving from at least one ethylenically unsaturated fluorinated monomer free from -SO2X group; wherein X is halogen (e.g. F) or -OXa, and Xais H, ammonium group, a monovalent alkali metal or a combination thereof; preferably Na or K.

[0078] The ethylenically unsaturated functional monomers containing -SO2X groups for the preparation of the polymeric dispersant may be selected from those described above as functional monomers suitable for forming the fluorinated polymer of the present invention. For example, suitable ethylenically unsaturated monomers are those of general formula F2C=CF- R1-SO2F and general formulae (M1) to (M4) detailed above.

[0079] The ethylenically unsaturated functional monomer free from -SO2X groups suitable for the preparation of the polymeric dispersant may be selected from those described above as comonomers suitable for forming the fluorinated polymer of the present invention, such as C2-C8 perfluoroolefins, C2-C8 hydrogen-containing fluoroolefins, C2-C8 chloro- and / or bromo- and / or iodo-containing fluoroolefins, fluorooxyalkylvinyl ethers and fluorodioxoles.

[0080] Preferably, the polymeric dispersant has weight-average molecular weight (Mw) within the range from 15000 to 600000, more preferably from 150000 to 400000. Preferably, the polymeric dispersant has number-average molecular weight (Mn) within the range from 25000 to 400000, more preferably from 50000 to 250000.

[0081] Herein, Mw and Mn of polymers are meant to be determined by Gel Permeation Chromatography analysis with respect to polystyrene standards, using dimethylacetamide as eluent and a Refractive Index detector (concentration of the polymer in the testing solution was 0.5% wt / vol).

[0082] Preferably, the amount of ionic groups (ion exchange capacity) in the polymeric dispersant is within the range 1 .00 - 2.50 meq / g, more preferably within the range 1 .40 - 2.00 meq / g.

[0083] In general, the use of polyfunctional fluorinated polymer as polymeric dispersant has the advantage that, at the end of the polymerization reaction to obtain the fluorinated polymer of the present invention, there is no need to remove the polymeric dispersant from the fluorinated polymer since the18 / 28 SSPI 2024_023 polymeric dispersant has a molecular weight and composition comparable to that of the fluorinated polymer and therefore does not substantially affect the properties of the fluorinated polymer. The polymeric dispersant is therefore a valuable environmental-friendly alternative to the fluorinated, non-polymeric surfactants used in the state of the art.

[0084] Preferably, the emulsion or suspension polymerization is carried out in the absence of non-polymeric compounds, i.e. having a weight average Mw lower than 3000, acting as emulsifier or suspending agents.

[0085] As per the aqueous medium in which the polymerization is carried out, the polymerizable mixture contains water, possibly in combination with an organic liquid. The organic liquid may be, for example, an alcohol such as methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, tert-butyl alcohol, sec-butyl alcohol, pentanol or octyl alcohol, an ether alcohol such as methyl cellosolve, ethyl cellosolve, isopropyl cellosolve, butyl cellosolve or diethylene glycol monobutyl ether or a fluorine-type solvent such as trichlorofluoroethane. Such organic liquids may be used alone or in combination as a mixture of two or more.

[0086] Optionally, the polymerizable mixture may comprise other constituents, such as chain transfer agents, pH buffers, paraffin waxes as antifoulant or complex-formers.

[0087] The polymerization may be carried out at a temperature within the range from 10°C to 75°C, preferably from 30°C to 65°C.

[0088] The polymerization may be carried out at a pressure within the range from 2 to 60 bar, more preferably 3 to 45 bar.

[0089] The temperature of the polymerizable mixture may be varied during the polymerization, for example to influence the molecular weight distribution of the final fluorinated polymer, e.g., to obtain a broad molecular weight distribution or to obtain a bimodal or multimodal molecular weight distribution.

[0090] The pH of the polymerizable mixture may be in the range of pH 1-10, preferably 2-10.

