A solid-state C-13 NMR study of the cure of C-13-labeled phenylethynyl end-capped polyimides

The cure reactions of phenylethynyl end-capped polyimides were investigated using solid-state C-13 magic-angle spinning (MAS) nuclear magnetic resonan ce (NMR). A C-13-labeled model compound (C-13-PEPA-3,4'-ODA) and an imide o ligomer (C-13-PETI-5) were synthesized and characterized. The thermal cure process for C-13-PEPA-3,4'-ODA was followed over the temperature range 318- 380 degrees C and for(13)C-PETI-6 over the temperature range from 350 to 40 0 degrees C. Our NMR results showed that, for the model compound, as curing proceeded, the percentage of polymeric structures containing double-bonded and single-bonded carbon increased while the percentage of triple-bonded c arbon gradually decreased and finally disappeared at the elevated temperatu res. The PETI-5 cure process was very similar to the PEPA-3,4'-ODA cure pro cess, and the percentage of double-bonded carbon structure of PETI-5 increa sed during the cure process as the percentage of triple-bonded carbon decre ased. Moreover, for the PETI-5 resin system, a weak broad C-13 Signal due t o a single-bonded structure was observed after cure. The carbonyl groups re mained relatively constant during the curing reactions for both the model c ompound and PETI-5 resin. The appearance of single-bonded structures in the cure of the model compound and PETI-5 can be derived from polyene structur es by a further intra- or intermolecular Diels-Alder reaction to form cyclo olefinic ring or branched structures. On the basis of the chemical shift da ta of several low molecular weight compounds with aromatic ring structures and polyene structures, we cannot exclude the formation of substituted arom atic ring structures from PEPA-3,4'-ODA or from PETI-5.