FR3161910A1 - Anion exchange membrane and its method of production. - Google Patents

Abstract

The invention relates to an anion exchange membrane containing a polymer ionomer bearing pendant imidazolium ionic groups attached to the polymer via a group resulting from a cycloaddition reaction between a 1,3-dipolar compound and an unsaturated polymer. The 1,3-dipolar compound is an aromatic nitrile oxide substituted with an imidazole functional group, and the unsaturated polymer is a polymer containing diene monomer units. The membrane, intended for use in a fuel cell or electrolyzer, has the advantage of being prepared by a process involving steps that can be carried out in the absence of solvent.

Description

anion exchange membrane and its method of production.

The field of the present invention is that of anion exchange membranes containing an ionomer and intended for use in an electrolyzer or a fuel cell.

The core of a fuel cell and an electrolyzer consists of two electrodes, an anode and a cathode, an electrolytic layer separating the two electrodes, and a catalyst located at the interfaces of the electrolytic layer with each electrode. Fuel cells and electrolyzers include a membrane that constitutes the electrolytic layer. One of the membrane's components is the ionomer, a polymer that carries ionic or ionizable groups.

A key step in membrane preparation is shaping the ionomer into a film. The process generally described for forming an ionomer film is a coating process that requires dissolving the ionomer.

Imidazolium-containing polymers are described as good candidates for anion-exchange membranes. These ionomers, which are generally prepared from a halogenated polymer and an imidazole compound, are poorly soluble or even insoluble in many solvents. As described in patent application WO 2019010290, the very low solubility or insolubility of these ionomers, attributed to ionic interactions within the ionomer, makes the coating process problematic. To overcome this issue, the film is prepared by coating not from the ionomer itself, but from its precursor, the halogenated polymer. The halogenated polymer film is then immersed in a bath, a solution of the imidazole compound, to induce the formation of the ionomer film. This series of steps must be followed by further steps of rinsing the film and drying before using the membrane.

The inventors have discovered a new membrane which contains an ionomer whose ionic groups are imidazoliums and whose chemical structure makes it possible to overcome the difficulties mentioned in the preparation of membranes of the prior art.

An object of the invention relates to an anion exchange membrane containing an ionomer, a polymer bearing pendant imidazolium ionic groups which are attached to the polymer via a group resulting from the cycloaddition reaction between a 1,3-dipolar compound and an unsaturated polymer, the 1,3-dipolar compound being an aromatic nitrile oxide substituted by an imidazole function, the unsaturated polymer being a polymer containing monomeric units of a diene.

Another object of the invention is a method for preparing an anion exchange membrane according to the invention, which method comprises the following steps a), b), c) and d):
- a) the preparation of a polymer bearing pendant imidazole functions by a cycloaddition reaction of the 1,3-dipolar compound and the unsaturated polymer,
- b) the incorporation of a haloalkane into the polymer bearing pendant imidazole functions by thermomechanical mixing,
- c) a heat treatment of the mixture obtained in step b) to convert the imidazole functions into imidazolium ionic groups,
- d) shaping the ionomer obtained at the end of step c) into a film,
steps b), c) and d) being carried out in bulk.

The invention also relates to an ionomer, a polymer bearing pendant imidazolium ionic groups which are attached to the polymer via a group resulting from the cycloaddition reaction between a 1,3-dipolar compound and an unsaturated polymer, the 1,3-dipolar compound being an aromatic nitrile oxide substituted by an imidazole function, the unsaturated polymer being a polymer containing monomeric units of a diene.

The invention also relates to a process for preparing an ionomer according to the invention which includes steps a) to c) of the process for preparing an anion exchange membrane according to the invention.

The invention also relates to a fuel cell or electrolyzer which contains a membrane according to the invention.

Detailed description

In this description, any range of values designated by the expression "between a and b" represents the range of values greater than "a" and less than "b" (i.e., bounds a and b excluded) while any range of values designated by the expression "from a to b" means the range of values from "a" to "b" (i.e., including the strict bounds a and b).

The polymers mentioned in the description can be of fossil origin or bio-based. In the latter case, they can be partially or entirely derived from biomass or obtained from renewable raw materials derived from biomass. Similarly, they can also come from the recycling of previously used materials; that is, they can be partially or entirely produced through a recycling process, or obtained from raw materials themselves derived from a recycling process.

The terms "membranes" and "films" are well known to those skilled in the technical field. It is well understood that a membrane is a structure as defined by IUPAC in "IUPAC Recommendations 1996." Similarly, and in accordance with the definition provided by IUPAC, the term "film" is understood according to the definition given by IUPAC in "IUPAC Recommendations 1996."

The anion exchange membrane according to the invention has as its essential characteristic the presence of an ionomer which is a polymer bearing pendant imidazolium ionic groups. Preferably, the counterions of the imidazolium ionic groups are halide anions, preferably bromides, or hydroxide anions.

The pendant imidazolium ionic groups are attached to the constituent polymer of the membrane via a group resulting from the cycloaddition reaction between an aromatic nitrile oxide substituted with an imidazole function and a polymer containing monomeric units of a diene.

The polymer containing monomer units of a diene, an unsaturated polymer useful for the purposes of the invention, is a polymer that exhibits well-known carbon-carbon double bonds. The unsaturated polymer may be a homopolymer of a diene or a copolymer of a diene.

The diene is preferentially a 1,3-diene. The unsaturated polymer is preferentially a homopolymer of a 1,3-diene or a copolymer of a 1,3-diene. The unsaturated polymer is then preferentially chosen from homopolymers of a 1,3-diene, copolymers of several 1,3-dienes, copolymers of a 1,3-diene and a vinylaromatic monomer, and copolymers of a 1,3-diene and ethylene. When the unsaturated polymer is a copolymer of a 1,3-diene, it can be statistical, random, block, or tapered.

