Preparation of linear polyorganosiloxanes

Polyorganosiloxanes (organosiloxane elastomers) with linear molecular chains have the following formula  

They are prepared by the hydrolytic condensation or cocondensation of difunctional organochlorosilanes and subsequent polymerisation of cyclic diorganosiloxanes. 

During the hydrolytic condensation diorganosiloxanes have a great tendency to form cycles. For example, dimethyldichlorosilane is hydrolysed with water (in the absence of solvents) by the mechanism of polycondensation, forming a mixture of linear and cyclic dimethylsiloxanes. 

The formation of cyclic compounds increases with the size of the organic radicals bound with silicon. For example, in the process of hydrolytic condensation methylphenyldichlorosilane and diphenyldichlorosilane form mostly cyclic products. The conditions of the hydrolysis of dior-ganodichlorosilanes, especially pFl of the medium, are essential for cycli-sation. As pFl increases, i.e. the acidity of the medium decreases, the process of ring formation can be reduced but cannot be completely avoided. 

During the cationic polymerisation, e.g. with sulfuric acid, the process is the following at the initial stage of initiation, when organocyclosiloxanes interact with sulfuric acid, the acid proton attacks the oxygen atom of the siloxane cycle. As a result of the redistribution of the electron density, the =Si-0 bond breaks, opening the cycle and forming an active centre at the end of the chain  

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