ELUCIDATION OF THE CROSS-LINK STRUCTURE OF NADIC-END-CAPPED POLYIMIDES USING NMR OF C-13-LABELED POLYMERS

Solid NMR of C-13 isotope-labeled samples of PMR-15 was used to follow the cross-linking reaction of the nadic end cap. Some samples were la beled on one of the carbon atoms of the nadic end cap, and others on t he methylene carbon atom of the methylenedianiline portion of the poly mer. NMR spectra were run on these samples both before and after cross -linking. In this way, direct evidence of the major products of cross- linking under normal cure conditions is provided. The majority (simila r to 85%) of the cross-linking derives from olefin polymerization thro ugh the double bond of the end cap. Approximately 15% of the products could come from a pathway involving a retro-Diels-Alder reaction. Howe ver, all of the products could be explained by a biradical intermediat e without a retro-Diels-Alder reaction. Evidence is also presented tha t the methylene moiety in the methylenedianiline part of the polymer c hain also participates in the cross-linking, albeit to a small extent, by a radical transfer reaction. Different cure conditions (higher tem peratures, longer times) could change the relative distribution of the products.