A solid-state 13C NMR study of the cure of 13C-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 resonance (NMR). A C-13-labeled model compound (C-13-PEPA-3,4'-ODA) and an imide oligomer (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 400 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 carbon gradually decreased and finally disappeared at the elevated temperatures. The PETI-5 cure process was very similar to the PEPA-3,4'-ODA cure process, and the percentage of double-bonded carbon structure of PETI-5 increased during the cure process as the percentage of triple-bonded carbon decreased. Moreover, for the PETI-5 resin system, a weak broad C-13 Signal due to a single-bonded structure was observed after cure. The carbonyl groups remained relatively constant during the curing reactions for both the model compound 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 structures by a further intra- or intermolecular Diels-Alder reaction to form cycloolefinic ring or branched structures. On the basis of the chemical shift data of several low molecular weight compounds with aromatic ring structures and polyene structures, we cannot exclude the formation of substituted aromatic ring structures from PEPA-3,4'-ODA or from PETI-5.