Chemistry Reference
In-Depth Information
O
Cl
O
OH
HO
+
Cl
Cl
+
Cl
O
O
O
2
O
O
O
O
O
O
O
O
O
m
n
2
The copolymers show little degradation at temperatures up to 400
C. In addition, the copolymers
with high phenoxailin content are soluble in several solvents [
228
].
There are also reports in the literature of attempts to modify polysiloxane backbones to increase
heat stability. These modifications consist of making changes in the electronic character of the Si-O
bonds to prevent rearrangements at high temperatures to low molecular weight cyclic products. It is
known that by making the bonds more ionic than covalent increases heat stability [
191
]. A metal that
is more electropositive, therefore, is used to form metal-oxygen linkages and form polymers that are
more ionic in character. Such metals are aluminum, titanium, tin, and boron. The results, however, so
far are disappointing.
7.17.5 Direct Condensation Polymers
Many polymers with enhanced heat stability can be prepared simply by direct condensation. These
aromatic polymers often contain heterocyclic unit. The materials are high melting, somewhat
infusible, and usually low in solubility. Many aromatic polyimides belong here. Polyimides, as a
separate class of polymers, were discussed in an earlier section, because many are common commer-
cial materials. On the other hand, the materials described in this section might be considered special
and, perhaps, at this points, still too high priced for common usage.
7.17.5.1 Polyimidazoles, Polybenzthiazoles, and Related Polymers
Many
are prepared by direct condensation. They are colored polymers that
mostly melt above 400
C. One such material is formed from 3,3
0
-diaminobenzidine and diphenyl
isophthalate by heating the two together at 350-400
C in an inert atmosphere [
206
]:
polybenzimidazoles
O
Δ
H
2
N
H
2
N
NH
2
NH
2
O
n
+
n
O
O
N
N
N
N
n
