COPOLYMERS AND MEMBRANES

Technical field

The present invention relates to soluble branched addition copolymers which may be cured post synthesis to form membranes. More specifically the present invention relates to the soluble branched addition copolymers which may be cured post synthesis to form ion-exchange membranes with improved properties, methods for their preparation, compositions comprising such soluble branched addition copolymers and their use in membrane preparation, most specifically ion-exchange membranes.

Background of the invention

The present invention relates to novel soluble branched addition copolymers which may be cured via a cross-linking reaction and their use as membranes, most specifically ion-exchange membranes wherein the membranes exhibit improved hydrolytic stability.

The inventors have previously shown in WO 201 1 /033261 that branched addition copolymers may be successfully used to prepare ion-exchange membranes with high ion selectivity, low electrical resistance and low swelling. Additionally, as the polymers used to form the membranes are branched in nature they may be dissolved and cast at high concentration in organic solvents leading to reduced solvent usage.

The inventors have now also surprisingly found that certain novel branched addition copolymers may be used to form membranes with improved properties. These novel copolymers may be used to resolve or mitigate many of the problems associated with existing polymeric membranes. For example, there is a requirement in electrochemical desalination devices for the manufacture of membranes with suitably improved hydrolytic stability which may be used to withstand extremes of pH and which are particularly effective as ion-exchange membranes in such devices.

An ion-exchange membrane may be described as a charged, ion-selective barrier or component for use in devices where the removal or concentration of one ion species over another is required. Such ion-exchange membranes have a net positive or negative charge and as such are selective for anions (Anion Exchange Membrane - AEM) or cations (Cation Exchange Membrane - CEM) respectively. Ion-exchange membranes are typically polymeric in nature and may be prepared either from polymerising suitable monomers and or via post-functionalising or crosslinking of a pre-formed polymer or oligomer to achieve the desired membrane material.

In order to provide charge discrimination an ion-selective membrane must possess a net positive or negative charge. These charged groups may be present in the monomer or pre-formed polymer structure or may be introduced via a post- modification step upon formation of the polymer or membrane. It is preferential to cast or fabricate an ion-exchange membrane from a polar or non-polar organic solvent, more preferably a polar organic solvent as casting charged ion-exchange membranes from water can lead to undesirable swollen membrane structures.

Most charged monomers and/or polymers are only soluble in highly polar organic or aqueous solvents, which can lead to issues when fabricating membranes or indeed polymerising or post-functionalising the polymers. A particularly useful process involves the introduction of charged groups while crosslinking the polymer. In this way, the membrane is formed by the reaction of a suitable pre-formed polymer with a functional co-reactant to achieve both crosslinking and introduction of the necessary charge in one step. An example of such a process would be in the preparation of an anion exchange membrane (AEM) from a basic nitrogenous polymer which is then reacted with a dihalo alkane, such as in the reaction of a pyridine-functional polymer with a diiodo or dibromo alkane. An alternative method involves reacting a polymer possessing a leaving group, such as a halogen atom, with a nucleophilic amine or polyamine molecule; such as the reaction between a polymer possessing benzyl chloride moieties with a tertiary diamine.

The charged species on the ion-exchange membrane is also important with regard to the selectivity and performance of the membrane. As previously mentioned in order to be selective, the membrane must have a net positive or negative charge. The charged species must be stable, advantageously permanent and accessible in order to have affinity for the appropriate ion. In order for the membrane to be anion-selective (an AEM) the material must have a net positive charge and contain cationic species, preferred cationic groups tend to be formed from, for example, charged nitrogen or phosphorous species including quaternary ammonium, pyridinium, imidazolidinium, guanidinium or phosphonium moieties.

In order for the membrane to be cation-selective (a CEM) the material must have a net negative charge and contain anionic species, preferred anionic groups tend to be strongly acidic species such as sulfonic or phosphoric acid moieties. Weakly acidic groups such as carboxylic acid groups may also be used although in certain applications such groups may become irreversibly fouled with divalent cations such as magnesium or calcium. This is particularly true for example in the desalination or purification of seawater.

Although ion-exchange membranes can be prepared directly via polymerising suitable functional monomers, a particularly attractive route is through the preparation of a functional polymer or oligomer wherein the polymeric material is used to fabricate an ion-exchange membrane. It is usual to crosslink the polymer in order to obtain a stable, robust membrane. Crosslinking is obtained through the reaction of a first functional group on the polymer with a suitable second group. This crosslinking may be through an inter- or intra- molecular reaction. That is, the polymer can be designed to react with itself during a membrane casting or curing process or, the polymer may be designed to react with an additional molecule, wherein the additional molecule is either for example a polymer or a small molecule.

Suitable curing reactions include the polymerisation of, for example; a pendant alkene unit such as a vinyl or allyl unit; or alternatively, the reaction may be between two reactive units to form a covalent bond, such as the formation of an ester or amide link; the ring opening of an epoxide; formation of a urethane or urea bond; nucleophilic substitution or addition; electrophilic substitution or addition or via the formation of an ionic linkage, for example through the formation of a salt bridge.

Crosslinking reactions may take place at ambient temperature or through thermal means or via a photochemical reaction, typically via a UV source. Additional initiators may also be used, for example a free radical initiator where the reactive species is an alkene unit. Catalysts may also be used to accelerate the curing step such as, for example, a strong acid in the case of the preparation of an ester or amide linkage, or a metal compound in the case of urethane or urea formation.

Crosslinked membranes have the advantage of being more environmentally resilient than uncured materials due to the insoluble nature of the crosslinked network, that is, the curing mechanism renders the material essentially intractable, that is, insoluble, hence the requirement for pre-formation into the desired form of the end product prior to the crosslinking step.

In the cases listed above a number of reactive chemistries may be exploited. Essentially any reaction that can form a covalent or ionic bond between two molecules may be utilised. Here follows a non-exclusive list of the functional groups and reactions that may be performed to provide a cured copolymer.

In all of the cases below, the functional group may be incorporated into the polymer structure via the use of functional monomers or alternatively the reactive moiety may be introduced through a further reactive step onto a pre-formed polymer.

Alkene polymerisation.

An unsaturated carbon-carbon unit in the form of for example an alkene bond may be essentially polymerised, usually via a free radical procedure. In such a mechanism the polymerisation occurs via the introduction of a free radical initiator which is then dissociated thermally, by the use of UV radiation or via a chemical means such as a redox reaction, to generate free radicals which react with the unsaturated units and provide a cured polymer, or alternatively via a transition metal catalyst "drier" in the case of alkyd systems. Allyl, vinyl or alkyd-functional polymers are typically used in this type of curing.

In the following cases the mutually reactive units described can be present within the same polymer structure or, on two or more polymers, or, on one polymer and one small molecule, wherein the complementary functionalities on each polymer or molecule may react. Ester or amide formation.

Alcohol or amine and carboxylic acid functionalities may be reacted to provide an ester or an amide linker unit respectively. These linking reactions are typically thermally propagated in the presence of a strong acid catalyst. Another route to these types of linkages is the reaction of an alcohol or amine with an anhydride an acid chloride or azlactone; or through the transesterification or transamidation of an activated ester such as that found in the monomer methyl acrylamidoglycolate methyl ether.

Epoxide ring-opening

In this case a compound possessing an epoxide ring is reacted with a nucleophilic material, usually a primary or secondary amine or alkoxide unit. The amine epoxy reaction may be catalysed by a hydroxylic species such as phenols and alcoholic solvents. Epoxides may also react with other nucleophilic species such as thiols or carboxylic acids, in the presence of a tri- alkyl or aryl phosphine catalyst. The epoxide may also be homopolymerised via the use of a Lewis or Bronsted acid such as boron tri-fluoride or tri-fluoromethane sulfonic acid.

Isocyanate chemistry

In this case, an isocyanate group is reacted with a group possessing an active hydrogen such as a hydroxyl group, a thiol or an amine. The polymer usually possesses the active hydrogen nucleophile and is reacted with a smaller molecular weight di- or poly-isocyanate, such as tolylenediisocyanate. Blocked isocyanates, where the isocyanate unit has been reacted with a labile monofunctional active hydrogen compound may also be used, in which case, the isocyanate is rendered less reactive and the formulation may be stored as a stable one-pack formulation.

Thiol-ene chemistry

In thiol-ene chemistry, the radical reaction between a thiol functionality and an electron-rich olefin is utilised to form a thioether linkage. These reactions are typically initiated by photochemical means.

Nucleophilic substitution. These reactions involve the substitution of a labile leaving group with a suitable nucleophile. An example of such a reaction involves the substitution of an alkyl halide with an amine or alkoxide. In the case of the reaction of the alkyl halide with an amine, a charged species is formed which may be advantageous in the formation of a charged membrane.

Electrophilic addition.

In this case, an electrophile is reacted with a suitable electron-rich moiety. An example of this crosslinking reaction is the reaction of an activated aryl unit with an electrophile such as an acid chloride, usually in the presence of a Lewis acid catalyst.

Disulfide curing

The reaction of two thiol units to form a disulfide may be undertaken through oxidation, for example by the use of hydrogen peroxide.

Silicone curing systems

The formation of siloxane linkages may be achieved through the reaction of an alkyloxysilane functionality where the curing proceeds via the elimination of a carboxylic acid, for example acetic acid in the case of an acetoxysilyl unit.

Membrane/Copolymer Hydrolysis

Many applications of ion-exchange membranes take place in aqueous environments; the pH of the environment may vary from acidic to basic. This is particularly true where the ion-selective membranes are being used for metal ion concentration in acidic waste streams or where they are being used in the electrodialysis of water. In the latter case highly basic conditions may be generated at the cathode due to so- called water splitting, this is due to device inefficiency where water is electrolysed in addition to the potential difference between the two electrodes being used to separate ions.

In both of these cases the hydrolytic stability of the polymer membrane is important, as hydrolysis of the crosslinked membrane and/or polymer and/or functional units can reduce the performance or mechanical strength of the membrane. Additionally, if the crosslinking units are hydrolysed, the membrane may break-up or swell in-use which is highly undesirable.

Where there is a quaternary ammonium group present in an anion exchange membrane (AEM) where the quaternary ammonium groups possess labile beta hydrogen groups, a Hoffman elimination reaction can occur liberating an amine or ammonium compound and leaving an alkene unit on the polymer. This results in reduction of the ionic functionality of the membrane with a corresponding reduction in membrane performance.

