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Fuel Cell Membranes Based on Functional Fluoropolymers

Final Report Summary - FLUPOL (Fuel cell membranes based on functional fluoropolymers)

Fluoropolymers are performance materials with unique properties. Key characteristics are their advantageously high thermostability and chemical inertness combined with low refractive index, friction coefficient, dielectric constant, dissipation factor as well as water absorptivity. These high value products have therefore found applications in various fields of advanced technology (e.g. chemical and automobile industries, engineering, microelectronics, optics, textile finishing and aeronautics). Fluorinated aromatic polymers belong to a highly desirable sub-category of fluorinated polymers but only few examples of copolymerisation of fluorine-functionalised aromatic monomers with fluoroolefins have been reported to date.

With the objective to prepare new functional fluoropolymers for fuel cell membranes, we have developed convenient and cost-effective routes to access various non-commercially available fluorinated styrenic monomers of increasing fluorine content. The a-monofluoromethyl styrene I (FMST), a-difluoromethyl styrene II (DFMST) and a-trifluoromethyl styrene III (TFMST) either unsubstituted at the aryl group or possessing a bromo substituent on the para position, were identified as prime candidates for further studies.

Polymerisation of these monomers could deliver material suitable for post-modification (e.g. sulfonylation or phosphonylation). Alternatively if this strategy suffers from the lack of control over the degree and location of functionalisation, these monomers can be manipulated prior to polymerisation.

The monofluorinated monomers 3a and 3b were prepared using a simple two-step process involving a palladium cross-coupling reaction of allyltrimethylsilane with corresponding triflates followed by a key fluorodesilylation (SE2') of the functionalised allylsilanes with selectfluor in acetonitrile.

Studies aimed at probing the efficiency of this synthetic route for the preparation of larger quantities of material (up to 40 and 30 mmol scale) indicated that the overall yields for this sequence decreased significantly, with both the synthesis of allylsilane and the fluorodesilylation becoming more problematic.

The attempts to improve the synthesis of allylphenylsilane derivatives 2 using ionic liquid-promoted palladium catalysed coupling of halobenzenes with allyltrimethylsilane failed. Additionally a one-step direct electrophilic fluorination of a-methylstyrene with Selectfluor reported recently by Luo and Loh was evaluated on large-scale synthesis of 3a. Carried out on a 2 mmol scale, reaction was found reproducible this and delivered 3-fluoro-2-phenylpropene 3a in 75 % isolated yield. When using up to 38 mmol of a-methylstyrene, the fluorination led to the formation of significant amount of by-products identified as the isomerised (E)- and (Z)-fluoroalkenes. The subsequent purification proved difficult and this was directly reflected in the low isolated yield of the pure desired product 3a (35%). This route was therefore abandoned.

At this stage, the availability of both diethylaminosulphur trifluoride (DAST) and 4-tert-butyl-2,6-dimethylphenylsulphur trifluoride (Fluolead), two reagents suitable for nucleophilic fluorination, encouraged further work. Ionic liquid-promoted Pd-catalysed coupling of various halobenzenes with allylalcohol,9 Ni-catalysed cross coupling of alkoxide-containing vinyl halides with Grignard reagents10 or metal-free synthesis have been investigated due to prepare phenylallylic alcohols. The ultimately successful preparation began with aldehydes 4a and 4b.