Field of Art

The present invention relates to a block copolymer - poly(styrene-block-C2-C4-alkylene-stat- C2-C4-alkylene-block-styrene) sulfomethylated on benzene cores of styrene units, to polymer cation exchange membranes consisting of or containing the said copolymer, to a process for preparation thereof and to use thereof in separation processes, fuel cells, batteries etc.

Background Art

Today, ion-exchange polymer membranes are used on a laboratory as well as industrial scale. The most important applications include electrochemical desalination of seawater and brackish water, separation of electrolytes from non-electrolytes in electrochemical devices, purification of pharmaceutical preparations or preparation of solid electrolytes. Ion exchange membranes are currently produced either in the form of homogeneous membranes, which are single-phase ion exchange systems, or as heterogeneous membranes, which consist of a dispersion of ion exchange particles in a hydrophobic polymeric binder (J. Schauer, L. Brozova, Journal of Membrane Science 250 (2005) 151). Majority of known cation exchange membranes are prepared by sulfonation of the benzene core of crosslinked polystyrene or by grafting styrene onto another polymer and subsequent sulfonation. Further to polystyrene, other aromatic polymers are used for the preparation of sulfonated aromatic membranes, for example poly(2,6- dimethylphenylene oxide), polysulfone or poly(ether ether ketone).

Drawbacks of membranes with a sulfo group bound directly to the benzene core include their low stability in oxidizing environments, high water sorption and a lower ionic conductivity relative to the total ion exchange capacity. A major cause of these undesired properties is the statistical distribution of sulfone groups in said polymers. However, if a block copolymer of styrene and olefins, such as poly(styrene-block-ethylene-stat-butylene-block-styrene)(PSE BS), is used to prepare the cation exchange membranes sulfonated on the benzene core, a higher membrane conductivity is achieved at the same ion exchange capacity. This is mainly due to its microheterogeneous structure (J. Schauer, J. Llanos, J. Zitka, J. Hnat, K. Bouzek, J., Appl. Polym. Sci., 124 (2012) E66). Although the use of the PSEBS copolymer for the preparation of cation exchange membranes significantly improves their mechanical properties, the drawbacks associated with the sulfo groups bound directly to the aromatic nucleus are not eliminated (S.- Y. Jang, S.-H. Han, J. Membr. Sci. 444 (2013) 1).

The inventors have previously described chloromethylated membranes based on styrene block copolymers (PSEBS) as intermediates for the preparation of anion exchange membranes (L. Kook, J. Zitka, P. Bakonyi, P. Takacs, L. Pavlovec, M. Otmar, R. Kurdi, K. Belafi-Bako, N. Nemestothy, Sep. Purif. Technol. 237 (2020) 116478; R. Cardena, J. Zitka, L. Kook, P. Bakonyi, L. Pavlovec, M. Otmar, N. Nemestothy, G. Buitron, Bioelectrochemistry, 133 (2020) 107479; J. Zitka, J. Schauer, M. Bleha, K. Bouzek, M. Paidar, J. Hnat, CZ305138), or polymer membranes for the separation of enantiomers (M. Otmar, J. Gaalova, J. Zitka, L. Brozova, P. Cufinova, M. Kohout, S. Hovorka, J. E. Bara, B. Van der Bruggen, J. Jirsak, P. Izak, Eur. Polym. J., 122 (2020) 109381). Merrifield crosslinked polystyrene (homopolymeric) resins bearing a mercaptomethyl group with a possible use for the removal of heavy metals from wastewater have also been described (R. Tank, U. Pathak, A. Sinng, A. Gupta, D. C. Gupta, React. Funct. Polymer 69 (2009) 224). However, their mercaptomethyl group was not further oxidized to a sulfomethyl group, nor were these resins used to prepare polymeric membranes. These documents thus do not provide cation exchange membranes based on styrene block copolymers which would comprise sulfo groups bound to the benzene core via a methylene linker (i.e. sulfomethyl(styrene) block copolymers). CN 1951969 discloses sulfomethylated triblock PSEBS copolymer for use in cation exchange membranes. These copolymers have, according to the document, the value equivalent weight (EW) of 1036-1085, which corresponds to ion exchange capacity (IEC) of 0.965 to 0.922 mmol.g' ¹ . IEC corresponds to the molar content of sulfomethylated groups in the sulfomethylated PSEBS.