As 1,3-dienes, 1,3-dienes with 4 to 20 carbon atoms are preferentially suitable, more preferably 1,3-butadiene and isoprene, and even more preferably 1,3-butadiene.

Styrene, or styrenes substituted at the alpha, ortho, meta, or para positions by an alkyl group, are preferentially suitable as vinylaromatic monomers. Styrene may be substituted by one or more alkyl groups, which may be identical or different. The alkyl group may contain one or more carbon atoms, particularly one to four carbon atoms. The vinylaromatic monomer is preferably styrene.

Copolymers of several 1,3-dienes, that is, copolymers of at least two 1,3-dienes, are known to be copolymers whose constituent monomer units are the monomer units of said several 1,3-dienes. Examples of copolymers of several 1,3-dienes include copolymers of 1,3-butadiene and isoprene, copolymers of 1,3-butadiene and piperylene, and copolymers of isoprene and piperylene.

As is well known, homopolymers of a single 1,3-diene and copolymers of several 1,3-dienes can be prepared in solution, dispersion, or emulsion. They can be prepared by anionic polymerization, by coordination polymerization (for example, in the presence of a Ziegler-Natta catalyst or a metallocene), or by radical polymerization.

Copolymers of a 1,3-diene and a vinylaromatic monomer are well-known copolymers, as are their preparation methods. They can be prepared in solution, dispersion, or emulsion, notably in solution or dispersion by anionic polymerization or in emulsion by radical polymerization. Examples include 1,3-butadiene-styrene copolymers, isoprene-styrene copolymers, and 1,3-butadiene-isoprene-styrene copolymers. The percentage of vinylaromatic monomer units, preferably styrene, in the copolymer can vary considerably and is adjusted by those skilled in the art according to the desired properties of the copolymer. It is typically greater than 0% and less than 95% by mol, a percentage calculated relative to the number of moles of the copolymer. It is preferably less than 75% by mole, more preferably less than 50% by mole, percentage calculated in relation to the number of moles of the copolymer.

Copolymers of 1,3-diene and ethylene are also known polymers, as are their synthesis processes, for example, described in patent applications EP 1 092 731, WO 200754223, WO 200754224, WO 2017103543, and WO 2017103544. Copolymers of 1,3-diene, preferably 1,3-butadiene, and ethylene preferably contain more than 50 mole percent of ethylene. Advantageously, they contain between 50 and 90 mole percent of ethylene units.

The 1,3-dipolar compound useful for the purposes of the invention is an aromatic nitrile oxide substituted with an imidazole function. Aromatic nitrile oxides are well-known compounds that react via cycloaddition, specifically [3+2], with double bonds, as described in "Nitrile Oxides, Nitrones and Nitronates in Organic Synthesis, Novel Strategies in Synthesis," ^2nd Edition, Henry Feuer, 2008, Wiley-Interscience. Aromatic nitrile oxides substituted with an imidazole group are known compounds from patent application WO 2015059269. Aromatic nitrile oxides substituted with an imidazole group are also known from patent application WO 2015059269 for reacting with the carbon-carbon double bonds of an unsaturated polymer via cycloaddition. The cycloaddition reaction allows imidazole groups to be grafted onto the polymer, as illustrated in the following diagram on a monomeric unit of 1,3-butadiene. IMIDAZ designates the imidazole functional group. The wavy line in the diagram of the 1,3-dipolar compound symbolizes the attachment of the imidazole group to the aromatic ring substituted by the nitrile oxide. The imidazole functional groups are attached to the modified polymer via the group formed by the cycloaddition reaction between the 1,3-dipolar compound and the unsaturated polymer.

The imidazole functional group is preferably of the formula 1H- imidazol-1-yl or 2-alkyl- 1H- imidazol-1-yl. The alkyl group substituting for the carbon atom at position 2 of the imidazole ring is preferably an alkyl group having 1 to 6 carbon atoms, more preferably an alkyl group having 1 to 3 carbon atoms, and even more preferably a methyl group. Advantageously, the imidazole functional group is of the formula 1H - imidazol-1-yl or 2-alkyl- 1H- imidazol-1-yl, which is preferably 2-methyl- 1H -imidazol-1-yl.

The 1,3-dipolar compound preferentially corresponds to formula (I) in which one of the symbols R ₁ to R ₅ represents a group containing the imidazole function, the other symbols a hydrogen atom or an alkyl, knowing that one of the R ₁ and R ₅ is not a hydrogen atom.

Preferably, _R4 represents the group containing the imidazole function. _R4 is preferably a ( 1H- imidazol-1-yl)alkyl or (2-alkyl- 1H- imidazol-1-yl)alkyl group, more preferably a ( 1H- imidazol-1-yl)methyl or (2-alkyl- 1H- imidazol-1-yl)methyl group. _R4 is even more preferably a 1H- imidazol-1-yl)methyl or (2-methyl- 1H- imidazol-1-yl)methyl group.

Preferably, in formula (I), _R1 , _R3 , and _R5 are each an alkyl group, and _R2 is a hydrogen atom. The alkyl groups of _R1 , _R3 , and _R5 are preferably methyl or ethyl groups, more preferably methyl groups.

The 1,3-dipolar compound is advantageously the compound 2,4,6-trimethyl-3-((2-methyl-1 H -imidazol-1-yl)methyl)benzo-nitrile oxide or the compound 2,4,6-triethyl-3-((2-methyl-1 H -imidazol-1-yl)methyl)benzo-nitrile oxide, respectively of formula (IIa) and (IIb).

