Polyphosphazene

Polyphosphazenes include a wide range of hybrid inorganic-organic polymers with a number of different skeletal architectures that contain alternating phosphorus and nitrogen atoms. Nearly all of these molecules contain two organic or organometallic side groups attached to each phosphorus atom. These include linear polymers with the formula (N=PR¹R²)_n, where R¹ and R² are organic or organometallic side groups. The linear polymers are the largest group, with the general structure shown schematically in the picture. Other known architectures are cyclolinear and cyclomatrix polymers in which small phosphazene rings are connected together by organic chain units. Other architectures are available, such as block copolymer, star, dendritic, or comb-type structures. More than 700 different polyphosphazenes are known, with different side groups (R) and different molecular architectures. Many of these polymers were first synthesized and studied in the research group of Harry R. Allcock at The Pennsylvania State University.

The method of synthesis depends on the type of polyphosphazene. The most widely used method for linear polymers is based on a two-step process. In the first step a cyclic small molecule phosphazene, known as hexachlorocyclotriphosphazene, with the formula (NPCl₂)₃, is heated in a sealed system at 250 °C to convert it to a long chain linear polymer with typically 15,000 or more repeating units. In the second step the chlorine atoms linked to phosphorus in the polymer are replaced by organic groups through reactions with alkoxides, aryloxides, amines or organometallic reagents. Because many different reagents can participate in this macromolecular substitution reaction, and because two or more different reagents may be used, a large number of different polymers can be produced, each with a different combination of properties. Variations to this process are possible using poly(dichlorophosphazene) made by condensation reactions.

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