Modern inorganic chemistry

190 GROUP IV

chloride is passed over a copper-silicon mixture at about 600 K, but

they can be prepared by other methods. Hydrolysis then gives, for

example:

(CH 3 ) 2 SiCl 2 + 2H 2 O -> (CH 3 ) 2 Si(OH) 2 + 2HC1

The resultant compound then polymerises by losing water thus:

CH 3 CH 3

HO-Si—I OH + Hi O

CH 3 CH 3

CH 3 CH 3 CH 3 CH 3

-» HO--Si--O-Si--O-Si----O-Si—OH

CH 3 CH 3 CH 3 CH 3

Note that in the compound (CH 3 ) 2 Si(OH) 2 the silicon atom can

hold two OH groups, unlike carbon. It is this property that makes

the existence of silicones possible. By variation of the compounds

and conditions of hydrolysis, straight chains, rings and cross-linked

polymers are obtained, for example:

O

These are the silicones. According to the degree of cross-linking
and length of the chain, they can be obtained in the form of oils or
rubber-like solids. The silicone oils are not volatile on heating and
can be heated to high temperatures without decomposition (and so
are useful for high vacuum pumps and high-temperature oil baths)