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

Silicones
consists of about 80% tetramer (Il= 4) (b.p. 176 C). together with some trimer
(n = 3) (b.p. 134°C) and larger rings (n = S-1 0). These materials can be
separated by fractional distillation or by gas liquid or high pressure liquid
chromatography. The cyclic trimer has a planar six-membered ring both in the
vapour (electron diffraction) and in the solid state (X-ray diffraction). The ring in
the cyclic tetramer however is puckered and adopts a saddle shaped conformation.
Puckered rings are also present in ali the higher oligomers.
The cyclic trimer is unstable with respect to the tetramer by 16-20 kj mol- 1 • on
account of ring strain in the former. Its presence in hydrolysates is due to its rapid
formation in the early stages of the reaction under kinetic control. Cyclic and
linear siloxanes can be interconverted using either acidic or basic catalysts. Iflong
chain linear materials. for example. are heated with potassium hydroxide.
(Me 2 Si0)" (n = 3,4) can be distilled away. Conversely cyclic siloxanes are
polymerized by OH-, OR- as follows:
polymer
\
Si-Oi K Si"-. -
Me2 \ ~ie2 0)2 Mez O -
Heating to 80°C with 1-2% ofsulphuric acid which contains up to 8<}{) oleum has
a similar effect. Polymethylsiloxanes produced in this way stiU carry end groups
which are susceptible to further condensation. To stabilize the linear polymers,
their ends can be blocked by Me,Si groups. (Me,Si) 2 0 is added to the polymerizing
mixture; the relative proportion of Me 1 Si- and Me 1 Si= units determine the
average chain length of the resulting siloxane polymer. The polymers thus
obtained do not ali have the same chain length. but there is a Gaussian
distribution about the mean.
Silicones are manufactured on a very large scale; in 19 7 5 about 24 000 ton nes
of silicon were converted into chlorosilanes for this purpose in the USA alone.
About halfthe production is of methylsilicone tluids Me ,SiO(SiMe 2 0)"SiMe, (n up
to 3000). The rest consists ofresins, rubbers and some composites which include
copolymers, blends and laminates.
Methyl and phenyl silicones exhibit very good thermal and oxidative stability
in comparison with hydrocarbons. Methylsilicone oils (linear polydimethylsilox-
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