Chemistry Reference
In-Depth Information
All of the above materials maintain useful properties up to 300
C in air and can be formed into fibers.
Some polymeric materials are completely free from hydrogens. An example is
polysulfodiazole
[
212
],
a
polyimide
prepared
from pyrazine-1,2,4,5-tetracarboxylic
acid
anhydride
and
diaminothiazine [
213
]. This material exhibits particularly good heat stability [
213
]:
O
O
N
N
N
+
O
O
H
2
N
NH
2
S
N
O
O
H
O
O
H
N
N
O
O
N
N
N
-2H
2
O
N
C
N
C
N
N
N
S
S
n
N
n
O
O
HO
C
N
C
OH
O
O
Films from this polysulfodiazole maintain their strength and stability at 592
C. Preparations of
several other, similar polyimides was reported [
219
]. A polyimide, however, prepared from
diaminothiazole with pyromellitic dianhydride chars at 320
C in air. The chemistry and preparations
of the principal types of polyheteroarylenes were review by Krongauz [
214
].
A series of thermally stable, organic solvent-soluble polyimides were synthesized by reacting
3,7-diaminophenothiazinium chloride (thionine) with four different dianhydrides [
243
]. These
polyimides can be illustrated as follows:
O
O
N
N
N
N
S
Cl
O
O
n
Many polymers that are described in this section can be prepared in either one or in two steps.
In the one-step process, polyphosphoric acid is often employed as a solvent. It is a proton donor,
promotes condensation, and acts as a cyclodehydrating agent, frequently yielding polymers of high
molecular weights. Shaping the resulting polymers, however, can be a problem due to infusibility
and insolubility. When prepared in two steps, the polymer can be shaped into films or fibers first,
before much cyclization takes place, while the material is still fusible and solvent-soluble. This is
followed by further heating to complete the process. Cyclization in this process occurs in solid
polymers that become increasingly rigid as the reaction progresses. The products, however,
prepared by this procedure are not as fully cyclized as are those formed in the one-step process
in phosphoric acid solution.
7.17.6 Oligomers with Terminal Functional Groups
The above-described two-step processes yield polymers that evolve volatiles upon further heating
because the condensations continue. To overcome this drawback, prepolymers were developed
that undergo addition-type reactions at fairly moderate time-temperature schedules [
215
,
216
].