In the process, another object of the invention, for preparing the membrane according to the invention, step a) consists of preparing a polymer bearing imidazole functional groups by a grafting reaction of the 1,3-dipolar compound onto the unsaturated polymer. The grafting reaction between a polymer containing diene monomer units and a 1,3-dipolar compound such as a nitrile oxide is a well-known [3+2] cycloaddition reaction of the 1,3-dipolar compound onto the carbon-carbon double bonds of the diene monomer units of the polymer. Since the 1,3-dipolar compound used in the present invention is a compound that, in addition to the dipole, bears a chemical functional group, in this case an imidazole functional group, the reaction allows the grafting of pendant chemical functional groups, imidazole functional groups, onto the polymer. The grafting reaction is typically carried out at a temperature above ambient (23°C), preferably at a temperature above 60°C.

According to a first embodiment of the invention, the grafting reaction is carried out in solution. The polymer thus modified can be separated from its solution by any type of means known to those skilled in the art, and in particular by a steam stripping operation.

According to a second embodiment of the invention, the grafting reaction is carried out in bulk, for example in internal mixers, extruders, ovens, or presses. It is then generally preceded by bulk mixing to incorporate the 1,3-dipolar compound into the unsaturated polymer. Step a) can be carried out by incorporating the 1,3-dipolar compound into the unsaturated polymer at a mixer temperature below 60°C, and then conducting the grafting reaction in a press or oven at temperatures ranging from 80°C to 200°C. Alternatively, step a) can be carried out by incorporating the 1,3-dipolar compound into the unsaturated polymer at a mixer temperature above 60°C, with the grafting reaction occurring simultaneously with the incorporation. When the grafting reaction is carried out in bulk, it is preferably performed in the presence of an antioxidant for the unsaturated polymer.

According to a first preferred embodiment of the second embodiment, step a) is a reactive extrusion step of a mixture of the unsaturated polymer and the 1,3-dipolar compound, at the end of which the 1,3-dipolar compound is grafted onto the polymer. Step a) is then typically a reactive extrusion process as described in patent application WO 2018115703. In this embodiment, the unsaturated polymer and the 1,3-dipolar compound feed a twin-screw extruder that conventionally comprises a barrel, a feeding zone, a mixing zone, a set of two worm screws, and a die. The extrusion temperature, the setpoint temperature applied inside the extruder, particularly to the barrel, is preferably above 100°C. Preferably, the extrusion temperature in the area extending from the mixing zone to the end of the screw assembly closest to the die is between 110 and 140°C. This temperature range provides the best compromise between grafting efficiency and productivity. Productivity is governed by the polymer flow rate in the extruder, which is itself adjusted according to the desired residence time of the polymer in the extruder from the mixing zone to the extruder die. Residence times are typically short, at most 5 minutes, and preferably less than 5 minutes. Residence times ranging from 30 seconds to 2 minutes can be sufficient to achieve both good grafting efficiency and precise control of the grafting rate, thus ensuring good process reproducibility. Reactive extrusion can be carried out without the need for specific atmospheric conditions inside the barrel. Typically, extrusion takes place in ambient air. At the extruder outlet, after passing through the die, the polymer is recovered, in which some or all of the diene monomer units have reacted with the 1,3-dipolar compound.

According to a second variant of the second embodiment, step a) is carried out in two phases: a first phase in which the unsaturated polymer and the 1,3-dipolar compound are mixed, typically at a temperature below 60°C, for example, in an external mixer such as a roller tool; and a second phase in which the mixture resulting from the first phase is heated to a temperature of 80°C to 200°C, preferably 100°C to 170°C, in a press or oven, for the time necessary to perform the grafting. At the end of the second phase, the polymer is recovered, in which some or all of the diene monomer units have reacted with the 1,3-dipolar compound. Such a process is described, for example, in patent application WO 2015059269.

According to a third variant of the second embodiment, step a) is carried out by thermomechanical mixing of the unsaturated polymer and the 1,3-dipolar compound, for example in an internal mixer, until a maximum mixing temperature of 110 to 180°C, preferably between 140 and 170°C, is reached. Following thermomechanical mixing, the polymer is recovered, in which some or all of the diene monomer units have reacted with the 1,3-dipolar compound. After this step, the modified polymer can be extruded to form granules, in order to facilitate its storage before use in step b), as described, for example, in patent application WO 2018115704.

Step a) is advantageously carried out in bulk. The grafting reaction of step a) is preferably carried out in bulk according to the second embodiment, advantageously according to the first variant, the second variant or the third variant, most advantageously according to the first variant which proves to be more efficient with regard to grafting yield and also more productive.

The amount of 1,3-dipolar compound needed in step a) can vary considerably. It is indexed to the grafting yield and the number of imidazole groups desired to be grafted onto the unsaturated polymer. The grafting yield, which is the ratio of the amount of 1,3-dipolar compound grafted onto the unsaturated polymer to the amount of 1,3-dipolar compound used in step a), is typically greater than or equal to 50% and can reach values greater than or equal to 80%, particularly when step a) is implemented according to the first variant, where the amount of 1,3-dipolar compound grafted onto the unsaturated polymer can be determined by NMR analysis. The number of imidazole groups desired to be grafted onto the unsaturated polymer is typically greater than or equal to 0.5 mmol per gram of unsaturated polymer. Preferably, it is greater than 0.5 mmol per gram of unsaturated polymer and less than 3.5 mmol per gram of unsaturated polymer. The amount of the 1,3-dipolar compound used in step a) is preferably greater than 0.5 milliequivalents of imidazole function per gram of unsaturated polymer, more preferably greater than 0.6 milliequivalents of imidazole function per gram of unsaturated polymer. It is also preferably less than 4.2 milliequivalents of imidazole function per gram of unsaturated polymer.

