Chemistry Reference
In-Depth Information
planar all-
trans
form of the chain approximate a series of closed polygons,
as illustrated in figure 1.1.
75
As a result, siloxane chains exhibit a number
of interesting configurational characteristics that impact properties and
associated applications.
The major categories of homopolymers and copolymers to be discussed
are
11
(i) linear siloxane polymers [-SiRR'O-] (with various alkyl and aryl
R,R' side groups), (ii) sesquisiloxane polymers possibly having a ladder
structure, (iii) siloxane-silarylene polymers [-Si(CH
3
)
2
OSi(CH
3
)
2
(C
6
H
4
)
m
-]
(where the skeletal phenylene units are either
meta
or
para
), (iv) silalkylene
polymers [
-
Si(CH
3
)
2
(CH
2
)
m
-], and (v) random and block copolymers, and
blends of some of the above. Table 1.1 provides some illustrative examples.
Important topics include the structure, flexibility, transition tempera-
tures, permeability, and other physical properties. Applications include
high-performance fluids, elastomers, and coatings; surface modifiers; gas
separation membranes; photoresists; soft contact lenses; body implants;
and controlled-release systems. Also of interest are the use of sol-gel tech-
niques to convert silicon-containing materials to novel reinforcing fillers
or to polymer-modified ceramics, and the use of pyrolysis to form high-
performance fibers.
1.2 HISTORY
The first reaction of relevance in silicon chemistry was the conversion of
elemental silicon to silicontetrachloride, SiCl
4
, and to trichlorosilane,
SiHCl
3
. These purely inorganic substances were then converted into orga-
nometallic species such as RSiX
3
, by reaction with diethyl zinc and related
compounds.
1, 16, 23, 29, 33
Organosilicon chemistry blossomed with Kipping's preparation of such
compounds by the more convenient Grignard process. The resulting si-
lanes turned out to be of paramount importance since they hydrolyzed
readily to form compounds containing Si-O bonds, both linear and
cyc lic.
4, 29, 33
These new materials were first called silicoketones or “sili-
cones” by analogy with ketones R-(C = O)-R' in the organic area. Struc-
tural studies, however, showed that they did not contain the Si = O double
bond. Thus, the silicone name is a misnomer, but it has persisted, at least
in casual usage. However, those in the field prefer the terms “siloxanes”
and “polysiloxanes.”
Kipping
68
prepared the earliest hybrid inorganic/organic material con-
taining Si-C bonds and reported in a series of papers over the period
1899-1944. Around 1930, the Corning Glass Company prepared an
