Stereoisomeric effects in thermo-remendable polymer networks based on Diels-Alder crosslink reactions

Understanding the Effect of Side Reactions on the Recyclability of Furan-Maleimide Resins Based on Thermoreversible Diels-Alder Network

We studied the effect of side reactions on the reversibility of epoxy with thermoreversible Diels-Alder (DA) cycloadducts based on furan and maleimide chemistry. The most common side reaction is the maleimide homopolymerization which introduces irreversible crosslinking in the network adversely affecting the recyclability. The main challenge is that the temperatures at which maleimide homopolymerization can occur are approximately the same as the temperatures at which retro-DA (rDA) reactions depolymerize the networks. Here we conducted detailed studies on three different strategies to minimize the effect of the side reaction. First, we controlled the ratio of maleimide to furan to reduce the concentration of maleimide groups which diminishes the effects of the side reaction. Second, we applied a radical-reaction inhibitor. Inclusion of hydroquinone, a known free radical scavenger, is found to retard the onset of the side reaction both in the temperature sweep and isothermal measurements. Finally, we employed a new trismaleimide precursor that has a lower maleimide concentration and reduces the rate of the side reaction. Our results provide insights into how to minimize formation of irreversible crosslinking by side reactions in reversible DA materials using maleimides, which is important for their application as novel self-healing, recyclable, and 3D-printable materials.

Designing New Sustainable Polyurethane Adhesives: Influence of the Nature and Content of Diels-Alder Adducts on Their Thermoreversible Behavior

With a view to the development of new sustainable and functional adhesives, two Diels-Alder (DA) adducts are incorporated as a third component into the curing process of solvent-based and solvent-free polyurethanes in this study. The influence of the nature and content of the DA molecules on the retro-DA (rDA) reaction and its reversibility and cyclability is investigated. It is demonstrated that the bonding/debonding properties of the adhesives are mainly controlled by the concentration of the DA adducts, with a minimum thermoreversible bond (TB) content required that depends on the system and the total ratio between all the diols in the formulation. For the solvent-based system, rDA/DA reversibility can be repeated up to ~20 times without deterioration, in contrast to the solvent-free system where a gradual loss in the DA network reconstruction efficiency is observed. Despite this limitation, the solvent-free system presents clear advantages from an environmental point of view. The changes observed in the physical properties of these new thermoreversible adhesives are of great relevance for recycling strategies and, in particular, their potential for separating multilayered film packaging materials in order to recycle the individual polymer films involved.

The Effect of Molecular Weight on the (Re)-Processability and Material Properties of Bio-Based, Thermoreversibly Cross-Linked Polyesters

A (partially) bio-based short-chain polyester is prepared through interfacial polycondensation of furan-functionalized diphenolic acid with terephthalic chloride. The furan groups along the backbone of the obtained polyester are able to form a covalent network (PE-fur/Bism) with various ratios of 1,1′-(methylenedi-4,1-phenylene)bismaleimide via the thermoreversible Diels–Alder (DA) reaction. Several techniques have been employed to characterize the polyester network, including 1H-NMR, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical thermal analysis (DMTA). The polyester base polymer displays a glass transition temperature of 115 °C, whereas the temperatures at which the retro-Diels–Alder (rDA) reaction takes place lie above 130 °C for the various polyester/bismaleimide networks. Excellent thermoreversibility and recyclability of the polyester resin have been shown through DSC and DMTA measurements.

Bio-Based Aromatic Polyesters Reversibly Crosslinked via the Diels–Alder Reaction

Diphenolic acid is functionalized with furfuryl amine and subsequently incorporated in a (partly) bio-based polyester through interfacial polycondensation with terepthalic chloride. The furan groups present in the resulting polyester are able to form a thermoreversible covalent network with different bismaleimide moieties via the Diels–Alder (DA) reaction. Our analysis of the polymer network by 1H-NMR clearly shows the formation of both possible stereoisomers (endo and exo) from the Diels–Alder coordination of furan and maleimide. Furthermore, it was found that these isomers can be reversibly interchanged at temperatures below the reported retro Diels–Alder reaction temperature, a phenomenon often claimed but, until present, never directly observed, for thermally reversible polymeric systems. Finally, a proof of principle for reversibility and recyclability is shown.

