Several substituted 1,2,3,6-tetrahydrophthalic anhydride end caps, including the 3-phenyl,
3-methoxy, 3-trimethylsilyloxy, and 3,6-diphenyl analogues, were synthesized via the Diels−Alder
condensation of the corresponding butadienes and maleic anhydride. These anhydrides, as well as the
commercially available 3-hydro and 4-methyl analogues, were each ground up together with methylenedianiline in a 2:1 ratio and heated gradually from 204 to 371 °C, with the thermolysis followed by NMR.
Generally speaking, a transformation via monoimide to bisimide was observed in the lower temperature
range, followed by competition between cross-linking and aromatization. We believe that this competition
produces a substantial percentage of aromatic product, with the concomitant lowering of the relative
amount of cross-linking and is responsible for both improved thermooxidative stability of tetrahydrophthalic end-capped polyimides and their substantial frangibility. The thermolysis of the tetrahydrophthalimides under an inert atmosphere dramatically lowers the amount of aromatization; hence, the
mechanism for aromatization is an oxidative one.
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