The aim of the present invention is to provide a styrene copolymer with cation-exchange properties, and cation exchange membranes consisting of or containing the said copolymer, having a high ionic conductivity and excellent mechanical and chemical stability. Disclosure of the Invention

A first aspect of the present invention is a block copolymer - poly(styrene-block-C2-C4- alkylene- tat-C2-C4-alkylene- block-styrene), wherein the benzene cores of the styrene units are substituted by sulfomethyl groups. The molar content of the sulfomethyl groups is within the range of 1.2 to 3.2 mmol.g' ¹ , more preferably 1.4 to 1.9 mmol.g' ¹ , even more preferably 1.42 to 1.9 mmol.g' ¹ , relative to dry weight of the sulfomethylated block copolymer.

The block copolymer can be schematically represented by formula I, wherein the substituents R ¹ and R ² are H, methyl, or ethyl.

Preferably, the content of the styrene units is within the range of 10 to 70 wt. %, more preferably 20 to 40 wt. %, and the content of alkylene units of each type is within the range of 10 to 50 wt. %, relative to the weight of the copolymer poly(styrene-block-C2-C4-alkylene-stat-C2-C4- alkylene-block-styrene) without sulfomethyl groups (non-sulfomethylated copolymer).

Preferably, the alkylene is butylene and/or ethylene.

Due to the synthetic procedure of preparation of the block copolymer, the copolymer of the invention may contain unreacted or incompletely reacted functional groups -H, -CH ₂ CI, - CH ₂ CS(NH ₂ ) ₂ ⁺ Cl , and/or -CH ₂ SH on the benzene cores, or the styrene units in the polymer may be partially cross-linked to form in particular disulfide bridges -CH ₂ -S-S-CH ₂ - and their incompletely oxidated derivatives -CH ₂ -SOx-SOy-CH ₂ - wherein x = 0-2, y = 0-2.

A second aspect of the present invention is a cation exchange membrane consisting of or containing a block copolymer - poly(styrene-block-C2-C4-alkylene-stat-C2-C4-alkylene- block-styrene), wherein the benzene cores of the styrene units are substituted by sulfomethyl groups. Such membrane has a high ionic conductivity, good mechanical properties even in dry state, and can be used, for example, in applications for ion exchange materials, such as, in particular, solid electrolytes, ion exchange membranes, ion exchange binders and catalyst supports. The term „high ionic conductivity ^" should be understood as a conductivity of at least 10 mS.cm' ¹ at 25 °C.

Preferably, the content of the styrene units is within the range of 10 to 70 wt. %, more preferably 20 to 40 wt. %, and the content of alkylene units of each type is within the range of 10 to 50 wt. %, relative to the weight of the copolymer poly(styrene-block-C2-C4-alkylene-stat-C2-C4- alkylene-block-styrene) without sulfomethyl groups (non-sulfomethylated copolymer).

Preferably, the alkylene is butylene and/or ethylene.

Preferably, the molar content of the sulfomethyl groups in within the range of 0.8 to 3.2 mmol.g- ¹ , more preferably 0.9 to 1.9 mmol.g' ¹ , relative to dry weight of the sulfomethylated block copolymer. Even more preferably, the molar content of the sulfomethyl groups is within the range of 1.2 to 3.2 mmol.g' ¹ , or 1.4 to 1.9 mmol.g' ¹ , yet more preferably 1.42 to 1.9 mmol.g' ¹ , relative to dry weight of the sulfomethylated block copolymer.

A third aspect of the present invention is a method of preparation of the block copolymer poly(styrene-block-C2-C4-alkylene-stat-C2-C4-alkylene-Zblock -styrene) in which the benzene cores in the styrene groups are substituted by sulfomethyl groups, said method comprising the following steps:

- poly(styrene-block-C2-C4-alkylene-stat-C2-C4-alkylene-Zblock -styrene) is subjected to chloromethylation to form chloromethyl substituents on the benzene cores in the styrene units; - the chloromethylated poly(styrene-block-C2-C4-alkylene-stat-C2-C4-alkylene-block- styrene) obtained in the previous step is reacted with thiourea to form isothiouronium salt; and the isothiouronium salt is subsequently hydrolyzed by alkali metal hydroxide or alkaline earth metal hydroxide solution or by alkali metal Cl-C4-alcoholate to form mercaptomethyl substituents on the benzene cores in the styrene units;

- the mercaptomethylated poly(styrene-block-C2-C4-alkylene-stat-C2-C4-alkylene-block- styrene) obtained in the previous step is subsequently subjected to oxidation by an oxidation agent to form the block copolymer poly(styrene-block-C2-C4-alkylene-stat-C2-C4-alkylene- block-styrene), in which the benzene cores in the styrene groups are substituted by sulfomethyl groups.

