Acetylene-terminated polyimide cure studies using carbon-13 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 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.

Synthesis and Characteristic of Novel Soluble Triazoleimide Oligomers with Terminated Arylacetylene

“…[23][24][25][26][27][28] Solid-state 13 C-NMR, FT-IR and ESR analyses have brought some insight into the cure mechanism of arylacetylene group. The results demonstrated that the arylacetylene group underwent cyclotrimerization, hydrogen addition, DielsAlder cycloaddition, etc., and formed a complex crosslinked structure like condensed polycyclic aromatic structure during the cross-linking process.…”

Mechanism of Step-Growth Thermal Polymerization of Arylacetylene

“…Solid state 13 C NMR cross polarization magic angle spinning data, obtained from 13 C isotope enriched samples, allowed Swanson et al (13) to suggest that the polymerization can occur according to different reaction pathways: cyclotrimerization, biradical mechanism, Glaser and Straus coupling, FriedelCraft addition and Diels-Alder cycloaddition. Diyne and enyne linear dimers formed from Glaser and Straus coupling reactions might be produced during the first steps of the thermal reaction (14,15).…”

Synthesis and characterization of polyarylacetylene for use in the monolithic vitreous carbon processing

“…The cyclotrimerization is usually a nickel-catalyzed reaction in which three ethynyl groups are mostly linked together to form a benzene ring, as presented in Figure 1 [13][14][15] . The cyclotrimerization produces aromatic rings stabilized by resonance that improve the thermal stability of the system and increase the carbon yield during pyrolysis [14,16,17] .…”