Exploiting exo and endo furan-maleimide Diels-Alder linkages for the functionalization of organoruthenium complexes

Diels-Alder cycloadditions involving furans and maleimides are extensively used in organic chemistry and materials synthesis. Given the promising advances of organoruthenium complexes in therapy, we explored the possibility of exploiting such Diels-Alder linkages as a mean to modulate their biological properties.

Diels–Alder Cycloadditions of Bio-Derived Furans with Maleimides as a Sustainable «Click» Approach towards Molecular, Macromolecular and Hybrid Systems

This mini-review highlights the recent research trends in designing organic or organic-inorganic hybrid molecular, biomolecular and macromolecular systems employing intermolecular Diels–Alder cycloadditions of biobased, furan-containing substrates and maleimide dienophiles. The furan/maleimide Diels–Alder reaction is a well-known process that may proceed with high efficiency under non-catalytic and solvent-free conditions. Due to the simplicity, 100% atom economy and biobased nature of many furanic substrates, this type of [4+2]-cycloaddition may be recognized as a sustainable “click” approach with high potential for application in many fields, such as fine organic synthesis, bioorganic chemistry, material sciences and smart polymers development.

Preparation and Characterization of Self-Healing Polyurethane Powder Coating Using Diels-Alder Reaction

Although powder coating systems offer many environmental, ecological and energy related benefits over liquid based coatings, in the case of uretdione based polyurethane systems, high curing temperature is still an issue. On the other hand, powder coating systems make it possible to reduce the costs and enhance the process of forming complex 3D structures using the deep drawing method by pre-coated metal substrates. During this processing method, there is a probability of micro crack formation in the coated film due to strain impact on the coating layer. A powder coating with self-healing ability is an ultimate solution to face not only this kind of fraction but also any other possible ones (such as defects caused by any impact on film surface during processing, transporting or even service). Here, a single molecule that is prepared via Diels–Alder cycloaddition reaction and retro Diels–Alder cleavage reaction was utilized as a self-healing additive to achieve self-healing ability in the powder coating system that is based on a commercially available uretdione cross-linker and OH-polyester resin. Coatings were prepared through melt mixing of components in a lab mixer, milling, sieving, and then application on the metal substrate through the electrostatic spraying method. To illustrate the role of self-healing additive, various concentrations (4 and 9% wt.) in combination with different curing temperatures (80 °C to 200 °C) were investigated. Both samples containing HA showed self-healing ability at elevated temperature around 120 °C for about 30 min with acceptable roughness and surface properties. Hardness measurement of cured film as well as thermal investigation indicate the chemical reaction of HA in a cross-linked network of cross-linker and resin. In addition, using HA leads to a 40 K drop in curing temperature of the system without using any catalyst. A 2.58% improvement in hardness values at a lower curing temperature and healing time of around 12.5 min at 120 °C to recover 100% of initial scratch (more than 10 cycles) in the sample containing 9% wt. HA was observed.

Intermolecular Diels-Alder Cycloadditions of Furfural-Based Chemicals from Renewable Resources: A Focus on the Regio- and Diastereoselectivity in the Reaction with Alkenes

A recent strong trend toward green and sustainable chemistry has promoted the intensive use of renewable carbon sources for the production of polymers, biofuels, chemicals, monomers and other valuable products. The Diels-Alder reaction is of great importance in the chemistry of renewable resources and provides an atom-economic pathway for fine chemical synthesis and for the production of materials. The biobased furans furfural and 5-(hydroxymethyl)furfural, which can be easily obtained from the carbohydrate part of plant biomass, were recognized as "platform chemicals" that will help to replace the existing oil-based refining to biorefining. Diels-Alder cycloaddition of furanic dienes with various dienophiles represents the ideal example of a "green" process characterized by a 100% atom economy and a reasonable E-factor. In this review, we first summarize the literature data on the regio- and diastereoselectivity of intermolecular Diels-Alder reactions of furfural derivatives with alkenes with the aim of establishing the current progress in the efficient production of practically important low-molecular-weight products. The information provided here will be useful and relevant to scientists in many fields, including medical and pharmaceutical research, polymer development and materials science.

