Synthesis of Poly[bis(phosphazo)phosphazenes] Bearing Aryloxy and Alkoxy Side Groups

The synthesis of a new class of phosphazene polymers through the thermal ring-opening polymerization of 1,1-bis(trichlorophosphazo)tetrachlorocyclotriphosphazene, gem-(NPCl₃)₂N₃P₃Cl₄ (1), is reported. The resultant polymer possesses controlled short-chain branching sites along the phosphazene backbone. The chlorine atoms in this polymeric intermediate were replaced by organic side groups to yield novel organic-inorganic polymers that are stable to ambient conditions. The macromolecules were characterized by ³¹P, ¹H, and ¹³C NMR spectroscopy, elemental analysis, differential scanning calorimetry, and gel permeation chromatography. Glass transition temperatures of these new materials in general are higher than those of classical poly(organophoshazenes) that bear similar side groups, and the tendency for crystallization is lower. Computer simulations of 1 were carried out to provide structural information about the cyclic trimer. The results suggested that the molecule contains a strained phosphazene ring. Kinetic studies showed that the activation energy for the initiation of the polymerization of 1 is about 46 kcal/mol, which is lower than some of the values suggested for the polymerization of hexachlorocyclotriphosphazene, [NPCl₂]₃. This feature, combined with the unusual structural characteristics of 1, appears to contribute to the relatively mild polymerization conditions required for this molecule. A mechanism is proposed for the polymerization of 1.