[0091] The aqueous emulsion or suspension polymerization generally results in an aqueous dispersion of fluorinated polymer particles (latex). The particle size19 / 28 SSPI 2024_023 of the fluorinated polymer, expressed as volume average diameter, is typically within the range from 40 nm to 400 nm. The particle size of the polymer in the polymer latex is expressed as volume average diameter and is intended to be determined by Dynamic Light Scattering Analysis according to the method ISO 22412:2017.

[0092] The amount of polymer solids in the dispersion may vary within a wide range, for example between 10 wt.% and 70 wt.%, and can be adjusted as needed or desired. Known concentration techniques may be used, including ultrafiltration and thermal concentration.

[0093] The fluorinated polymer may be isolated from the continuous phase by any suitable technique, such as oven drying, spray drying, shear or acid coagulation or freeze thawing followed by drying, or it can be kept in the aqueous media for subsequent application or use.

[0094] The fluorinated polymer may be obtained as a powdery material having ion exchange groups or precursor groups thereof. Precursor groups of the ion exchange groups are -SO2X groups in which X is F, Cl, Br or I. When the fluorinated polymer is obtained with precursor groups, the precursor groups can be converted into ion exchange groups — SO3Z (Z = H, NH4+or alkali metal) by appropriate known hydrolysis treatments,

[0095] The fluorinated polymer in -SO3Z form, wherein Z is NH4+or alkali metal, is typically prepared from the fluorinated polymer in -SO2X form, preferably -SO2F form, by treating it with a strong base (e.g. an aqueous solution of NaOH or KOH, NH3).

[0096] The fluorinated polymer in — SO3H form can be obtained by treatment of the corresponding salified-SOsZ form, wherein Z is NH4+or alkali metal, of the polymer with a concentrated acid solution (e.g. aqueous solution of nitric acid) or using an acid ion exchange resin.

[0097] A process for the recovery of the polymer in powder form from an emulsion polymerization latex is described in WO 2020 / 094563A1 and WO 2022 / 224105A1.

[0098] Advantageously, the fluorinated polymer has an equivalent weight (EW) of at least 600 g / eq, preferably at least 650 g / eq.20 / 28 SSPI 2024_023

[0099] Advantageously, the fluorinated polymer has an equivalent weight (EW) of at most 1200 g / eq, more at most 1100 g / eq.

[0100] Herein, by equivalent weight (EW) of the polymer it is meant the amount (expressed in grams) of ionomer comprising one equivalent of ion exchange groups. The EW can be determined by FTIR measurements , as described for example in Fluorinated Ionomers (Plastics Design Library), Second Edition, Walther Grot, Elsevier Science, 2011.

[0101] When the fluorinated polymer comprises recurring units derived from SFVE, the selection of EW in the range between 1100 and 650 g / eq is particularly advantageous for the purpose of obtaining a good ionic conductivity and valuable mechanical properties and thermal resistance.

[0102] The use of the redox initiating systems according to the present invention allows to polymerize TFE, ethylenically unsaturated functional monomers and optionally other monomers with high polymerization rates at relatively low temperatures leading to a final fluorinated polymer having a low amount of undesired — COOH end groups.

[0103] Advantageously, the fluorinated polymers obtained with the method of the present invention exhibit durability (i.e. endurance of the polymer as a membrane), when submitted to Open Circuit Voltage (OCV) decay test, which is comparable to or better than that of the corresponding polymers obtained using conventional thermal initiators (e.g. ammonium persulfate) or redox initiating systems used in the prior art for the preparation of fluorinated copolymers of TFE. The OCV decay tests are described in the examples section. An acceptable failure value for the OCV test under the conditions described in the examples is 300 hours or higher.

[0104] The fluorinated polymer is suitable for use in the manufacturing of components of an electrochemical device, such as fuel cells and electrolysis cells. Notably, the fluorinated polymer is suitable for use as proton exchange membrane or binder material in electrode layers or in the whole membraneelectrode assembly.

[0105] The fluorinated polymer can be formed into membranes using any conventional method such as but not limited to extrusion, solution or dispersion film casting techniques or impregnation of preformed supports. The membrane thickness can be varied as desired for a particular21 / 28 SSPI 2024_023 application. Typically, the membrane thickness is less than about 350 pm, more typically in the range of about 10 pm to about 175 pm.