Trends in the Diels-Alder reaction in polymer chemistry

The Diels-Alder (DA) reaction is regarded as quite a useful strategy in organic and macromolecular syntheses. The reversibility of this reaction and the advent of self-repair technology, as well as other applications in controlled macromolecular architectures and crosslinking, have strongly boosted the research activity, which is still attracting a huge interest in both academic and industrial research. The DA reaction is a simple and scalable toolbox. Though it is well-established that furan/maleimide is the most studied diene/dienophile couple, this perspective article reports strategies using other reversible systems with deeper features on other types of diene/dienophile pairs being either petro-sourced (cyclopentadiene, anthracene) or bio-sourced (muconic and sorbic acids, myrcene and farnesene derivatives, eugenol, cardanol). This review is composed of four sections. The first one briefly recalls the background on the DA reactions involving cyclodimerizations, dienes, and dienophiles, parameters affecting the reaction, while the second part deals with the furan/maleimide reaction. The third one deals with petro-sourced and bio-sourced (or products becoming bio-sourced) reactants involved in DA reactions are also listed and discussed. Finally, the authors' opinion is given on the potential future of the crosslinking-decrosslinking reaction, especially regarding the process (e.g., key temperatures of decrosslinking) or possibly monocomponents. It presents both fundamental and applied research on the DA reaction and its applications.

Diels-Alder Cross-Linked, Washing-Free Hydrogel Films with Ordered Wrinkling Patterns for Multicellular Spheroid Generation

Three-dimensional (3D) multicellular spheroids are a new generation in vitro cell model, however, their applications are severely limited by difficulties in their generation. Here patterned poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel films were synthesized for their generation. Instead of polymerization of HEMA monomers in the presence of a cross-linker, here the PHEMA films were synthesized by cross-linking furan-functionalized linear PHEMA, PHEMA-furan, and maleimide-functionalized linear PHEMA, PHEMA-mal, via Diels-Alder (DA) reaction between furan and maleimide groups. A thermal treatment temperature of 75 °C was chosen for the cross-linking reaction. The occurrence of DA reaction was confirmed by IR spectra. Using this method, cross-linked PHEMA films with smooth surface were successfully synthesized in situ in the well of cell culture plates. The films were then patterned by simply adding water to swell them. Highly ordered, honeycomb-like wrinkling patterns were successfully obtained by adjusting the furan and maleimide contents in the precursor linear polymers. The patterned hydrogel films were used to generate multicellular spheroids. Guided by the patterns, 3D spheroids with narrow size distribution, tunable size, and high cell viability were successfully obtained. The patterned PHEMA films reported here exhibited a lot of advantages. The patterning method was quite simple and required no template or special equipment. They were synthesized in situ in commercial cell culture plates. Particularly, thanks to the clean nature of the DA reaction, no low molecular weight monomer, cross-linker, initiator, or catalyst, which were potentially cytotoxic, was involved in the film synthesis, and no byproduct was produced and left in the film. The resulting films presented a high biocompatibility, allowing the avoidance of the tedious washing step. The films synthesized here were expected to have high potential for massive production of well-defined multicellular spheroids.

