Backbone polymers phosphonic acid

All phosphoms oxides are obtained by direct oxidation of phosphoms, but only phosphoms(V) oxide is produced commercially. This is in part because of the stabiUty of phosphoms pentoxide and the tendency for the intermediate oxidation states to undergo disproportionation to mixtures. Besides the oxides mentioned above, other lower oxides of phosphoms can be formed but which are poorly understood. These are commonly termed lower oxides of phosphoms (LOOPs) and are mixtures of usually water-insoluble, yeUow-to-orange, and poorly characteri2ed polymers (58). LOOPs are often formed as a disproportionation by-product in a number of reactions, eg, in combustion of phosphoms with an inadequate air supply, in hydrolysis of a phosphoms trihahde with less than a stoichiometric amount of water, and in various reactions of phosphoms haUdes or phosphonic acid. LOOPs appear to have a backbone of phosphoms atoms having —OH, =0, and —H pendent groups and is often represented by an approximate formula, (P OH). LOOPs may either hydroly2e slowly, be pyrophoric, or pyroly2e rapidly and yield diphosphine-contaminated phosphine. LOOP can also decompose explosively in the presence of moisture and air near 150° C. 

Figure 23.10 Proton conductivity of a few prototypical proton conducting separator materials Nafion as a representative of hydrated acid ionomers (see also Fig. 23.2(a) [43, 78], a complex of PBI (polybenzimidazole) and phosphoric acid as a representative of adducts of basic polymers and oxo-acids (see also Fig. 23.2(b)) [16], phosphonic acid covalently immobilized via an alkane spacer at a siloxane backbone (see also Fig. 23.2(c)) [127], the acid salt CsHSO, [125] and an Y-doped BaZrOj [126].

Scheme 13.2 Selected phosphonated aromatic backbone polymers (a) phospho-nated pol5 aiylene ether),(b) phosphonated poly(phenyl sul-fone), (c) poly(m-phenylene-5-phosphonic acid), (d) PAES with a phosphonic acid group directly attached to the aromatic backbone, (e) methylenephosphonic acid functionalized PAES, (f) PAES with a phosphonated aUgrl chain, (g) PAES bearing a bis(phosphonic acid) on short alkyl chains, and (h) PAES with a pendant phenyl-CF2P03H2 group. ...

Various phosphonated block and graft copolymers have been prepared in order to obtain membranes with high local concentrations of phosphonic acid. The block copolymers have mainly been based on phosphonated and non-phosphonated vinyl monomers, whereas the graft copolymers have been based on polymers with aromatic backbones such as PAESs, poly(phenylene oxide) (PPO), and PBI, from which phosphonated vinyl monomers have been grafted. 

Kerres et al., among others, developed the acid-base blend membranes from sulfonated polymers and aminated or other basic polymers [98] and concluded that the protonation of the basic groups is incomplete if the base is too weak [99]. Very recently, Frutsaert et al. [70] synthesized novel polymers for the development of high temperature PEMFC membranes comprising a blend of s-PEEK and a fluorinated copolymer bearing imidazole functions as pendant groups. The extensive work on intermediate temperature fuel cell membranes are well reviewed in Chap. 4 of this book including polybenzimidazole as the basic component and sulfonated and phosphonated ionomers of either nonfluorinated or partially fluorinated backbones as the acidic component. 

Several synthetic strategies have been developed to prepare phosphonated aromatic polymers, where the acid groups are attached either directly " or via spacers to an aromatic backbone. " These polymers can be prepared either by post-phosphonation of prepolymers via, e.g. transition metal catalyzed Michaelis-Arbuzov reactions and lithiation chemistiy, or by direct polymerization of phosphonated monomers via polycondensation. Both synthetic strategies then require hydrolysis of the esters to obtain the free acid. The former strategy requires the formation of C-P bonds in the polymer structure, and the latter necessitates the synthesis and purification of suitable monomers. 

By using hydrophobic fluorinated polymer backbones, the phase separation can be made more distinct in phosphonated membranes, which in turn can enhance the proton conductivity. DesMarteau and co-workers have studied the proton transport characteristics of model perfluoroacid compounds functionalized with phosphonic, phosphinic, sulfonic, and carbo)q lic acids. The results indicated that the proton transfer in phosphonic and phosphinic acids occurs via structural diffusion rather than by a vehicle mechanism. The findings suggested that fluoroallqrlphosphonic and -phosphinic acids are good candidates for further development as anhydrous, high-temperature proton conductors. 

Progressive neutralization with NaOH gave the sodium salt analogs shown in Figure 20.56. Neutralization of the most acidic protons produced the hemi-sodium salt of the polymer for which 39% of N-atoms were oxidized. It was reported that protonation occurred solely by internal proton transfer. Further neutralization to the monosodium salt resulted in 26% self-doping. Upon further titration, neutralization was complete, resulting in the disodium salt, for which the phosphonic moiety is doubly ionized, offering no internal protons for protonation of die polymer backbone. 

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