The Diels-Alder reaction is one of the most studied thermoreversible equilibrium reactions. This reaction is a [4+2] cyclocaddition reaction between a conjugated diene and a dienophile to form a six membered cyclic adduct, also known as the Diels-Alder cycloadduct. The equilibrium is such that at low temperatures the Diels-Alder cycloadduct is formed predominantly, whereas at higher temperatures the equilibrium shifts towards the reactants. The most studied Diels-Alder reaction is the reaction between a furan (diene) and a maleimide (dienophile). Their fast reaction kinetics and high conversion at room temperature make them suitable candidates for thermoresponsive materials, e.g., thermoremendable and self-healing networks, which can be reshaped and healed at elevated temperatures due to the thermo-reversible crosslinking. In previous work, these materials have been employed in self-healing coatings and self-healing soft robotic actuators. In the current work, the reaction kinetics of the Diels-Alder reaction between furan and maleimide is thoroughly investigated in view of mentioned applications.

Two stereoisomeric cycloadducts can be formed depending on the suprafacial approach between the furan and maleimide functional groups. The exo isomer, formed when the bulkier sides of the diene and dieneophile lie away from each other during the cycloaddition reaction, is the more thermodynamically stable adduct since less steric hindrance is occurring. The endo isomer, formed when the bulkier sides lie above each other, is formed faster due to a lower activation energy barrier. Although the stereochemistry is well known, its effect on the reactivity, thermal reversibility and kinetics is often overlooked in literature especially with regard to the popular use for self-healing materials, remendability and advanced processing. The few kinetics studies determining kinetics parameters for both isomers separately are performed in a solvent, affecting the reaction kinetics.

In this study an amorphous, liquid bismaleimide is used allowing a thorough kinetic evaluation of the Diels-Alder reaction between furan and maleimide without the interference of a solvent in the reaction kinetics. For the modelling an in-house developed Matlab software was used, which optimizes the natural logarithm of the rate constants at a reference temperature and the activation energies to fit the isothermal (microcalorimetry) and non-isothermal (Differential Scanning Calorimetry) experimental data. The obtained kinetic parameters have been confirmed using proton Nuclear Magnetic Resonance by following the evolution of the concentration profiles of the monomers and both adducts as a function of time during reaction.
Original languageEnglish
Publication statusPublished - 28 May 2018
EventAnnual meeting of the Belgium Polymer Group 2018 - Blankenberge, Belgium
Duration: 28 May 201829 May 2018

Conference

ConferenceAnnual meeting of the Belgium Polymer Group 2018
CountryBelgium
CityBlankenberge
Period28/05/1829/05/18

ID: 38004001