Phosphonated polyphosphazene

Allcock et al. also have investigated the use of phosphonated polyphosphazenes as potential membrane materials for use in direct methanol fuel cells (Figure A2) Membranes were found to have lEC values between 1.17 and 1.43 mequiv/g and proton conductivities between 10 and 10 S/cm. Methanol diffusion coefficients for these membranes were found to be at least 12 times lower than that for Nafion 117 and 6 times lower than that for a cross-linked sulfonated polyphosphazene membrane. 

Figure 42. Synthesis of polyphosphazenes bearing phenyl phosphonic acid side groups.

Two methods have been presented for the phosphonation of poly(bro-moaryloxyphosphazenes), viz. via sodium dialkyl phosphite as reagent or via a treatment with Bu Li in combination with dialkyl chlorophosphate. The reactions are visualized by the product formation of (206) and (208) from (205) and (207), respectively. Polyphosphazenes with diphenyl phosphonate groups (209) have been synthesized by the reaction of the bromophenoxy substituted polymer (207) with Bu Li followed by a rapid addition of diphenyl chlorophosphate in thf. Only 50% of the available bromophenoxy groups are converted to diphenyl phosphonate esters groups. ... 

Phosphonated polymers have been proposed for fuel cells with the expectation of being thermally more stable and better retaining water than sulfonic groups [210, 211]. Phosphonated poly(phenylene oxide) [212], poly(4-phenoxy-benzoyl-l,4-phenylene) [213] and polysulfones [214, 215] have been reported. Phosphonated fluoromonomers were polymerized [164]. Characterization of phosphonated films in terms of their proton conductivity has been reported for some of the phosphonated polymers polyphosphazene [216], trifluoropolysty-rene [217], poly(4-phenoxybenzoyl-l,4-phenylene) [218]. Relatively low conductivity values were reported for most of the polymers prepared up to now. The values for polyphosphazene [216] and for perfluorocarbon polymers [219] were quite encouraging. Phosphonated poly(phenylene oxide) [211] was evaluated in fuel cell-tests. 

Two types of polyphosphazene (PPh) group such as sulfonated and phosphonated PPh have been developed recently for DMFC application (Gao and Sammes 1999 AUock et al. 2002 Zhou et al. 2003). Zhou et al. reported that the methanol crossover of the sulfonated membrane was about eight times lower than that of Nafion 117 at room tanperature even though the values were comparable at HO C (Zhou et al. 2003). The methanol permeability of the phosphonated PPh derivative was about 40 times lower than that of Nafion 117 at room temperature and about 9 times lower at 120 C. This is a significant improvement over the behavior of Nafion 117. Although the conductivities of both PPh membranes were lower than that of the Nafion 117 manbrane, they are still practical for DMFC applications since the selectivity of both sulfonated PPh (at temperature <85°C) and phosphonated PPh (at temperature 22°C-125°C) is higher than that of Nafion 117. 

Polyphosphazenes are flexible inorganic phosphorus-containing polymers which have been phosphonated and studied as electrolyte membranes. For example, brominated poly(aiylo g hosphazene) has been phosphonated via lithiation and reaction with diphenyl chlorophosphonate (Scheme 13.1h). Membranes cast from N,Af-dimethylformamide (DMF) had proton conductivities between 10 and 100 mS cm after equilibration in water. " ... 

A significant amoimt of research was devoted to post-functionahzation of aryloxy- or methyl-substituted polyphosphazenes. In the first case, electrophihc aromatic substitution reactions were used to obtain sulfonated (113-116), car-boxylated (117) nitrated (aminated) (118,119) and phosphonated (120,121) products (Fig. 7a). Alternatively, methyl substituents were deprotonated with n-BuLi and subsequently treated with a desired nucleophile (Fig. 7b). Using this method, polyphosphazenes bearing carboxylic (122), alcohol (123,124), ester (125), and other (63,126) groups became available. Numerous studies have been devoted... 

The synthesis of polyphosphazenes with phosphonic acid functionalities was studied by Allcock and co-workers [36-38]. In initial experiments [36], dialkyl phosphonate units were incorporated into poly[(aryloxy)phosphazenes] containing either bromomethanephenoxy or lithiophenoxy (via lithiation of bromophenoxy) side groups. Later, the approach was extended to poly[(aryl-... 

A new analytical approach involving time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to probe biomineralization process utilizing phosphoester/phosphonic acid containing polyphosphazenes (155) after exposure to stimulated body fluid. ... 

FIGURE 7.21 Synthesis of phenylphosphonic acid-substituted polyphosphazenes. (Reprinted from J. Membr. Sci., 201, Allcock, H.R., Hofmann, M.A., Ambler, C.M., Lvov, S.N., Zhou, X.Y., Chalkova, E., and Weston, J., Phenyl phosphonic acid functionalized poly[aryloxyphosphazenes] as proton-conducting membranes for direct methanol fuel cells, 47-54, Copyright 2002, with permission from Elsevier.)... 

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