Therefore there is a requirement for the manufacture of copolymers, and ultimately membranes with suitably improved hydrolytic stability to withstand the rigours of current polymer applications. More specifically, there is a requirement for copolymers with improved hydrolytic stability for use in ion-exchange applications.

By the term 'hydrolytic stability' used herein is meant in-use functional group stability where the membrane is utilised in an aqueous environment at extremes of low and high pH and is stable to hydrolysis for example via potential saponification or Hoffman elimination reactions.

In addition, the term 'hydrolytic stability' in the context of the present application relates to soluble branched copolymers which when used in the formation of a membrane, the permselectivity and electrical resistance of the membrane does not change by more than 50 % when subjected to hydrolysis at 25 °C for 48 hours and to extremes of pH such as greater than pH 12 and less than pH 3. More preferably the permselectivity and electrical resistance of the membrane does not change by more than 40 % when subjected to hydrolysis at 25 °C for 48 hours and extremes of pH above and below pH 12 and pH 3 respectively. Even more preferably the permselectivity and electrical resistance of the membrane does not change by more than 25 % when subjected to hydrolysis at 25 °C for 48 hours and extremes of pH above and below pH 12 and pH 3 respectively. Most preferably the permselectivity and electrical resistance of the membrane does not change by more than 10 % when subjected to hydrolysis at 25 °C for 48 hours and extremes of pH such as greater than pH 12 and less than pH 3.

Avoiding membrane hydrolysis.

Membrane hydrolysis can be reduced by preparing the membrane or polymer from hydrolytically stable units. By hydrolytically stable units is meant stability in-use through potential membrane degradation reactions or reduction in functional groups via for example Hoffman elimination reactions or saponification reactions of the functional species.

This can be undertaken by synthesising the polymer using monomers or functional groups which are stable to hydrolysis, either in acidic or basic conditions depending upon the final application. Where the polymer is being used in an alkali environment then it is desirable to avoid labile ester functionality, either in the polymer or in the membrane crosslinker, thus hydrolytically unstable ester-functional polymers are to be avoided as these moieties are susceptible to cleavage.

As previously mentioned, Hoffman elimination reactions can occur in AEMs where a quaternary ammonium group is beta to a labile hydrogen, in order to maintain cationic charge in these membranes and polymers while avoiding these elimination reactions, pyridinium, or imidazolidinium functional moieties are preferred.

Applications.

Ion-selective membranes are used for a number of industrial applications. Essentially the membranes are used where ions are required to be separated, concentrated or detected from a solution. Ion-selective membranes can be simply utilised in place of conventional ion-exchange resins, for ion separation, removal or concentration applications whereby the laminar format of a membrane is preferable to a bead morphology. Ion-selective membranes also find uses in a number of electrically driven ion-separation devices, where the ion-selective membrane acts as a charged barrier in order for the device to operate or to improve performance. Ion-exchange membranes can be used for electrodialysis which is a process whereby ions are removed from water or an aqueous solution via the use of a electrical potential difference. In this process, unlike pressure driven processes such as ultrafiltration or reverse osmosis, charged ions as opposed to water molecules pass through a membrane driven by an external electrical potential. Ion-selective membranes are used in these devices acting as a charge barrier and allowing only one ion species to traverse the membrane barrier, thereby enabling the device to act efficiently. Additional electro-purification devices are known in the art, and again ion- exchange membrane barriers are known to improve the efficiency when utilised in the device. For these applications, in order to be effective, the membranes require a high ion permselectivity, low electrical resistance, low swelling in water, high physical strength and resistance to hydrolysis.

Types of polymers used to prepare ion-selective membranes.

Linear polymers are commonly used in many applications due to their high solubility and ease of preparation. Due to their architectures these polymers can give rise to high viscosity solutions or melts, in addition they can be extremely slow or difficult to dissolve or melt to give isotropic liquids. The high viscosity of these solutions can be problematic in membrane formulation where a large amount of solvent is required in order to provide a workable formulation. Where the solvent is organic in nature this can lead to a large amount of volatile organic compound (VOC) being necessary to use the linear polymer effectively. Increasing legislation to decrease the VOC levels of many formulations makes this undesirable. Linear addition polymers typically also have a functional group pendant to the main chain of the polymer. This situation may give rise to slow curing reactions due to the inaccessibility of functional groups within the interior of the polymer structure during the curing reaction. This in turn leads to longer cure times and higher cure temperatures in thermally mediated reactions.

Linear polymers can also give rise to incomplete curing. Due to the architecture of these materials the membrane can also swell significantly in formulations leading to poor substrate adhesion and poor membrane properties. Swelling of a membrane during use is particularly problematic as it can lead to failure of the polymer membrane properties or the device itself.

The use of linear polymers can also lead to poorly cross-linked or open networks when cured into a membrane. Where highly dense membranes are required, or where a high concentration of functionalities or charge is required in the finished membrane, this can be unfavourable. This can also lead to poorer mechanical strengths for membranes prepared using linear polymers.

The curing rate of a linear polymer system is proportional to the molecular weight of the macromolecule concerned. Ideally, high molecular weight materials are preferred. However due to the sharp increase in solution or melt viscosity of the formulation with increasing molecular weight a compromise in molecular weight must be achieved to avoid high amounts of solvent (typically a VOC) in the formulation, or temperature, in the case of melt processed systems.

It has now been found that these disadvantages, namely the high viscosity of polymer systems, low cure rate, low density of functional groups, poor mechanical strength or incomplete cross-linking can however be addressed by using a branched architecture.

Branched Copolymers.

Branched copolymers are polymer molecules of a finite size in which the backbone is branched. Branched copolymers differ from cross-linked polymer networks which tend towards an infinite size having interconnected molecules and which are generally not soluble. In some instances, branched polymers have advantageous properties when compared to analogous linear polymers. For instance, solutions of branched copolymers are normally less viscous than solutions of analogous linear polymers. Moreover, higher molecular weights of branched copolymers may be solubilised more easily than those of corresponding linear polymers. In addition, as branched polymers tend to have more end groups than a linear polymer they generally exhibit strong surface-modification properties. Thus, branched polymers are useful components of many compositions and may be utilised in the formation of polymer membranes. Branched and hyperbranched copolymers can also be used in curable systems. Unlike dendrimers branched copolymers typically show non-ideal branching in their structure and can possess polydisperse structures and molecular weights. The preparation of branched copolymers however is much easier than their dendrimer counterparts and although the final structure may not be perfect or monodisperse, branched copolymers are more suitable for a number of industrial applications.

Branched copolymers are usually prepared via a step-growth mechanism via the polycondensation of suitable monomers and are usually limited by the choice of monomers, the chemical functionality of the resulting polymer and the molecular weight. In addition polymerisation, a one-step process can be employed in which a multifunctional monomer is used to provide functionality in the polymer chain from which polymer branches may grow. However, a limitation on the use of a conventional one-step process is that the amount of multifunctional monomer must be carefully controlled, usually to substantially less than 0.5% w/w in order to avoid extensive cross-linking of the polymer and the formation of insoluble gels. It is difficult to avoid cross-linking using this method, especially in the absence of a solvent as a diluent and/or at high conversion of monomer to polymer.

WO 99/46301 discloses a method of preparing a branched polymer comprising the steps of mixing together a monofunctional vinylic monomer with from 0.3 to 100% w/w (of the weight of the monofunctional monomer) of a multifunctional vinylic monomer and from 0.0001 to 50% w/w (of the weight of the monofunctional monomer) of a chain transfer agent and optionally a free-radical polymerisation initiator and thereafter reacting said mixture to form a copolymer. The examples of WO 99/46301 describe the preparation of primarily hydrophobic polymers and, in particular, polymers wherein methyl methacrylate constitutes the monofunctional monomer. These polymers are useful as components in reducing the melt viscosity of linear poly(methyl methacrylate) in the production of moulding resins.

WO 99/46310 discloses a method of preparing a (meth)acrylate functionalised polymer comprising the steps of mixing together a monofunctional vinylic monomer with from 0.3 to 100 % w/w (based on monofunctional monomer) of a polyfunctional vinylic monomer and from 0.0001 to 50 % w/w of a chain transfer agent, reacting said mixture to form a polymer and terminating the polymerisation reaction before 99 % conversion. The resulting polymers are useful as components of surface coatings and inks, as moulding resins or in curable compounds, for example curable moulding resins or photoresists.

WO 02/34793 discloses a rheology modifying copolymer composition containing a branched copolymer of an unsaturated carboxylic acid, a hydrophobic monomer, a hydrophobic chain transfer agent, a cross linking agent, and, optionally, a steric stabilizer. The copolymer provides increased viscosity in aqueous electrolyte- containing environments at elevated pH. The method for production is a solution polymerisation process. The polymer is lightly cross-linked, less than 0.25%.

US 6,020,291 discloses aqueous metal working fluids used as lubricant in metal cutting operations. The fluids contain a mist-suppressing branched copolymer, including hydrophobic and hydrophilic monomers, and optionally a monomer comprising two or more ethylenically unsaturated bonds. Optionally, the metal working fluid may be an oil-in-water emulsion. The polymers are based on poly(acrylamides) containing sulfonate-containing and hydrophobically modified monomers. The polymers are cross-linked to a very small extent by using very low amount of bis-acrylamide, without using a chain transfer agent.

Stamatialis and co-workers (Journal of Membrane science 310 (2008) 512-521)

Discloses the use of a hyperbranched polyester (Boltorn H40) to increase the gas permeability coefficients for polyimide membranes. The hyperbranched material is cast together with the polyimide to give the corresponding hybrid membrane. Although the Boltorn material is not covalently linked in any way in the membrane there is a marked increase in the permeability coefficients for nitrogen and oxygen with a modest (1 % w/w) incorporation of the hyperbranched polymer.

Shi and co-workers (Journal of Membrane science 245 (2004) 35-40) describe the formation of an organic-inorganic hybrid membrane utilising the dendritic polyol Boltorn H20 which had been post-functionalised with acetoxysily units capable of undergoing a sol-gel curing process with a further poly(alkoxysilane) and phosphoric acid. The membrane showed excellent proton conductivity at high temperature and humidity which the authors attributed to the dendritic component of the cured membrane structure.

WO 03/104327A1 describes the formation of a highly gas impermeable film via the use of a functionalised hyperbranched polyester amide (HBPEA). The HBPEA is incorporated in a preferably post-cured film in conjunction with a quantity of poly(vinylalcohol) or derivative thereof. The film is then post-cured by incorporation of a further reactive molecule capable of reacting covalently with hydroxyl groups. The films were found to be effective barriers against oxygen.

