Phosphonated Aromatic Backbone Polymers

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. ...

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. 

Phosphonated polymers with fluorinated backbones have been obtained by reacting fluorinated poly(atylene ether)s with a H-phosphonate diester in the presence of Pd(PPh3)4 (Scheme 13.3d). Using fluorinated aromatic polymers as the backbone can be expected to improve the dimensional stability of the membranes compared to aromatic polymers with hydrocarbon backbones because of the increased hydrophobicity induced by the presence of the fluorocarbon. The phosphonated polymers with fluorinated aromatic backbones reached a conductivity of 6 mS cm at 95% RH at 80 °C and 2.6 mS cm in water at room temperature, which was below that of Nafion 117. 

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. 

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