retro-DA Reaction Research Articles

Self-Healing Thermal-Reversible Low-Temperature Polyurethane Powder Coating Based on Diels-Alder Reaction.

This work focused on obtaining a low-temperature powder coating characterized by self-healing properties. To achieve this, acrylic resin, blocked polyisocyanates (bPICs) with 1,2,4-triazole, and unsaturated commercial resin were used. The synthesis of bPICs with triazole enabled the low-temperature curing and reversible Diels-Alder (DA) reaction at 160 °C. The chemical structure of bPICs was confirmed using 1H-NMR. The occurrence of the DA and retro-DA (rDA) reactions in the crosslinked polymer, at temperatures of 60-85 °C and 90-130 °C, respectively, was confirmed using Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), and FT-IR spectroscopy. The self-healing properties of the powder coating were examined using polarized optical microscopy. Additionally, the occurrence of the DA and rDA reactions between triazole and unsaturated polyester resin was investigated through repeated self-healing tests.

Multiple Force-Triggered Downconverted and Upconverted Emission in Polymers Containing Diels-Alder Adducts.

Fluorescent mechanophores can indicate the deformation or damage in polymers. The development of mechanophores with multi-triggered response is of great interest. Herein, Diels-Alder (DA) adducts are incorporated into linear poly(methyl acrylate) PMA-BA and network poly(hexyl methacrylate) (PHMA) as mechanophores to detect the stress caused by ultrasound, freezing, and compression. The DA mechanophores undergo retro-DA reaction to release 9-styrylanthracene chromophore upon applying force, resulting in cyan fluorescence. The dissociation ratio of the DA mechanophore after pulsed ultrasonication of PMA-BA solution for 240 minutes is estimated to be 52 % by absorption spectra and 1H NMR. Additionally, the rate constant of mechanical cleavage is calculated to be 1.2×10-4 min-1⋅kDa-1 with the decrease in molecular weight from 69 to 22 kDa measured by gel permeation chromatography. Freezing of PHMA gels as well as compression of PHMA bulk samples turn-on the DA mechanophores, revealing the microscale fracture. Photon upconversion responses toward various force stimuli are also achieved in both polymer solutions and bulk samples by doping platinum octaethylporphyrin (PtOEP) or palladium meso-tetraphenyltetrabenzoporphyrin (PdTPTBP) sensitizers with multiple excitation wavelengths.

Evidence through Thermal Analysis of Retro Diels-Alder Reaction in Model Networks Based on Anthracene Modified Polyester Resins.

The present work is focused on polyester resins obtained from the diglycidyl ether of bisphenol A and anthracene modified 5-maleimidoisophthalic acid. Because the maleimide-anthracene Diels-Alder (DA) adduct is stable at high temperatures, it is considered a good option for high performance polymers. However, the information related to the retroDA reaction for this type of adduct is sometimes incoherent. A detailed thermal study (conventional TGA, HiRes TGA, MTGA, DSC, MDSC) was performed in order to establish whether the rDA reaction can be revealed for this type of anthracene modified polyester resins. The TGA method confirmed the cleavage of the anthracene-maleimide DA adduct, while the DSC demonstrated the presence of anthracene in the system. At high temperatures, unprotected maleimide homopolymerizes and/or reacts with allyl groups according to the -ene reaction. Therefore, the thermal DA reaction is not displayed anymore upon the subsequent cooling, and the glass transition region is registered at a higher temperature range during the second heating. The use of sample-controlled thermal analysis (HiRes TGA) and MTGA improved the TGA result; however, it was not possible to separate the very complex degradation processes that are interconnected.

Convolution Neural Networks for Motion Detection with Electrospun Reversibly-Cross-linkable Polymers and Encapsulated Ag Nanowires.

