Thermally Curing Aryl−Ethynyl End-Capped Imide Oligomers:  Study of New Aromatic End Caps

Two new aryl−ethynyl end-cap model compounds, N-phenyl-4-(1-naphthylethynyl)phthalimide (1) and N-phenyl-4-(2-naphthylethynyl)phthalimide (3), were synthesized and characterized. The single-crystal molecular structure of 1 reveals an alternating “head to tail” packing mode whereas the “heads and tails” of 2 are aligned and not alternating. The thermal cure kinetics of 1 and 3 were analyzed using NMR and FT-IR spectroscopy, respectively, and compared to N-phenyl-4-phenylethynylphthalimide (2). In both cases the naphthyl−ethynyl model compounds reacted faster than the phenyl−ethynyl derivative. The end-capped imide oligomers analogues of 1 and 2, NETI-5 and PETI-5, respectively, were synthesized and their thermal cure kinetics were analyzed by DSC. The thermal cure kinetics of 1 and 2 best fit a first-order rate law. Compound 1 cures at faster rates than 2, although the calculated E a values of 1 and 2 are statistically indistinguishable. The thermal cure kinetics of 2 and 3 also fit a first-order rate law and show that the E a of 3 is higher than that of 2, although 3 thermally cures at slightly faster rates than 2, but slower than 1 (1H NMR) at the temperatures studied. The E a values determined for 2 by 1H NMR and by FT-IR differ significantly. The DSC thermal cure kinetics of NETI-5 and PETI-5 best fit a first-order rate law and revealed a rate acceleration trend similar to that observed for the model compounds 1 and 2. Effectively, NETI-5 cures at the same rate as PETI-5 but at approximately a 30 °C lower curing temperature.