A fourth aspect of the present invention is a method of preparation of a membrane consisting of the block copolymer poly(styrene-block-C2-C4-alkylene-stat-C2-C4-alkylene-block- styrene) in which the benzene cores in the styrene groups are substituted by sulfomethyl groups, said method comprising the following steps:

- poly(styrene-block-C2-C4-alkylene-stat-C2-C4-alkylene-Zblock -styrene) is subjected to chloromethylation to form chloromethyl substituents on the benzene cores in the styrene units;

- the chloromethylated poly(styrene-block-C2-C4-alkylene-stat-C2-C4-alkylene-block- styrene) obtained in the previous step is casted into the form of a membrane;

- the chloromethylated poly(styrene-block-C2-C4-alkylene-stat-C2-C4-alkylene-block- styrene) in the form of the membrane is reacted with thiourea to form isothiouronium salt; and the isothiouronium salt is subsequently hydrolyzed by alkali metal hydroxide or alkaline earth metal hydroxide solution or by alkali metal Cl-C4-alcoholate to form mercaptomethyl substituents on the benzene cores in the styrene units;

- the mercaptomethylated poly(styrene-block-C2-C4-alkylene-stat-C2-C4-alkylene-block- styrene) in the form of the membrane, obtained in the previous step, is subsequently subjected to oxidation by an oxidation agent to form the block copolymer poly(styrene-block-C2-C4- alkylene- tat-C2-C4-alkylene-block-styrene), in which the benzene cores in the styrene groups are substituted by sulfomethyl groups, in the form of the membrane.

The chloromethylation may be carried out using a procedure described in the art (J. Zitka, J. Schauer, M. Bleha, K. Bouzek, M. Paidar, J. Hnat, CZ305138). The procedure involves subjecting a starting block copolymer poly(styrene-block-C2-C4-alkylene-stat-C2-C4- alkylene-block-styrene) to a reaction with dimethoxymethane, a chlorinating agent selected from a group consising of PCh, SOCh and SiCh, and ZnCh catalyst, preferably at a temperature within the range of 10 °C to 65 °C, more preferably for a period of at least 24 h.

In a preferred embodiment, the starting block copolymer poly(styrene-block-C2-C4-alkylene-stat-C2-C4-alkylene-block- styrene) has a number average molar weight Mn within the range of 10 000 to 1 000000 g.mol' ¹ and the content of styrene within the range of 10 to 70 % by weight, preferably 20 to 40 % by weight. The content of each type of alkylene is preferably within the range of 10 to 50 % by weight.

In a preferred embodiment, the copolymer poly(styrene-block-C2-C4-alkylene-stat-C2-C4- alkylene-block-styrene) is a copolymer poly(styrene-block-ethylene- tat-butylene-block- styrene).

In a preferred embodiment, suitable solvents for the step of the reaction of the chloromethylated polymer or membrane with thiourea include water and C1-C4 alcohols. These solvents provide for a good solubility of the reagents and a good wettability of the membrane, but avoid dissolution of the membrane.

In a preferred embodiment, the reaction with thiourea involves immersing the polymer, optionally in the form of membrane, into the reaction mixture and leaving to react at a temperature in the range of 10 to 95 °C for 1 hour to 72 hours.

In a preferred embodiment, the reaction with sodium, lithium or potassium hydroxide or with alkali metal alcoholate involves immersing the polymer, optionally in the form of membrane, into the reaction mixture and leaving to react at a temperature in the range of 20 to 60 °C for 12 hours to 48 hours.

In a preferred embodiment, the oxidation of the thiolmethyl (mercaptomethyl) group involves immersing the polymer, optionally in the form of membrane, into the reaction mixture containing the oxidation agent and leaving to react at a temperature in the range of 5 to 50 °C for 1 hour to 72 hours, preferably at a temperature in the range of 5 to 30 °C for for 1 hour to 48 hours. Preferably, the oxidation agent may be performic acid.

