Solubility properties polyphosphazenes

The molecular architecture of a polyphosphazene has a profound influence on properties. For example, linear and tri-star trifluoroethoxy-substituted polymers with the same molecular weight (1.2 x 104 or higher) have strikingly different properties.138 The linear polymers are white, fibrous materials that readily form films and fibers, whereas the tri-arm star polymers are viscous gums. One is crystalline and the other is amorphous. Cyclolinear polymers are usually soluble and flexible. Cyclomatrix polymers are insoluble and rigid. Linear polymers can be crystalline, but graft or comb polymers are usually amorphous. 

Figure 2 Properties in polyphosphazenes are determined hy (1) the backbone bonds that control the inherent flexibility of the polymer via their influence on bond torsional freedom, and also provide photo-and thermo-oxidative stahihty (2) the side groups control polymer solubility, reactivity, thermal stability, crystallinity, cross-linking, and (indirectly) polymer flexibility (3) free volume between the side groups affects polymer motion, solvent penetration, membrane behavior, and density (4) functional groups (usually introduced hy secondary reactions) affect soluhihty, biological behavior, proton conduction, cross-hnking, and many other properties...

The presence of the phosphorus-nitrogen backbone confers to these kind of polymers thermo-oxidative stability, fire resistance, very high torsional mobility (low barrier to skeletal bond twisting), high refractive index, and hydrophilicity. On the other hand, the side groups in polyphosphazenes control other properties such as solubility, secondary reaction chemistry, thermal decomposition, and resistance to hydrolysis. The possibility of tuning the properties of polyphosphazenes thanks to their synthetic flexibility has led to enormous interest in their applications in several areas of research. 

Polyphosphazenes are a diverse class of polymers with an inorganic backbone of phosphorus and nitrogen atoms. Because these polymers have two substituents at each phosphorus, it is possible to widely vary the properties of this polymer system by simple changes in these substituents. In fact, several hundred such polymers are known and their potential applications include materials that are flame retardant, resistant to UV radiation and chemicals, semiconductors, conductors, insulators, water soluble, biologically inert or active, etc. (1,2)... 

In order to further control the properties of the polyphosphazenes, we are now investigating the materials obtained by derivatizing the copolymer [Me(Ph)PN][Me2PN]y, 2, where x = y Tg = ca. 0°C). The decreased solubility of this polymer in THF relative to the homopolymer 1 necessitates longer reaction times for deprotonation. Both the silylated (eq 4) and carboxylated (eq 5) derivatives have very different solubilities relative to the homopolymer derivatives. For example, the silyl derivatives are partially soluble in hexane (24) and both the 50% substituted acid and the lithium salt of the 25% substituted acid are... 

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