[0106] The MEAs of the invention comprising the stabilized fluorinated polymer obtained by the method as above described can be produced by standard techniques well-known to those skilled in the art; for instance a paste or ink comprising the stabilized fluorinated polymer obtained by the method as above described and a suitable metal catalyst (e.g. Pt, Pt alloys, Iridium Oxide) is casted on a membrane prepared as above described.

[0107] Otherwise a multilayer extrusion process can be carried out so as to coextrude the membrane and the electrode layers.

[0108] The present invention will be now described in more details by reference to the following examples, whose purpose are merely illustrative and do not limit the scope of the present invention.

[0109] 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.

[0110] EXAMPLES

[0111] Materials usedTBHP = t-butylhydroperoxide by Aldrich;Bruggolite® = Bruggolite® E28 manufactured by Bruggermann-Group (aqueous solution of 2-hydroxy-2-sulfinatoacetic acid disodium salt and 2- hydroxy-2-sulfonatoacetic acid disodium salt, cone. 85 wt%);MBS = sodium metabisulfite by Aldrich;Triflinate = sodium triflinate (CFsSC^Na) by Aldrich;Cysteine = L-cysteine by Aldrich.

[0112] Quantitative determination of the unstable polymer end-groupsA polymer sample, submitted to a preliminary drying at 90°C until constant weight, is compression moulded into a film having an average thickness between 50 and 300 pm. An FT-IR spectrum between 4000 cm-1 and 400 cm-1 is recorded, e.g. by using a Nicolet® Nexus FT-IR equipment (256 scannings, resolution 2 cm-1 ), from said film.The optical densities of absorption bands in the spectral region between 1900 and 1700 cm-1 are measured and converted into values expressed as22 / 28 SSPI 2024_023 mmol / kg of polymer using the extinction coefficients reported in Table 1 , page 73 of the report by PIANCA, M., et al. End groups in fluoropolymers. J. Fluorine Chem., 1999, vol.95, p.71-84. The sensitivity limit of this method is 0.05 mmol / Kg.

[0113] Preparative Example 1 - Preparation of the polymeric dispersant (D)Step 1 - Polymerization

[0114] A polymeric dispersant (D) was prepared as described below.

[0115] In a 5 L autoclave the following reagents were charged:1 .8 L of demineralized water;533 g of the monomer with formula: CF2=CF-O-CF2CF2-SO2F (SFVE);89 g of a 46 wt% aqueous solution of the ammonium salt of the fluorocompound of formula:with Xa being NFU.The autoclave, stirred at 650 rpm, was heated at 58°C. A water based solution with 16 g / L of ammonium persulfate was added in a quantity of 90 mL. The pressure was maintained at a value of 8.5 bar (abs.) by feeding 8.2 bar of tetrafluoroethylene (TFE). After the initial addition of 83 g, SFVE was added portion-wise (23 g) each 5 wt% of TFE converted. The reaction was stopped after 200 min by stopping the stirring, cooling the autoclave and reducing the pressure by venting the TFE; a total of 340 g of TFE was fed into the autoclave. Overall, 0.12 grams of surfactant for each gram of converted TFE were used.The latex thus obtained was degassed for 48 h with air flow to remove monomer’s residuals and then coagulated through freeze-thawing. The powder was washed with deionized water (4 x 1 L) for 30 min and dried in a vent oven at 120°C overnight.23 / 28 SSPI 2024_023A copolymer was obtained having an equivalent weight (EW) of 720 g / eq and possessing the following composition: TFE: 81.5 mol%; SFVE: 18.5 mol% as determined by FT-IR measurements.The polymer had a number-average molecular weight (Mn) of 93000 and a weight-average molecular weight (Mw) of 241000. Substantially no fraction having a molecular weight below 3000 was detected by GPC.

