Re: dimethylpolysiloxane

From: russ (tupiloking_at_hotmail.com)
Date: 01/30/05 

Date: 29 Jan 2005 22:09:32 -0800

I wish to expand on what the others said. It is a several step process,
suitable for a lab.

1) Mix the Me2SiCl2 with water. If excess water is used, it is
exothermic due to the heat of dilution of HCl (g) in the water. If
deficient water is used, the HCl will be liberated as a gas and it is
endothermic. The product is termed a hydrolyzate. It will still be
acidic.
2) Wash several time with water. You will now have a mixture of
HO(SiMe2)xOH (linears) and (SiMe2)y y= mostly 4, 5, and 6 cyclics.
3) Mix above product with some hexamethyldisiloxane (Me3SiOSiMe3), or
more preferably, a short linear such as Me3SiO(SiMe2O)2SiMe3.
4) Add potassium hydroxide such that the amount of potassium is about
10-50 ppm above the amount of residule chloride remaining in the
hydrolyzate after washing.
5) Heat with agitation to about 140 C with water removal. (Slight N2
purge will suffice) for about 6 hours.
6) Cool.
7) Add acetic acid, stir awhile and filter.

The product is a mixture of linears and cyclics. The cyclics are about
15% w/w. The length of the linear (or the viscosity) is controlled by
the Me3Si to Me2Si ratio.

8) Remove the cyclics by heating under vacuum with removal of
distillate. 150 C at 10-20 mm Hg should remove most of the cyclics.

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