Poly(phenylene)-based anion exchange membrane

#The present invention relates to anion exchange membranes and, more particularly, to anion exchange membranes based on a poly(phenylene) backbone prepared by a Diels-Alder polymerization reaction.

#Anion exchange membranes (AEMs) have been used mainly in electrodialysis for the desalination of brackish water and for the production of table salt from sea water. Interest has grown in more demanding applications such as alkaline fuel cells and electrolysis. Hydrogen fueled alkaline fuel cells consisting of a liquid electrolyte (KOH) have demonstrated the ability to generate high power densities. The main advantage of the alkaline media as compared to other low temperature fuel cells is that the kinetics at the cathode oxygen reduction reaction and anode process are much more facile. In addition many non-noble metals and their corresponding oxides are stable in alkaline media. For these reasons platinum based catalysts are not needed and non-noble metal catalysts can be employed. Other technical issues have prevented the alkaline liquid electrolyte fuel cell from being a viable commercial power generator. Many of these issues are associated with the liquid electrolyte. For example, ultra pure oxidants must be used to prevent electrode degradation caused by the formation of carbonate species. Liquid electrolyte cells (and particularly the circulated electrolyte ones) were shown to be complex in design and operation. Over the last decade attempts have been made to develop alkaline anion-exchange membranes and eliminate the issues related to the liquid electrolyte. The main concern with alkaline anion-exchange membranes is the long term stability. Under operation at elevated temperatures membrane degradation occurs by beta-hydrogens undergoing the Hoffmann elimination reaction.

#Useful would be a robust alkaline anion exchange membrane immune to such degradation. Unlike cation-exchange membranes where highly durable perfluorinated membranes dominate the market, commercially-available AEMs are typically based on crosslinked polystyrene and are not very stable in alkaline environments. In addition, they are generally blended with other inert polymers and fabric supports that limit their ionic conductivities. Thus there is a need to develop new AEMs that not only have high conductivities and ion selectivities, but that also exhibit excellent chemical stability at high pH values and elevated temperatures.

#The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one or more embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating one or more preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings:

#FIG. 1 shows a structural representation of the parent poly(phenylene) used to make the anion exchange membranes.

#FIG. 2 shows the reaction of two oligomers to form the multiblock copolymer of the present invention.

#The present invention is an anion exchange membrane based on the parent poly(phenylene) as depicted in FIG. 1. The ratio of m to n in FIG. 1 can be varied from zero to nine and the copolymers can be prepared with either random or multiblock structures. When m:n is zero, the resulting poly(phenylene) is a homopolymer in which the repeat unit has a structure comprised of four sequentially connected phenyl rings with a total of 2 pendant phenyl groups and 4 pendant tolyl groups. When m:n is not zero, the resulting poly(phenylene) is a copolymer in which one of the repeat units has the structure previously described and the other repeat unit has a structure comprised of four sequentially connected phenyl rings with a total of 6 pendant phenyl groups. Each R group can be a group selected from R═CH3, CH2Br, and CH2N(CH3)3Br. When at least one R group is CH2N(CH3)3Br, the material functions as an anion exchange membrane. The pendant aryl rings are bulky and prevent the chains from packing closely together in the solid state. These factors result in space between the polymer chains which can hold water molecules when the membranes made from these polymers becomes hydrated. The greater water uptakes lead to the higher ionic conductivities.

#As shown in FIG. 2, the multiblock copolymer of the present invention is synthesized from two oligomers, one oligomer, referred to as Compound 1, having cyclopentadienone end groups and the other oligomer, referred to as Compound 2, having ethynyl end groups. The Compound 1 oligomer has a repeat unit a structure comprised of four sequentially connected phenyl rings with a total of 6 pendant phenyl groups and it has cyclopentadienone end groups. The Compound 1 oligomer has an average number of m repeat units where 20≧m≧7. The Compound 2 oligomer has a repeat unit a structure comprised of four sequentially connected phenyl rings with a total of 2 pendant phenyl groups and 4 pendant tolyl groups and it has ethynyl end groups. The Compound 2 oligomer can have an average number of n repeat units where 18≧n≧6. These two oligomers are combined at elevated temperature to produce the Compound 3 block copolymer shown in FIG. 2 where /b/ designates the junction of two different blocks rather than two random repeat units.

#The synthesis of the parent poly(phenylenes) of the present invention can be initiated from a variety of precursor compounds. In the general synthesis, the first step is to prepare via a known synthetic method a bis(cyclopentadienone) Compound 4 by a condensation reaction of a dibenzylketone with a bisbenzil compound (see FIG. 3) as a preparation for making a monomer for the parent poly(phenylene). In this preparation, every R1 designated on the dibenzylketone and Compound 4 structures can be either a hydrogen atom or a methyl (CH3) group. The poly(phenylene) Compound 5a is generally synthesized via a Diels-Alder reaction as shown in FIG. 4. In one embodiment R1═CH3. In another embodiment R1═H. In another embodiment, random copolymers have been produced using a mixture of monomers where R1═H on some of the bis(cyclopentadienone)s and R1═CH3 on the remaining bis(cyclopentadienone)s. The Diels-Alder polymerization to form Compound 5a was conducted by heating equimolar amounts of Compound 4 and 1,4-diethynylbenzene in diphenyl ether with the loss of one carbon monoxide molecule for each Diels-Alder reaction. It is this loss of CO and the accompanying formation of a highly stable aromatic ring that makes the polymerization irreversible. The product, a Diels-Alder poly(phenylene), is a tough, off-white thermoplastic. All prepared samples could be cast into clear, creasible films.