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Harry R. Allcock coauthored Phosphonitrilic Compounds. VI. High Molecular Weight Poly(alkoxy- and aryloxyphosphazenes) by Harry R. Allcock and others.

Polyphosphazenes have a backbone of alternating phosphorus and nitrogen atoms with substituents attached to phosphorus. Changing those substituents alters properties such as flexibility and chemical resistance without changing the basic backbone, which is central to the poly(alkoxy- and aryloxyphosphazene) materials described in the publication.

Huai Sun coauthored research on polyphosphazene.^[ref]

A polymer with an alternating phosphorus–nitrogen backbone and typically two substituents at phosphorus. Polar bonding and negative hyperconjugation differ from a continuously conducting conjugated carbon backbone.

The chemical requirement for permanence is retention of the backbone and network in a wet environment. A suitably prepared polysiloxane has strong Si–O bonding, relatively slow backbone cleavage under mild aqueous conditions, and flexible chains that can be retained in a cross-linked elastic network. In the comparison intended here, hydrolysis of susceptible phosphazene substituents and P–N linkages leads to chain scission, reduced molar mass and loss of mechanical integrity, which conflicts with permanence.

This is a comparison of chemical durability, not a universal statement that every polyphosphazene degrades rapidly or that every polysiloxane is indefinitely inert. phosphazene side groups can strongly change access of water and hydrolysis rates; siloxanes can also be cleaved under sufficiently acidic or basic conditions. Those structural and kinetic qualifications matter even before any biological suitability is considered.