Chemistry Reference
In-Depth Information
by the structural features of the chains. As expected, the
p
-silphenylene
group has a larger stiffening effect, increasing the
T
g
to -18°C and giving
rise to crystallinity with a
T
m
of 148°C. The resulting polymer is thus a
ther-
moplastic
siloxane. Apparently,
ortho
(
o
) silphenylene units have not been
introduced in this way; they are probably much harder to incorporate be-
cause of steric problems. Even if they could be incorporated, such groups
would not be expected to have much of a stiffening effect on the chain.
Silarylene polymers that contain more than one phenylene group in the
repeat unit could be of considerable interest because of the various
meta,
para
combinations that could presumably be synthesized. Cyclohexylene
(C
6
H
10
) groups can switch between boat and chair forms and should thus
be less stiff than phenylene groups. Both
meta-
and
para
-groups are of in-
terest in this regard. Table 5.1 summarizes some of these modifications
and their likely effects. Figure 5.3 also has a sketch showing the use of
bulky side groups to make a chain stiffer, such as by replacing one of the
Table 5.1.
SOME POLYSILOX ANE STRUCTUR AL CHANGES
AND EXPECTED CHANGES IN PROPERTIES
Expected changes
in properties
Part of repeat unit
Structural change
Backbone
Replacement of O by CH
2
Decrease in polarity and
tendency to cyclize
Insertion of
p
-phenylene
Large decrease in flexibility
Insertion of
m
-phenylene
Smaller decrease in flexibility
Insertion of
p
-cyclo-hexylene
Small decrease in flexibility
Insertion of
m
-cyclo-hexylene
Small decrease in flexibility
Side group
(CH
2
)
m
CH
3
, m > 0
Tendency to form liquid-
crystalline phases
Phenyl (atactic placements)
Suppression of crystallinity,
and increase in radiation
resistance
Fluorinated groups
Increase in solvent resistance
Bulky groups in general
Decrease in flexibility
H atoms
Capacity for cross linking by
vinyl groups
Vinyl groups
Capacity for cross linking by
H atoms
Hydrophilic groups
Increase in water solubility
Chiral groups, e.g.,
-CH(CH
3
)C
2
H
5
Generation of optical activity
