Acetylene-terminated polyimide cure studies using ¹³C Magic-Angle Spinning NMR on Isotopically Labeled Samples

Abstract

The solid-state cure mechanisms of an acetylene-terminated polyisoimide have been studied using solid-state 13C cross-polarization magic-angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectroscopy on samples which had been selectively labeled using 13C labeled precursors. The polymer backbone was labeled at sites which were expected to be involved in the cure chemistry: the benzophenone carbonyl carbon, the isoimide/imide carboxyl carbon, the C-1 (quaternary acetylene) ethynyl carbon, or the C-2 (terminal acetylene) ethynyl carbon. After preparing and subsequently curing the selectively labeled and unlabeled (control) resins identically, difference spectroscopy techniques were used to subtract the resonances due to the natural abundance 13C nuclei, thereby resulting in spectra which were due to the selective label alone. Combining this technique with delayed decoupling experiments in which one allows the 13C nuclei that are coupled to protons (1H) to relax, those resonances which are protonated and nonprotonated were identified. The results were correlated with model and related compounds. The carbonyl function remained unchanged in the cured product. The isoimide/imide carboxyl carbons underwent an isomerization reaction to produce the expected imide structure. The solid-state ethynyl cure products were found to contain aromatic structures, condensed polycyclic aromatic structures, backbone addition and bridge structures. Steric factors and population densities of reactive sites in the polymer could influence the ratio of these products. © 1992, American Chemical Society. All rights reserved.