This paper presents the design, fabrication, and implementation of a novel composite film, a polybutadiene-based urethane (PBU)/AgNW/PBU sensor (PAPS), demonstrating remarkable mechanical stability and precision in motion detection. The sensor capitalizes on the integration of Ag nanowire (AgNW) electrodes into a neutral plane, embedded within a reversibly cross-linkable PBU polymer. The meticulous arrangement confers pore-free and interfaceless sensor formation, resulting in an enhanced mechanical robustness, reproducibility, and long-term reliability. The PBU polymer is subjected to an electrospinning process, followed by sequential Diels-Alder (DA) and retro-DA reactions to produce a planarized encapsulation layer. This pioneering technology, based on electrospinning, allows for more flawless engineering of the neutral plane as compared to conventional film lamination or layer-by-layer spin-coating processes. This encapsulation, matching the thickness of the preformed PBU film, effectively houses the AgNW electrodes. The PAPS outperforms conventional AgNW/PBU sensors (APS) in terms of mechanical stability and bending insensitivity. When affixed to various body parts, the PAPS generates distinctive signal curves, reflecting the specific body part and degree of motion involved. The PAPS sensor's utility is further magnified by the application of machine learning and deep learning algorithms for signal interpretation. K-means clustering algorithm authenticated the superior reproducibility and consistency of the signals derived from the PAPS over the APS. Deep learning algorithms, including a singular 1D convolutional neural network (1D CNN), long short-term memory (LSTM) network, and dual-layered combinations of 1D CNN + LSTM and LSTM + 1D CNN, were deployed for signal classification. The singular 1D CNN model displayed a classification accuracy exceeding 98%. The PAPS sensor signifies a pivotal development in the field of intelligent motion sensors.

Thermally reversible prototype adhesive via the furan–maleimide Diels–Alder reaction

The development of structural epoxy adhesives with the capacity to bond and de-bond on-demand would open many opportunities for the assembly, inspection, repair, and end of life disposal of structural bonded assemblies. Herein, a reversible cross-linking mechanism enabling bonding and debonding of an epoxy adhesive based on Diels–Alder (DA) chemistry is investigated and demonstrated. The Diels–Alder incorporated multifunctional diamine cross-linker was initially synthesised, followed by forming a covalently cross-linked network with DGEBA epoxy at low temperature (80 °C) that breaks via cleavage of DA adducts at elevated temperature (150 °C). Upon cooling the covalent cross-links reform enabling the epoxy to retain adhesive properties, and the bonding/de-bonding effect was performed over two complete cycles advancing a thermally reversible adhesive based on the retro-DA mechanism. The viscoelastic properties of the prototype adhesive including viscosity, storage and loss modulus were retained after bonding and de-bonding cycles, affirming retro-DA reaction occurs at about 150 °C upon heating cycle and DA reaction upon cooling cycle. Mechanical testing demonstrates maximum of 39.5% reduction in bond strength in debonding cycle, followed by maximum of 100% bond strength in re-bonding cycle with no additional application of the prototype adhesive, revealing excellent adhesive performance.

C(alkyl)–C(vinyl) bond cleavage enabled by Retro-Pallada-Diels-Alder reaction

Activation and cleavage of carbon–carbon (C–C) bonds is a fundamental transformation in organic chemistry while inert C–C bonds cleavage remains a long-standing challenge. Retro-Diels-Alder (retro-DA) reaction is a well-known and important tool for C–C bonds cleavage but less been explored in methodology by contrast to other strategies. Herein, we report a selective C(alkyl)–C(vinyl) bond cleavage strategy realized through the transient directing group mediated retro-Diels-Alder reaction of a six-membered palladacycle, which is obtained from an in situ generated hydrazone and palladium hydride species. This unprecedented strategy exhibits good tolerances and thus offers new opportunities for late-stage modifications of complex molecules. DFT calculations revealed that an intriguing retro-Pd(IV)-Diels-Alder process is possibly involved in the catalytic cycle, thus bridging both Retro-Diels-Alder reaction and C–C bond cleavage. We anticipate that this strategy should prove instrumental for potential applications to achieve the modification of functional organic skeletons in synthetic chemistry and other fields involving in molecular editing.

Highly Thermally Stable, Reversible, and Flexible Main Chain Type Benzoxazine Hybrid Incorporating Both Polydimethylsiloxane and Double-Decker Shaped Polyhedral Silsesquioxane Units through Diels-Alder Reaction.

This work synthesizes a new bifunctional furan derivative (PDMS-FBZ) through a sequence of hydrosilylation of nadic anhydride (ND) with polydimethylsiloxane (PDMS), reaction of the product with p-aminophenol to form PDMS-ND-OH, and its subsequent Mannich reaction with furfurylamine and CH2 O. Then, the main chain-type copolymer PDMS-DABZ-DDSQ is prepared through a Diels-Alder (DA) cycloaddition of PDMS-FBZ with the bismaleimide-functionalized double-decker silsesquioxane derivative DDSQ-BMI. Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy confirm the structure of this PDMS-DABZ-DDSQ copolymer; differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA) reveal it to have high flexibility and high thermal stability (Tg = 177 °C; Td10 = 441 °C; char yield = 60.1 wt%); contact angle measurements reveal a low surface free energy (18.18 mJ m-2 ) after thermal ring-opening polymerization, because the inorganic PDMS and DDSQ units are dispersed well, as revealed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). This PDMS-DABZ-DDSQ copolymer possesses reversible properties arising from the DA and retro-DA reactions, suggesting its possible application as a functional high-performance material.

