Preparation of low-molecular polydimethylsiloxane elastomers

Low-molecular polydimethylsiloxane elastomers can be obtained by the polymerisation of a dimethylcyclosiloxane mixture in the presence of alkali  

The molecular weight of the elastomer depends on the amount of the catalyst since the molecular weight is inversely proportionate to the concentration of the catalyst, the molecular weight of the polymer can be reduced by increasing the concentration, and vice versa. 

The preparation of low-molecular polydimethylsiloxane elastomers often requires a considerable quantity of alkali, which calls for additional neutralisation. That is why in some cases it is advisable to use special substances (so-called chain growth regulators), which can take part in the reaction of chain transition. The part of regulators can be played by oli-gomethylsiloxanes, which do not have active end groups. Below we give the consumption of some regulators (g, per 1 tonne of polydimethylsiloxane)  

When SKTN elastomers are produced, dimethylcyclosiloxanes are polymerised with 2 N potassium hydroxide solution (0.005% of the amount of the polymerised substance) the molecular weight of the polymer is regulated by water supply (diluting the alkali). The raw stock for polymerisation can be the depolymerisate obtained at the depolymerisation stage of the product of hydrolytic condensation of dimethyldichlorosilane in the production of polydimethylsiloxane elastomer SKT. 

Raw stock depolymerisate, a mixture of dimethylcyclosiloxanes (60-85% of tetramer, up to 5% of trimer, 13-20% of pentamer, the allowable content of trifunctional impurities and moisture does not exceed 0.01%) potassium hydroxide (at least 95% of KOH) Aerosil or carbon white U-333. The production of this elastomer relies on the hermeticity of the equipment and service lines. The pressure is raised with nitrogen (0.3 MPa) and held for 1 hour. Then the pressure is reduced to atmospheric by releasing nitrogen through an air vent. Only then can polymerisation take place. 

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