Chemistry Reference
In-Depth Information
CHAPTER 3
Types of Polysiloxanes
3.1 HOMOPOLYMERS
The polysiloxane of greatest commercial importance and scientific interest
is poly(dimethylsiloxane) (PDMS), [Si(CH
3
)
2
-O-]
x
, a member of the sym-
metrical dialkyl polysiloxanes, with repeat unit [SiR
2
-O-]
x
. This polymer is
discussed extensively in the following chapters, particularly in chapter 5.
Other members of this series are poly(diethylsiloxane) [Si(C
2
H
5
)
2
-O-]
x
,
and poly(di-
n-
propylsiloxane) [Si(C
3
H
7
)
2
-O-]
x
. An example of an aryl
member of the symmetrically substituted series is poly(diphenylsiloxane),
with repeat unit [Si(C
6
H
5
)
2
-O-]
x
.
1-17
This polymer is unusual because of its
very high melting point and the mesophase it exhibits. The closely related
polymer, poly(phenyl/tolylsiloxane), has also been prepared and studied.
18
The unsymmetrically substituted polysiloxanes have the repeat unit
[SiRR'O-]
x
, and are exemplified by poly(methylphenylsiloxane) [Si(CH
3
)
(C
6
H
5
)-O-]
x
19-24
and poly(methylhydrosiloxane) [Si(CH
3
)(H)-O-]
x
.
25
In
some cases, one of the side chains has been unusually long, for example
C
6
H
13
, C
16
H
33
, and C
18
H
37
,
26-27
including a branched side chain—CH(CH
3
)-
(CH
2
)
m
-CH
3
.
27
Another example has methoxy-substituted aromatic frag-
ments as one of the two side chains in the repeat unit.
28
Such chains have
stereochemical variability in analogy with the vinyl polymers such as poly-
propylene [CH(CH
3
)-CH
2
-]
x
and vinylidene polymers such as poly(methyl
methacrylate) [C(CH
3
)(C = OOCH
3
)-CH
2
-]
x
.
29
One can also introduce opti-
cally active groups as side chains, the simplest example being the
secondary
butyl group—CH(CH
3
)(C
2
H
5
). Another example involves redox-active den-
dritic wedges containing ferrocenyl and carbonylchromium moieties.
30
Other substituents have included phenylethenyl groups,
31
cyclic silox-
ane groups,
32
and Cr-bound carbazole chromophores.
33
In a reversal of
