Polyorganophosphazenes

Polyorganophosphazenes are a class of inorganic–organic polymers characterized by a phosphorus–nitrogen backbone with alternating P and N atoms. Each phosphorus atom bears two organic substituents, which define the properties of the polymer. The parent material, poly(dichlorophosphazene), [-N=PCl2-]n, is prepared by polymerization of hexachlorophosphazene or by cyclotrimerization of phosphorus–nitrogen rings, after which the reactive P–Cl bonds are replaced by nucleophiles to give polyorganophosphazenes with diverse side chains. In the common substitution scheme, both chlorines on every phosphorus are replaced, giving repeating units [-N=P(R)(R')-]n, where R and R' are the chosen organic groups; partial substitution is also possible.

The substituent design allows tuning of thermal stability, mechanical properties, solubility, and hydrolytic behavior. The polymer backbone is generally thermally stable and chemically robust; the P–N bond provides flexibility; certain side groups render the polymer soluble in common solvents; hydrolyzable substituents confer degradability under aqueous conditions, enabling environmentally or biologically compatible materials.

Applications include degradable biomaterials for drug delivery, tissue engineering scaffolds, hydrogels, coatings and elastomers; flame retardant materials; specialty polymers for optoelectronic applications.

Research continues to expand the scope of side-chain chemistry and to develop controlled polymerization methods, such as living polymerization, to tune molecular weight and architecture.