[0116] Step 2 - Hydrolysis and dissolution in water250 g of the powder obtained in Step 1 was treated at 70°C with a mixture of 10 L / h of fluorine and 4 L / h of nitrogen for 3 h end-capping the backbone with -CF3functional groups and removing the undesired carboxylic acid groups. The powder was treated under stirring with a 1.5 L solution of NaOH / H2O (20 wt%) and heated at 80°C. Complete conversion of the originally comprised -SO2F groups to ionic -SOsNa groups was confirmed by solid state NMR. The amount of ionic groups in the polymer was 1.39 meq / g as measured by FT-IR. After 10 h the powder was washed under stirring with deionized water (4x 1 L) for 30 min and dissolved in deionized water in a pressurized vessel heated at 250°C.

[0117] Example 1 - Preparation of a fluorinated polymer using a redox initiating system according to the inventionStep 1 - PolymerizationIn a 5 L autoclave the following reagents were charged: 1.8 L of demineralized water; 60 g of a 25 wt% aqueous dispersion of dispersant (D) (i.e. corresponding to 15 g of dispersant (D) obtained from Preparative Example 1), thus corresponding to a concentration of dispersant (D) of about 0.83 wt % based on the initial aqueous phase. The autoclave, stirred at 650 rpm, was heated at 45°C. Two water based solutions with 0.8 g / L of TBHP and 1.1 g / L of Bruggolite were continuously added with a feeding rate of 1 mL / min each. The pressure was maintained at a value of 7.3 bar (abs.) by feeding tetrafluoroethylene (TFE). 82 g of SFVE were fed in the reactor and the pressure of the autoclave was maintained at constant value of 7.8 bar by feeding TFE and feeding SFVE (23 g) each 5% of TFE conversion. After 100 minutes, when a quantity of 340 grams of TFE and 443 g of SFVE were fed, the TFE, TBHP and Bruggolite feeding were stopped. By keeping constant stirring of 650 rpm the autoclave was cooled to ambient24 / 28 SSPI 2024_023 temperature, the latex was discharged after being kept under air bubbling for 48 hours to strip away residual monomers from the polymerization, and then stored in a plastic tank. No latex coagulation / precipitation was observed. The latex so produced was characterized by laser light scattering for determining average particle size, which was found to be of 68 nm. The polymer had an equivalent weight (EW) of 828 g / mol, a concentration of COOH groups of 6 mmol / kg and a composition, measured through FT-IR, of TFE: 84.6 mol% and SFVE: 15.4 mol%.The latex was then cooled at -26°C for 72 h to let the powder coagulate. Once the coagulum was formed, the tank was heated up to room temperature and the obtained powder (ca. 500 g) washed in a 50 I stirred reactor. About 500 g of powder were placed in a reactor, stirred at 90 rpm at room temperature, and the empty space left in the tank was filled with water for 15 minutes to wash the polymer from residual impurities. This step was repeated 4 times for a better washing efficiency, and the obtained powder was dried in an oven at 80°C for 40h.

[0118] Step 2 - Hydrolysis and Dissolution in water

[0119] The SO2F pendant groups of the polymer were converted into SOsNa groups by contacting the powder from Step 1 in a 5 I reactor stirred at 45 rpm with 4 I of NaOH aqueous solution (20 wt.%) were added. The temperature was raised to 80°C and the reaction was conducted at this temperature for 8 h. Afterwards, the reactor was cooled down to room temperature and the NaOH solution was removed from the reactor, and then the powder washed 5 times with 5 L of distilled water for 30 min at 45 rpm for each time until reaching a pH = 7,5.

[0120] For the dissolution step, 60 g of the powder was placed in a 300 mL reactor together with 200 mL of distilled water. The mixture was stirred at 500 rpm at 250°C for 3h and autogenic pressure of 40 bar was generated into the reactor.The solution cooled at ambient temperature was fed to a column containing a Dowex Monosphere® 650C UPW resin previously treated with 1 M nitric acid and washed with demineralized water to remove cations (in particular excess Na+cations) and convert the SOsNa groups into -SO3H groups. In a next step the solution so obtained was fed in a second column containing25 / 28 SSPI 2024_023Dowex Monosphere® 550A previously treated with 1 M NaOH and washed with demineralized water to remove the fluoride generated from the -SO2F conversion and other anionic species present in the dispersion. After these treatments, the pH of the solution was found to be around 2.