In WO 2011/033261 the inventors described the use of soluble branched addition copolymers, capable of crosslinking, in the preparation of thin films and membranes. In particular, the membranes could be cast or cured with ionic functionality and be used in ion-separation applications. The inventors also showed that these materials could be cast from organic solvent where the ionic group was introduced during the crosslinking reaction to achieve an ion-exchange membrane. The materials produced showed high ion permselectivities, low electrical resistances, high tensile strength and low swelling in aqueous solutions.

Detailed Description.

Polymers capable of undergoing a subsequent curing or cross-linking reaction are used in many everyday applications. Typically these polymers are of a linear architecture where the functional groups are either pendant to the polymer main chain or at the termini of the macromolecule. The polymers can be natural, synthetic or hybrid in composition and can either react via an intra or intermolecular mechanism. In the case of addition polymers the functionality is usually either preformed within the polymer structure through choice of suitable reactive monomers or incorporated through a further chemical reaction. In these cases the functionality is placed along the carbon main-chain of the material. The concentration and location of the functionality can be tuned through the ratios of functional monomers or by using a controlled technique respectively. Problems associated with using linear molecules to form membranes.

It has been now been found that the use of soluble curable branched copolymers in the formation of membrane has a number of advantages over linear systems. The architecture of branched copolymers means that these polymers give rise to solutions or melts of lower viscosity enabling higher solids compositions to be formulated. This then enables less solvent to be used which can be problematic where volatile organic solvents (VOCs) are employed. In many curable systems there is a growing trend toward high solids formulations, the presence of organic solvents is something of a liability as they impart flammability, high cost and in many cases toxicity and are almost entirely lost in the final cured system. Since the solvent usually plays no part in the curing mechanism, and in many cases hinders it, the removal of the solvent is preferential. The ability to formulate at high solids level is particularly attractive since it can lead to compositions with a higher concentration of active curable polymer thus leading to faster cure rates during film or membrane manufacture. In many applications cure rate is crucial in the preparation of films or membranes and where this is thermally initiated, a number of cost savings can be made. Due to the polyvalency in branched copolymer systems there is also a greater availability of functional groups within the polymer structure and once more this can lead to faster cure times.

Due to this high accessibility of functional groups and fast onset of gelation during curing there is typically greater formulation-substrate interaction leading to greater substrate adhesion, lower swelling, greater permselectivity and higher tensile strength; particularly desirable for membranes. These membranes may be used in applications under hydrostatic, concentration or electrical potential differences.

Dendritic polymers are prepared via a multi-step synthetic route and are limited by chemical functionality and ultimate molecular weight, being prepared at a high end cost. Such molecules have therefore only limited high-end industrial applications. Branched copolymers are typically prepared via a step-growth procedure and again are limited by their chemical functionality and molecular weight. However, the reduced cost of manufacturing such polymers makes them more industrially attractive. Due to the chemical nature of both of these classes of macromolecules (that is, such molecules typically possess ester or amide linkages), problems arising from their miscibility with olefin-derived polymers have been observed. This may be circumvented by the use of hydrocarbon-based, star-shaped polymers prepared via anionic polymerisation or the post-functionalisation of pre-formed dendrimers or branched species although this again leads to an increased cost in the materials.

The inventors have previous shown that soluble branched copolymers of high molecular weight may be prepared via a one-step process using commodity monomers. Through specific monomer choices the chemical functionality of these polymers can be tuned depending on the specific application. These benefits therefore give advantages over dendritic or step-growth branched polymers. Since these polymers are prepared via an addition process from commodity monomers, they may be tuned to give good miscibility with equivalent linear addition polymers. Since branched copolymers comprise a carbon-carbon backbone they are not susceptible to thermal or hydrolytic instability unlike ester-backbone-based dendrimers or step-growth branched copolymers. It has been observed that these polymers also dissolve faster than equivalent linear polymers.

The branched soluble copolymers of the present invention are branched, non-cross- linked addition copolymers and include statistical, block, graft, gradient and alternating branched copolymers. The copolymers of the present invention comprise at least two chains which are covalently linked by a bridge other than at their ends, that is, a sample of said copolymer comprises on average at least two chains which are covalently linked by a bridge other than at their ends. When a sample of the copolymer is made there may be accidentally some polymer molecules that are un- branched, which is inherent to the production method (addition polymerisation process). For the same reason, a small quantity of the polymer may not have a chain transfer agent (CTA) on the chain end.

When manufacturing a membrane the choice of copolymer is also an important consideration. Since branched copolymer formulations give rise to lower solution or melt viscosities, they may be applied more easily. This is particularly true in the case where the formulation is spray applied during manufacture, once more leading to significant cost savings. Applications:

The following is a non-exhaustive list of potential applications for copolymer membranes which includes:

medical separation and diagnostics applications, ion-exchange applications, desalination, water purification, gas separation, electrodialysis, fuel cells, pervaporation, energy generation, energy storage, filtration and sensors. In each application field the membrane composition may be tuned through a choice of the monomers and the curing of the material.

In summary, the advantages of using soluble branched curable copolymers over linear systems are considerable, for example branched curable copolymers lead to; higher solids content formulations may be achieved; low viscosity formulations may be prepared; less volatile organic compounds (VOCs) are required in the final formulation; faster cure rates may be achieved leading to faster processing times; greater substrate adhesion may be obtained; higher density of functionalities or charge may be achieved; denser cross-linked structures may be obtained; greater mechanical strength may be achieved; thinner robust membranes may be prepared; higher permselectivities may be achieved; lower electrical resistances may be obtained and a lower swelling of the final polymer membrane with increased hydrolytic stability. That is, it has now been found that certain soluble branched copolymers with increased hydrolytic stability are able to withstand the demands of ion-selective membranes.

The branched addition copolymers of the present invention are in contrast to insoluble polymer gels. Simply put, a gel is formed when a cross-linked non-soluble polymer is mixed with a solvent which would normally dissolve the polymer backbone if it were a linear and non crosslinked analogous polymer, that is, the same as those chains, which when linked together, form the cross-linked network. The chains are compatible with the solvent but cannot form an isotropic homogenous solution because they are all linked together to form the network and are thus physically constrained. Hence the system is swollen to form a gel. The rigidity or amount of swelling depends on the amount (or degree) of cross-linking present; the more cross-linking the more rigid and less swollen the gel will be. In contrast, a soluble branched addition copolymer will mix completely with a solvent to form a totally homogeneous isotropic solution. Soluble branched addition copolymers are differ from microgels which are lightly cross-linked polymers, that is, small pieces of crosslinked networks that still appear soluble to the naked eye as when mixed with a good solvent they appear to form free flowing isotropic solutions. A test for judging whether a polymer has dissolved to form a solution is to take a small amount of the polymer (around 0.2 g of greater than 50 weight % solution or dry material) and dilute with 10 g of a good solvent of the polymer. If this very dilute solution then passes through an in-line microdisc 0.2 micron pore size syringe filter then the polymer is truly soluble. A highly diluted micro gel will not pass though such a filter.

Therefore according to a first aspect of the present invention there is provided the use of a soluble branched addition copolymer to form a cross-linked hydrolytically stable membrane wherein the soluble branched addition copolymer is cured after formation via an addition polymerisation process prior to formation of the cross- linked membrane; and wherein

the soluble branched addition copolymer is obtainable by an addition polymerisation process; and wherein the

branched addition polymer comprises a weight average molecular weight of 2,000 Da to 1 ,500,000 Da; and wherein

the branched addition copolymer comprises:

at least two chains which are covalently linked by a bridge other than at their ends; and wherein

the at least two chains comprise at least one ethyleneically monounsatu rated monomer, and wherein

the bridge comprises at least one ethyleneically polyunsaturated monomer; and wherein

the polymer comprises a residue of a chain transfer agent and optionally a residue of an initiator; and wherein

the mole ratio of polyunsaturated monomer(s) to monounsaturated monomer(s) is in a range of from 1 :100 to 1 :3 and wherein the branched copolymer is hydrolytically stable such that the permselectivity and electrical resistance of the membrane does not change by more than 50 % when subjected to hydrolysis at 25 °C for 48 hours and a pH of greater than 12.

Furthermore there is provided the use of a soluble branched addition copolymer according to the first aspect of the present invention wherein the soluble branched addition copolymer is obtainable by an addition polymerisation process; wherein the branched addition polymer comprises a weight average molecular weight of 2,000 Da to 1 ,500,000 Da; and wherein

the branched addition copolymer comprises:

at least two chains which are covalently linked by a bridge other than at their ends; and wherein

the at least two chains comprise at least one ethylenically monounsatu rated monomer, and wherein

the bridge comprises at least one ethylenically polyunsaturated monomer; and wherein

the polymer comprises a residue of a chain transfer agent and optionally a residue of an initiator; and wherein

the mole ratio of polyunsaturated monomer(s) to monounsaturated monomer(s) is in a range of from 1 :100 to 1 :3 and wherein the branched copolymer is hydrolytically stable such that the permselectivity and electrical resistance of the membrane does not change by more than 40 % when subjected to hydrolysis at 25 °C for 18 hours and a pH of greater than or equal to 12.

When in use the branched addition copolymer may be cured by the addition of a reactive polymer, oligomer or small molecular weight reactive molecule and wherein the copolymer comprises at least 50 mole percent aromatic, heteroaromatic, amide, ether or urethane residues derived from the monofunctional monomer and/or the multifunctional monomer and/or the chain transfer agent.

When in use the branched addition copolymer may be cured by the addition of a reactive polymer, oligomer or small molecular weight reactive molecule and the copolymer may comprise at least 50 mole percent aromatic or heteroaromatic residues derived from the monofunctional monomer and/or the multifunctional monomer and/or the chain transfer agent. When in use the branched addition copolymer may be cured by means of thermal, photolytic, oxidative, reductive reaction or nucleophilic or electrophilic substitutions or addition or by the addition of a catalyst or initiator.

When in use the branched addition copolymer may be prepared from monomers comprising one or more of the following groups: hydroxyl, amino, carboxylic, epoxy, isocyanate, pyridinyl, imidazolyl, sulfonic acid, vinyl, allyl, (meth)acrylate and styrenyl.

The branched addition copolymer may be cured by means of the reaction of reactive functional groups provided by the monomers and the reactive functional groups may react via an inter or intra molecular process.

Preferably the soluble branched addition copolymer comprises less than 1 % monomer impurity.

Also, the branched addition polymer preferably comprises a weight average molecular weight of 3,000 Da to 900,000 Da.

