_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Silicones
chain (MeSiHCl 2 is one product from the 'direct synthesis' ). Linkage can occur by
reaction with OH groups on the surface, for example:
:::::si-H + M-OH ---> :::=si-0-M + H 2
Silicones confer anti-stick properties on surfaces. Moulds for rubbers, as in tyre
manufacture, or for plastics are usually silicone treated to give easy release.
Silicones are also useful surfactants and anti-foam agents.
Rubbers in general are polymers with long chain lengths in which there are
occasional cross links. They can thus be deformed under stress without flow.
Increase in the proportion of cross links results in increasing rigidity which leads
ultimately to hard resinous materials.
Elastomers designed for fabrication under high pressure consist of linear
polymers R 3 Si(OSiR 2 ) 2 0SiR 1 with n about 4000, cross linked to forma network
with about 300 monomer units between the links. A filler such as finely divided
silica is also included. Cross links are introduced by heating after the mixture has
been formed into the desired shape by rolling, extrusion or moulding. Radical
sources such as benzoyl peroxide are often used to initia te cross linking (Fig. 4.6 ).
Better control over the structure is achieved by introducing a few vinyl
substituents, which are more susceptible to attack by free radicals. Rubbers which
cure at room temperature are also available. They are produced by mixing two
components. In one type the first component is a polysiloxane with some vinyl
side chains plus a trace ofhydrosilation catalyst (H 2 PtC1 6 ). The second component
consists of a polysiloxane which has a few Si-H units. Cross linking occurs
through hydrosilation of Si-CH=CH 1 by Si-H. The vinyl precursors are also
made in the same way. Addition of Si-H bonds to alkenes or alkynes is
catalysed at ambient temperatures by platinum complexes present in very low
(PhCOOl2 -
2 Ph• + 2 C02
+ H 0 etc
"' .
5 ./ Cotolyst
/SI-CH=CH2 + H- \
Fig. 4.6 Cross-linking of siloxane chains.
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