At the end of step a), a polymer is therefore obtained which carries pendant imidazole functions in a content which is preferably greater than or equal to 0.5 mmole per gram of polymer, more preferably greater than 0.5 mmole per gram of polymer and less than 3.5 mmoles per gram of polymer.

Step b) consists of incorporating a haloalkane, also called a haloalkane, into the polymer resulting from step a), a polymer bearing pendant imidazole groups. Incorporation is carried out by bulk thermomechanical mixing, for example in an internal or external mixer, which aims to distribute the haloalkane as uniformly as possible within the polymer bearing pendant imidazole groups along its chain without the use of a solvent. At the end of step b), a mixture is obtained containing the polymer bearing pendant imidazole groups and the haloalkane.

The amount of haloalkane used in step b) is indexed to the rate of imidazole function grafting of the functional polymer obtained at the end of step a), in other words, to the number of imidazole functions carried by the polymer at the end of step a), or to the amount of 1,3-dipolar compound used in step a). Preferably, in step b) the amount of haloalkane is 1 to 2 molar equivalents of imidazole functions carried by the polymer at the end of step a) or is 1.2 to 2.4 molar equivalents of 1,3-dipolar compound used in step a).

The haloalkane can be an iodoalkane, a chloroalkane, or a bromoalkane. It is preferentially a bromoalkane. The hydrocarbon chain constituting the haloalkane can be linear, branched, or cyclic. In the haloalkane, the halogen atom can substitute for any of the carbon atoms in the hydrocarbon chain constituting the haloalkane. The haloalkane is chosen by those skilled in the art based on the desired performance compromise of the process, which may include, for example, minimizing or avoiding the emission of volatile compounds during step b) or step c), controlling the amount of haloalkane to be introduced in step b), and obtaining the best ionomer yield. A person skilled in the art will be able to choose the haloalkane based on its physicochemical and chemical properties, such as its vapor pressure, boiling point, molar mass, density, and reactivity with imidazole groups, all of which influence this trade-off. Preferably, the haloalkane contains at least 6 carbon atoms. Advantageously, it contains fewer than 12 carbon atoms. Also preferably, the haloalkane contains only one halogen atom. Suitable haloalkanes include iodohexanes, bromohexanes, chlorohexanes, iodoheptanes, bromoheptanes, chloroheptanes, iodooctanes, bromooctanes, chlorooctanes, iodononanes, bromononanes, chlorononanes, iododecanes, bromodecanes, and chlorodecanes, preferably with a linear chain and the halogen atom at the end of the carbon chain. A single haloalkane or a mixture of haloalkanes may be used, preferably a single haloalkane, at least for the sake of process simplification.

Step c), which follows step b), consists of forming an ionomer by reacting the imidazole groups of the polymer with the haloalkane, a quaternization reaction. In this application, the quaternization reaction is defined as the reaction that forms an ionic group, the imidazolium cation, with the halide anion as the counter-ion. The heat treatment in step c) consists of heating the mixture obtained in step b) to a temperature sufficient to carry out the quaternization reaction. A person skilled in the art adjusts the temperature at which the heat treatment is carried out, taking into account the reactivity of the haloalkane to the quaternization reaction and the thermal stability of the reactants and reaction products. In step c), the heat treatment is carried out at a temperature generally above 10°C, preferably above 30°C. The heat treatment is preferably carried out at a temperature below 100°C. Step c) which follows step b) is also carried out in bulk.

According to one embodiment of the invention, steps b) and c) are carried out in the same device which enables steps b) and c) to be carried out. The device useful for this embodiment may be an internal mixer or an extruder which provides mechanical work to uniformly incorporate the haloalkane into the polymer bearing pendant imidazole functions and which also provides heat to the mixture formed from the polymer bearing pendant imidazole functions and the haloalkane to enable the quaternization reaction.

According to another embodiment of the invention, steps b) and c) are carried out in different devices, a first device such as a roller tool or an extruder allowing the haloalkane to be incorporated into the polymer bearing pendant imidazole functions at a temperature below the quaternization reaction temperature, a second device which brings the mixture formed of the polymer bearing pendant imidazole functions and the haloalkane to a temperature allowing the quaternization reaction.

At the end of step c), an ionomer is obtained whose ionic groups are imidazoliums. The content of the imidazolium ionic groups is preferably greater than or equal to 0.5 mmol per gram of ionomer, more preferably greater than 0.5 mmol per gram of ionomer and less than 3.5 mmol per gram of ionomer.

Step d) of the process according to the invention consists of forming the ionomer, a polymer obtained at the end of step c), into a film intended to form part or all of an anion exchange membrane. Step d) is also carried out in bulk. Any device known for forming a polymer into a film in the absence of a solvent can be used. Examples include calenders, roller nozzles located at the exit of an extruder, and presses. To facilitate its formation into a film, the ionomer can be heated to soften it. The film preferably has a thickness of less than 1 mm, more preferably a thickness greater than 10 µm, and preferably less than 500 µm.

As is well known to those skilled in the art, a membrane is primed with water before being put into operation in a fuel cell or electrolyzer assembly. This membrane primer is generally part of the break-in or activation process. Typically, during this primer, the halides of the membrane's constituent ionomer can be replaced by hydroxide anions, which are known to have much better ionic mobility than halide anions. See, for example, the article Energy Environ Sci 2014, 7, 3135.

The replacement of halide counterions with hydroxide anions can be achieved by impregnating the membrane's ionomer film with an aqueous solution. This can be done, for example, by exposing the ionomer film to an aqueous solution containing hydroxide anions, such as a strong base. Suitable strong bases include potassium hydroxide and sodium hydroxide, with potassium hydroxide being the preferred choice. Ideally, the replacement of halide counterions with hydroxide anions is carried out during the membrane's break-in or activation process.