Thermally Reversible Polymeric Networks from Vegetable Oils

Low cross-link density thermally reversible networks were successfully synthesized from jatropha and sunflower oils. The oils were epoxidized and subsequently reacted with furfurylamine to attach furan groups onto the triglycerides, preferably at the epoxide sites rather than at the ester ones. Under the same reaction conditions, the modified jatropha oil retained the triglyceride structure more efficiently than its sunflower-based counterpart, i.e., the ester aminolysis reaction was less relevant for the jatropha oil. These furan-modified oils were then reacted with mixtures of aliphatic and aromatic bismaleimides, viz. 1,12-bismaleimido dodecane and 1,1'-(methylenedi-4,1-phenylene)bismaleimide, resulting in a series of polymers with T_g ranging between 3.6 and 19.8 °C. Changes in the chemical structure and mechanical properties during recurrent thermal cycles suggested that the Diels-Alder and retro-Diels-Alder reactions occurred. However, the reversibility was reduced over the thermal cycles due to several possible causes. There are indications that the maleimide groups were homopolymerized and the Diels-Alder adducts were aromatized, leading to irreversibly cross-linked polymers. Two of the polymers were successfully applied as adhesives without modifications. This result demonstrates one of the potential applications of these polymers.

The Use of Click-Type Reactions in the Preparation of Thermosets

Click chemistry has emerged as an effective polymerization method to obtain thermosets with enhanced properties for advanced applications. In this article, commonly used click reactions have been reviewed, highlighting their advantages in obtaining homogeneous polymer networks. The basic concepts necessary to understand network formation via click reactions, together with their main characteristics, are explained comprehensively. Some of the advanced applications of thermosets obtained by this methodology are also reviewed.

Tuning the thermoreversible temperature domain of PTMC-based networks with thermosensitive links concentration

In this work, thermoreversible poly(trimethylene carbonate) (PTMC) based networks with different crosslinking densities were obtained by Diels-Alder (DA) reaction between furan-functionalized PTMC precursors and a bismaleimide. Furan-grafted PTMC with various functionalities determined by ¹H-NMR analyses were prepared from telechelic PTMC oligomer, glycerol, 4,4'-methylenebis(cyclohexyl isocyanate) (H₁₂MDI) and furfuryl alcohol. The formation of network structures by DA reaction between furan and maleimide groups were proved by Fourier-transform infrared spectroscopy (FT-IR). Although both exo and endo DA adduct forms exist, the thermally more stable exo form dominates. The thermoreversibility of networks was evidenced by FT-IR, solubility, differential scanning calorimetry (DSC) and rheology experiments at different temperatures. By increasing furan functionality or node concentration, denser and stiffer networks could be formed with higher Young's modulus and true stress at break in tensile tests, as well as higher crossover temperature, which indicates a nominal transition from elastic behavior to viscous state. The disruption of networks was found to occur in high temperature ranges from 130 to 160 °C, depending on their crosslinking density.

Stereoisomeric furan/maleimide adducts as latent monomers for one-shot sequence-controlled polymerization

Two stereoisomeric latent monomers were used for one-shot sequence-controlled polymerization to create diverse sequence structures.

Structural and solvent control over activation parameters for a pair of retro Diels-Alder reactions

We report the temperature dependent NMR of two Diels-Alder adducts of furan: one formed with maleic anhydride and the other with N-methylmaleimide. These adducts are the products of so-called 'click' reactions, widely valued for providing simple, reliable, and robust reactivity. Under our experimental conditions, these adducts undergo a retro Diels-Alder reaction and we use our temperature dependent NMR to determine the rates of these reactions at multiple temperatures-ultimately providing estimates of the activation parameters for the reversion. We repeat these measurements in three solvents. We find that, in all solvents, the barrier to reversion is larger for the adduct formed with N-methylmaleimide. The barrier to reversion for this adduct is relatively insensitive to changes in solvent while the adduct formed with maleic anhydride responds more strongly to changes in solvent polarity. The differences in reaction barrier and solvent dependence arises because the adduct formed with N-methylmalemide is more stable-leading to a larger barrier to reversion-while the adduct formed with maleic anhydride experiences a larger change in dipole during the reaction-leading to a larger solvent dependence.

