molecules Communication Maleimide Self-Reaction in Furan/Maleimide-Based Reversibly Crosslinked Polyketones: Processing Limitation or Potential Advantage? Felipe Orozco 1 , Zafarjon Niyazov 1, Timon Garnier 1, Nicola Migliore 1 , Alexander T. Zdvizhkov 1, Patrizio Raffa 1 , Ignacio Moreno-Villoslada 2, Francesco Picchioni 1 and Ranjita K. Bose 1,* 1 Department of Chemical Engineering, Product Technology, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands; [email protected] (F.O.); [email protected] (Z.N.); [email protected] (N.M.); [email protected] (A.T.Z.); [email protected] (P.R.); [email protected] (F.P.) 2 Laboratorio de Polímeros, Instituto de Ciencias Químicas, Facultad de Ciencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile; [email protected] * Correspondence: [email protected] Abstract: Polymers crosslinked via furan/maleimide thermo-reversible chemistry have been exten- sively explored as reprocessable and self-healing thermosets and elastomers. For such applications, it is important that the thermo-reversible features are reproducible after many reprocessing and healing cycles. Therefore, side reactions are undesirable. However, we have noticed irreversible changes in the mechanical properties of such materials when exposing them to temperatures around 150 ◦C. Citation: Orozco, F.; Niyazov, Z.; In this work, we study whether these changes are due to the self-reaction of maleimide moieties Garnier, T.; Migliore, N.; Zdvizhkov, that may take place at this rather low temperature. In order to do so, we prepared a furan-grafted A.T.; Raffa, P.; Moreno-Villoslada, I.; polyketone crosslinked with the commonly used aromatic bismaleimide (1,10-(methylenedi-4,1- Picchioni, F.; Bose, R.K. Maleimide phenylene)bismaleimide), and exposed it to isothermal treatments at 150 ◦C. The changes in the Self-Reaction in Furan/Maleimide- 1 Based Reversibly Crosslinked chemistry and thermo-mechanical properties were mainly studied by infrared spectroscopy, H- Polyketones: Processing Limitation or NMR, and rheology. Our results indicate that maleimide self-reaction does take place in the studied Potential Advantage? Molecules 2021, polymer system. This finding comes along with limitations over the reprocessing and self-healing 26, 2230. https://doi.org/10.3390/ procedures for furan/maleimide-based reversibly crosslinked polymers that present their softening molecules26082230 (decrosslinking) point at relatively high temperatures. On the other hand, the side reaction can also be used to tune the properties of such polymer products via in situ thermal treatments. Academic Editor: Teresa Pinho e Melo Keywords: Diels-Alder; thermo-reversibly crosslinked polymers; maleimide self-reaction; maleimide and Maria Isabel L. Soares homopolymerization; self-healing polymers Received: 25 February 2021 Accepted: 7 April 2021 Published: 13 April 2021 1. Introduction Publisher’s Note: Maleimide chemistry has long been used in the preparation of polymer resins [1–4]. MDPI stays neutral ◦ with regard to jurisdictional claims in At temperatures above 200 C, maleimides undergo homopolymerization in bulk via free published maps and institutional affil- radicals, forming a backbone of succinimide units (Figure1a) [ 2,3,5]. This self-reaction 0 iations. is also known to occur for 1,1 -(methylenedi-4,1-phenylene)bismaleimide (MDP-BMI) (Figure1b) at lower temperatures (from 180 ◦C[2,5]) even without free-radical initiators [5]. However, such chemistry presents a drawback; it is irreversible. Therefore, reprocessing this sort of polymer product is overly complicated. Nowadays, in the polymer field, MDP-BMI receives much more attention as a Diels- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Alder (DA) dienophile, with furan as its diene counterpart (Figure1c) [ 4,6–14]. This DA This article is an open access article pair reacts thermo-reversibly, which allows for preparation of polymer networks with distributed under the terms and crosslinking points that cleave and form thermodynamically. The DA pair also presents conditions of the Creative Commons fast kinetics and forms and cleaves at practical temperatures, at approximately 60 and ◦ Attribution (CC BY) license (https:// 110 C, respectively [8,14]. Over the last two decades, many furan/maleimide-based creativecommons.org/licenses/by/ thermo-reversibly crosslinked polymers (FM-TRCP) have been explored as reprocessable 4.0/). thermosets and elastomers, and self-healing materials [6–17]. Molecules 2021, 26, 2230. https://doi.org/10.3390/molecules26082230 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x 2 of 9 MoleculesMolecules2021 2021,,26 26,, 2230x 22 of 89 Figure 1. (a) Maleimide self-reaction into succinimide repetitive