The membrane according to the invention can be used in a fuel cell or an electrolyzer, preferably in an electrolyzer.

The ionomer used to prepare the membrane is prepared by steps a), b), and c) of the process according to the invention relating to the preparation of an anion exchange membrane, step a) advantageously being by mass. The ionomer is a polymer whose essential characteristic is to bear pendant imidazolium ionic groups, preferably in a concentration greater than or equal to 0.5 mmol per gram of polymer, more preferably greater than 0.5 mmol per gram of polymer and less than 3.5 mmol per gram of polymer. The pendant imidazolium ionic groups are attached to the polymer via a group resulting from the cycloaddition reaction between the 1,3-dipolar compound and the unsaturated polymer. The counterions of the imidazolium ionic groups are halide anions, preferably bromide anions.

In the preparation of the membrane according to the invention, the elimination of the step of immersion in a solution of a haloalkane to carry out the quaternization reaction is made possible thanks to the chemical structure of the ionomer and its method of preparation which can be carried out in bulk.

In summary, the invention can be implemented according to any one of embodiments 1 to 33:

Mode 1: Anion exchange membrane containing an ionomer, polymer bearing pendant imidazolium ionic groups which are attached to the polymer via a group resulting from the cycloaddition reaction between a 1,3-dipolar compound and an unsaturated polymer, the 1,3-dipolar compound being an aromatic nitrile oxide substituted by an imidazole function, the unsaturated polymer being a polymer containing monomeric units of a diene.

Mode 2: Membrane according to mode 1 in which the unsaturated polymer is a homopolymer of a 1,3-diene or a copolymer of a 1,3-diene.

Mode 3: Membrane according to mode 1 or 2 in which the unsaturated polymer is selected from homopolymers of a 1,3-diene, copolymers of two or more 1,3-dienes, copolymers of a 1,3-diene and a vinylaromatic monomer, and copolymers of a 1,3-diene and ethylene.

Mode 4: Membrane according to mode 3 in which the vinylaromatic monomer is styrene.

Mode 5: Membrane according to any one of modes 1 to 4 in which the diene is 1,3-butadiene or isoprene.

Mode 6: Membrane according to any one of modes 1 to 5 in which the diene is 1,3-butadiene.

Mode 7: Membrane according to any one of modes 1 to 6 in which the imidazole function is of formula 1 H -imidazol-1-yl.

Mode 8: Membrane according to any one of modes 1 to 6 in which the imidazole function is of formula 2-alkyl-1 H- imidazol-1-yl.

Mode 9: Membrane according to mode 8 in which the imidazole function has the formula 2-methyl-1 H -imidazol-1-yl.

Mode 10: Membrane according to any one of modes 1 to 9 in which the 1,3-dipolar compound corresponds to formula (I) in which one of the symbols R ₁ to R ₅ represents a group containing the imidazole function, the other symbols a hydrogen atom or an alkyl, knowing that one of the R ₁ and R ₅ is not a hydrogen atom.

Mode 11: Membrane according to mode 10 in which R ₄ represents the group containing the imidazole function.

Mode 12: Membrane according to mode 10 in which R ₄ is a (1 H -imidazol-1-yl)alkyl group.

Mode 13: Membrane according to mode 10 in which R ₄ is a (2-alkyl-1 H -imidazol-1-yl)alkyl group.

Mode 14: Membrane according to mode 10 in which R ₄ is a (1 H -imidazol-1-yl)methyl or (2-alkyl-1 H -imidazol-1-yl)methyl group.

Mode 15: Membrane according to any one of modes 10 to 14 in which _R1 , _R3 and _R5 are each an alkyl and _R2 is a hydrogen atom and _R4 represents the imidazole function.

Mode 16: Membrane according to any one of modes 10 to 15 in which _R1 , _R3 and _R5 are methyl or ethyl.

Mode 17: Membrane according to any one of modes 1 to 16 in which the 1,3-dipolar compound is the compound 2,4,6-trimethyl-3-((2-methyl-1 H -imidazol-1-yl)methyl)benzo-nitrile oxide or the compound 2,4,6-triethyl-3-((2-methyl-1 H -imidazol-1-yl)methyl)benzo-nitrile oxide.

Mode 18: A process for preparing an anion exchange membrane defined in any one of modes 1 to 17 which comprises the following steps a), b), c) and d):
- a) the preparation of a polymer bearing pendant imidazole functions by a cycloaddition reaction of the 1,3-dipolar compound and the unsaturated polymer, ,
- b) the incorporation of a haloalkane into the polymer bearing pendant imidazole functions by thermomechanical mixing,
- c) a heat treatment of the mixture obtained in step b) to convert the imidazole functions into imidazolium ionic groups,
- d) shaping the ionomer obtained at the end of step c) into a film,
steps b), c) and d) being carried out in bulk.

Mode 19: Process according to mode 18 in which step a) is carried out in bulk.

Mode 20: Process according to mode 18 or 19 in which the unsaturated polymer is defined in mode 2 or 3.

Mode 21: Process according to any one of modes 18 to 20 in which the diene is 1,3-butadiene or isoprene.

Mode 22: Process according to any one of modes 18 to 21 in which the haloalkane is a bromoalkane.

Mode 23: Process according to any one of modes 18 to 22 in which the haloalkane contains a single halogen atom.

Mode 24: A process according to any one of modes 18 to 23 in which the amount of the 1,3-dipolar compound used in step a) is greater than 0.5 milliequivalent of imidazole function per gram of unsaturated polymer and less than 4.2 milliequivalents of imidazole function per gram of unsaturated polymer.

