Methylated tetrahydrophthalic anhydrides as end caps in addition polyimides

Thermolysis of methylenedianiline (MDA) 1,2,3,6-tetrahydrophthalic (THP) bismides up to 371 °C, produces aromatic product, with a concomitant lowering in cross-linking. This aromatization is responsible for both the improved thermal oxidative stability of THP end-capped polyimides and their substantial frangibility. In the hope of inhibiting precuring aromatization, 2,3-dimethyl, 3,3-dimethyl, and 1,2,3-trimethyl THP analogues were synthesized and reacted with MDA in a 2:1 ratio, heating gradually from 204 to 371 °C. In the lower temperature range, monoimide transforms to bisimide. At temperatures above 320 °C, cross-linking predominates-generally reaching ∼30% at 371 °C, though it was as much as 69% for 3,3-dimethyl THP. The surprising facility of cross-linking in the latter was correlated to the double bond length of the trans imide formed via thermal enolization. The aromatization (2-25%) observed in these methylated systems at higher temperatures presumably results from oxidative decarboxylation of the pendant methyls, which is the preferred pathway for degradation after cross-linking.