Synthesis of novel polyimides from dianhydrides with flexible side chains - Polyimides and Other High Temperature Polymers - page 71

“semiaromatic polyimides” [22]. The introduction of an alicyclic unit into the
polyimide backbone would reduce polymer-polymer interaction and enhance the
solubility in organic solvents. These polyimides showed excellent thermal stabil-
ity with no significant weight loss up to approximately 400°C and the 5% weight-
loss temperature in a N 2 atmosphere was over 450°C; some of them had glass
transition temperatures over 380°C. The high-temperature stability can also be
explained by the introduction of an alicyclic structure, which would foster less
probability of main-chain scission because of the presence of multibonds and
would also increase main-chain rigidity. Of course, the use of alicyclic polymer
constituents implies that the ultimate end use of such materials is targeted for ap-
plications with less stringent thermal requirements than those of aromatic polyim-
ides. Alicyclic polyimides, however, have attracted much attention in recent years
because of their potential applications including use as liquid crystal orientation
layers, nonlinear optics buffer layers, or low dielectric materials.
On the other hand, there are only a few studies on polyimides containing various
kinds of side groups as well as other functional groups such as alicyclic units. To
achieve these aims, a proper design of new functionalized monomers is required.
An effective functionalization of a monomer depends on its physical form, chemical
reactivity and stability. These factors are crucial in the design of new materials and
can be influenced by the conditions employed during their preparation.
This article reports on recent developments in polyimides having flexible side
groups for heat resistance or LC alignable materials with particular emphasis on
their synthesis and properties as well as the new structure-property relationships
for novel polyimides.
2. SYNTHESIS OF POLYIMIDES FROM NEW DIANHYDRIDES HAVING
ALKYLOXY PENDENT GROUP
2.1. Synthesis of new dianhydride monomers
Two different kinds of dianhydrides having various alkyl lengths were prepared.
For easier comprehension, we have listed their chemical structures and codes in
Figure 2.
First of all, as shown in Figure 3, the two (n-alkyloxy)phenyloxy side chains
[23] were successfully introduced into pyromellitimide by nucleophilic substitu-
tion reaction of bromine atoms by (n-alkyloxy)phenoxide anions in pyridine at
0 o C followed by hydrolysis of the imide groups. The dianhydrides blocked by ani-
line, 3,6-bis(n-alkyloxy)phenyloxy-N,N'-diphenylpyromellitimides, were hydro-
lyzed to obtain the corresponding tetracarboxylic acids. This hydrolysis was car-
ried out by a two-step process: basic hydrolysis reaction using NaOH followed by
acidic hydrolysis reaction through HCl/DMSO system. The resulting tetraacids
were cyclodehydrated by treating with acetic anhydride. Irrespective of the side
chain length, all the dianhydride monomers could be purified to polymerization
grade by recrystallization from toluene.