Furan-maleimide Diels-Alder Research Articles

Modular design of Diels-Alder reversible networks for the facile production of highly tunable materials

Covalent adaptable networks (CANs) have attracted significant attention due to their potential to form crosslinked, yet reprocessable networks. Their ability to rearrange upon exposure to specific stimuli, in combination with properties such as self-healing can advance the development of novel materials, including for additive manufacturing. Thorough understanding of structure-property relations and processing potential will aid future application-driven research in network design as well as material selection. Therefore, a multitude of CANs were synthesized herein, by crosslinking epoxide-based oligomers via the furan-maleimide Diels-Alder reaction, to evaluate small systematic variations of (co)monomer composition, crosslinker length/flexibility and crosslinking density as vectors of tuning the CANs' thermomechanical properties. Networks with glass transition temperatures (Tg) spanning from <−40 °C up to >20 °C and Young's Moduli spanning from 0.2 MPa to >500 MPa were readily attainable. Crosslinker length/flexibility had a profound impact on the tensile properties, while changes in backbone composition provided insight into the impact of secondary interactions versus rigid moieties on mechanical performance. Self-healing at ambient conditions was demonstrated for elastomeric networks, with healing efficiency being enhanced when using longer crosslinkers. Finally, a cell viability and metabolic activity assay provided a preliminary in vitro demonstration of the biocompatibility of these materials.

Reversible Postsynthetic Modification in a Metal-Organic Framework.

Postsynthetic modification (PSM) of metal-organic frameworks (MOFs) provides access to functional materials and advanced porous solid engineering. Herein, we report the reversible PSM of a multivariate isoreticular MOF by applying dynamic furan-maleimide Diels-Alder (DA) chemistry. The key step involves incorporating a furan group into the MOF via "click" PSM, which can then undergo repeated cycles of modification and de-modification with maleimides. The structural integrity, crystallinity, and porosity of the furan-appended MOF remained intact even after three consecutive PSM/de-modification cycles using three different functionalized maleimides.

Beyond Diels-Alder: Domino reactions in furan-maleimide click networks

The furan-maleimide Diels-Alder chemistry has emerged as an important tool to design thermo-reversible click networks. This not only preserves the strong and robust properties of thermo-sets and rubbers, but also makes it possible to cleave crosslinks at non-degradative temperatures, or in other words to recycle them. In this work a new reaction is reported in furan-maleimide click networks, which is the Double-Diels-Alder reaction (DDA), also known as domino Diels-Alder. This forms extra linkages between Diels-Alder adducts and non-reacted furan groups and evidently leads to stronger materials, but also prevents efficient thermal (re)cycling. This work shows with nuclear magnetic resonance (NMR) characterization, differential scanning calorimetry (DSC) and rheology that the DDA reaction can occur both in intramolecular and intermolecular fashion, but also that it exhibits reversibility just like the regular Diels-Alder reaction. This reaction can be easily overlooked creating what at first sight might be inexplicable reactivity when analyzing the properties of furan-maleimide click networks.

Interfacial and matrix healing of thermally reversible bismaleimide infused Graphene Nano Platelets reinforced polymer nanocomposite through Diels-Alder bonding

The interfacial and matrix healing capability have been studied in this work for thermally reversible self-healing furan maleimide Diels Alder (FMDA) based hybrid polymer composite. FMDA composite with various weight ratios (0.5, 1.0, and 1.5 wt%) of bismaleimide infused Graphene Nano Platelets (GNPBm) was fabricated and characterized. FMDA with varying epoxy, GNPBm wt% were considered for mechanical and morphological features. Amine functionalization was used to address GNPBm aggregation, precise interface management. Thermal stability of functionalized GNPs were revealed from TG-DTA data. FE-SEM images attributes homogenous distribution, bismaleimide moieties infusion on GNPs. Successful integration of DA and retro-DA bonds of FMDA composite was confirmed with TG-DSC through exo-endothermic peaks. TDCB specimens were fabricated and tested to evaluate FMDA's self-healing efficiency. An improvement in Breaking load, Tensile strength, Elastic modulus, and Self-healing efficiency was observed in 45FMDA(Gr-1.0), these being 30 %, 162 %, 42 %, and 90 % respectively, when compared to pristine sample.