The soluble branched addition copolymer may be used in the membrane in the application areas selected from the group comprising:

medical separation and diagnostics applications, industrial purification and separation, ion-exchange applications, desalination, water purification, gas separation, electrodialysis, pervaporation, fuel cells, energy generation, energy storage, filtration and sensors.

The use of the soluble branched addition copolymer may be where the membrane comprises an anion exchange membrane and the branched addition copolymer comprises monomer residues selected from the groups consisting of:

i) monofunctional monomers A or C;

ii) optionally a neutral hydrophobic monofunctional monomer (N); and iii) optionally a hydrophilic monofunctional monomer (H); and

iv) a multifunctional monomer (M); and

v) a chain transfer agent (CTA); and wherein the copolymers formed from monofunctional monomers A or C are reacted with compounds selected from the groups B or D respectively below; and wherein A, C , B and D are selected from the groups comprising:

A: 2-vinyl pyridine, 4-vinyl pyridine, N-vinyl imidazole, N-vinyl carbazole, dimethylaminoethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminopropyl (meth)acrylamide,

B : α,ω-dihalodoalkanes such as α,ω-diiodoalkanes such as 1 ,4-diiodobutane, 1 ,5-diiodopentane, 1 ,6-diiodohexane, 1 ,7-diiodoheptane, 1 ,8-diiodooctane, 1 ,9- diiodononane and 1 ,10-diiododecane; α,ω-dibromoalkanes such as 1 ,4- dibromobutane, 1 ,5-dibromopentane, 1 ,6-dibromohexane, 1 ,7-dibromoheptane, 1 ,8- dibromooctane, 1 ,9-dibromononane and 1 ,10-dibromodecane; α,ω- dichloromoalkanes such as 1 ,4-dichlorobutane, 1 ,5-dichloropentane, 1 ,6- chlorohexane, 1 ,7-dichloroheptane, 1 ,8-dichlorooctane, 1 ,9-dichlorononane and 1 ,10-dichlorodecane; most preferred are dibromoalkanes such as 1 ,5-diiodopentane and diiodoalkanes such as 1 ,8-diiodooctane.

C: Vinyl benzyl chloride, and

D: α, ω -tertiaryalkydiaminoalkanes such as: N,N,N'N'-tetramethyl-1 ,6- diaminohexane, N,N'-diisopropylethylenediamine, 4,4'- bis(dimethylamino)benzophenone and bis[2-(dimethylamino)ethyl]ether.

The use of the soluble branched addition copolymer according may be where the membrane comprises a cation exchange membrane and a negative charge is required the branched addition copolymer comprises monomer residues selected from the groups consisting of:

i) a component from S wherein component S comprises a monomer with a permanently negative charge, selected from the group comprising: vinylsulfonic acid, styrene sulfonic acid or 2-acrylamido 2- methylpropanesulfonic acid;

ii) optionally a neutral hydrophobic monofunctional monomer (N); and iii) optionally a hydrophilic monofunctional monomer (H); and

iv) a multifunctional monomer (M); and

v) a chain transfer agent (CTA).

The neutral, hydrophilic monomers, (N) may be selected from the group comprising: hydroxyl-containing monomers such as hydroxyethyl(meth)acrylate, hydroxylpropyl(meth)acrylate, amides such as N-vinyl pyrollidine, (dimethyl)(meth)acrylamide, or ether-functional monomers such as poly or oligo(ethyleneglycol)(meth)acrylate and vinylacetate.

The monofunctional monomers A, C or S may be selected from the group comprising: 2-vinyl pyridine, 4-vinyl pyridine, N-vinyl imidazole, N-vinyl carbazole, dimethylaminoethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminopropyl (meth)acrylamide, vinyl benzyl chloride, vinylsulfonic acid, styrene sulfonic acid or 2-acrylamido 2-methylpropanesulfonic acid;

The hydrophobic monomers (H) may be selected from the group comprising: styrene, vinyl naphthalene, alkl (meth)acrylates, such as methyl (meth)acrylate, ethyl methacrylate, propyl(meth)acrylate, isomers of butyl(meth)acrylate, 2-ethyl hexyl(methacrylate), lsobornyl(meth)acrylate, N-isopropyl(meth)acrylate, N- butyl(meth)acrylamide.

The hydrophobic monomers (H) may be preferably selected from the group comprising: styrene, vinyl naphthalene, isomers of butyl(meth)acrylate and 2-ethyl hexyl(methacrylate).

The multifunctional monomers may be preferably selected from the group comprising: divinylbenzene, ethyleneglycol di(meth)acrylate, 1 ,4- butanediol(meth)acrylate, poly(ethyleneglycol)di(meth)acrylate, 1 ,3,5-triallyl-1 ,3,5- triazine-2,4,6(1 H,3H,5H)-trione.

The chain transfer agents are preferably selected from the group comprising: dodecane thiol, hexane thiol, 2-mercaptoethanol, 2-ethylhexyl thioglycolate and 2,4- diphenyl-4-methyl-1 -pentene.

According to a second aspect of the present invention there is provided an ion- exchange membrane comprising a cured branched addition copolymer as described in relation to the first aspect of the invention wherein the membrane further comprises a hardener selected from: α, ω -diiodoalkanes such as 1 ,4-diiodobutane, 1 ,5-diiodopentane, 1 ,6-diiodo hexane, 1 ,7-diiodoheptane, 1 ,8-diiodooctane and 1 ,10-diiododecane; α, ω - dibromoalkanes such as 1 ,4-dibromobutane, 1 ,5-dibromopentane, 1 ,6-dibromo hexane, 1 ,7-dibromoheptane, 1 ,8-dibromooctane and 1 ,10-dibromodecane; α, ω- tertiaryalkydiaminoalkanes such as: N,N,N'N'-tetramethyl-1 ,6-diaminohexane, Ν,Ν'- diisopropylethylenediamine, 4,4'-bis(dimethylamino)benzophenone and bis[2- (dimethylamino)ethyl]ether, tolylene diisocyanate and hexamethylene diisocyanate.

The membrane may further comprise a support material.

The ion-exchange membrane may comprise a permselectivity of at least 80 %.

More preferably the ion-exchange membrane may comprise a permselectivity of at least 90 %.

The ion-exchange membrane may be in a film or membrane and may be comprised an electrical resistance of less than 5 Ω.ατι ² .

According to a third aspect of the present invention there is provided a soluble branched addition copolymer for use in the formation of an ion exchange membrane or film as described in relation to aspects one or two of the present invention wherein the soluble branched addition copolymer is obtainable by an addition polymerisation process; wherein the

branched addition polymer comprises a weight average molecular weight of 2,000 Da to 1 ,500,000 Da; and wherein

the branched addition copolymer comprises:

at least two chains which are covalently linked by a bridge other than at their ends; and wherein

the at least two chains comprise at least one ethylenically monounsatu rated monomer, and wherein

the bridge comprises at least one ethylenically polyunsaturated monomer; and wherein

the polymer comprises a residue of a chain transfer agent and optionally a residue of an initiator; and wherein the mole ratio of polyunsaturated monomer(s) to monounsaturated monomer(s) is in a range of from 1 :100 to 1 :3 and wherein the branched copolymer is hydrolytically stable such that the permselectivity and electrical resistance of the membrane does not change by more than 50 % when subjected to hydrolysis at 25 °C for 48 hours and a pH of less than 3 and greater than 12.

The soluble branched addition copolymer may be a soluble branched addition copolymer obtainable by an addition polymerisation process; wherein the

branched addition polymer comprises a weight average molecular weight of 2,000 Da to 1 ,500,000 Da; and wherein

the branched addition copolymer comprises:

at least two chains which are covalently linked by a bridge other than at their ends; and wherein

the at least two chains comprise at least one ethylenically monounsaturated monomer, and wherein

the bridge comprises at least one ethylenically polyunsaturated monomer; and wherein

the polymer comprises a residue of a chain transfer agent and optionally a residue of an initiator; and wherein

the mole ratio of polyunsaturated monomer(s) to monounsaturated monomer(s) is in a range of from 1 :100 to 1 :3 and wherein the branched copolymer is hydrolytically stable such that the permselectivity and electrical resistance of the membrane does not change by more than 40 % when subjected to hydrolysis at 25 °C for 18 hours and a pH of greater than or equal to 12.

At least 50 mole percent of the monofunctional monomers may comprise aromatic or heteroaromatic monofunctional monomers.

The aromatic monofunctional monomers may be selected from the group consisting of vinyl pyridine and styrene.

At least 10 mole percent of copolymer may comprise an aromatic chain transfer agent. At least 10 mole percent aromatic chain transfer agent may comprise 2,4-diphenyl-4- methyl-1 -pentene.

At least 5 mole percent of the copolymer may comprises aromatic multifunctional monomer.

The aromatic multifunctional monomer preferably comprises divinyl benzene.

At least 60% of the monofunctional monomers may comprise aromatic monofunctional monomers.

Alternatively, at least 70% of the monofunctional monomers comprise aromatic monofunctional monomers.

The chain transfer agent (CTA) is a molecule which is known to reduce molecular weight during a free-radical polymerisation via a chain transfer mechanism. These agents may be any thiol-containing molecule and can be either monofunctional or multifunctional. The agent may be hydrophilic, hydrophobic, amphiphilic, anionic, cationic, neutral, zwitterionic or responsive. The molecule may also be an oligomer or a pre-formed polymer containing a thiol moiety. (The agent may also be a hindered alcohol or similar free-radical stabiliser). Catalytic chain transfer agents such as those based on transition metal complexes such as cobalt bis(borondifluorodimethyl-glyoximate) (CoBF) may also be used. Suitable thiols include but are not limited to: C ₂ to C-| ₈ branched or linear alkyl thiols such as dodecane thiol, functional thiol compounds such as thioglycolic acid, thio propionic acid, thioglycerol, cysteine and cysteamine. Thiol-containing oligomers or polymers may also be used such as for example poly(cysteine) or an oligomer or polymer which has been post-functionalised to give a thiol group(s), such as poly(ethyleneglycol) (di)thio glycollate, or a pre-formed polymer functionalised with a thiol group. For example, the reaction of an end or side-functionalised alcohol such as polypropylene glycol) with thiobutyrolactone, to give the corresponding thiol- functionalised chain-extended polymer. Multifunctional thiols may also be prepared by the reduction of a xanthate, dithioester or trithiocarbonate end-functionalised polymer prepared via a Reversible Addition Fragmentation Transfer (RAFT) or Macromolecular Design by the Interchange of Xanthates (MADIX) living radical method. Xanthates, dithioesters, and dithiocarbonates may also be used, such as cumyl phenyldithioacetate. Alternative chain transfer agents may be any species known to limit the molecular weight in a free-radical addition polymerisation including alkyl halides, ally-functional compounds and transition metal salts or complexes. More than one chain transfer agent may be used in combination. Non-thiol chain transfer agents such as 2,4-diphenyl-4-methyl-1 -pentene may also be used.

