An Ionically Cross-Linkable Polyphosphazene: Poly[bis(carboxylatophenoxy)phosphazene] and Its Hydrogels and Membranes

Hexachlorocyclotriphosphazene and poly(dichlorophosphazene) react with the sodium salt of ethyl p-hydroxybenzoate to give small molecule cyclic and high polymeric phosphazenes with aryloxy ester side groups. The structures and physical properties of both classes of compounds were investigated by³¹P NMR,¹H NMR, and infrared spectroscopies and by thermal analysis. Reaction of these compounds with potassium tert-butoxide brought about complete hydrolysis of the ester groups to yield aqueous media-soluble, carboxylic acid bearing cyclic and high polymeric phosphazenes. The carboxylic acid bearing high polymer formed ionic cross-links when treated in aqueous media with salts of di- or trivalent cations, such as calcium chloride, copper bromide, copper sulfate, or aluminum acetate, to yield hydrogels and membranes. Aluminum ions proved to be more efficient crosslinking reagents than the divalent cations. The cross-linked gels were stable in neutral or strongly acidic aqueous media, but the crosslinking process was reversed in basic aqueous solutions of excess monovalent cations.