Data on production and characterization of melamine-furan-formaldehyde particles and reversible reactions thereof

The data present in this article affords insides in the characterization of a newly described bi-functional furan-melamine monomer, which is used for the production of monodisperse, furan-functionalized melamine-formaldehyde particles, as described in https://doi.org/10.1016/j.eurpolymj.2019.04.006 Urdl et al., 2019. In the related research article Urdl et al., 2019 data interpretations can be found. The furan-functionalization of particles is necessary to perform reversible Diels-Alder reactions with maleimide (BMI) crosslinker to form thermoreversible network systems. To understand the reaction conditions of Diels-Alder (DA) reaction with a Fu-Mel monomer and a maleimide crosslinker, model DA reaction were performed and evaluated using dynamic FT-IR measurements. During retro Diels-Alder (rDA) reactions of the monomer system, it was found out that some side reaction occurred at elevated temperatures. The data of evaluating the side reaction is described in one part of this manuscript. Additional high resolution SEM images of Fu-Mel particles are shown and thermoreversible particle networks with BMI2 are shown. The data of different Fu-Mel particle networks with maleimide crosslinker are presented. Therefore, the used maleimide crosslinker with different spacer lengths were synthesized and the resulting networks were analyzed by ATR-FT-IR, SEM and DSC.

Simultaneous Recovery of Matrix and Fiber in Carbon Reinforced Composites through a Diels-Alder Solvolysis Process

Efficient and comprehensive recycling of fiber-reinforced thermosets is particularly challenging, since the irreversible degradation of the matrix component is necessary in order to separate the fiber component in high purity. In this work, a new approach to fully recyclable thermoset composites is presented, based on the thermal reversibility of an epoxy-based polymer network, crosslinked through Diels-Alder (DA) chemistry. Carbon fiber composites, fabricated by compression molding, were efficiently recycled through a simple solvolysis procedure in common solvents, under mild conditions, with no catalysts. Specifically, the purity of reclaimed fibers, assessed by thermogravimetric analysis and scanning electron microscopy, was very high (>95%) and allowed successful reprocessing into second generation composites. Moreover, the dissolved matrix residues were directly employed to prepare smart, thermally healable coatings. Overall, DA chemistry has been shown to provide a convenient strategy towards circular economy of thermoset composites.

The influence of stereochemistry on the reactivity of the Diels–Alder cycloaddition and the implications for reversible network polymerization

The differences in reactivity and thermal stability of the stereoisomers define the thermal properties and responsiveness of the reversible polymer network.

Partially bio-based aromatic poly(ether sulfone)s bearing pendant furyl groups: synthesis, characterization and thermo-reversible cross-linking with a bismaleimide

A fully bio-based bisphenol, namely, 4,4′-(furan-2-ylmethylene)bis(2-methoxyphenol) was synthesized and its utility for synthesis of aromatic poly(ether sulfone)s bearing clickable pendant furyl groups was demonstrated.

Control of Interfacial Diels-Alder Reactivity by Tuning the Plasma Polymer Properties

Functionalizing the surface of a material with a smart plasma polymer coating is an interesting alternative strategy to obtain a thermoresponsive material without changing its formulation. On the basis of a low-pressure plasma polymerization process, the present work first aims to fabricate polymer thin films that react via the well-known thermoreversible Diels-Alder (DA) reaction (diene/dienophile cycloaddition). A two-step surface modification process based on (pulsed) plasma polymerization enables the design of functional coatings that contain furan (diene) groups. The reactivity of these surfaces with maleic anhydride (dienophile) in solution is thoroughly investigated, mainly by studying the kinetics of the DA reaction by advancing contact angle measurements. The determination of rate constants of reactions at various temperatures leads to the quantification of thermodynamic parameters such as the activation energy of the reaction as well as the enthalpy and entropy of activation related to the formation of the transition-state complex involved in the DA reaction. More interestingly, the design of furan-functionalized coatings with various physicochemical properties enables the understanding of the role played by the density of functional groups and the cross-linking rate of the polymer on the interfacial reactivity. Thus, we show in this work how to control the interfacial DA reaction on plasma coatings by tailoring the operating conditions of plasma polymerization.

