Chemistry Reference
In-Depth Information
Also,
N
,
N
-bis(ethoxycarbonyl) pyromelitimide condenses with diamines to yield polyimides [
100
]:
O
O
O
N
N
O
+
C
2
H
5
C
2
H
5
H
2
N
NH
2
O
O
O
O
O
O
O
O
H
H
C
2
H
5
N
NO
O
N
N
C
2
H
5
n
H
H
O
O
O
O
O
O
O
+
N
O
N
2n
C
2
H
5
O
NH
2
n
O
O
The first step in the above preparation takes place in solution. After casting a film, the second step
takes place at 240
C under vacuum. Interesting polyimides also form from reactions of 2,2
0
,6,6
0
-
biphenyltetracarboxylic acid anhydride [
101
] with aromatic diamines, like 4,4
0
-diaminodiphenyl ether:
O
O
+
O
O
H
2
N
O
NH
2
O
O
O
O
N
N
O
O
O
n
As in the previous cases, the polyamic acid forms first in solution at approximately 40
C. It is
converted to the polyimide by heating in acetic anhydride at reflux for 18 h.
Heterocyclic dianhydrides, like pyrazinetetracarboxylic dianhydride, also react with diamines to
form polyimides [
102
]. Such polymers are harder to form, however.
Two types of thermoset polyimides are currently prepared commercially. They are based on low
molecular weight bis imides such as bis maleimides or bis-5-norbornene-2,3-dicarboximides. Due to
unsaturations, the materials cross-link by free-radical mechanism into tight networks. Michael type
additions of primary and secondary amines to the bis maleimides are often used to chain-extend them
before cross-linking. This reduces the cross-linking density and the brittleness [
115
]. The materials
are designated by the term PMR, for polymerizable monomeric reactants.
Ueda et al. [
102
] synthesized a polyimide from 3,6-bis-(4-aminophenylenesulfanyl)pyridazine.
O
O
S
S
N
N
N
N
S
S
n
S
O
O
The polymer has a high refractive index, high transparency, and is aimed at optical devices.
