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Keywords:

  • Diels–Alder reactions;
  • self-healing polymer networks;
  • thermoreversible reactions

Abstract

The Diels–Alder (DA) reaction between maleimide and furan moieties possessing various substitutions was performed as a means for developing predictive capabilities for temperature and conversion-dependent material properties in networks comprised of DA moieties. Using HNMR spectroscopy, the reactions of maleimide- and furan-containing molecules further functionalized with carboxylic acids were monitored to ascertain the impact that substitutional changes had on the thermodynamic and kinetic behavior of the DA reaction. The reaction rate and equilibrium conversion of the furan and maleimide increased when the carboxylic acid functional group directly connected to the furan ring was moved from the two to the three position. When an aliphatic two-carbon spacer was used, such that the π-electrons of the carboxylic acid and furan were no longer conjugated, the reaction rate increased further. We also report the reactivity effect on the distance between the carboxylic acid functional group and the maleimide, which yielded little impact on the reaction rate but exhibited increased equilibrium conversion with increasing distance. Additionally, the impact on the kinetic and thermodynamic properties of coupling the carboxylic acid to another molecule, tert-butyl glycine, was also determined. When the carboxylic acid was coupled to an amine, the DA reaction between the furan and maleimide was generally found to have similar kinetic and thermodynamic behavior as compared to their uncoupled, carboxylic acid equivalents. Thus, the characterized and tabulated kinetic and thermodynamic data presented herein enables the prediction of a broad set of temperature-dependent chemical and material properties. Finally, we discuss practical limitations and nuances of the DA reaction, such as the potential for the maleimide to ring-open in aqueous media via hydrolysis. © 2012 American Institute of Chemical Engineers AIChE J, 2012