Mode 25: Process according to mode 24 in which the amount of the 1,3-dipolar compound used in step a) is greater than 0.6 milliequivalent of imidazole function per gram of unsaturated polymer.

Mode 26: A process according to any one of modes 18 to 25 in which at step b) the amount of haloalkane is 1 to 2 molar equivalents of imidazole functions carried by the polymer at the end of step a) or is 1.2 to 2.4 molar equivalents of 1,4-dipolar compound used in step a).

Method 27: A process according to any one of methods 18 to 26 in which, in step c) the mixture obtained in step b) is brought to a temperature above 30°C.

Mode 28: Ionomer, polymer bearing pendant imidazolium ionic groups which are attached to the polymer via a group resulting from the cycloaddition reaction between a 1,3-dipolar compound and an unsaturated polymer, the counterions of the imidazolium ionic groups being halide anions, the 1,3-dipolar compound being an aromatic nitrile oxide substituted with an imidazole function, the unsaturated polymer being a polymer containing monomeric units of a diene.

Mode 29: Ionomer according to mode 28 in which the 1,3-dipolar compound is defined in any one of modes 10 to 17.

Mode 30: Ionomer according to mode 28 or 29 in which the unsaturated polymer is defined at any one of modes 2 to 3.

Mode 31: Ionomer according to any one of modes 28 to 30 in which the diene is 1,3-butadiene or isoprene.

Method 32: A process for preparing an ionomer defined according to any one of the methods 28 to 31, which process includes steps a) to c) defined in any one of the methods 18 to 27.

Mode 33: Fuel cell or electrolyzer that contains a membrane defined in any one of modes 1 to 17.

The aforementioned features of the present invention, as well as others, will be better understood upon reading the following description of several examples of embodiments of the invention, given by way of illustration.

Examples

In the examples, the 1,3-dipolar compound substituted with an imidazole function is 2,4,6-trimethyl-3-((2-methyl- 1H- imidazol-1-yl)methyl)benzonitrile oxide. It is prepared according to the procedure described in patent application WO 2015059269.

Nuclear magnetic resonance (NMR):
The prepared polymers are characterized by ^1H and ^13C NMR spectroscopy. The NMR spectra are recorded on a Brüker Avance III 500 MHz spectrometer equipped with a 5 mm BBFOz-grad "broadband" cryo-probe. The quantitative ^1H NMR experiment uses a single 30° pulse sequence and a 5-second repetition delay between each acquisition. 64 to 256 accumulations are performed. The quantitative ^13C NMR experiment uses a single 30° pulse sequence with proton decoupling and a 10-second repetition delay between each acquisition. 1024 to 10240 accumulations are performed. The two-dimensional ^1H / ^13C experiments are used to determine the structure of the copolymers. The ^1H chemical shift axis is calibrated with respect to the protonated impurity of the solvent ( _CDCl3 ) at _δ1H = 7.20 ppm. The ^13C chemical shift axis is calibrated with respect to the solvent signal ( _CDCl3 ) at _δ13C = 77 ppm.

SEC 3D Analysis:
Size exclusion chromatography (SEC) separates macromolecules in solution according to their size using columns filled with a porous gel. The macromolecules are separated according to their hydrodynamic volume, with the largest being eluted first.
Combined with three detectors (3D), a refractometer, a viscometer, and a 90° light scattering detector, SEC allows for the determination of the absolute molar mass distribution of a polymer. The various absolute molar masses, average by number (Mn), by weight (Mw), and the dispersity ( Đ = Mw/Mn) can also be calculated.

Each sample is solubilized in tetrahydrofuran (+ 1% vol. diisopropylamine + 1% vol. triethylamine) at a concentration of approximately 1 g/L. The solution is then filtered through a 0.45µm porosity filter before injection.

The equipment used is a WATERS Alliance e2695 chromatograph. The elution solvent is tetrahydrofuran (+ 1% vol. diisopropylamine + 1% vol. triethylamine), the flow rate is 1 mL/min, and the system temperature is 35°C. A set of three columns, "PL Gel Mixed B-LS" from Agilent, is used.
The injected volume of the polymer sample solution is 100 µL. The detection system used, "MALS DAWN 8+, Viscostar and T-Rex" from WYATT, is composed of a differential refractometer, a differential viscometer and a 90° light scatter detector.

The haloalkane is TCI's 1-bromooctane (99% purity), the press is a Carver brand, the microextruder is a twin-screw recirculating extruder, Xplore brand, equipped with a barrel with an internal volume of 15 cm³the chamber volume being 3 cm³, the speed of the screws being 100 rpm (revolutions per minute).

Example 1: preparation of an ionomer, polybutadiene bearing pendant imidazolium ionic groups (or imidazolium functions)

Preparation of a polybutadiene bearing imidazole functions (step a):

The polymer containing monomer units of a diene is a strong cis polybutadiene containing 97% cis 1,4-butadiene units, commercial product "BR Synteca 63" from the company Synthos.

The 1,3-dipolar compound (1.308 g with a mass purity of 60%, corresponding to 3.1 mmol of compound) is incorporated in several stages (5 times) into polybutadiene (1.251 g – 23.1 mmol of butadiene monomer units) in a microextruder for a residence time in the barrel of 3 min at 120°C. The mixture is collected at the extruder outlet in the form of a rod.
This mixing phase is followed by heat treatment at 120°C for 10 minutes under pressure of 3 to 4 bar. A functional imidazole polybutadiene is obtained, the polybutadiene modified by the grafting reaction with the 1,3-dipolar compound.

