Thermodynamic and Kinetic Study of Diels–Alder Reaction Between Furfuryl Alcohol and N-Hydroxymaleimides—An Assessment for Materials Application
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molecules Article Thermodynamic and Kinetic Study of Diels–Alder Reaction between Furfuryl Alcohol and N-Hydroxymaleimides—An Assessment for Materials Application Jamerson Carneiro de Oliveira 1,2, Marie-Pierre Laborie 1,2 and Vincent Roucoules 3,* 1 Chair of Forest Biomaterials, Faculty of Environment and Natural Resources, University of Freiburg, 79085 Freiburg, Germany; [email protected] (J.C.d.O.); [email protected] (M.-P.L.) 2 Freiburg Materials Research Center, University of Freiburg, 79104 Freiburg, Germany 3 IS2M, UMR 7361, CNRS, Université de Haute-Alsace, Université de Strasbourg, F-68100 Mulhouse, France * Correspondence: [email protected] Received: 5 December 2019; Accepted: 2 January 2020; Published: 7 January 2020 Abstract: The study of Diels–Alder reactions in materials science is of increasing interest. The main reason for that is the potential thermoreversibility of the reaction. Aiming to predict the behavior of a material modified with maleimido and furyl moieties, 1H NMR and UV-Vis solution studies of the Diels–Alder reaction between furfuryl alcohol and two N-hydroxymaleimides are explored in the present study. Rate constants, activation energy, entropy, and enthalpy of formation were determined from each technique for both reacting systems. Endo and exo isomers were distinguished in 1H NMR, and the transition from a kinetic, controlled Diels–Alder reaction to a thermodynamic one could be observed in the temperature range studied. A discussion on the effect of that on the application in a material was performed. The approach selected considers a simplified equilibrium of the Diels–Alder reaction as the kinetic model, allowing materials scientists to evaluate the suitability of using the reacting molecules for the creation of thermoresponsive materials. The proposed approach determines the kinetic constants without the direct influence of the equilibrium constant value, thereby allowing a more objective data analysis. The effects of the selection of kinetic model, analytical method, and data treatment are discussed. Keywords: Diels–Alder; materials; kinetic; thermodynamic 1. Introduction Diels–Alder (DA) reaction is a well-known [4+2]cycloaddition between a conjugated diene and a dienophile. That addition reaction generally produces two diasteroisomers (endo and exo). The formation and the proportion of the isomers depend on several conditions such as the solvent, temperature, and catalyst [1,2]. The study of Diels–Alder reaction kinetics is extensively explored in literature [3–5]. Depending on the diene and dienophile selected and the reaction conditions, Diels–Alder reaction can be considered irreversible—meaning that the retro-Diels–Alder reaction is neglectable. That consideration is used in several publications [6–9]. The complexity of the kinetic model is increased when the reaction equilibrium and isomers are taken into account [10]. The possible application of the Diels–Alder/retro-Diels–Alder in responsive materials resulted in greater interest for the consideration of the equilibrium reaction in kinetic models for materials applications [11–15]. The use of Diels–Alder reaction in materials science is not a new research topic [16], and nowadays, it covers a significant range including polymer synthesis, pharmaceutical, and biomedical Molecules 2020, 25, 243; doi:10.3390/molecules25020243 www.mdpi.com/journal/molecules Molecules 2020, 25, 243 2 of 12 Molecules 2020, 25, x FOR PEER REVIEW 2 of 12 engineering[17–21]. In that [17 field,–21]. the In thatcomplication field, the of complication considering ofthe considering equilibrium the reaction equilibrium with the reaction kinetic model with the of kineticthe Diels–Alder model of thereaction Diels–Alder can be reaction often minimized, can be often depending minimized, on depending the system on the studied system and studied the andmethodology the methodology selected. selected. For example, For example, Vauthier Vauthier et al. et [9] al. [evaluated9] evaluated the the reaction reaction between between a a plasma polymer surface functionalized with furan moieties an andd maleic anhydride in in so solution.lution. In In their their study, study, the authors found that thethe considerationconsideration of anan equilibriumequilibrium reaction did notnot considerablyconsiderably change the values obtained for for the the rate rate constant constant of of the the forwar forwardd Diels–Alder Diels–Alder reaction. reaction. That That could could be beexplained explained by bythe thefact fact that that the the amount amount of ofspecies species present present in inth thee solution solution was was much much higher higher