units. (b) MDP-BMI. (c) Diels- Alder (DA) thermo-reversible reaction between furan and maleimide. Nowadays, in the polymer field, MDP-BMI receives much more attention as a Diels- Alder (DA) dienophile, with furan as its diene counterpart (Figure 1c) [4,6–14]. This DA pair reacts thermo-reversibly, which allows for preparation of polymer networks with crosslinking points that cleave and form thermodynamically. The DA pair also presents fast kinetics and forms and cleaves at practical temperatures, at approximately 60 and 110 °C, respectively [8,14]. Over the last two decades, many furan/maleimide-based thermo- FigureFigurereversibly 1.1. ((a)) crosslinked MaleimideMaleimide self-reactionself-reaction polymers into(FM-TRCP)into succinimide succinimide have repetitive repetitive been explored units. units. ( b(b) as) MDP-BMI. MDP-BMI. reprocessable (c ()c Diels-Alder) Diels- thermo- (DA)Aldersets thermo-reversibleand (DA) elastomers, thermo-reversible and reaction self-healing reaction between between furanmaterials and furan maleimide. [6–17]. and maleimide. The reversibility of these FM-TRCP is remarkable—for example, Zhang et al. pre- paredTheNowadays, a polyketone reversibility in the thermoset of polymer these FM-TRCP that field, could MDP-BMI is be remarkable—for reprocessed receives many much example, times more Zhang without attention et al.considerable as prepared a Diels- aAlderchanges polyketone (DA) in dienophile,the thermoset mechanical that with could properties furan be as reprocessed its [6]. dien However,e counterpart many timeswhile without(Figure working 1c) considerable with [4,6–14]. different This changes FM-DA inpairTRCP, the reacts mechanical we thermo-reversibly,noticed properties that irreversible [ 6which]. However, changesallows while for in preparationthe working thermo-mechanical with of polymer different propertiesnetworks FM-TRCP, withtake we noticedcrosslinkingplace when that handling irreversiblepoints that the cleave changes materials and in atform the high thermo-mechanical th teermodynamically.mperatures (140–170 properties The °C).DA Here,pair take also we place preparedpresents when ◦ handlingfasta crosslinked kinetics the and materials MDP-BMI forms atand furan-grafted high cleaves temperatures at practical polyketone (140–170 temperatures, (FigureC). Here, 2), andat weapproximately explored prepared if athis crosslinked 60 irreversi- and 110 MDP-BMI°C,ble respectivelyeffect was furan-grafted caused [8,14]. byOver polyketone the the MDP-BMI last (Figuretwo reactingdecades,2), and with many explored itself furan/ma ifat this theseleimide-based irreversible rather low effect tempera-thermo- was causedreversiblytures. In by order thecrosslinked MDP-BMI to do so, polymers reactingwe studied with(FM-TRCP) the itself therma at ha thesevel stability been rather explored lowof the temperatures. system as reprocessable at 150 In order°C bythermo- toFTIR, do ◦ 1 so,sets1H-NMR, we and studied elastomers, and therheology. thermal and self-healing stability of thematerials system [6–17]. at 150 C by FTIR, H-NMR, and rheology. The reversibility of these FM-TRCP is remarkable—for example, Zhang et al. pre- pared a polyketone thermoset that could be reprocessed many times without considerable changes in the mechanical properties [6]. However, while working with different FM- TRCP, we noticed that irreversible changes in the thermo-mechanical properties take place when handling the materials at high temperatures (140–170 °C). Here, we prepared a crosslinked MDP-BMI furan-grafted polyketone (Figure 2), and explored if this irreversi- ble effect was caused by the MDP-BMI reacting with itself at these rather low tempera- tures. In order to do so, we studied the thermal stability of the system at 150 °C by FTIR, 1H-NMR, and rheology. FigureFigure 2. 2.Prepared Prepared FM-TRCP:FM-TRCP: furan-graftedfuran-grafted polyketonepolyketone thermo-reversiblythermo-reversibly crosslinked with MDP- BMI.BMI. R R stands stands for for hydrogen hydrogen and and methyl methyl groups. groups. 2.2. ResultsResults FTIRFTIR spectra spectra of of the the studiedstudied FM-TRCP FM-TRCP were were taken taken at at isothermal isothermal conditions conditions over over three three hourshours (Figure(Figure3 a).3a). The The spectra spectra taken taken at at the the beginning beginning of of the the measurements measurements display display the the thermo-reversible equilibrium already displaced towards the reactants (free furan and maleimide groups), since the retro-DA kinetics are quite fast [8,14]. Figure3a shows the −1 peaks at 1180 and 1146 cm corresponding to the CNC bending mode of the succin- ◦ imideFigure and2. Prepared maleimide FM-TRCP: rings, furan-grafted respectively polyketone [3,18–20].