POLYMORPHISM IN POLY(DIETHYLPHOSPHAZENE) AND ITS CYCLIC TRIMER - A MAS NMR AND X-RAY-DIFFRACTION STUDY

Poly(diethylphosphazene) (PDEP), contrary to other polyphosphazenes, i s hardly soluble in standard organic solvents and presents a melting p oint 80 degrees C higher than that of the dimethyl analog. Stimulated by this specificity, we have investigated the polymorphic behavior of PDEP and its cyclic trimer by X-ray diffraction, MAS NMR, and differen tial scanning calorimetry. The crystal structure of the native room te mperature form of the cyclic trimer has been solved. The resulting arr angement of the ethyl groups has provided the basis to interpret the M AS NMR spectra of the trimer itself and of the polymer in the native m odification, which present remarkable similarities. The MAS NMR data s how analogous averaging of the ethyl side-chain conformations in the m odifications obtained heating samples above the transition temperature of both the trimer and the polymer. X-ray and thermal data indicate t hat extensive tridimensional order must be maintained in these forms. Both modifications of PDEP obtained after thermal treatment present ve ry significant side-chain mobility, coupled, in one of them, with only minor perturbations of the main-chain conformation and of the packing as compared to the native form. These unusual features imply for the disordered phases of PDEP a model substantially differing from the hit herto reported polyphosphazenic mesophases, which may account for the specificity of crystalline PDEP.