 Solution

Prepare a chlorophosphazene ring from phosphorus pentachloride and ammonium chloride, isolate the cyclic trimer, open the rings to make the linear chloropolymer, and finally replace P–Cl by P–OEt:The first stage can produce a mixture of cyclic oligomers, so isolation of hexachlorocyclotriphosphazene is important. Controlled thermal ring-opening polymerization, conventionally around $250{}^{\circ }\mathrm{C}$ for the classical route, gives soluble linear polydichlorophosphazene before excessive cross-linking. Moisture exclusion avoids uncontrolled hydrolysis of the reactive P–Cl precursor.

The target is poly[bis(ethoxy)phosphazene]. The final step is nucleophilic substitution with an ethoxide salt in a compatible dry medium, replacing two chlorides per phosphorus. Polymerize the chlorinated ring first, then substitute the linear polymer: complete ethoxylation of the isolated ring does not by itself produce the requested high polymer. The primary macromolecular-substitution work explicitly includes the ethoxy derivative.^[ref]