Poly(ary1 ether phtha1azine)s were found to undergo an exothermic reaction at a temperature range of 360-44OOC. We elucidated the origin of the exothermic reaction and the physiochemical phenomena associated with it, based on thermal analyses, model compound studies, and 13C solid-state NMR studies. At elevated temperatures, polymers containing a diphenylphthalazine moiety 4 underwent extensive thermal crosslinking reactions as a result of a nitrogen elimination reaction of the phthalazine moiety. However, polymers containing the tetraphenyl or hexaphenyl phthalazine moiety 5 and 6 were found to undergo principally a backbone rearrangement reaction, in which the phthalazine moiety rearranged to a quinazoline. Utilizing this efficient thermal rearrangement of polyphenylated phthalazines, we have prepared a novel activated difluoride, 2,4-bis(4-fluoropheny1)-,5,6,7,8-tetraphenylquinazoline 9d, which underwent high-temperature solution polycondensation with BPA to give the quinazoline containing poly(ary1 ether) 14. Polymer 14 is amorphous, has a glass transition temperature of 264"C, and has high thermooxidative stability with 5% weight loss being recorded a t 514OC in nitrogen.