Enantiotropic conformational polymorphism in 2,2,4,4-bis-(2′,2′-dimethylpropane-1′,3′-dioxy)-6,6-dichlorocyclotriphosphazene

Conformational polymorphism has previously been observed in 2,2-dichloro-4,4:6,6-bis(2,2'-dimethlypropane-1,3-dioxy)-cyclotriphosphazene and the crystal structures of two forms were reported. In that study the melting points of both forms (alpha and beta) were reported as 211-212 degrees C and no investigation was made on the stability of, or the thermodynamic relationships between the polymorphs. In the present work the two forms were characterized by various analytical techniques, such as single-crystal X-ray diffraction, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), thermomicroscopy (TM) and variable temperature single crystal and powder X-ray diffraction. Thermal analysis suggests that the polymorphs are enantiotropically related and it is found that the solid-solid transformation of the beta to alpha form may start with defects of alpha form inside the beta form followed by a nucleation and growth type mechanism involving at least three main molecular centred events; conformational change, rotation and tilting.