Chemistry Reference
In-Depth Information
The resultant polymers maintain useful mechanical properties for up to 3,000 h in air at 250
C.
Fluorinated epoxy resins are another group of materials that might potentially possess improved
thermal stability. This, however, has not been demonstrated. It was shown [
190
] though that these
materials can be key intermediates in organic coating and plastics that require special properties, like
hydrophobicity, oleophobicity, light stability, and low friction.
7.17.2 Polyphenylene
This polymer is completely aromatic in character [
182
]. Polymerization of benzene to polyphenylene
was, therefore, investigated quite thoroughly [
184
,
185
]. Benzene [
186
] and other aromatic structures
[
184
,
185
] polymerize by what is believed to be a radical-cationic mechanism. In this type of
polymerization, benzene polymerizes under mild conditions in the presence of certain Lewis acids
combined with oxidizing agents [
186
-
188
]:
AlCl
3
+
+
3n CuCl
2
+
n
3n CuCl
2
H
H
n
2n HCl
The yield of polymers reaches a maximum value (close to quantitative) at an aluminum chloride to
cupric chloride molar ratio of 2:1 [
180
]. Solvents, concentrations, and temperatures affect the
molecular weights of the products [
189
]. Other Lewis acids that are effective in benzene
polymerizations are MoCl
5
, FeCl
3
, and MoOCl
4
. The products, however, possess greater degrees
of structural irregularity [
183
].
Theoretical considerations indicate [
183
] that during the polymerization the benzene rings become
associated in a stacked end to end arrangement. As a result, the radical-cation becomes delocalized
over the entire chain:
C
6
H
6
C
6
H
6
C
6
H
6
C
6
H
6
C
6
H
6
C
6
H
6
[O], (H , -e)
n
n
bond formation shown above can also be accompanied by simultaneous depropagation and
loss of benzene molecules. Chain buildup stops when the radical-cation on the terminal phenyl group
becomes too small to promote further association.
The polymer is very stable up to 500-600
C and oxidizes very slowly. It is, however, quite
insoluble with a very high melting point that makes is difficult to process and even to determine its
molecular weight. Introduction of irregularity into the polymeric structure by copolymerizing
terphenyl, biphenyl, or triphenylbenzene with benzene results in formation of soluble products.
Their molecular weights, however, are low up to 3,000 [
192
]. They melt between 300 and 400
C
and contain phenyl branches and some fused rings. The copolymers can be cross-linked with xylylene
glycol or with benzene-1,3,5-trisulfonyl chloride. Soluble alkyl substituted poly(
p
-phenylene)s were
prepared [
193
] by a coupling process, using palladium catalysts:
The
s
