Polyphosphazene

Polyphosphazenes are a class of inorganic polymers with the repeat unit (-RR'P=N-). The substituents R,R' are usually alkoxy, amino, (R₂N), or halogens (such as chloride or fluoride).

Ball-and-stick model of part of a poly(difluorophosphazene) chain, (NPF₂)_n.

Polyphosphazenes are typically prepared by the thermal ring-opening polymerization of hexachlorophosphazene (Cl₂PN)₃ followed by esterification of the intermediate poly(dichlorophosphazene) with either amines or sodium salts of alcohols:

n (Cl₂PN)₃ → (Cl₂PN)_3n

(Cl₂PN)_3n + Nu^- → (Nu₂PN)_3n + 6n Cl^-

where Nu^- = NR₂^-, OR^-, F^-

These polymers were popularized by the research group of Harry R. Allcock at Pennsylvania State University.

Properties

The properties of polyphosphazenes depend on the nature of the side groups. Most polyphosphazenes are colorless and exhibit high thermal stability, although they are somewhat sensitive to base hydrolysis. For certain alkoxides (e.g. OCH₂CF₃, derived from trifluoroethanol), the polymers are highly flexible with very low glass transition temperatures. In this respect the polymers resemble the commercially significant siloxanes.

Polyphosphazenes containing polyether side chains are good ionic conductors for lithium triflate. They are among the few polymers that will dissolve such salts. Similarly, the incorporation of acidic functional units into the side groups of polyphosphazene yields proton conductors.

Occurrence

Exist as glasses, flexible or rubbery solids with low tendency for crystallization.

Use

Waterproofing, flame retardance and fuel hoses.

Further information

"H. R. Allcock Research Group". 2007-01-22. http://research.chem.psu.edu/hragroup/. Retrieved 2007-10-26.