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The solid-state cure mechanisms of an acetylene-terminated polyisoimide have been studied using solid-state l3C cross-polarixation 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 isoimidelimide 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 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 ('H) 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 isoimidelimide 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.

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SynopsisPurified insoluble elastin samples labeled with [l-13C]valine, [l-13C]alanine, and [1-13C]-lysine were prepared from chick aorta in culture. The molecular mobility at the labeled sites was investigated using 13C-lH magnetic double-resonance spectroscopy. Linewidths, 2'1, and nuclear Overhauser effect (NOE) values of the labeled carbons alone were obtained from dipolar decoupled difference spectra. Analysis of these parameters together with signal intensity measurements showed that essentially all the valyl residues, ca. 75% of the alanyl residues, and ca. 60% of the lysyl residues were characterized by rapid backbone motions having ? = 65 nsec. Resonances due to the remaining alanyl and lysyl residues were detected in cross-polarization experiments, which enhance the signals of motionally restricted carbons. Since lysyl and alanyl residues are found in the crosslink regions of elastin, whereas valyl residues are not, we conclude that crosslinks rather than secondary structures in the extensible region of the protein are the main source of motional restrictions in the protein. Elastin chain mobility was monitored by linewidth measurements over the range -90 to +70"C. When the swelling solvent (0.15M NaCl) was fixed at 0.6 g/g of elastin, a rapid monotonic reduction in chain mobility was observed as the temperature was lowered from 50 to 5°C. Liquidlike mobility was completely lost at 5°C. In contrast, the same sample in contact with excess solvent retained its liquidlike molecular mobility until -13"C, where it abruptly became rigid. The molecular mobility of this sample was temperature insensitive in the physiologically interesting range, 20-40°C, as a consequence of the opposing influences of temperature and swelling. Taken together these nmr data indicate that under physiological conditions, elastin is a network of mobile chains whose motions are strongly influenced by protein-solvent interactions.