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.

The nitration of tetraiodoethylene (7) yields 1,1-diiodo-2,2-dinitroethylene (8). The latter reacts with alkylamines 9 or alkyldiamines 11 to give the corresponding acyclic 1,1-diamino-2,2-dinitroethylenes 10 or their cyclic analogs 12, respectively. On the basis of liquid and solid-state (13)C and (15)N NMR data, x-ray analysis and ab initio calculations, we suggest that the title compounds are always zwitterionic and that the C(A)-C(N) bond is not a true double bond.

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