Hydrophobic CTAs include but are not limited to: linear and branched alkyl and aryl (di)thiols such as dodecanethiol, octadecyl mercaptan, 2-methyl-1 -butanethiol and 1 ,9-nonanedithiol. Hydrophobic macro-CTAs (where the molecular weight of the CTA is at least 1000 Daltons) may be prepared from hydrophobic polymers synthesised by RAFT (or MADIX) followed by reduction of the chain end, or alternatively the terminal hydroxyl group of a preformed hydrophobic polymer may be post functionalised with a compound such as thiobutyrolactone.

Hydrophilic CTAs typically contain hydrogen bonding and/or permanent or transient charges. Hydrophilic CTAs include but are not limited to: thio-acids such as thioglycolic acid and cysteine, thioamines such as cysteamine and thio-alcohols such as 2-mercaptoethanol, thioglycerol and ethylene glycol mono- (and di-)thio glycollate. Hydrophilic macro-CTAs (where the molecular weight of the CTA is at least 1000 Daltons) can be prepared from hydrophilic polymers synthesised by RAFT (or MADIX) followed by reduction of the chain end, or alternatively the terminal hydroxyl group of a preformed hydrophilic polymer can be post functionalised with a compound such as thiobutyrolactone.

Amphiphilic CTAs may also be incorporated in the polymerisation mixture, these materials are typically hydrophobic alkyl-containing thiols possessing a hydrophilic function such as but not limited to a carboxylic acid group. Molecules of this type include mercapto undecylenic acid.

Responsive macro-CTAs (where the molecular weight of the CTA is at least 1000 Daltons) may be prepared from responsive polymers synthesised by RAFT (or MADIX) followed by reduction of the chain end, or alternatively the terminal hydroxyl group of a preformed responsive polymer, such as polypropylene glycol), may be post functionalised with a compound such as thiobutyrolactone.

The residue of the chain transfer agent may comprise 0.05 to 80 mole % of the copolymer (based on the number of moles of monofunctional monomer). More preferably the residue of the chain transfer agent comprises 0.05 to 50 mole %, even more preferably 0.05 to 40 mole % of the copolymer (based on the number of moles of monofunctional monomer). However, most especially the chain transfer agent comprises 0.05 to 30 mole %, of the copolymer (based on the number of moles of monofunctional monomer).

The initiator is a free-radical initiator and may be any molecule known to initiate free-radical polymerisation such as for example azo-containing molecules, persulfates, redox initiators, peroxides, benzyl ketones. These may be activated via thermal, photolytic or chemical means. Examples of these include but are not limited to: 2,2'-azobisisobutyronitrile (AIBN), azobis(4-cyanovaleric acid), benzoyl peroxide, diisopropyl peroxide, tert-butyl peroxybenzoate (Luperox® P, Trigonox C), di-tert- butyl peroxide (Luperox® Dl, Trigonox B), cumylperoxide, 1 -hydroxycyclohexyl phenyl ketone, hydrogenperoxide/ascorbic acid. Iniferters such as benzyl-N,N- diethyldithiocarbamate may also be used. In some cases, more than one initiator may be used. The initiator may be a macroinitiator having a molecular weight of at least 1000 Daltons. In this case, the macroinitiator may be hydrophilic, hydrophobic, or responsive in nature.

The residue of the initiator in a free-radical polymerisation may comprise from 0 to 10% w/w of the copolymer based on the total weight of the monomers. Preferably, the residue of the initiator in a free-radical polymerisation comprises from 0.001 to 15% w/w of the copolymer based on the total weight of the monomers. More preferably the residue of the initiator in a free-radical polymerisation comprises from 0.001 to 10% w/w of the copolymer based on the total weight of the monomers. Especially the residue of the initiator in a free-radical polymerisation comprises from 0.001 to 8% w/w, of the copolymer based on the total weight of the monomers. The use of a chain transfer agent and an initiator is preferred. However, some molecules can perform both functions.

Hydrophilic macroinitiators (where the molecular weight of the pre-formed polymer is at least 1000 Daltons) may be prepared from hydrophilic polymers synthesised by RAFT (or MADIX), or where a functional group of a preformed hydrophilic polymer, such as terminal hydroxyl group, may be post-functionalised with a functional halide compound, such as 2-bromoisobutyryl bromide, for use in Atom Transfer Radical Polymerisation (ATRP) with a suitable low valency transition metal catalyst, such as CuBr Bipyridyl.

Hydrophobic macroinitiators (where the molecular weight of the preformed polymer is at least 1000 Daltons) may be prepared from hydrophobic polymers synthesised by RAFT (or MADIX), or where a functional group of a preformed hydrophilic polymer, such as terminal hydroxyl group, may be post-functionalised with a functional halide compound, such as 2-bromoisobutyryl bromide, for use in Atom Transfer Radical Polymerisation (ATRP) with a suitable low valency transition metal catalyst, such as CuBr Bipyridyl.

Responsive macroinitiators (where the molecular weight of the preformed polymer is at least 1000 Daltons) may be prepared from responsive polymers synthesised by RAFT (or MADIX), or where a functional group of a preformed hydrophilic polymer, such as terminal hydroxyl group, may be post-functionalised with a functional halide compound, such as 2-bromoisobutyryl bromide, for use in Atom Transfer Radical Polymerisation (ATRP) with a suitable low valency transition metal catalyst, such as CuBr Bipyridyl.

The monofunctional monomer may comprise any carbon-carbon unsaturated compound which can be polymerised by an addition polymerisation mechanism, for example vinyl and allyl compounds. The monofunctional monomer may be hydrophilic, hydrophobic, amphiphilic, anionic, cationic, neutral or zwitterionic in nature. Preferably the monofunctional monomer is hydrolytically stable. The monofunctional monomer may be selected from but not limited to monomers such as: vinyl acids, vinyl acid esters, vinyl aryl compounds, vinyl acid anhydrides, vinyl amides, vinyl ethers, vinyl amines, vinyl aryl amines, vinyl nitriles, vinyl ketones, and derivatives of the aforementioned compounds as well as corresponding allyl variants thereof.

Other suitable monofunctional monomers include: hydroxyl-containing monomers and monomers which can be post-reacted to form hydroxyl groups, acid-containing or acid-functional monomers, zwitterionic monomers and quaternised amino monomers. Oligomeric, polymeric and di- or multi-functionalised monomers may also be used, especially oligomeric or polymeric (meth)acrylic acid esters such as mono(alkyl/aryl) (meth)acrylic acid esters of polyalkyleneglycol or polydimethylsiloxane or any other mono-vinyl or allyl adduct of a low molecular weight oligomer. Mixtures of more than one monomer may also be used to give statistical, graft, gradient or alternating copolymers.

Vinyl acids and derivatives thereof include: (meth)acrylic acid, fumaric acid, maleic acid, itaconic acid vinyl sulfonic acid, vinyl phosphoric acid, 2-acrylamido 2- methylpropane sulfonic acid, and acid halides thereof such as (meth)acryloyl chloride. Vinyl acid esters and derivatives thereof include: Ci to C ₂ o alkyl(meth)acrylates (linear and branched) such as for example, methyl (meth)acrylate, stearyl (meth)acrylate and 2-ethyl hexyl (meth)acrylate; aryl(meth)acrylates such as for example benzyl (meth)acrylate; tri(alkyloxy)silylalkyl(meth)acrylates such as trimethoxysilylpropyl(meth)acrylate; and activated esters of (meth)acrylic acid such as N-hydroxysuccinamido (meth)acrylate. Vinyl aryl compounds and derivatives thereof include: styrene, acetoxystyrene, styrene sulfonic acid, 2- and 4-vinyl pyridine, vinyl naphthalene, vinylbenzyl chloride and vinyl benzoic acid. Vinyl acid anhydrides and derivatives thereof include: maleic anhydride. Vinyl amides and derivatives thereof include: (meth)acrylamide, N-(2- hydroxypropyl)methacrylamide, N-vinyl pyrrolidone, N-vinyl formamide, (meth)acrylamidopropyl trimethyl ammonium chloride, [3-

((meth)acrylamido)propyl]dimethyl ammonium chloride, 3-[N-(3-

(meth)acrylamidopropyl)-N,N-dimethyl]aminopropane sulfonate, methyl

(meth)acrylamidoglycolate methyl ether and N-isopropyl(meth)acrylamide. Vinyl ethers and derivatives thereof include: methyl vinyl ether. Vinyl amines and derivatives thereof include: dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, diisopropylaminoethyl (meth)acrylate, mono-t-butylaminoethyl (meth)acrylate, morpholinoethyl(meth)acrylate and monomers which can be post- reacted to form amine groups, such as N-vinyl formamide. Vinyl aryl amines and derivatives thereof include: vinyl aniline, 2- and 4-vinyl pyridine, N-vinyl carbazole and vinyl imidazole. Vinyl nitriles and derivatives thereof include: (meth)acrylonitrile. Vinyl ketones or aldehydes and derivatives thereof including acrolein.

Hydroxyl-containing monomers include: vinyl hydroxyl monomers such as hydroxyethyl (meth)acrylate, 1 - and 2-hydroxy propyl (meth)acrylate, glycerol mono(meth)acrylate and sugar mono(meth)acrylates such as glucose mono(meth)acrylate. Monomers which may be post-reacted to form hydroxyl groups include: vinyl acetate, acetoxystyrene and glycidyl (meth)acrylate. Acid-containing or acid functional monomers include: (meth)acrylic acid, styrene sulfonic acid, vinyl phosphonic acid, vinyl benzoic acid, maleic acid, fumaric acid, itaconic acid, 2- (meth)acrylamido 2-ethyl propanesulfonic acid, mono-2-((meth)acryloyloxy)ethyl succinate and ammonium sulfatoethyl (meth)acrylate. Zwitterionic monomers include: (meth)acryloyl oxyethylphosphoryl choline and betaines, such as [2- ((meth)acryloyloxy)ethyl] dimethyl-(3-sulfopropyl)ammonium hydroxide. Quaternised amino monomers include: (meth)acryloyloxyethyltri-(alkyl/aryl)ammonium halides such as (meth)acryloyloxyethyltrimethyl ammonium chloride.