[0121] Examples 2 to 3 - Preparation of fluorinated polymers according to the inventionThe polymer powders of Examples 2 and 3 were prepared following the same method of Example 1 , except for using the couples TBHP (0.8 g / L water-based solution) / sodium metabisulfite (MBS - 1.1 g / L water-based solution) and TBHP (0.2 g / L water-based solution) / sodium triflinate (0.2 g / L water-based solution), respectively, as redox initiating systems that were added at a feeding rate of 1 mL / min each.The average particle size, equivalent weight (EW), concentration of COOH groups and composition (TFE / SFVE) of these two polymers are listed in Table 1.

[0122] The latex, thus obtained, was treated as described in Example 1 preparing the corresponding dispersions in acidic form.

[0123] Comparative Examples 4 to 6 - Preparation of fluorinated polymers not according to the inventionFor comparison purposes, the polymer powders of Examples 4 to 6 were prepared following the same method of Example 1 , except for using the following couples as redox initiating systems (the concentration of the aqueous solution used for each component of the redox system is indicated between parenthesis), instead of the redox system of Example 1 , which were added at a feeding rate of 1 mL / min each:- Example 4: APS (13 g / L water-based solution) / MBS (53 g / L water-based solution);- Example 5: APS (40 g / L water-based solution) / Na thiosulfate (100 g / L water-based solution);- Example 6: APS (25 g / L water-based solution) / Na sulfite (75 g / L waterbased solution).Notably, the redox initiating system of Examples 5 and 6 did not result in the starting of any polymerization reaction.26 / 28 SSPI 2024_023The average particle size, equivalent weight (EW), concentration of COOH groups and composition (TFE / SFVE) of the polymer of Example 4 are listed in Table 1.

[0124] Comparative Example 7 - Preparation of fluorinated polymers not according to the inventionIn a 5 L autoclave the following reagents were charged: 1.8 L of demineralized water; 60 g of a 25 wt% aqueous dispersion of dispersant (D) (i.e. corresponding to 15 g of the dispersant (D) obtained from Preparative Example 1), thus corresponding to a concentration of dispersant (D) of about 0.83 wt % based on the initial aqueous phase. The autoclave, stirred at 650 rpm, was heated at 58 C. A water based solution with 16 g / L of ammonium persulfate was added in a quantity of 90 mL. The pressure was maintained at a value of 76 bar (abs.) by feeding tetrafluoroethylene (TFE). 82 g of SFVE were fed in the reactor and the pressure of the autoclave was maintained at constant value of 7.8 bar by feeding TFE and feeding SFVE (23 g) each 5% of TFE conversion. After 230 minutes, when a quantity of 340 grams of TFE and 443 g of SFVE were fed, the TFE feeding was stopped. By keeping constant stirring of 650 rpm the autoclave was cooled to ambient temperature the latex was discharged after being kept under air bubbling for 48 hours to strip away residual monomers from the polymerization, and then stored in a plastic tank. No signals of latex coagulation / precipitation were observed. The latex so produced was characterized by laser light scattering for determining average particle size, which was found to be of 107 nm. The polymer thus obtained had an equivalent weight (EW) of 758 g / mol, a concentration of COOH groups of 15 mmol / kg and a composition, measured through FT-IR, of TFE: 82.7 mol% and SFVE: 17.3 mol%.The latex, thus obtained, was treated as described in Example 1 preparing the corresponding dispersions in acidic form.

