Characterization of the chemical structure of thermosetting resins by high-resolution solid-state carbon-13 nuclear magnetic resonance spectrometry

The combination of proton-dipolar decoupling, magic-angle spinning and cross-polarization techniques results in high-resolution ¹³C-nuclear magnetic resonance (n.m.r.) spectra of solid-state compounds. In homogeneous materials, using a relatively low external field and in the absence of motional line broadenings, the chemical structure of totally insoluble curec polymer resins can be estimated quantitatively, and the chemical mechanisms leading to their formation can be elucidated. The capability of high-resolution solid-state ¹³C-n.m.r. for solving such problems is illustrated by the example of the polystyrylpyridine system obtained by reaction of terephthalic aldehyde with pyridine. The decreasing amounts of aldehyde and methyl groups and the appearance of network crosslinkages were studied as a function of both the temperature and the duration of the curing treatment. The results are interpreted quantitatively in terms of two chemical mechanisms: first, the addition of a methyl group from the collidine molecule to the aldehyde function of terephthalic aldehyde followed by elimination of water; secondly, a cross-linking reaction consisting of the addition of a collidine methyl group to the double bonds formed in the first process. The high resolution of the ¹³C-n.m.. spectra also makes it possible to observe the different reactivities of the o- and p-methyl substituents of the collidine molecule. p-Methyl reactions occur only when most of the o-methyl functions have disappeared. Thus, high-resolution solid-state ¹³C-n.m.r. provides a precise characterization of the polystyrylpyridine resins in terms of both their quantitative chemical composition and their reaction mechanisms.