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.

Injection moldable, self-healable, and recyclable rubber-bonded NdFeB magnets with the magnetic particulates content up to 90 wt%

To apply the facile injection molding technique commonly used in thermoplastics processing to rubber-bonded magnets, typical highly filled thermosetting composites, an approach based on Diels–Alder (DA) chemistry is proposed by the authors. In the proof-of-concept experiments, the oligomers of hydroxyl-terminated polybutadiene rubber are successively functionalized by furyl and trifluoromethyl, and the magnetic NdFeB particles are modified by maleimide. By taking advantage of the reversible DA reaction, cross-linkages are established among the treated polybutadiene oligomers and NdFeB particles. The composite exhibits very low viscosity due to de-cross-linking when heated to retro-DA reaction temperature, which provides the highly filled system with necessary thermal processibility. In addition, the fluorine groups of the rubber phase are aggregated surrounding the particulate fillers allowing for well separation of the latter in the melt state. During the subsequent cooling, the DA bonds are re-built up in the rubber and at the particles/rubber interface, respectively, offering the robustness of traditional thermosets. As a result, the polybutadiene-bonded magnets with NdFeB content up to 90 wt% can be injection molded and possess rather high magnetic properties, mechanical properties, self-healability, and recyclability. The present work combines the merits of thermoplastics and thermosets and satisfies the contradictory requirements of high filling, efficient manufacturing, and balanced performance of functional polymer composites.

Self- and Cross-Fusing of Furan-Based Polyurea Gels Dynamically Cross-Linked with Maleimides.

Bio-based polyureas (PUs) with main-chain furan rings were synthesized by the polyaddition of 2,5-bis(aminomethyl)furan with various diisocyanates, such as methylene diphenyl diisocyanate. Several PU's were soluble in polar organic solvents, and were cast to form thermomechanically stable films with softening temperatures of over 100 °C. The furan rings of the PU main chains underwent a dynamic Diels-Alder (DA) reaction with bismaleimide (BMI) cross-linkers. While the mixed solution of PU and BMI did not show any apparent signs of reaction at room temperature, the DA reaction proceeded to form gels upon heating to 60 °C, which became a solution again by further heating to 80 °C (retro-DA reaction). The solution phase was maintained by rapid quenching from 80 °C to room temperature, while the gel was reformed upon slow cooling. The recovered gels exhibited self-healing properties. A scratch made by a hot knife at temperatures above 80 °C disappeared spontaneously. When two different gels were cut using a knife at room temperature, placed in contact with each other, and heated to 60 °C, they fused. The ability to control the DA/retro-DA reaction allowed gels of varying composition to heal.

How Bases Catalyze Diels-Alder Reactions.

We have quantum chemically studied the base-catalyzed Diels-Alder (DA) reaction between 3-hydroxy-2-pyrone and N-methylmaleimide using dispersion-corrected density functional theory. The uncatalyzed reaction is slow and is preceded by the extrusion of CO2 via a retro-DA reaction. Base catalysis, for example, by triethylamine, lowers the reaction barrier up to 10 kcal mol-1 , causing the reaction to proceed smoothly at low temperature, which quenches the expulsion of CO2 , yielding efficient access to polyoxygenated natural compounds. Our activation strain analyses reveal that the base accelerates the DA reaction via two distinct electronic mechanisms: i) by the HOMO-raising effect, which enhances the normal electron demand orbital interaction; and ii) by donating charge into 3-hydroxy-2-pyrone which accumulates in its reactive region and promotes strongly stabilizing secondary electrostatic interactions with N-methylmaleimide.

