macro-molecular structure of polysilazanes. Silazane copolymers are normally produced from ammonolyses of chlorosilane mixtures. In this chemical reaction, different chlorosilanes react at similar rates so that the monomers are statistically distributed in the copolymer. Preparation Ammonia and chlorosilanes, both readily available and low-priced, are used as starting materials in the synthesis of polysilazanes. In the ammonolysis reaction, large quantities of ammonium chloride are produced and must be removed from the reaction mixture. R2SiCl2 + 3 NH3 → [R2Si–NH] + 2 NH4Cl In the laboratory, the reaction is normally carried out in a dry organic solvent (polysilazanes decompose in the presence of water or moisture) and the ammonium chloride is removed by filtration from the reaction mass. Because the filtration step is both time-consuming and cost-intensive, several production methods were developed in which no solid materials are formed during the final reaction step. The liquid-ammonia-procedure was developed by Commodore/KiON for polysilazane synthesis. It calls for adding chlorosilane or chlorosilane mixtures simultaneously to an excess of liquid ammonia. The resulting ammonium chloride dissolves in the liquid ammonia and phase separates from the polysilazane. Two immiscible liquids form. This allows for the simple isolation of pure polysilazane from the liquid ammonia/ammonium chloride solution. The patented procedure is used today by Merck KGaA , formerly AZ Electronic Materials in the manufacture of polysilazanes. Previously, Hoechst AG manufactured VT 50 and ET 70 (now discontinued) as polysilsesquiazane solutions. Synthesis took place in two steps: first a trichlorosilane was reacted with dimethylamine and the resulting monomeric aminosilane was separated from dimethylammoniumchloride. In a subsequent salt-free step, the aminosilane was treated with ammonia to afford a polymer solution. If hexamethyldisilazane (HMDS) is used as a nitrogen source instead of ammonia, transamination takes place. The chlorine atoms liberated from the chlorosilane are tied to the trimethylsilyl groups of HMDS so that no chlorine-containing solid salts are formed. This procedure was used by Dow Corning to manufacture the hydridopolysilazane HPZ. Numerous additional procedures for the synthesis of Si–N based polymers have been described in the literature (for example a dehydrogenation coupling between Si–H and N–H or ring-opening polymerizations) but none are currently employed commercially. For the industrial manufacture of perhydropolysilazane [H2Si–NH]n, ammonolysis in a solvent is the standard process. Although this results in a higher price, the material is routinely used as a coating in the electronics industry due to its special properties (insulating effect in thin layers). The