Self-healable ultrahydrophobic modified bio-based elastomer using Diels-Alder ‘click chemistry’

Inclusion of self-healing and water-repellant properties within a commercially available elastomer could be of great significance in prolonging its longevity and sustainability. Herein, Diels-Alder (DA) ‘click’ reaction was employed to introduce self-healing and ultrahydrophobic features in commercially available ENR. In this case, the furfuryl groups were grafted onto the backbone of the pristine elastomer, followed by their DA ‘click’ modification with polyhedral oligomeric silsesquioxane isobutyl maleimide (POSSMI) molecules. Besides the provision of dynamic covalent linkages, the DA modified elastomer showed significantly improved hydrophobicity [water contact angle (WCA) > 140°], because of the presence of nano-sized POSS materials on the surface. In contrast to pristine ENR, the DA adduct POSS hybrid elastomer showed remarkably improved thermal stability, surface hardness, and adhesion to metal substrates. On heating at 120 °C, the furan-maleimide DA covalent linkages cleaved, which re-installed on annealing at 60 °C. In consequence, the DA elastomers exhibited relatively higher mechanical and healing efficacy than the pristine ENR.

Self-healing UV-curable polymer network with reversible Diels-Alder bonds for applications in ambient conditions

The ambient-temperature self-healing potential of a poly(methacrylate) network, containing reversible furan-maleimide Diels-Alder crosslinks, is studied. A reversible bis(methacrylate) monomer, containing Diels-Alder bonds, is synthesized and characterized by means of 1H NMR spectroscopy. Subsequent polymerization via UV-cure yields a reversible polymer network, consisting of thermoplastic poly(methacrylate), crosslinked by Diels-Alder bonds. Characterization of the polymer network via high-resolution solid-state 13C NMR indicates that undesirable side reactions, such as the formation of irreversible maleimide homo- and copolymers, are prevented by protecting the maleimide functionality in the Diels-Alder adduct. The thermal reversibility of the polymer network is studied by means of Fourier transform infrared spectroscopy and differential scanning calorimetry, and it is confirmed that the reversibility of the Diels-Alder reaction remains during several thermal cycles between −40 °C and 85 °C. Dynamic mechanical analysis confirms that the reversible polymer network maintains sufficient mechanical properties even at elevated temperatures up to 110 °C, at which retro Diels-Alder reaction is favored. Self-healing of the polymer network at ambient temperature, even in the fully vitrified state at 20 °C, is demonstrated by grinding the polymer material into a powder and then healing the powder (after compression) to form a rectangular bar with recovered mechanical properties. Ambient temperature healing without human intervention is therefore feasible, making this material suitable as e.g. encapsulant in photovoltaic modules for outside applications.

Mechanically Triggered Small Molecule Release from a Masked Furfuryl Carbonate.

Stimuli-responsive polymers that release small molecules under mechanical stress are appealing targets for applications ranging from drug delivery to sensing. Here, we describe a modular mechanophore design platform for molecular release via a mechanically triggered cascade reaction. Mechanochemical activation of a furan-maleimide Diels-Alder adduct reveals a latent furfuryl carbonate that subsequently decomposes under mild conditions to release a covalently bound cargo molecule. The computationally guided design of a reactive secondary furfuryl carbonate enables the decomposition and release to proceed quickly at room temperature after unmasking via mechanical force. This general strategy is demonstrated using ultrasound-induced mechanical activation to release a fluorogenic coumarin payload from a polymer incorporating a chain-centered mechanophore.

Stereochemical effects on the mechanochemical scission of furan-maleimide Diels-Alder adducts.

Clarifying the correlation between the chemical structure of mechanophores and their mechanical reactivity informs the design of mechanochemical systems. One specific correlation that has received much recent attention is that between stereoisomerism and mechanical reactivity. Here, we report previously unobserved differences in the mechanical reactivity of furan-maleimide Diels-Alder (DA) stereoisomers. We evaluated the internal competition between the mechanically triggered retro-DA reaction and the mechanochemical ring opening of gem-dichlorocyclopropane mechanophores in the pulsed sonication of polymer solutions. The relative extent of the two sonomechanochemical reactions in the same polymer shows that the endo DA isomer exhibits greater mechanical lability than its exo isomer. This result contrasts with recent measurements of the relative rates of scission in a similar system and points to potential enhanced sensitivity obtained through the use of internal competition as opposed to absolute rates in assessing mechanical reactivity in sonication studies.