Preparation of a polybutadiene bearing imidazolium functions (steps b) and c)):

Functional imidazole polybutadiene (1.95 g) is introduced into a microextruder. 1-Bromooctane (0.75 g – 3.9 mmol) is then added to the screw head of the microextruder to be incorporated into the functional imidazole polybutadiene. The residence time in the barrel is 17 h at a temperature between 39°C and 49°C. At the extruder outlet, polybutadiene bearing imidazolium groups is recovered as a rod, as confirmed by NMR analysis. The results are shown in Table 1. The polymer contains 0.54 mmol of imidazolium ionic groups per gram of polymer.

Example 2: Preparation of a polybutadiene ionomer bearing imidazolium groups

Steps a) and b) are carried out as in Example 1. Step c) differs from step c) of Example 1 in that the residence time in the sheath is 45 h at a temperature between 37°C and 45°C. A polybutadiene bearing imidazolium functional groups is recovered, as confirmed by NMR analysis. The results are shown in Table 1. The polymer contains 0.71 mmol of imidazolium ionic groups conforming to formula 1 per gram of polymer.

Formula (1):

Table 1: Molar content in the polymer (% mol) at the end of step c)
Example 1 at the end of step c)
Example 2 cis 1,4-butadiene 91.3 86.4 imidazole function grafted 4.1 6.3 imidazolium function grafted 4.6 7.3

Example 3 : preparation of an ionomer, poly(1,3-butadiene-co-ethylene) bearing pendant imidazolium ionic groups (or imidazolium functions)

Preparation of poly(1,3-butadiene-co-ethylene), a polymer containing monomer units of a 1,3-diene:

The ethylene-1,3-butadiene copolymer (EBR) is prepared according to the following procedure:
Metallocene [{Me ₂ SiFlu ₂ Nd(µ-BH ₄ ) ₂ Li(THF) }] ₂ is prepared according to the procedure described in patent application WO 2007054224.
BOMAG butyl methylmagnesium (20% in heptane, at 0.88 mol L ^-1 ) comes from Chemtura and is stored in a Schlenk tube under an inert atmosphere.
The ethylene, of N35 grade, comes from the company Air Liquide and is used without prior purification.
1,3-Butadiene is purified over alumina guards.
The methylcyclohexane (MCH) solvent from BioSolve is dried and purified on an alumina column in a solvent fountain from mBraun and used under an inert atmosphere.
The polymerization reaction is carried out under an inert atmosphere in a 90 L stainless steel reactor equipped with a stainless steel stirring paddle. Temperature control is achieved via a thermostatically controlled oil bath connected to a double-insulated jacket. This reactor has all the necessary inlets and outlets for the process.
In a 90 L stainless steel reactor, 64 L of MCH and a solution of BOMAG (27 mmol) in MCH (0.01 mol/L) are introduced. The reactor is heated to 80°C, and the monomers are added at a controlled rate to maintain a constant monomer mixture composition in the polymerization medium. The ethylene flow rate is set at 40 g/min, butadiene is injected independently, and its flow rate is controlled by the ethylene flow rate at a butadiene/ethylene mass ratio of 1.26. When the reactor reaches a pressure of 8.8 bar, the catalytic system (5.16 mmol of Nd), prepared according to the protocol described below, is introduced into the polymerization medium. The polymerization reaction, conducted at 80°C, is stopped with methanol when approximately 5 to 6 kg of polymer have formed; the polymer is then recovered after a stripping step. The polymer is then dried on a screw conveyor equipped with a single screw at 150°C.

The catalytic system is a preformed catalytic system. It is prepared in methylcyclohexane from the metallocene, [ _Me₂Si (Flu) _₂Nd (μ- _BH₄ ) _₂Li (THF)], the cocatalyst, butylloctylmagnesium (BOMAG), and a preforming monomer, 1,3-butadiene. It is prepared according to a preparation method conforming to paragraph II.1 of patent application WO 2017093654 A1:
In an 80 L reactor containing 54.5 L of methylcyclohexane previously degassed with nitrogen, 936 mL of a butylmagnesium in heptane solution (0.933 M, 870 mmol) and 254 g of the complex {( _Me₂Si ( _C₁₃H₈ ) _₂ )Nd( _-BH₄ ) _₂Li (THF)} _₂ (number of moles of Nd, nNd = 399 mmol), prepared according to patent application WO2007/054224 (complex 1), are _successively introduced. 1.9 kg of 1,3-butadiene (also referred to hereafter as butadiene) are added to the reactor at 50°C. The reactor is then heated to 80°C for 5 h with stirring. The resulting catalytic solution is stored at -5°C.

The copolymer thus prepared is analyzed by SEC and NMR. Table 2 shows:
-the microstructure of the copolymer, the units being expressed as a molar percentage calculated relative to the total number of moles of ethylene, butadiene and 1,2-cyclohexane units (designated in Table 2 as "cycle"),
- its number-average molar mass, Mn, expressed in g/mol and the dispersity, Đ .

Table 2: Units
1,4-Butadiene Units
1,2-butadiene Unit
cycle Unit
ethylene Mn
(DRY) IP 10 21 6 61 236200 1.4

Preparation of a poly(1,3-butadiene-co-ethylene) bearing imidazole functions (step a):

The 1,3-dipolar compound (72 g) of 60% purity by mass is incorporated in several stages (4 times) into the copolymer (57 g) on a roller tool (external mixer 23°C) by carrying out a total of 12 wallet passes to obtain a homogeneous mixture.
This mixing phase is followed by heat treatment at 120°C for 10 minutes in the press under 10 t (tonnes) of charge. A functional imidazole copolymer is recovered, the copolymer modified by the grafting reaction with the 1,3-dipolar compound.