Reaction intermediates may be schematically represented by formula II: wherein:

R ¹ = H or CH ₃ or CH2CH3;

R ² = H or CH ₃ or CH ₂ CH ₃ ;

R ³ = H for the starting copolymer;

R ³ = CH2CI for chloromethylated copolymer;

R ³ = CH ₂ SC(NH ₂ )2 ⁺ Cl for isothiuronium salt of the copolymer;

R ³ = CH2SH for mercaptomethylated copolymer.

A fifth aspect of the invention is use of the copolymer of the present invention and/or of the membrane consisting of or containing the copolymer for the preparation of homogeneous or microheterogeneous membranes, for impregnation of electrodes in electrochemical devices, as catalyst supports, for the preparation of ion exchange membranes and binders in electrochemical devices, such as solid electrolytes, in ion exchange applications and/or in catalytic systems. Examples of carrying out the Invention

Example 1 - Preparation of the membrane The process for the preparation of the membrane consisting of the block copolymer of the invention is schematically shown in Scheme I.

Scheme I Chloromethylated polymer prepared using the procedure described in: J. Zitka, J. Schauer, M. Bleha, K. Bouzek, M. Paidar, J. Hnat, patent CZ305138, example 2, from the starting poly(styrene-block-ethylene-stat-butylene-Zblock-styrene), M _w = 178 100 g mol' ¹ , Mn= 193 900 g mol' ¹ , containing 29 wt. % styrene units, 43 wt. % ethylene units and 28 wt. % butylene units, was dissolved in toluene to form a 5 % w/w solution. The solution was cast on a teflon (PTFE) pad and covered by a Petri dish in order to slow down the evaporation of the solvent. The solvent was evaporated for 48 hours at room temperature in a laminar box. The thus prepared membrane having the weight of about 10 g was then immersed into 1 kg of 10% w/w solution of thiourea in methanol and reacted for 48 hours at 60 °C. The membrane was then removed from the reaction mixture and rinsed with ethanol. Elemental analysis of the resulting membrane showed a sulfur content of 3.04 wt. %, nitrogen content 2.60 wt. %, chlorine content 5.08 wt. %. The membrane was then immersed into a IM solution of NaOH in methanol and reacted at 60 °C for 48 hours. The membrane was then removed from the reaction mixture and rinsed with methanol. The resulting sulfur content was 2.37 wt. %, nitrogen content 0 wt.%, chlorine content 0.2 wt. %. Subsequently, the membrane was immersed in a mixture of 315 ml of 88% w/w formic acid and 135 ml of 30% w/w hydrogen peroxide at 5 °C. The membrane was allowed to react for 48 hours. It was then rinsed with demineralized water and air dried. The resulting sulfur content was 1.94 wt. %, nitrogen content 0 wt. %, chlorine content 0.2 wt. %. The ionic conductivity at 25 °C in 0.1 M KC1 is 18.0 mS.cm' ¹ . The molar amount of sulfomethyl groups per 1 g of dry membrane weight is 1.42 mmol g' ¹ .

Example 2 - Oxidative stability

Fenton test for oxidative stability of a cation exchange membrane consisting of sulfomethylated poly(styrene-block-C2-C4-alkylene-stat-C2-C4-alkylene-Zblock -styrene) block copolymer prepared according to Example 1 was performed so that the membrane having the weight of 0.50 g and the thickness of 220 micrometers was immersed in 50 g (1 : 100) of 3% w/w H2O2 with 4 ppm anhydrous FeSCU, and after an initial heating time of 15 minutes the membrane was maintained at 80 °C for 1 hour at neutral pH. After this time, the membrane was removed, dried and weighed, and a weight loss of 0.01 grams was found.

Industrial Applicability

Ion exchange membranes are currently used on a laboratory as well as industrial scale. The most important applications include electrochemical desalination of seawater and brackish water, separation of electrolytes from non-electrolytes in electrochemical devices, purification of pharmaceutical preparations, use as solid electrolytes and use in other electrochemical processes such as electrodialysis, electrolysis and fuel cells. The materials of the present invention are intended for the preparation of homogeneous (microheterogeneous) membranes and for use in devices using ion exchange membranes.