= 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:
$$
\mathrm{3PCl_5+3NH_4Cl\longrightarrow(NPCl_2)_3+12HCl},
$$
$$
\mathrm{m(NPCl_2)_3\xrightarrow{\text{controlled heating}}[NPCl_2]_{3m}},
$$
$$
\mathrm{[NPCl_2]_n+2nNaOEt\longrightarrow[NP(OEt)_2]_n+2nNaCl}.
$$
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>. \b[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.<Phosphonitrilic Compounds. VI. High Molecular Weight Poly(alkoxy- and aryloxyphosphazenes) by Harry R. Allcock and others>[Inorganic Chemistry 5, pp. 1709–1715; alkoxide substitution of linear polydichlorophosphazene, including ethoxy derivatives]{ref}
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