Polyphosphazene sulfonates

Polyphosphazenes sulfonates XIX with the anion covalently attached to the polymer are a new class of cation conductors that have been synthesized by Shriver [625]. They were obtained by reaction of Na0C2H4S03Na with an excess of polydichlorophosphazene in the presence of 15-crown-5, followed by the reaction of the partially substituted product with the sodium salt of poly(ethylene glycol methyl ether). The conductivity at 80 °C of the polymer with x=1.8, m=7.22 is 1.7x10 S cm This low conductivity can be attributed to an extensive ion pair formation between the sodium and sulfonate ions. 

Polyphosphazene sulfonic acids [200, 201] offer a unique combination of inorganic backbone with high stability, high ionic density and structural diversity with crosslinking alternatives. Polyphosphazenes with sulfonamide functionalization have been reported with high proton conductivity [202]. 

Fig. 7 Example structures of polyphosphazene sulfonic acids a sulfonated poly[(me-thylphenoxy)(phenoxy)phosphazene], b sulfonated poly[bis(3-methylphenoxy)(phenoxy)-phosphazene], c sulfonated poly[bis(phenoxy)phosphazene] and d sulfonated poly[bis(2-phenoxyethoxylphosphazene]...

FIGURE 7.19 Direct synthesis of sulfonated polyphosphazene with disodium salt of 2-hydroxyethanesulfonic acid. (Reprinted with permission from Ganapathiappan, S., Chen, K., and Shriver, D.F., A new class of cation conductors Polyphosphazene sulfonates. Macromolecules, 21, 2299. Copyright 1988 American Chemical Society.)... 

As already reported in Table 6, the solubility of phosphazene polymers is strongly influenced by the nature of the substituent groups attached at the phosphorus atoms along the -P=N- skeleton. Water-solubility, for instance, can be induced in polyphosphazenes by using strongly polar substituents (e.g. methylamine [84], glucosyl [495], glyceryl [496], polyoxyethylene mono-methylether [273] or sulfonic acid [497,498] derivatives), or may be promoted by acids or bases when basic (amino substituents like ethylamine [499]) or acid (e.g. aryloxy carboxylate [499] or aryloxy hydroxylate [295]) substituents are exploited. 

Cross-linked, sulfonic-acid-substituted, polyphosphazene-based PEMs have primarily been examined for potential use in DMFC applications due to their low MeOH crossover with reported values 2.5 times lower than that of Nafion. These materials have also been shown to display good thermomechanical and chemical stability (in a Fenton test). Sulfonamide-substituted polyphosphazenes have exhibited very high power densities that are comparable with Nation and may be suitable for use in PEMFC applications. ... 

Wycisk, R. and Pintauro, P. N. 1996. Sulfonated polyphosphazene ion-exchange membranes. Journal of Membrane Science 119 155-160. 

Tang, H., Pintauro, P. N., Guo, Q. and O Connor, S. 1999. Polyphosphazene membranes. ni. Solid-state characterization and properties of sulfonated poly[bis(3-methylphenoxy)phosphazene]. Journal of Applied Polymer Science 71 387-399. 

Andrianov, A. K., Marin, A., Chen, J., Sargent, J. and Corbett, N. 2004. Novel route to sulfonated polyphosphazenes Single-step synthesis using "noncova-lent protection" of sulfonic acid functionality. Macromolecules 37 4075 4080. 

Allcock s research led to the development of poly-phosphazene-based PEMs by his small molecule studies of the sulfonation of cyclic trimeric phosphazenes and the surface chemistry of polyphosphazene macromolecules. In a 1993 report, he described the sulfonation of aminophosphazenes with 1,3—propanesultone. While these specific materials are not necessarily ideal as PEMs, this study demonstrated a novel technique for creating sulfonated polyphosphazene materials that may provide more control over the sulfonated polymer product than wholesale sulfonation of a base polymer by a strong sulfonating agent. 