[0125] Fuel cell durability tests in OCV (open circuit voltage decay)Membranes were obtained by casting the dispersions prepared in Examples 1 - 4 and 7. To this end, the dispersions of each example were formulated with n-propanol and dimethyl sulfone. The formulated dispersions had the following composition: polymer material: 20 wt%, water: 40.5 wt%, n-27 / 28 SSPI 2024_023 propanol: 35 wt% and dimethyl sulfone; 4.5 wt%. Dispersion casting was carried out using a doctor blade and an automatic film applicator on a tempered glass support. After deposition, the film underwent a 3 steps heating cycle in a vent oven: 1 h at 65 °C, 1 h at 90 °C and 1 h at 190 °C. The membrane was then peeled off from the glass using demineralized water and dried in a vent oven at 80 °C overnight. Membrane thickness was 54 ± 1 micrometers.Membrane Electrode Assemblies (MEAs) were produced using the each membrane. For the electrodes a commercial Pt / C with a loading of 0.45 mgPt / cm2was used as a catalyst. Each MEA was obtained by hot pressing the catalyst layer with wet-proofed carbon fiber paper (SGL 25BC) with microporous layer (MPL) as gas diffusion layer (GDL) at 200°C and 8 barA. During OCV test, the fuel cells were maintained in open circuit conditions, as detailed below, by supplying hydrogen and oxygen to the electrodes. The decreasing OCV trend was monitored v. time and the threshold value of 0.7 V was considered as the membrane failure condition. The chemical durability of the membrane was then ranked according to the amount of hours needed to reach the aforementioned failure condition.

[0126] Operating conditions:- Reactants flows: 250 NCCM oxygen on cathode side - 250 NCCM hydrogen on anode side (pure hydrogen 5.5 grade);- Reactant humidity level: 90%;- Cell temperature: 90°C;- Operating pressure: 1 Bar abs;- Load current: 0 Amps.Table 128 / 28 SSPI 2024_023comparative examples

[0127] The results compiled in T able 1 show that the redox initiating systems of the invention (Examples 1 - 3) allowed to obtain adequate polymerization rates at relatively lower temperatures compared to examples employing thermal initiators (example 7), thus reducing the energy consumption of the synthesis process.

[0128] Moreover, the membranes produced with the fluoropolymer of the invention (Examples 1 - 3) exhibit a low amount of unstable -COOH end-groups; these amounts are substantially lower than that of the reference Example 7 produced with a thermal initiator.

[0129] The membrane prepared with the fluoropolymer of the comparative Examples 4 allowed to obtain a low amount of unstable COOH groups. The OCV durability tests, however, did not achieve the acceptable threshold value of 300 hours.

Claims

1 / 4 SSPI 2024_023Claims1. A method for preparing a fluorinated polymer comprising ion exchange groups or precursor groups thereof, said method comprising polymerizing a polymerizable mixture comprising:- tetrafluoroethylene,- an ethylenically unsaturated functional monomer having at least one SO2X group, where X is halogen or — OZ group, Z being a hydrogen atom, an ammonium group or an alkali metal atom; and- a redox initiating system comprising at least one reducing agent having a one or more sulfinic or sulfinate groups and at least one organic hydroperoxide as oxidizing agent.

2. The method according to claim 1 , wherein the at least one reducing agent comprises at least one compound of formula (I)wherein:- M is a hydrogen atom, an ammonium ion, a monovalent metal ion;- R1 is OH or N(R4)(Rs) where each of R4 and Rs, identical or different from one another, are hydrogen atom or linear or branched alkyl group having from 1 to 6 carbon atoms;- R2 is hydrogen atom linear or branched alkyl group having from 1 to 6 carbon atoms, 5- or 6-membered cycloalkyl group, 5- or 6-membered aryl group;- R3is a group -COOM, -SO3M, -C(=O)R4, -C(=O)N(R4)(Rs), -C(=O)OR4, wherein M, R4 and Rs are as defined above.

3. The method according to claim 2, wherein the at least one reducing agent further comprises at least one compound of formula (II)2 / 4 SSPI 2024_023 wherein:- M is a hydrogen atom, an ammonium ion, a monovalent metal ion;- Ri is OH, or N(R4)(Rs) where each of R4 and Rs, identical or different from one another, are hydrogen atom or linear or branched alkyl group having from 1 to 6 carbon atoms;- R2 is hydrogen atom linear or branched alkyl group having from 1 to 6 carbon atoms, 5- or 6-membered cycloalkyl group, 5- or 6-membered aryl group;- R3is -COOM, -SO3M, -C(=O)R4, -C(=O)N(R4)(RS), -C(=O)OR4, wherein M, R4 and Rs are as defined above.