Achieving Controlled Biomolecule-Biomaterial Conjugation

The conjugation of biomolecules can impart materials with the bioactivity necessary to modulate specific cell behaviors. While the biological roles of particular polypeptide, oligonucleotide, and glycan structures have been extensively reviewed, along with the influence of attachment on material structure and function, the key role played by the conjugation strategy in determining activity is often overlooked. In this review, we focus on the chemistry of biomolecule conjugation and provide a comprehensive overview of the key strategies for achieving controlled biomaterial functionalization. No universal method exists to provide optimal attachment, and here we will discuss both the relative advantages and disadvantages of each technique. In doing so, we highlight the importance of carefully considering the impact and suitability of a particular technique during biomaterial design.

Chemical Design of Functional Polymer Structures for Biosensors: From Nanoscale to Macroscale

Over the past decades, biosensors, a class of physicochemical detectors sensitive to biological analytes, have drawn increasing interest, particularly in light of growing concerns about human health. Functional polymeric materials have been widely researched for sensing applications because of their structural versatility and significant progress that has been made concerning their chemistry, as well as in the field of nanotechnology. Polymeric nanoparticles are conventionally used in sensing applications due to large surface area, which allows rapid and sensitive detection. On the macroscale, hydrogels are crucial materials for biosensing applications, being used in many wearable or implantable devices as a biocompatible platform. The performance of both hydrogels and nanoparticles, including sensitivity, response time, or reversibility, can be significantly altered and optimized by changing their chemical structures; this has encouraged us to overview and classify chemical design strategies. Here, we have organized this review into two main sections concerning the use of nanoparticles and hydrogels (as polymeric structures) for biosensors and described chemical approaches in relevant subcategories, which act as a guide for general synthetic strategies.

Synthesis of switchable intelligent molecularly imprinted polymers with selective adsorption of ethyl carbamate and their application in electrochemical sensor analysis

A cyclodextrin aldehyde based molecularly imprinted polymer with thermally responsive Diels–Alder (DA) linkages of grafted furan-type dienes was polymerized. The synthesized DA-MIP has dienophile characteristics and the specific absorption of ethyl carbamate (EC) can be switched on or off simply by thermal adjustment to 130 °C and 60 °C, respectively. The imprinting factors (α) of the MIP and rDA-MIP to EC were 6.2 and 5.0, and the selection factors (β) were 5.2 and 4.0, respectively. The restoration of the molecular target ratio was 88%, as determined by absorption and desorption experiments. The thermal restoration ratio, determined by thermal cycling experiments, was 78%. A new electrochemical sensor was prepared using the DA-MIP and its responsiveness for detecting trace amounts of EC was investigated. The results indicate that the electrode response has good affinity and excellent specific recognition performance for template molecular chemicals.

A cyclodextrin aldehyde based molecularly imprinted polymer with thermally responsive Diels–Alder (DA) linkages of grafted furan-type dienes was polymerized. The synthesized DA-MIP has dienophile characteristics and the specific adsorption of ethyl carbamate (EC) can be switched on or off simply by thermal adjustment.

Synthesis of a self-healing siloxane-based elastomer cross-linked via a furan-modified polyhedral oligomeric silsesquioxane: investigation of a thermally reversible silicon-based cross-link

We have successfully prepared an elastomeric material exhibiting excellent temperature-controlled self-healing characteristics.

A minimalist furan–maleimide AB-type monomer and its thermally reversible Diels–Alder polymerization

An original approach to prepare and polymerize the shortest Diels–Alder AB monomer.