Post-Sulfonated Polymers

Sakaguchi, Y, Kaji, A., Kitamura, K., Takase, S., Omote, K., Asako, Y, Kimura, K., Polymer electrolyte membranes derived from novel fluorine-containing polyfarylene ether ketone)s by controlled post-sulfonation. Polymer, 2012,53,4388-4398. 

One alternative approach to obtain sulfonated aromatic polymers is the direct synthesis via aromatic nucleophilic substitution reaction from the sulfonated-functionalized monomers. This direct synthesis process has been proven to be more advantageous than post-sulfonation, namely ... 

Y. Sakaguchi, A. Kaji, K. Kitamura, S. Takase, K. Omote, Y. Asako, K. Kimura, Polymer electrolyte membranes derived from novel fluorine-containing poly(arylene ether ketone)s by controlled post-sulfonation.Po/ymer2012,53(20), 4388-4398. 

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... 

Recent efforts in the synthesis of sulfonated aromatic polymers are directed to the polymerization of sulfonated monomers (such as (b), (d), (g), (j), (k), and (1) shown in Scheme 3) [14,15,53,54,96-102] or coupling reactions of sulfonated compoimds with fimctional groups attached to a polymer backbone [ 103,104]. In post-sulfonation, attachment of the sulfonic acid group is restricted to the activated position ortho to the aromatic ether bond, as indicated in Scheme 4a, while in direct polymerization of sulfonated monomers, the sulfonic acid groups are attached to the deactivated site on the ring (Scheme 4b). An enhancement of stabUity toward desulfonation and a modestly higher acidity are expected for the structure shown in Scheme 4b. Recently, polymerization of sulfonated monomers was also used to obtain sulfonated polysulfone (m) via oxidation of a sulfonated polysulfide-polysulfone copolymer [105]. 

Sulfonated polyimide can be also prepared by post-sulfonation of polyimide polymers, but the former procedure is generally adopted because of the existence of commercially available sulfonated aromatic diamines. 

H. Komber, S. Chakraborty, B. Voit, S. Baneijee, Degree of sulfonation and micro-structure of post-sulfonated polyethersulfone studied by NMR spectroscopy, Polymer 53 (2012) 1624-1631. 

Miyatake et al. have combined two strategies in a single polymer architecture (multiblock copolymers containing sulfonic acid clusters in their hydrophilic blocks) [62]. They have successfully synthesized a series of multiblock poly (arylene ether sulfone ketone)s (SPESKs) containing fully sulfonated fluorenylidene biphenylene units (Fig. 7.23) [63, 64]. The well-controlled post-sulfonation reaction of the precursor polymers enabled preferential sulfonation on... 

Because of good thermal and hydrolytic stability, excellent mechanical and chemical stability, low cost, and commercial availability of sulfonated aromatic hydrocarbon polymers, recent research has focused on the synthesis and development of sulfonated aromatic hydrocarbon polymers specifically for high-temperature PEMFCs. Typical examples include sulfonated poly(ether ether ketone) (SPEEK) or poly(ether ketone ketone) (SPEKK) [1,2], sulfonated poly(ether sulfone) (SPSE) [3], alkyl sulfonated polybenzimidazole (PBI), sulfonated naphthalenic polyinrides (sNPl) [4-6], sulfonated polyCphenylene sulfide) [7,8]. Both post- and pre-sulfona-tion methods have been used in the past. Other than the post-sulfonation modification of aromatic polymers, recently, efforts have been dedicated to direct polycondensation from sulfonic acid containing monomers to synthesize sulfonated polymers [9]. The latter approach, namely pre-sulfonation, is widely applied because of the ease of controlling sulfonation degree and deactivated sites in the arylene backbones, which further avoid side reactions such as decomposition and hydrolysis of polymers resulted from the post-sulfonation method. 

Polymer membranes have low proton conductivity until they contain enough proton conductive functional groups, e.g., sulfonic acid, in polymer backbones or side chains. Sulfonic acid in polymer chains shows a high dissociation constant, resulting in high proton conductivity in aqueous environments compared with other acids therefore, most DMFC membranes use the sulfonic moiety as the proton transfer medium. Sulfonation methods of polymer membranes are generally classified as post-sulfonation in the presence of polymers and in situ sulfonation through co-polymerization of sulfonated monomers and nonsulfonated monomers. 

The basic concept to use block co-polymer for the application to the DMFC is that ordered hydrophilic/hydrophobic phase separations offer a route for the selective transport of proton ions with reduced methanol crossover in the hydrophilic domains, because block co-polymers can be selectively sulfonated using post-sulfonation methods, and the block co-polymers can be verified over a wide range of structures during anionic polymerization. For example, methanol transport behaviors of a triblock co-polymer ionomer, sulfonated poly(styrene-isobutylene-styrene) (S-SIBS), were compared with Nafion to determine whether the sulfonated block co-polymer could serve as a viable alternative membrane for application to the DMFC [62]. The S-SIBS membranes showed approximately 5-10 times more methanol selectivity than that of Nafionll , although the S-SIBS membranes exhibited low conductivity compared with Nafion 117. 

Synthesis of the polymers by post-sulfonation is straightforward whenever there are electron-rich phenyl rings present in the polymer backbone or in side chains. This is, for example, the case in many poly(arylene ether ketone)s and poly(arylene ether sulfone)s, poly(phenylene oxide), poly(phenylene sulphide), certain poly(paraphenylene)s, poly(phenylquinoxaline)s, and others. However, the exact chenucal structure, such as the exact level of activation or deactivation of the aromatic groups, or the presence of acid labile groups, as well as the solubility of the starting material and the end product, deter-... 

Sometimes, when the aromatic groups of the polymer are too activated, simple post sulfonation can lead to crosslinking. The arylsulfonic add groups formed by sulfonation are then able to act as sulfonating agent themselves. 

Both methods, the post-sulfonation of preformed polyfarylene ether sul-fone)s and the preparation of functionalized polymers by the use of sulfonated monomers, have been widely described in the hterature and will be discussed in the following sections. 

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