In another report polyphosphazene copolymers were synthesized from bis (2-methylphenoxy) phosphazene, which was sulfonated after polymerization. Polymers such as polyvinylidene fluoride, polyhexa-fluoropropylene, and polyacrylonitrile were used to produce a blended membrane system. Polymer blends, cross-linking, and other means of re-enforcement are... 

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. 

Wycisk, R., Lee, J.K., and Pintauro, P.N., Sulfonated polyphosphazene-polybenzimid-azole membranes for DMFCs, J. Electrochem. Soc., 152, A892, 2005. 

Guo, Q. et al., Sulfonated and crosslinked polyphosphazene-based proton-exchange membranes, J. Membr. Sci., 154, 175, 1999. 

Studies of membranes based on polyphosphazenes, bearing groups other that substituted phenoxy groups, have been continued, in particular to develop materials for specific apphcations. Ion-exchange membranes of cross-linked and non-crosslinked sulfonated [NP(OC6H4Me-3)2]n have been... 

Diffusion coefficients for methanol (concentration range 1.0-5.0 M) in ion-exchange membranes of UV-crosslinked sulfonated poly(bis-3-methoxyphenoxyphosphazene) (134) have been reported to be much smaller than those in Nafion perfluorosulfonic ion-exchange membranes. Application of polyphosphazene-based membranes in methanol-based fuel cells has been reported. ... 

H. Tang, P.N. Pintauro, Q. Guo and S. O Connor, Polyphosphazene membranes. III. Solid-state characterization and properties of sulfonated poly[bis(methylphenoxy)-phosphazene], J. Appl. Polym. Sci., 1999, 71, 389-399. 

R. Wycisk, P.N. Pintauro, Sulfonated polyphosphazene ion-exchange membranes, J. Membr. Sci., 1996, 119, 155 Q. Guo, P.N. Pintauro, Sulfonated and cross-linked polyphosphazene-based proton-exchange membranes, J. Membr. Sci., 1999, 154, 175. 

Sulfonation Via the Use of Sultones. Direct sulfonation is generally not possible when aliphatic side chains are present. However, sulfonated aliphatic side chains can be produced via the reactions of polyphosphazenes that bear aliphatic amino side groups widi sultones (55) (Scheme IV). The pendent terminal amino units must be introduced by a protection-deprotection route (see later). 

Sulfonated aromatic polymers have been widely studied as alternatives to Nafion due to potentially attractive mechanical properties, thermal and chemical stability, and commercial availability of the base aromatic polymers. Aromatic polymers studied in fuel cell apphcations include sulfonated poly(p-phenylene)s, sulfonated polysulfones, sulfonated poly(ether ether ke-tone)s (SPEEKs), sulfonated polyimides (SPIs), sulfonated polyphosphazenes, and sulfonated polybenzimidazoles. Representative chemical structures of sulfonated aromatic polymers are shown in Scheme 3. Aromatic polymers are readily sulfonated using concentrated sulfuric acid, fuming sulfuric acid, chlorosulfonic acid, or sulfur trioxide. Post-sulfonation reactions suffer from a lack of control over the degree and location of functionalization, and the... 

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. 

Q. Guo, P.N. PiNTAURO, H. Tang, S. O Connor, Sulfonated and crosslinked polyphosphazene-based proton-ex-change membranes. Journal of Membrane Science 154 (1999) 175. 

A. K. Rianov, a. Marin, ). Chen,. Sargent, N. Corbett, Novel Route to Sulfonated Polyphosphazenes Single-Step Synthesis Using Noncovalent ProtectionT of Sulfonic Acid Enmctional-ity. Macromolecules 37 (2004) 4075-4080. 

Wycisk R, Lee JK, Pintauro PN (2005) Sulfonated polyphosphazene-polybenzimidazole membranes for DMFCs. J Electrochem Soc 152 A892-A898... 

---