Vinyl acetate and derivatives thereof may also be utilised.

Oligomeric and polymeric monomers include: oligomeric and polymeric (meth)acrylic acid esters such as mono(alk/aryl)oxypolyalkyleneglycol(meth)acrylates and mono(alk/aryl)oxypolydimethyl-siloxane(meth)acrylates. These esters include for example: monomethoxy oligo(ethyleneglycol) mono(meth)acrylate, monomethoxy oligo(propyleneglycol) mono(meth)acrylate, monohydroxy oligo(ethyleneglycol) mono(meth)acrylate, monohydroxy oligo(propyleneglycol) mono(meth)acrylate, monomethoxy poly(ethyleneglycol) mono(meth)acrylate, monomethoxy poly(propyleneglycol) mono(meth)acrylate, monohydroxy poly(ethyleneglycol) mono(meth)acrylate and monohydroxy poly(propyleneglycol) mono(meth)acrylate. Further examples include: vinyl or allyl esters, amides or ethers of pre-formed oligomers or polymers formed via ring-opening polymerisation such as oligo(caprolactam), oligo(caprolactone), poly(caprolactam) or poly(caprolactone), or oligomers or polymers formed via a living polymerisation technique such as poly(1 ,4- butadiene).

The corresponding allyl monomers to those listed above may also be used where appropriate.

Examples of monofunctional monomers are:

Amide-containing monomers such as (meth)acrylamide, N-(2- hydroxypropyl)methacrylamide, N,N'-dimethyl(meth)acrylamide, N and/or N'-di(alkyl or aryl) (meth)acrylamide, N-vinyl pyrrolidone, [3-((meth)acrylamido)propyl] trimethyl ammonium chloride, 3-(dimethylamino)propyl(meth)acrylamide, 3-[N-(3- (meth)acrylamidopropyl)-N,N-dimethyl]aminopropane sulfonate, methyl

(meth)acrylamidoglycolate methyl ether and N-isopropyl(meth)acrylamide; (Meth)acrylic acid and derivatives thereof such as (meth)acrylic acid, (meth)acryloyl chloride (or any halide), (alkyl/aryl)(meth)acrylate; functionalised oligomeric or polymeric monomers such as monomethoxy oligo(ethyleneglycol) mono(meth)acrylate, monomethoxy oligo(propyleneglycol) mono(meth)acrylate, monohydroxy oligo(ethyleneglycol) mono(meth)acrylate, monohydroxy oligo(propyleneglycol) mono(meth)acrylate, monomethoxy poly(ethyleneglycol) mono(meth)acrylate, monomethoxy poly(propyleneglycol) mono(meth)acrylate, monohydroxy poly(ethyleneglycol) mono(meth)acrylate, monohydroxy poly(propyleneglycol) mono(meth)acrylate, glycerol mono(meth)acrylate and sugar mono(meth)acrylates such as glucose mono(meth)acrylate;

vinyl amines such as aminoethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, diisopropylaminoethyl (meth)acrylate, mono-t- butylamino (meth)acrylate, morpholinoethyl(meth)acrylate; vinyl aryl amines such as vinyl aniline, vinyl pyridine, N-vinyl carbazole, N-vinyl imidazole, and monomers which may be post-reacted to form amine groups, such as N-vinyl formamide; vinyl aryl monomers such as styrene, vinyl benzyl chloride, vinyl toluene, cc-methyl styrene, styrene sulfonic acid, vinyl naphthalene and vinyl benzoic acid; vinyl hydroxyl monomers such as hydroxyethyl (meth)acrylate, hydroxy propyl (meth)acrylate, glycerol mono(meth)acrylate or monomers which may be post- functionalised into hydroxyl groups such as vinyl acetate, acetoxy styrene and glycidyl (meth)acrylate; acid-containing monomers such as (meth)acrylic acid, styrene sulfonic acid, vinyl sulfonic acid, vinyl phosphoric acid, 2-acrylamido 2-methylpropane sulfonic acid, vinyl phosphonic acid, vinyl benzoic acid, maleic acid, fumaric acid, itaconic acid, 2- (meth)acrylamido 2-ethyl propanesulfonic acid and mono-2-((meth)acryloyloxy)ethyl succinate or acid anhydrides such as maleic anhydride; zwitterionic monomers such as (meth)acryloyl oxyethylphosphoryl choline and betaine-containing monomers, such as [2-((meth)acryloyloxy)ethyl] dimethyl-(3- sulfopropyl)ammonium hydroxide; quaternised amino monomers such as (meth)acryloyloxyethyltrimethyl ammonium chloride.

Vinyl acetate or vinyl butanoate or derivatives thereof.

The corresponding allyl monomer, where applicable, may also be used in each case. Functional monomers, that is monomers with reactive pendant groups which can be pre or post-modified with another moiety following polymerisation may also be used such as for example glycidyl (meth)acrylate, tri(alkoxy)silylalkyl(meth)acrylates such as trimethoxysilylpropyl(meth)acrylate, (meth)acryloyl chloride, maleic anhydride, hydroxyalkyl (meth)acrylates, (meth)acrylic acid, vinylbenzyl chloride, activated esters of (meth)acrylic acid such as N-hydroxysuccinamido (meth)acrylate and acetoxystyrene.

Macromonomers (monomers having a molecular weight of at least 1000 Daltons) are generally formed by linking a polymerisable moiety, such as a vinyl or allyl group, to a pre-formed monofunctional polymer via a suitable linking unit such as an ester, an amide or an ether. Examples of suitable polymers include: monofunctional poly(alkylene oxides) such as monomethoxy[poly(ethyleneglycol)] or monomethoxy[poly(propyleneglycol)], silicones such as poly(dimethylsiloxane)s, polymers formed by ring-opening polymerisation such as poly(caprolactone) or poly(caprolactam) or mono-functional polymers formed via living polymerisation such as poly(1 ,4-butadiene).

Preferred macromonomers include: monomethoxy[poly(ethyleneglycol)] mono(methacrylate), monomethoxy[poly(propyleneglycol)] mono(methacrylate) and mono(meth)acryloxypropyl-terminated poly(dimethylsiloxane).

Hydrophilic monofunctional monomers include: (meth)acryloyl chloride, N- hydroxysuccinamido (meth)acrylate, styrene sulfonic acid, maleic anhydride, (meth)acrylamide, N-(2-hydroxypropyl)methacrylamide, N-vinyl pyrrolidinone, N-vinyl formamide, quaternised amino monomers such as (meth)acrylamidopropyl trimethyl ammonium chloride, [3-((meth)acrylamido)propyl]trimethyl ammonium chloride and (meth)acryloyloxyethyltrimethyl ammonium chloride, 3-[N-(3-

(meth)acrylamidopropyl)-N,N-dimethyl]aminopropane sulfonate, methyl

(meth)acrylamidoglycolate methyl ether, glycerol mono(meth)acrylate, monomethoxy and monohydroxy oligo(ethylene oxide) (meth)acrylate, sugar mono(meth)acrylates such as glucose mono(meth)acrylate, (meth)acrylic acid, vinyl phosphonic acid, fumaric acid, itaconic acid, 2-(meth)acrylamido 2-ethyl propanesulfonic acid, mono- 2-((meth)acryloyloxy)ethyl succinate, ammonium sulfatoethyl (meth)acrylate, (meth)acryloyl oxyethylphosphoryl choline and betaine-containing monomers such as [2-((meth)acryloyloxy)ethyl] dimethyl-(3-sulfopropyl)ammonium hydroxide.

Hydrophilic macromonomers may also be used and include: monomethoxy and monohydroxy poly(ethylene oxide) (meth)acrylate and other hydrophilic polymers with terminal functional groups which may be post-functionalised with a polymerisable moiety such as (meth)acrylate, (meth)acrylamide or styrenic groups.

Hydrophobic monofunctional monomers include: Ci to C ₂₈ alkyl (meth)acrylates (linear and branched) and (meth)acrylamides, such as methyl (meth)acrylate and stearyl (meth)acrylate, aryl(meth)acrylates such as benzyl (meth)acrylate, tri(alkyloxy)silylalkyl(meth)acrylates such as trimethoxysilylpropyl(meth)acrylate, styrene, acetoxystyrene, vinylbenzyl chloride, methyl vinyl ether, vinyl formamide, (meth)acrylonitrile, acrolein, 1 - and 2-hydroxy propyl (meth)acrylate, vinyl acetate, 5- vinyl 2-norbornene, Isobornyl methacrylate and glycidyl (meth)acrylate. Hydrophobic macromonomers may also be used and include: monomethoxy and monohydroxy poly(butylene oxide) (meth)acrylate and other hydrophobic polymers with terminal functional groups which may be post-functionalised with a polymerisable moiety such as (meth)acrylate, (meth)acrylamide or styrenic groups.

Responsive monofunctional monomers include: (meth)acrylic acid, 2- and 4-vinyl pyridine, vinyl benzoic acid, N-isopropyl(meth)acrylamide, tertiary amine (meth)acrylates and (meth)acrylamides such as 2-(dimethyl)aminoethyl (meth)acrylate, 2-(diethylamino)ethyl (meth)acrylate, diisopropylaminoethyl (meth)acrylate, mono-t-butylaminoethyl (meth)acrylate and N-morpholinoethyl (meth)acrylate, vinyl aniline, 2- and 4-vinyl pyridine, N-vinyl carbazole, vinyl imidazole, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, maleic acid, fumaric acid, itaconic acid and vinyl benzoic acid.

Responsive macromonomers may also be used and include: monomethoxy and monohydroxy polypropylene oxide) (meth)acrylate and other responsive polymers with terminal functional groups which may be post-functionalised with a polymerisable moiety such as (meth)acrylate, (meth)acrylamide or styrenic groups.

Monomers based on styrene or those containing an aromatic functionality such as styrene, a-methyl styrene, vinyl benzyl chloride, vinyl naphthalene, vinyl benzoic acid, N-vinyl carbazole, 2-, 3- or 4- vinyl pyridine, vinyl aniline, acetoxy styrene, styrene sulfonic acid, vinyl imidazole or derivatives thereof may also be used.

