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The oxidative degradation of PMR (for polymerization of monomeric reactants) polyimides at elevated temperatures was followed by cross-polarized magic angle spinning (CP-MAS) NMR. Labeling of selected sites in the polymers with 13C allowed for direct observation of the transformations arising from oxidation processes. The formation of several degradation products has been proposed to be occurring in the cross-links derived from the nadic end caps on oxidation. Model compounds have been synthesized and characterized by CPMAS NMR with both normal and delayed decoupling to distinguish between protonated and unprotonated carbons. Comparison of these spectra to predicted chemical shifts of proposed products for the aged polymer provides further insight to degradation occurring in the cross-linked moieties.

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We consider possible replacements for the norbornenyl end cap in addition polyimides that should favor bridge degradation and lead to lower weight loss in the resin system. Our preliminary evaluation demonstrates that molding powder made with 7-hydroxynorbornene-2,3-dicarboxylic acid, 17, as the end cap can be fully imidized at 200 °C. By differential scanning calorimetry (DSC), onset of cross-linking occurs approximately 50 °C lower than that for the unsubstituted end cap. The hydroxy-bearing carbon on 17 is more easily oxidized to carbonyl on aging in the polymer than the parent end cap. Therefore, the new end cap more highly favors bridge degradation. However, processing studies, long-term weight loss, and careful evaluation of degradation mechanisms must be carried out to fully assess 17 as a more thermo-oxidatively stable replacement for the parent norbornenyl end cap.

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