than than in in the the surface. However, in some cases, it might be necessary to consider the influen influencece of the retro-Di retro-Diels–Alderels–Alder on the kinetics of the system. That That is is true true for for the case of materials that will react in near-equimolar proportions or in whichwhich thethe reversibilityreversibility is planned to take placeplace inin near-equilibriumnear-equilibrium conditions. Diaz Diaz et et al. al. [14], [14], Stirn et al. [12], [12], and and Koehler Koehler et et al. al. [11] [11] modeled modeled the the kinetics kinetics of of their their system system through through different different approaches, approaches, but considered an an equilibrium equilibrium reaction reaction be betweentween diene, diene, dienophile, dienophile, and and one one adduct. adduct. Another factor to be considered in the case of evaluation of the Diels–Alder reaction and the reversibility for for materials materials application application is is the the ratio ratio between between the the isomeric isomeric products. products. As Asthe theendoendo andand exo exoproductsproducts are areknown known to toundergo undergo the the retro-Diels– retro-Diels–AlderAlder reaction reaction at at different different temperatures, the application of that reaction in a responsiveresponsive material—thatmaterial—that ideally will operate in cycles—should be carefully analyzed. Some Some works works have have been been dedicated to that issue. Froidevaux Froidevaux et et al. al. [13] [13] chose a kinetic approach that minimized the effect effect of the equilibriumequilibrium reaction by selectively analyzing the endo adduct decomposition reaction. In a solvent-free approach, Cuvellier et al. [15] [15] showed a strong agreement betweenbetween a kinetica kinetic model model used toused determine to determineendo/exo composition,endo/exo composition, obtained via obtained calorimetric via 1 experiments,calorimetric experiments, and experimental and experimental results obtained results via obtained1H NMR. via H NMR. The present work was oriented by the idea ofof predictingpredicting thethe Diels–AlderDiels–Alder/retro-Diels–Alder/retro-Diels–Alder reaction conditionsconditions forfor useuse in in a a suspended suspended material. material. The The fact fact that that the the suspension suspension should should be be analyzed analyzed in near-equilibriumin near-equilibrium conditions conditions requires requires the prediction the prediction of not only ofthe not kinetic, only butthe also kinetic, the thermodynamic but also the behaviorthermodynamic of the reaction. behavior Although of the reaction. the thermodynamic Although the behaviorthermodynamic of the suspended behavior particlesof the suspended could be theparticles limiting could factor be forthe thelimiting prediction factor of for the the suspension prediction behavior, of the suspension the present behavior, study provides the present information study necessaryprovides information to prove or discard necessary that hypothesis.to prove or Thediscar approachd that hypothesis. selected in the The present approach work selected aims to debatein the qualitativelypresent work theaims influence to debate of quendoalitatively/exo isomers the influence on the reversibility of endo/exo ofisomers the reaction. on the reversibility It also proposes of the a methodreaction. for It thealso determination proposes a method of the kinetic for the and determination thermodynamic of parametersthe kinetic that, and although thermodynamic the endo andparametersexo products that, arealthough not considered the endo in and the model,exo products offers theare possibilitynot considered of a more in the precise model, determination offers the ofpossibility those values of a more than similarprecise contributionsdetermination in of the thos field.e values than similar contributions in the field. 2. Results and Discussion The current current work work evaluated evaluated the the kinetic kinetic and andthermodynamic thermodynamic parameters parameters of two of different two di ffDiels–erent Diels–AlderAlder reactions. reactions. Hydroxyl Hydroxyl terminated terminated molecules molecules were selected were due selected to the due possibility to the possibilityof using that of usingfunction that for function future forgrafting future onto grafting the ontoparticles theparticles of the suspension. of the suspension. The reactions The reactions studied studied here were here werebetween between furfuryl furfuryl alcohol alcohol (FAL) (FAL) and two and N two-hydroxymaleimides:N-hydroxymaleimides: N-HydroxymaleimideN-Hydroxymaleimide (NHM) (NHM) and andN-(2-Hydroxyethyl)maleimideN-(2-Hydroxyethyl)maleimide (N2HM). (N2HM). The Thefollowing following Scheme Scheme 1 1shows shows the the chemicals chemicals and thethe expected reactionreaction product. product.