Preparation of polydimethylsiloxane elastomer

The production of polydimethylsiloxane elastomer comprises three main stages the hydrolytic condensation of dimethyldichlorosilane the depolymerisation of the products of hydrolytic condensation the polymerisaton of dimethylcyclosiloxanes. 

The hydrolytic condensation of dimethyldichlorosilane forms a mixture of linear and cyclic oligodimethylsiloxanes according to the scheme given below. 

To transform linear oligodimethylsiloxanes into cyclic oligodimethylsiloxanes, the mixture is subjected to depolymerisation. The process takes place in the presence of a catalyst (potassium hydroxide)  

The depolymerisation forms a mixture of dimethylcyclosiloxanes (60-85% of tetramer, 13-20% of pentamer, 0.5% of hexamer and up to 5% of trimer), which is then sent to polymerisation. 

The polymerisation of dimethylcyclosiloxanes is conducted in the presence of a catalyst, e.g. based on aluminum sulfate and sulfuric acid. The catalyst is used in the form of a paste of mixed dimethylcyclosiloxanes, anhydrous aluminum sulfate and sulfuric acid. The reaction proceeds thus  

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