Preparation of polymethyldimethylsilazanes and polymethyldimethylsilazane varnishes

Polymethyldimethylsilazanes are synthesised by the coammonolysis of a mixture of methyltrichlorosilane and dimethyldichlorosilane in the 3 1 mole ratio with gaseous ammonia in toluene solution  

After coammonolysis, to obtain varnish of required concentration (50-80%, depending on the brand), toluene should be distilled from the product. The distillation is accompanied by the polycondensation of the coammonolysis product and the formation of polymethyldimethylsilazane of the following composition  

The production process (Fig.77) comprises two main stages the coammonolysis of methyltrichlorosilane and dimethyldichlorosilane the preparation of varnish of a required concentration. 

Apparatus 1 is filled with methyltrichlorosilane, dimethyldichlorosilane and toluene in the required ratio. The mixture is agitated for 30 minutes and sent by nitrogen flow (0.07 MPa) into reactor 5, an enameled apparatus with an agitator, inverse condenser 6, a water vapour jacket and a bubbler to feed ammonia. To avoid explosive concentration of the mixture of ammonia with air, the whole system is blown with nitrogen before the start of synthesis. The inverse condenser is filled with salt solution, the reactor is filled with reactive mixture from the agitator, the jacket of the apparatus is filled with water. 

The sample is put in a special airproof metallic sampler, filtered from ammonia chloride gaseous ammonia is sent through the filtrate. The reaction is considered complete, if after the passing of ammonia there is no sediment of ammonia chloride. In case there is sediment, coammonolysis is continued and the sample is taken again after 1-2 hours. 

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