Page 422 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Organometallic and Inorganic–Organic Polymers 385

Thus, many of the metal-containing polycondensations can be considered as extensions of
organic polyesterfications, polyamination, etc. reactions.

11.3.1 POLYSILOXANES

The most important organometalloid polymers are the polysiloxanes based on the siloxane Si–O
linkage found in glass and quartz (Chapter 12). The polysiolxanes were incorrectly named silicones
by Kipping in the 1920s, but this name continues to be widely used. Originally it was wrongly
believed to have a structure similar to a ketone, hence, silicone.
The production of silicate glass is believed to be a transcondensation of the siloxane linkages in
silica. A comparable poly(silicic acid) is produced when silicon tetrachloride is hydrolyzed.

OH
SiCl Si(OH) 4 −(−Si−O−)−
4
(11.12)
OH
Silicic acid Poly(silicic acid)

The poly(silicic acid) further condenses producing a cross-linked gel. This cross-linking can
be prevented by replacing the hydroxyl groups in silicic acid with alkyl groups. Ladenburg pre-

pared the first silicone polymer in the nineteenth century by the hydrolysis of diethyldiethoxysilane.
Kipping, in the early 1940s, recognized that these siloxanes could also be produced by the hydroly-
sis of dialkyldichlorosilanes giving a poly(silicic acid)-like structure where the hydroxyl groups are
replaced by alkyl groups.
In 1945, Rochow discovered that a silicon–copper alloy reacted with organic chlorides forming
a new class of compounds called organosilanes.

CH Cl + Si(Cu) (CH ) SiCl + Cu
2
3 2
3
(11.13)
Dimethyldichlorosilane
These compounds react with water forming dihydroxylsilanes,

(CH ) SiCl + H O (CH ) Si(OH) + HCl (11.14)
3 2
2
2
2
3 2
and eventually dimeric, oligomeric, and finally polysiloxanes (Equation 11.15). Because of the tox-

icity of HCl, the chlorine groups are replaced by acetate groups leading to the familiar vinegar
smell for many silicon caulks and sealants. Branching and cross-linking is introduced through the
use of methyltrichlorosilane. Modern caulks and sealants are made using the tetrafunctional group
tetraethoxysilane to introduce cross-linking into the resin.
Polysiloxanes, also called silicones, are characterized by combinations of chemical, mechanical,
and electrical properties, which when taken together are not common to any other commercially
available class of polymers. They exhibit relatively high thermal and oxidative stability, low power
loss, high dielectric strength, and unique rheological properties, and are relatively inert to most ionic
reagents. Almost all of the commercially utilized siloxanes are based on polydimethylsiloxane with
trimethylsiloxy end groups. They have the widest use temperature range for commercial polymers
suitable for outdoor applications from the winter of Nome, Alaska, to the summer of south Florida
o
o
o
o
(about –80 F to 100 F; –60 C to 40 C). The first footprints on the moon were made with polysilox-

ane elastomeric boots.

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