Preferred monomers:

For Anion Exchange Membranes it is essential that the final membrane possess a net positive charge. In order to achieve this net positive charge the membrane may be formed from the reaction of a nitrogenous monomer (A) with a dihalo species (B). Alternatively, the net positive charge may be formed by the reaction of a polymer comprising a halide (C), or equivalent, leaving group and a nucleophilic amino compound (D). Examples of (A), (B), (C) and (D) are listed below: Examples of (A) include: 2-vinyl pyridine, 4-vinyl pyridine, N-vinyl imidazole, N-vinyl carbazole, dimethylaminoethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminopropyl (meth)acrylamide. Preferred non-hydrolysable monomers are 2- vinyl pyridine, 4-vinyl pyridine, N-vinyl imidazole and N-vinyl carbazole. Most preferred is 4-vinyl pyridine.

Examples of (B) include: α,ω-dihalodoalkanes such as α,ω-diiodoalkanes such as 1 ,4-diiodobutane, 1 ,5-diiodopentane, 1 ,6-diiodohexane, 1 ,7-diiodoheptane, 1 ,8- diiodooctane, 1 ,9-diiodononane and 1 ,10-diiododecane; α,ω-dibromoalkanes such as 1 ,4-dibromobutane, 1 ,5-dibromopentane, 1 ,6-dibromohexane, 1 ,7- dibromoheptane, 1 ,8-dibromooctane, 1 ,9-dibromononane and 1 ,10-dibromodecane; α,ω-dichloromoalkanes such as 1 ,4-dichlorobutane, 1 ,5-dichloropentane, 1 ,6- chlorohexane, 1 ,7-dichloroheptane, 1 ,8-dichlorooctane, 1 ,9-dichlorononane and 1 ,10-dichlorodecane. Most preferred are dibromoalkanes such as 1 ,5-diiodopentane and diiodoalkanes such as 1 ,8-diiodooctane. Most preferred are dibromoalkanes such as 1 ,5-diiodopentane and diiodoalkanes such as 1 ,8-diiodooctane.

Examples of (C) include: Vinyl benzyl chloride,

Examples of (D) include: α,ω-Tertiarydiaminoalkanes such as:

N,N,N',N'-tetramethyl-1 ,6-hexane diamine, N,N'-diisopropylethylenediamine,

4,4'-bis(dimethylamino)benzophenone and bis[2-(dimethylamino)ethyl]ether.

For cation-exchange membranes (CEMs) a net negative charge is required.

This is generally obtained by using a monomer with a permanent negative charge, due to issues with divalent ion fouling this monomer is generally a sulfonic acid such as for example: vinylsulfonic acid, styrene sulfonic acid or 2-acrylamido 2- methylpropanesulfonic acid. Additionally, CEMs are normally prepared by preparing a polymer with anionic functionality followed by membrane casting via a further crosslinking step. For example, the CEM may be prepared by the reaction of a diisocyanate with a hydroxyl functionality present in the polymer. Crosslinking may also be achieved by crosslinking a sulfonic acid unit through the formation of for example, a sulfonamide. Additionally Hydrophobic monomers can be used.

The multifunctional monomer (also known as a brancher) may comprise a molecule containing at least two vinyl groups which may be polymerised via addition polymerisation. The molecule may be hydrophilic, hydrophobic, amphiphilic, neutral, cationic, zwitterionic, oligomeric or polymeric. Such molecules are often known as cross-linking agents in the art and may be prepared by reacting any di- or multifunctional molecule with a suitably reactive monomer. Preferably the multifunctional monomer or brancher is hydrolytically stable, and as such avoids labile ester functionalities. Examples include: di- or multivinyl esters, di- or multivinyl amides, di- or multivinyl aryl compounds, di- or multivinyl alk/aryl ethers. Typically, in the case of oligomeric or polymeric di- or multifunctional branching agents, a linking reaction is used to attach a polymerisable moiety to a di- or multifunctional oligomer or polymer. The brancher may itself have more than one branching point, such as T- shaped divinylic oligomers or polymers. In some cases, more than one multifunctional monomer may be used.

The corresponding allyl monomers to those listed above may also be used where appropriate.

Preferred multifunctional monomers or branchers include but are not limited to: divinyl aryl monomers such as divinyl benzene; (meth)acrylate diesters such as and 1 ,3-butylenedi(meth)acrylate; trimethylolpropane tri(meth)acrylate, 1 ,3,5-triallyl-1 ,3,5- triazine-2,4,6(1 H, 3H,5H)trione and bisacrylamide.

Statistical copolymers.

In relation to the present invention, the polymers prepared and used comprise statistical copolymers. That is, a copolymer is a polymer derived from two (or more) monomeric monomers or residues. Since a copolymer comprises at least two types of monomer units / residues, copolymers may be classified according to how these monomers or residues are arranged in a polymer chain. These include: 1 . Alternating copolymers with regular alternating units of monomers A and B, that is, copolymers with A and B monomer units arranged in a repeating sequence for example, (A-B-A-B-B-A-A-A-A-B-B-B) _n

2. Statistical copolymers are copolymers in which the sequence of monomer units or residues follows a statistical rule. That is, if the probability of finding a given type of monomer residue at a particular point in the chain is equal to the mole fraction of that monomer residue in the chain, then the polymer may be referred to as a truly random copolymer.

The statistical distribution depends upon the likelihood of whether a monomer residue will react with another monomer residue of the same type or one of a different type. The statistical distribution of monomers in a copolymer is therefore a function of the reactivity ratios of the actual monomer residues and is described by the Mayo equation:

The Mayo-Lewis equation or copolymer equation in polymer chemistry describes the distribution of monomers in a copolymer:

Taking into consideration a monomer composition comprising two components M and M ₂ , there are four different reactions that may take place at a reactive chain end terminating in either monomer (M ^* ) with their reaction rate constants k:

the copolymer equation is given as Equation 1

Equation 1 : with the concentration of the components given in square brackets.

Equation 1 provides the copolymer composition at any instant during the polymerization.

3. Block copolymers comprise two or more homopolymer subunits linked by covalent bonds (4). The union of the homopolymer subunits may require an intermediate non-repeating subunit, known as a junction block. Block copolymers with two or three distinct blocks are called diblock copolymers and triblock copolymers, respectively.

From Equation 1 several limiting cases may be derived: r-i = r ₂ » 1 , with both reactivity ratios very high the two monomers M _; M ₂ have no inclination to react to each other except with themselves leading to a mixture of two homopolymers.

r-i = r ₂ > 1 , with both ratios larger than 1 , homopolymerization of component Mi is favoured (but in the event of a crosspolymerization by M ₂ the chain-end will continue as such giving rise to a block copolymer)

ri = r ₂ ~ 1 , with both ratios around 1 , Mi will react as fast with another Mi or M ₂ and a random copolymer results.

r-i = r ₂ ~ 0, with both values approaching 0 the monomers are unable to react in homopolymerization and the result is an alternating polymer.

r-ι» 1 » r ₂ , In the initial stage of the copolymerization monomer 1 is incorporated faster and the copolymer is rich in M-i . When this monomer gets depleted, more M ₂ segments are added. This is called composition drift. In terms of this invention, the term hydrolytic stability means that the membrane has a low tendency to lose selectivity at alkaline pH as result of hydrolysis taking place. If cationic groups (ammonium or pyridinium) present on the membrane are susceptible to attack by hydroxide ions then the cationic groups may be lost from the polymer (as a result of nucleophilic substitution or elimination reactions) thus reducing the overall (cationic) charge on the membrane (that is, the ion exchange capacity, I EC mmol/g) and hence its selectivity toward anionic species.

This effect also manifests as an increase in electrical resistance. It is known that even anion exchange membranes made from very hydrolytically stable backbones (like fluorocarbon based monomers) may still be unstable towards attack by hydroxide ions. It is known that thermal and chemical stability of cationic moieties increase across the series from sulfonium<phosphonium< ammonium.

A further measure of hydrolytic stability may be the assessment of the chemical stability (to hydrolysis) of the overall membrane structure to extremes of pH; that is, whether the membrane film itself is stable to exposure to such conditions or becomes mechanically weakened or disintegrates. This second indicator is relevant when the membrane already retains acceptable performance under such conditions.

The copolymers prepared and used in the membranes according to the present invention have a branched structure which is more compact. Therefore, when the copolymers are reacted to form a crosslinked membrane, the membrane comprises a dense and compact copolymer network with an increased concentration of functional groups. The nature of the functional groups ensures that the polymers have a reduced tendency to swelling and are therefore less susceptible to hydrolysis.

EXAMPLES

The present invention will now be explained in more detail by reference to the following non-limiting examples.

In the following examples, copolymers are described using the following nomenclature: (Monomer G) _g (Monomer J)j (Brancher L)i (Chain Transfer Agent) _d wherein the values in subscript are the molar ratios of each constituent normalised to give the monofunctional monomer values as 100, that is, g + j = 100. The degree of branching or branching level is denoted by I and d refers to the molar ratio of the chain transfer agent.

It is preferred that the molar ratio of chain transfer agent to multifunctional monomer is from 2 : 1 to 1 to 2. Most preferably the molar ratio of chain transfer agent to multifunctional monomer is from 1 .2 : 1 to 1 to 1 .2.

For example:

4-Vinyl pyridine ₅ o Styrene ₅ o Ethylene glycol dimethacrylate-i ₅ Dodecane thiol i ₅ would describe a polymer containing 4 vinyl pyridine : Styrene : Ethylene glycol dimethacrylate : dodecane thiol at a molar ratio of 50:50:15:15.

Abbreviations:

Monomers:

EHMA - 2-Ethylhexyl methacrylate

HEMA - 2-Hydroxyethyl methacrylate

HPMA - 2-Hydroxypropyl methacrylate

St-Styrene

VBC - 4-Vinylbenzyl chloride

4-VPy - 4-Vinylpyridine

Branchers:

DVB - Divinylbenzenes

(EGDMA - Ethyleneglycol dimethacrylate

Chain Transfer Agents (CTAs)

DDT - Dodecanethiol

DMP - 2,4-diphenyl-4-methyl-1 -pentene

Initiators

Luperox® Dl - diisobutyl peroxide Luperox® P - f-Butyl peroxybenzoate Solvents

THF - Tetrahydrofuran

BuOAc - n-Butyl acetate

Xyl - Xylene

Hardeners

BDMAE = Bis[2-(N,N-dimethylamino)ethyl]ether,

BDMAH = 1 ,6-Bis(N.N-dimethylamino)hexane,

DIH = 1 ,6-Diiodohexane

All materials were obtained from the Aldrich Chemical Company with the exception of Luperox® P and Dl which were obtained from Arkema Chemical Company.