4. The method according to claim 1 , wherein the at least one reducing agent having a sulfinic or sulfinate group is a fluoroaliphatic compound of formula (I)Rf-SO2M’i / x (IV) wherein Rf is monovalent fluoroaliphatic radical having from 1 to 20 carbon atoms, preferably 1 to 10 carbon atoms, even more preferably 1 to 4 carbon atoms; M’ is a hydrogen atom or a cation with valence x, x being 1 or 2.

5. The method according to claim 1 , wherein the at least one reducing agent having a sulfinic or sulfinate group is a metabisulfite salt.

6. The method according to any one of claims 1 to 5, wherein the organic hydroperoxide is selected from: t-butyl hydroperoxide, 1 ,1 ,3,3-tetramethylbutyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, p- menthane hydroperoxide, 2,5-dimethylhexane-2,5-dihydroperoxide and mixture thereof; preferably t-butyl hydroperoxide.

7. The method according to claim 1 , wherein the redox initiating system is selected from the following ones:(a) system comprising a compound of formula (I), optionally in combination with a compound of formula (II), and t-butyl hydroperoxide; wherein formula (I) and formula (II) are as defined in the claims 2 and 3;(b) system comprising a triflinate salt and t-butyl hydroperoxide;.(c) system comprising a metabisulfite salt and t-butyl hydroperoxide.

8. The method according to any one of claims 1 to 7, wherein the fluorinated monomer is a monomer of general formula F2C=CF-R1-SO2F, wherein R1is a branched or unbranched perfluoroalkyl, perfluoroalkoxy, or perfluoroether group, which contains one to fifteen carbon atoms and zero to four oxygen atoms.3 / 4 SSPI 2024_0239. The method according to any one of claims 1 to 8, wherein the fluorinated monomer is selected among:- sulfonated perfluoroolefin of formula (M1 )wherein n is an integer between 0 and 6 and X’ is a halogen atom or — OM group, wherein M is H, NH4, alkali metal or mixtures thereof;- sulfonated perfluorovinylethers of formula (M2):wherein m is an integer between 1 and 10 and X’ is a halogen atom or — OM group, wherein M is H, NH4, alkali metal or mixtures thereof;- sulfonated perfluoroalkoxyvinylethers of formula (M3):wherein w is an integer between 0 and 2; RF1 and RF2, equal or different from each other and at each occurrence, are independently F, Cl or a C1-C10 perfluoroalkyl group, optionally substituted with one or more ethereal oxygen atoms; y is an integer between 0 and 6 and X’ is a halogen atom or — OM group, wherein M is H, NH4, alkali metal or mixtures thereof;- sulfonated aromatic (per)fluoroolefins of formula (M4):4 / 4 SSPI 2024_023 wherein Ar is a C3-C15 aromatic or heteroaromatic moiety, preferably perfluorinated, and X’ is a halogen atom or — OM group, wherein M is H, NH4, alkali metal or mixtures thereof; preferably X’ is fluorine; and- mixtures thereof.

10. The method according to any one of claims 1 to 9, wherein the polymerizable mixture comprises at least one polyfunctional fluorinated polymer as emulsifier or suspending agent.

11. A fluorinated polymer comprising ion exchange groups or precursor groups thereof as obtainable by the method of claim 1 .

12. A membrane comprising a fluorinated polymer comprising ion exchange groups or precursor groups thereof according to claim 11 , wherein the polymer preferably comprises ion exchange groups of formula — SO3Z, where Z is a hydrogen atom, an ammonium group or an alkali metal atom, more preferably a hydrogen atom.

13. A membrane-electrode assembly comprising a membrane according to claim 12.

14. An electrochemical device, such as a fuel cell or an electrolyzer, comprising a membrane-electrode assembly according claim 13.

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