Coupling the Microscopic Healing Behaviour of Coatings to the Thermoreversible Diels-Alder Network Formation

While thermally reversible polymer network coatings based on the Diels-Alder reaction are widely studied, the mechanisms responsible for the heating-mediated healing of damage is still not well understood. The combination of microscopic evaluation techniques and fundamental insights for the thermoreversible network formation in the bulk and coating shed light on the mechanisms behind the damage healing events. The thermomechanical properties of thermoset and elastomer coatings, crosslinked by the furan-maleimide Diels-Alder cycloaddition reaction, were studied in bulk and compared to the thermal behaviour applied as coatings onto aluminium substrates. The damage sealing of thermoset (Tg = 79 °C) and elastomer (Tg = −49 °C) coatings were studied using nano-lithography and atomic force microscopy (AFM). The sealing event is studied and modelled at multiple temperatures and correlated to the changes in the network structure and corresponding thermomechanical properties.

Thermophysical characterization of a reversible dynamic polymer network based on kinetics and equilibrium of an amorphous furan-maleimide Diels-Alder cycloaddition

The equilibrium and kinetics of the furan-maleimide Diels-Alder (DA) reaction for the formation of a reversible network were studied by calorimetry between 25 °C and 90 °C using an amorphous bismaleimide eliminating solvent use. The relationship between the equilibrium conversion xeq with temperature and the effect of dilution were simulated. The glass transition-conversion relationship of the reversible network was established.The thermophysical properties of the reversible network were linked to the kinetics and equilibrium of the DA system, and studied by dynamic mechanical analysis (DMA), dielectric analysis (DEA) and rheometry. Below Tg the reversible network behaves like an irreversible network; however, above Tg, a decrease in the rubber tensile storage modulus was observed due to a reduction of cross-link density caused by the retro DA reaction. DEA revealed that above Tg, an interface is formed between released bismaleimide molecules and the remaining network. The rheological behavior is related to xeq and the lifetime of the reversible covalent bonds. When xeq is higher than the gelation conversion xgel (below 90 °C), the system gels at constant xgel. An elastic strengthening effect is observed in a transition region between 90 °C and 115 °C. Above 115 °C the system is reaching a viscous melt behavior. These observations are important for the application of this network as a self-healing material and as a recyclable elastomer or thermoset.

Effect of Polymer Network Architecture, Enhancing Soft Materials Using Orthogonal Dynamic Bonds in an Interpenetrating Network.

Doubly dynamic polymer networks were synthesized with two distinct exchangeable cross-linkers. The first linker is highly dynamic and rapidly exchanging hydrogen bonded 2-ureido-4[1H]-pyrimidinone (UPy) and the second is a thermoresponsive furan-maleimide Diels-Alder adduct (FMI). Two network architectures were considered: an interpenetrating network (IPN) where one network is cross-linked with the UPy linker and the other is cross-linked with the FMI linker, and a single network (SN) where both the UPy and FMI linkers are in the same single network. Remarkably, the IPNs were superior to the SNs with the same composition of the UPy and FMI cross-linkers when comparing peak stress, strain at break, fracture toughness, malleability, and self-healing. Both materials studied were stable and creep resistant under ambient conditions.

Thermally reversible and biodegradable polyglycolic-acid-based networks

The furan-maleimide Diels-Alder (DA) reaction was used to synthesize thermally reversible and biodegradable networks based on functionalized polyglycolic acid precursors. Hydroxyl-telechelic polyglycolic acid (PGA) with a well-controlled molar mass was first obtained by ring-opening polymerization (ROP) of glycolide catalyzed by Sn(Oct)2 and using 1,4-butanediol as initiator. Functional furan polyglycolic acids with different functionalities were prepared by the isocyanate-alcohol condensation reaction from multi-alcohols such as glycerol and pentaerythritol, H12MDI, PGA-diol and furfuryl alcohol. For a more effective control of the DA reaction conversion and before approaching the synthesis of the networks, a Diels-Alder adduct and a coupling agent bearing maleimide functions on its extremities were synthesized separately. At different stages, the obtained products were characterized by FTIR, NMR, DSC and SEC techniques. The thermoreversible networks were then obtained via two different methods: first, one in which the Diels-Alder adduct gives rise to the networks (method 1) and then via a one-shot process where the alcoholysis reaction of the coupling agent is responsible for the crosslinking (method 2). Thermomechanical analyses were used to verify the thermal reversible character of PGA-based networks. Furthermore, biodegradability studies such as in vitro degradation as well as aerobic biodegradability revealed that an increase in the polymer crosslinking density and a decrease in glycolide content typically lead to a slower degradation.