Triple Detection-Size Exclusion Chromatography was performed on a Viscotek triple detection instrument. The columns used were two ViscoGel HHR-H columns and a guard column with an exclusion limit for polystyrene of 10 ⁷ g.mol ^"1 .

THF was the mobile phase, the column oven temperature was set to 35 ^Q C, and the flow rate was 1 mL.min ^"1 . The samples were prepared for injection by dissolving 10 mg of polymer in 1 .5 ml_ of HPLC grade THF and filtered of with an Acrodisc® 0.2 μιη PTFE membrane. 0.1 ml_ of this mixture was then injected, and data collected for 30 minutes. The Omnisec software package was used to collect and process the signals transmitted from the detectors to the computer and to calculate the molecular weight.

General procedure:

Into a three-necked round bottom flask fitted in a DrySyn® Vortex overhead stirrer system and equipped with a condenser the required monomers and solvent were introduced. The solution was then degassed for one hour by bubbling nitrogen through it. The solution was then heated to the appropriate temperature and stirred at 320 revolutions per minute (rpm). Once the expected temperature had been reached, the initiator was added and the reaction was allowed to proceed for between 15 and 50 hours until the monomer conversion was found to be greater than 99% (measured by ¹ H NMR). The reaction mixture was cooled to room temperature and poured into a vessel. The polymers were characterised by Triple Detection-Size Exclusion Chromatography (TD-SEC).

Example 1 - 4-Vinylpyridine-containing polymer for use as an AEM.

4-Vinyl pyridine (10 g, 95.1 mmol), styrene (3.96g, 38.0 mmol), 2-hydroxyethyl methacrylate (7.42g, 57.0 mmol), divinyl benzene (2.48g, 19.05 mmol) and dodecane thiol (4.63g, 22.8 mmol) and were weighed in to 250 ml_ round bottomed flask, then a solvent, butyl acetate (66.47 g), was added and a condenser fitted before the system was flushed with nitrogen. Heat was applied and the flask was brought to reflux. Luperox® P (0.887g, 4.6 mmol) was then added in a single portion. Reflux was continued for seven hours then the temperature was reduced (to 1 15 °C) and a second aliquot of initiator added (0.22 g = 0.5 mol%) and the temperature maintained at 1 15 °C for a further 12 hours. The conversion at his stage was found to be 99% and once cooled the resultant dark brown solution was collected.

Table 1 below details the nature of the polymers tested in connection with the present application for soluble copolymers derived from 4-vinyl pyridine

13 50 25 25 - - 9 - 1 1 - 50

14 50 50 - - - - 5 - 10 50

15 50 50 - - - - 5 - 10 50

16 50 50 - - - - 5 - 10 50

17 50 50 - - - 5 - 10 50

18 50 50 - - - - 5 - 10 50

19 50 50 - - - - 5 - 10 50

20 64.5 35.5 - - - - 5 - 10 50

^* refers to the concentration of polymer components in solvent - butyl acetate (1 % weight/weight) during the polymerisation process.

Example 2 :Vinylbenzyl chloride-containing polymers for AEMs

Method: The previously described synthetic procedure was followed, using xylene as the solvent throughout and at 50 % weight/weight solids concentration.

19 6 100 50 900 8.3 0.5

20 5 800 1 1 100 1 .9 0.36

Mn/Da - Number average molecular weight

Mw/Da - Weight average molecular weight

PD - Mw/Mn - Polydispersity index

a - Mark-Houwink exponential factor

# - Theoretical CI value from monomer composition.

na - not available

Mark Houwink Equation

The Mark Houwink Equation relates to relationship between intrinsic viscosity and molecular weight and is specific to each polymer and solvent pairing.

Thus,

[η] = ΚΜ ^α Equation 2 where

[η] - is intrinsic viscosity;

M - is molecular weight (viscosity average)

K and a are the Mark-Houwink constants, the values of which are dependent on the type of polymer and solvent used as well as temperature at which the viscosity measurements were made. The Mark-Houwink constants are determined from a graph of log[r|] versus log M which provides a straight line with gradient equal to a and an intercept (y-axis) equal to log K.

The a value is a function of the polymer geometry and typical value ranges are as follows:

0 to 0.1 = sphere

0.35 to 0.80 = random coil

1 .5 to 2 = rigid rod structure.

Example 2 :Vinylbenzyl chloride-containing polymers for AEMs

Method: The previously described synthetic procedure was followed, using xylene solvent throughout and at 50 % weight/weight solids concentration.

Table 3 - Polymer examples based on vinylbenzyl chloride

# - Theoretical CI value from monomer composition.

Anion Exchange Membranes - AEMs

Preparation of the membranes

The solution of polymers 8 and 9 were concentrated (evaporation under vacuum) until solids content of the solution was 60.0 wt%. To this was added the hardener (crosslinker) 0.45 molar equivalents (with respect to the total moles of chlorine groups in the copolymer; that is to say, sufficient to react with 90 mol% of the total chloromethyl groups). The hardener was thoroughly mixed in to the polymer solution and a membrane or film of the mixture drawn over a piece of fabric mesh reinforcement (Sefar Petex 07/240-59) supported on a polished stainless steel plate. Once dry, the membrane films were cured in an oven at 60 °C for 14 hours then cooled and released from the stainless steel backing plates by immersion in a mixture of water and isopropanol (80:20 parts by volume) for one hour. The reinforced membranes were robust enough to be handled wet or could be allowed to air dry.

Membrane characterization

Electrical Resistance

The membrane under test was placed in a cell consisting of two measuring Haber- Lugin capillary electrodes placed adjacent to the membrane in order to measure the potential drop as a function of current density. The outer chambers contained the working electrodes and were circulated with 0.5 M sodium sulfate (Na ₂ S0 ) solution. Both buffer chambers adjacent to the electrodes contained 0.5 M sodium chloride (NaCI) solution to protect the inner chambers from the acid produced at the electrodes. The inner chambers were circulated with a different batch of 0.5 M NaCI. In these chambers, the two shielding and the two electrode compartments were paired to keep the concentration in the compartments constant.

A current was placed across the cell and the limiting current density (LCD) was measured by following the increase in resistance with increasing current density. The electrical resistance of the membrane was determined in relation to the limiting current density LCD

Permselectivitv

In addition to membrane resistance, the selectivity of the membranes is an important feature with respect to the efficiency of the process to which the membrane is applied. The permselectivity of the membrane can be determined via different methods like chronopotentiometry, Nernst potential and limiting current density (LCD) ratio. The inventors employed the Nernst potential method in this application. The permselectivity of the membranes was determined using a cell consisting of two compartments fitted with two Ag/AgCI reference electrodes separated by the membrane under test. Potassium chloride (KCI) 0.50 M was circulated through one chamber and potassium chloride (KCI) 0.10 M was circulated through the other chamber at 25 °C. Using potassium chloride (KCI) to measure the membrane potential ensured that no liquid polarization occurs as potassium ions (K ⁺ ) and chloride ions (CI-) have similar diffusion coefficients in water. The potential and the measured electrical potential can be linked directly to the apparent selectivity via equation 1 :

Ψ" ¹ = [ φ / φ '] x 100% Equation 3 wherein:

Ψ™ is the apparent permselectivity and φ and φ' are the measured and ideal electrical (Nernst) potential difference.

Using 0.50 M and 0.10 M potassium chloride (KCI) provides a theoretical voltage drop (<p) of 36.94 MV.

Table 4 - Test data for supported membranes

Measurement of hvdrolvtic stability- Membranes (4-VPv/DVB)

All membranes were prepared according to a standard protocol (that is, theoretical max quaternisation reaction of N groups = 95%: meaning a molar ratio of N : alkyl halide functionality of 1 : 0.95) using 1 ,6-diiodohexane (DIH) as the hardener (crosslinker). Films of the polymer / hardener mixture were then drawn down on to a Petex 07-240/59 mesh fabric (woven PET) and the membranes cured at 65 °C in an oven overnight.

The membranes were cast using the same method as has been described for the VBC based membranes (M1 to M4).

Hvdrolvtic stability

In order to determine the stability of the membranes at high pH a sample of the membrane was equilibrated for 18 hours in pH 14 (NaOH in demineralised water) at room temperature. The sample was then rinsed thoroughly with demineralised water and the permselectivity and electrical resistance of the sample compared to that obtained at pH 7. The results of the hydrolytic stability tests are shown in Table 5. Table 5 - Test data for supported membranes with improved hydrolytic stability

The permselectivity was determined via the Nernst potential at 25 °C between 0.1 0 M and 0.50 M KCI and pH = 7.0; for results at pH = 14.0, 1 .0 M sodium hydroxide was added to adjust the pH prior to commencing the measurements.

Electrical resistance was determined in 0.5 M NaCI at 25 °C and pH = 7.0; for results at pH = 14.0, 1 .0 M sodium hydroxide was added to adjust the pH prior to commencing the measurements.

The above data shows that an increase in pH (from 7 to 14) results in a loss in permselectivity for all the membranes. However, this loss is largest in the case of membrane M5 (falling from 76% to 39% which represents a drop of 49%). Under identical conditions, the membranes M6 and M7 show a drop of only 29% and 41 % respectively. A loss of permselectivity is known to be attributable to a loss of positively charged groups within the membrane, which is caused by hydrolysis of such groups when the membrane is exposed to a high pH (alkaline) environment.

Membranes M5, M6 and M7 were prepared from soluble copolymers comprising very similar molar quantities of nitrogen (amine) per gram (of dry copolymer), which are reacted with similar molar quantities of DIH crosslinker, therefore, it is assumed that the differences in permselectivity loss observed when the pH is increased must be due to the differences in the copolymer compositions themselves. Membrane M5 was obtained from a copolymer comprising the ester-containing multifunctional monomer residue EGDMA; whereas M6 and M7 were obtained from copolymers which have DVB as the multifunctional monomer: the absence of ester linkages in DVB means that the copolymers derived from this multifunctional monomer are less susceptible to hydrolysis and thus the membranes made from such copolymers are more stable to alkaline conditions than those containing an ester derived multifunctional monomer residue such as M5. Hence membranes such as M6 and M7 have improved in-use stability at high pH.

It can be seen that the membrane resistance increases with pH which is due to a lower protonation of the amine groups and therefore lower polymer cationic character at high pH.

Therefore, the branched addition copolymers of the present invention have been designed to contain non-hydrolysable units, that demonstrate an increased Ίη-use stability' where the membrane is being used at extremes of pH in an aqueous environment. A technical effect that has not previously been seen for ion-selective membranes prepared from soluble branched addition copolymers.