Tetrahydrophthalic Anhydrides as Addition Curing Polyimide End Caps: Thermal Isomerization of Methylendianiline 3,6-Diphenyltetrahydrophthalic Bisimides

摘要

Over the past decade, we have explored the mechanism of the thermal oxidative degradation of addition-curing polyimides, and have shown that it is the end cap which is responsible for much of the observed weight-loss. Further exploration focused on a variety of new end caps in the hope that they would prolong the useful lifetime of polyimide materials. We recently reported in this journal on our reevaluation of tetrahydrophthalic anhydrides as such an end cap. In particular, the preferred Diels-Alder endo-addition of butadienes with maleic anhydride conveniently yields these anhydrides in a cis, cis, cis configuration. When this anhydride is heated with methylenedianiline (MDA) to 204 degrees C, the primary product at this stage was bisimide, accompanied by a small amount (lesser than 2%) of the intermediate/precursor monoimide. Further heating of the bisimide to 371 degrees C in the presence of air, results in competititve crosslinking and aromatization. Under inert conditions, aromatization is inhibited and crosslinking is favored.