Design Paradigm Utilizing Reversible Diels–Alder Reactions to Enhance the Mechanical Properties of 3D Printed Materials

A design paradigm is demonstrated that enables new functional 3D printed materials made by fused filament fabrication (FFF) utilizing a thermally reversible dynamic covalent Diels-Alder reaction to dramatically improve both strength and toughness via self-healing mechanisms. To achieve this, we used as a mending agent a partially cross-linked terpolymer consisting of furan-maleimide Diels-Alder (fmDA) adducts that exhibit reversibility at temperatures typically used for FFF printing. When this mending agent is blended with commercially available polylactic acid (PLA) and printed, the resulting materials demonstrate an increase in the interfilament adhesion strength along the z-axis of up to 130%, with ultimate tensile strength increasing from 10 MPa in neat PLA to 24 MPa in fmDA-enhanced PLA. Toughness in the z-axis aligned prints increases by up to 460% from 0.05 MJ/m(3) for unmodified PLA to 0.28 MJ/m(3) for the remendable PLA. Importantly, it is demonstrated that a thermally reversible cross-linking paradigm based on the furan-maleimide Diels-Alder (fmDA) reaction can be more broadly applied to engineer property enhancements and remending abilities to a host of other 3D printable materials with superior mechanical properties.

Straightforward functionalization of acrylated soybean oil by Michael-addition and Diels–Alder reactions

A straightforward Michael-addition reaction of acrylated soybean oil (AESO) with different nitrogen and sulfur nucleophiles is reported. The reaction proceeded under mild conditions and the products were obtained in good yields. The reaction product of AESO with furfurylamine (FA) was used in Diels–Alder (DA) reactions with a bifunctional (1,1′-(methylenedi-4,1-phenylene) bismaleimide (BMI) and a monofunctional maleimide (N-phenylmaleimide, PMI), in different solvents. The 1H NMR spectroscopic analysis of the AESO-FA-PMI revealed the presence of both exo and endo DA adducts. The reverse DA reaction (rDA) of the AESO-FA-PMI (BMI) derivatives was shown to take place at temperatures between 103 and 134°C, with enthalpy changes ranging from 4.7 to 29.5J/g. The results indicate that the solvent used in the furan–maleimide DA reaction has an important role in determining the properties of the adduct formed.

Furan-maleimide thermoplast-thermoset merged polyimides

Novel thermoplast–thermoset merged polyimide system has been developed via Diels-Alder intermolecular reaction of bisfuran A namely, 2,5-bis(furan-2-ylmethyl carbamoyl)terephthalic acid with a series of bismaleimides B1–5 containing aliphatic and alicyclic groups in the main chain viz., B1: 1,2–bismaleimido ethane, B2: 2–methyl–1,5–bismaleimido pentane, B3: 1,6–bismaleimido hexane, B4: 2,2,4–trimethyl 1,6–bismaleimido hexane, B5: isophorone bismaleimide. The intermediate Furan—maleimide Diels-Alder adducts C1–5 were aromatized and imidized (i.e. cyclized) through carboxylic and amide groups to afford thermoplast-thermoset merged polyimides D1–5. Synthesized polyimides were characterized by elemental analysis, spectral features, number average molecular weight (\( \overline{\mathrm{Mn}} \)) and thermal analyses. Bulk polymerization was also carried out. Proof of structure was based mainly on a comparison of infrared spectra of polyimides with those of the corresponding model compound 4 prepared from phthalic anhydride and furan–2yl–methanamine in the similar way. FTIR spectral features of polyimides D1–5 were quite identical with the model compound 4. The ‘in situ’ void–free glass fiber reinforced composites GFRC1–5 were prepared and characterized by mechanical, electrical and chemical properties. The ‘in situ’ produced PIs show good adhesion to glass fibers. All the composites showed good mechanical and electrical properties and good resistance to organic solvents and mineral acids.

Solid-Phase Synthesis of Polymers Using the Ring-Opening Metathesis Polymerization

We report a general method for the solid-phase synthesis of polymers via the ring-opening metathesis polymerization (ROMP). The method involves polymerization in solution to form a block copolymer, immobilization of the polymer via reaction of one block with a resin-bound functional group, modification of the other block, and liberation of the polymer from the resin. We demonstrated the utility of this approach by generating a block copolymer with an N-hydroxysuccinimidyl ester-substituted block (for on-resin functionalization) and a maleimide-substituted block (for conjugation to the resin). We showed that the Diels-Alder reaction can be employed to immobilize the polymers and that amines of diverse structure can be used to modify the resin-bound polymers. The reversibility of the furan-maleimide Diels-Alder adduct was exploited to liberate the polymer from the support. Specifically, treatment of the resin with cyclopentadiene resulted in complete polymer release. The resulting polymers are functional: they were as potent in assays with the lectin concanavalin A as polymers generated by traditional solution routes. We anticipate that this method can be used for the rapid synthesis of diverse polymers via ROMP.