Water-soluble polyphosphazenes emerge as an important class of biologically active macromolecular compounds dictating the need for the development of their well-defined and controlled synthesis. The synthetic pathway leading to biologically active polyphosphazenes involves preparation and chemical transformation of inorganic macromolecular precursor-poly(dichlorophosphazene), PDCP. Synthesis, stabilization, and characterization of this hydrolytically sensitive, reactive intermediate are the focus of the present study. Ring-opening polymerization reaction leading to PDCP has been investigated under strictly controlled conditions by NMR, viscometry, and direct multiangle laser light scattering-GPC methods. A substantial dependence of the molecular weight on degree of conversion and the formation of branched polymer structures have been observed even at early stages of the polymerization process. A new approach has been suggested for the stabilization of PDCP solutions involving the use of diethylene glycol dimethyl ether (diglyme). This stabilization technique allows highly reliable direct analysis of PDCP using chromatographic methods and greatly simplifies PDCP conversion in the organo-substituted polymer.