Preparation of a poly(1,3-butadiene-co-ethylene) bearing imidazolium functions (steps b) and c)):

The imidazole functional copolymer (2.1 g) is introduced into the microextruder. 1-Bromooctane (0.90 g) is then added to the screw head of the microextruder to be incorporated into the imidazole functional copolymer. The residence time in the barrel is 30 minutes at 80°C. The resulting mixture is collected at the extruder outlet in the form of a rod.
This mixing phase is followed by heat treatment at 90°C for 17 hours in a press under 3 to 4 bars of pressure. A poly(1,3-butadiene-co-ethylene) containing imidazolium ionic groups is obtained as a film 180 µm thick. A test sample 180 µm thick is then prepared to form a membrane.
To determine the ionic conductivity of the membrane, its electrochemical impedance is measured across the membrane plane at 30°C and 30% relative humidity. The measurement parameters are a 50 mV amplitude change and a 0 V applied potential. Prior to this, the halide counterions are replaced by hydroxide anions according to the following procedure:
The ionomer film is immersed for 24 h at room temperature (23°C) in a 1M potassium hydroxide solution in demineralized water to perform ion exchange between bromide and hydroxide ions. It is then soaked for 15 min in demineralized water to remove excess hydroxide ions.
The water absorption is 100% by mass. The measured ionic conductivity is 13.45 mS/cm.

Claims (20)

Anion exchange membrane containing an ionomer, polymer bearing pendant imidazolium ionic groups which are attached to the polymer via a group resulting from the cycloaddition reaction between a 1,3-dipolar compound and an unsaturated polymer, the 1,3-dipolar compound being an aromatic nitrile oxide substituted with an imidazole function, the unsaturated polymer being a polymer containing monomeric units of a diene. Membrane according to claim 1 in which the unsaturated polymer is a homopolymer of a 1,3-diene or a copolymer of a 1,3-diene. Membrane according to claim 1 or 2 wherein the unsaturated polymer is selected from homopolymers of a 1,3-diene, copolymers of two or more 1,3-dienes, copolymers of a 1,3-diene and a vinylaromatic monomer and copolymers of a 1,3-diene and ethylene. Membrane according to any one of claims 1 to 3 wherein the imidazole function is of formula 1 H -imidazol-1-yl. Membrane according to any one of claims 1 to 3 wherein the imidazole function is 2-alkyl-1 H -imidazol-1-yl, preferably 2-methyl-1 H -imidazol-1-yl. Membrane according to any one of claims 1 to 5 wherein the 1,3-dipolar compound corresponds to formula (I) in which one of the symbols R ₁ to R ₅ represents a group containing the imidazole function, the other symbols a hydrogen atom or an alkyl, knowing that one of the R ₁ and R ₅ is not a hydrogen atom.
Membrane according to claim 6 in which R ₄ represents the group containing the imidazole function. Membrane according to claim 6 or 7 wherein R ₄ is a (1 H -imidazol-1-yl)alkyl or (2-alkyl-1 H -imidazol-1-yl)alkyl group, preferably a (1 H -imidazol-1-yl)methyl or (2-alkyl-1 H -imidazol-1-yl)methyl group. Membrane according to any one of claims 6 to 8 wherein _R1 , _R3 and _R5 are each an alkyl and _R2 is a hydrogen atom. Membrane according to any one of claims 6 to 9 wherein _R1 , _R3 and _R5 are methyl or ethyl. Membrane according to any one of claims 1 to 10 wherein the 1,3-dipolar compound is the compound 2,4,6-trimethyl-3-((2-methyl-1 H -imidazol-1-yl)methyl)benzo-nitrile oxide or the compound 2,4,6-triethyl-3-((2-methyl-1 H -imidazol-1-yl)methyl)benzo-nitrile oxide. A method for preparing an anion exchange membrane as defined in any one of claims 1 to 11, comprising the following steps: a), b), c) and d)
- a) the preparation of a polymer bearing pendant imidazole functions by a cycloaddition reaction of the 1,3-dipolar compound and the unsaturated polymer, ,
- b) the incorporation of a haloalkane into the polymer bearing pendant imidazole functions by thermomechanical mixing,
- c) a heat treatment of the mixture obtained in step b) to convert the imidazole functions into imidazolium ionic groups,
- d) shaping the ionomer obtained at the end of step c) into a film,
steps b), c) and d) being carried out in bulk. A process according to claim 12 in which the haloalkane, preferably a bromoalkane, contains a single halogen atom. A process according to any one of claims 12 to 13 wherein the amount of the 1,3-dipolar compound used in step a) is greater than 0.5 milliequivalent of imidazole function per gram of unsaturated polymer, preferably greater than 0.6 milliequivalent of imidazole function per gram of unsaturated polymer, and less than 4.2 milliequivalents of imidazole function per gram of unsaturated polymer. A process according to any one of claims 12 to 14 wherein in step b) the amount of haloalkane is 1 to 2 molar equivalents of imidazole functions carried by the polymer at the end of step a) or is 1.2 to 2.4 molar equivalents of the 1,4-dipolar compound used in step a). Ionomer, polymer bearing pendant imidazolium ionic groups which are attached to the polymer via a group resulting from the cycloaddition reaction between a 1,3-dipolar compound and an unsaturated polymer, the counterions of the imidazolium ionic groups being halide anions, the 1,3-dipolar compound being an aromatic nitrile oxide substituted with an imidazole function, the unsaturated polymer being a polymer containing monomeric units of a diene. Ionomer according to claim 16 in which the 1,3-dipolar compound is defined in any one of claims 6 to 11. Ionomer according to claim 16 or 17 wherein the unsaturated polymer is defined in any one of claims 2 to 3. A process for preparing an ionomer as defined in any one of claims 16 to 18, comprising steps a) to c) as defined in any one of claims 12 to 15. Fuel cell or electrolyzer which contains a membrane as defined in any one of claims 1 to 11.