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]. Such prepolymers are terminated by functional groups. Following is an example of one such material, an oligomer, polyquinoxaline terminated by acetylene groups:
No volatiles can be detected by mass spectrometry [215, 216] or by thermo gravimetric analyses during the cross-linking reaction. The thermo oxidative stability of the resultant polymers is at least equivalent to polyphenylquioxalines not terminated by acetylene. The cross-linking reaction was shown on a model compound to be an intramolecular cyclization [215-217]:
It is not necessary for the acetylenic groups to be on the terminal ends of the prepolymers. They can also be located as pendant structures [216].
Aromatic polyamides with terminal acetylenic groups [218] were formed from 2,2'- diiododiphenyl-4,4'-dicarbonyl chloride reacted with aromatic diamines. The phenylethynyl groups were introduced by reacting the iodine moieties with copper phenyl acetylide. Thermal treatment converted the prepolymers to 9-phenyl dibenzanthracene-based rigid-rod polymers that fail to melt below 500°C.
High molecular weight polyquinoxaline polymers were prepared from 3,3′,4,4'-tetraamino- biphenyl that was reacted with aromatic bis(2-diketones) and/or ethynyl-substituted aromatic bis (x-diketones) [219]. The polymers contain 0, 5, 10, 30, and 100% pendant groups. Also, ethynyl- substituted diketones were synthesized by the following procedure:
The synthesis is completed by condensation with a tetramine:
Above shown reactions yield polymers with high Tg values. The materials, however, exhibited lowered thermo oxidative stability. The same was found to be true when the ethynyl moieties were replaced by phenylethynyl groups [219]. Other functional groups that were investigated [229] are phenylethynyl, phenylbutadiynyl, phenylbutenyl, biphenylene, styryl, maleimide, nadimide (5-norbornene-2,3-dicarboximide), cyanate, and N-cyanourea [230]. The advantage of terminally capped prepolymers is that they melt at lower temperatures and can be dissolved in different solvent. Heating of these materials converts them to thermally stable polymeric networks. A recent paper reports preparation of quinoline oligomers that were end-capped with 4 acetylbenzocyclobutene, 6-acetyl-8-phenyl-1,2-dihydro-[3, 4] cyclobuta-[1,2-b] quinone (CBQ), and 8-acetyl-6b,10b-dihydrobenzo-[j]cyclobuta-[1,2-a]-acenaphthalene (BCBAN) [230]. The structures of the two cross-linking groups are:
It was reported [230] that the oligomer capped with BCBAN yields a cured film that exhibits good flexural moduli and superior heat stability in air at 400C.