Effect of monomer composition on thermal, mechanical, and self-healing properties of thermosets containing Diels-Alder adduct moieties and disulfide bonds

The Diels-Alder (DA) and thiol-maleimide reactions of difurfuryl disulfide (DFDS), 1,4-bismaleimidodiphenylmethane (BMI), pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), and thiol-terminated disulfide-containing polyether (LP) at various feed monomer ratios produced polymer networks containing both DA adduct moieties and disulfide bonds. Fourier transform infrared spectral analysis revealed that the DA and thiol-maleimide reactions occurred for all the cured products. A differential thermal analysis revealed that all the cured products exhibited an endothermal peak due to the retro-DA reaction at 115–124 °C. The tensile moduli and strengths of the cured products increased with increasing DFDS (or DA) and PETMP fractions. The elongations at break increased with increasing LP (or disulfide) fraction. Most of the cured products exhibited self-healing ability by annealing at 60 °C for 24 h, and/or at 120 °C for 1 h followed by additional annealing at 60 °C for 24 h. The cured product with a higher LP (or disulfide) fraction exhibited a better healing ability at 60 °C for 24 h by the disulfide metathesis. Also, the cured product with a higher DFDS fraction exhibited a better healing ability at 120 °C for 1 h and 60 °C for 24 h by the DA/retro-DA reaction.

The role of molecular architecture on the viscoelastic properties of thermoreversible polyurethane adhesives

New polyurethane (PUR) adhesives containing covalent Diels-Alder (DA) bonds that can break and form with temperature have been recently developed for sustainable multilayer packaging. The understanding of the mechanical properties and in particular of creep in terms of molecular architecture appears of outmost importance in view of their applications and potential reutilization. Oscillatory shear rheometry shows a modulus decrease with temperature associated to bond breakage and the enhancement found upon cooling can be associated with network reconstruction. Analysis of storage modulus values above the glass transition and below the retro-DA reaction suggests a lower network crosslinking density with increasing adduct content. Room temperature indentation and shear rheometry tests reveal that DA bonds improve the modulus and reduce the resistance to linear viscoelastic flow. The modulus improvement can be associated to the enhanced chain rigidity introduced by the DA moieties and the reduced creep resistance seems to be related to the decrease of crosslinking density.

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.

Concurrent and Mechanochemical Activation of Two Distinct and Latent Fluorophores via Retro-Diels-Alder Reaction of an Anthracene-Aminomaleimide Adduct.

Generally, a typical mechanochromophore produces color change through chemical transformation into one or two identical new chromophores/fluorophores under applied mechanical force. Herein, we introduce a novel mechanophore based on an anthracene-aminomaleimide Diels-Alder (DA) adduct featuring two distinct and latent fluorophores. This nonfluorescent mechanophore undergoes retro-DA reaction upon mechanochemical activation in solution and the solid state, generating the respective anthracene and aminomaleimide fragments simultaneously, both of which are highly emissive with different fluorescent colors. In addition, the aminomaleimide fluorophore exhibits sensitive fluorescence on-off response to protic solvents or polar solvents, which enables dual-color mechanochromism from this single mechanophore.

Improving creep resistance while maintaining reversibility of covalent adaptive networks via constructing reversibly interlocked polymer networks

Thermoresponsive covalent adaptive networks (CANs) have attracted increasing research attention because of their temperature-dependent reversibility, which endows the materials with versatile smart functionalities including intrinsic self-healability not available for traditional thermosets. Nevertheless, the associated reduction in creep resistance of this type of CANs limits their practical application. Here in this work, we demonstrate that the reversibility and dimensional stability can be combined via interlocking the thermoresponsive network with an ultraviolet-responsive network. The two types of networks are, respectively, crosslinked by orthogonal reversible Diels-Alder (DA) bonds and coumarin. Owing to the interlocked architecture, the latter single network can be uniformly distributed in the former, restricting the chain movement and enhancing the creep resistance even when the former is decrosslinked at elevated temperature as a result of retro-DA reaction. Meanwhile, the ultraviolet-responsive network plays the role of a photo-reversible switch. Its decrosslinking on exposure to 254 nm ultraviolet (UV) light and recrosslinking under a UV irradiation of 350 nm lead to repeated releasing and reimposing of the restraints on the neighboring thermoresponsive network. By using this smart habit, the material can either be conditionally self-healed with moderate healing efficiency or completely self-healed depending on whether only DA bonds or both DA bonds and coumarin are triggered. More importantly, the conflicting properties, that is, creep resistance and reversibility of the CANs are thus united. The proposed strategy provides a facile way for overcoming the weaknesses of CANs while maintaining the advantage.

Covalent adaptable networks impart smart processability to multifunctional highly filled polymer composites

To tackle the challenge of manufacturing highly filled composite materials via melt compounding and injection molding, the authors proposed a novel approach based on Diels-Alder (DA) chemistry. Polycaprolactone oligomers (PCLolig, Mw = 2000 g/mol) were firstly crosslinked by DA bonds, and then the resultant network PCLolig-DA was also connected to maleimide functionalized Al2O3 particles (Al2O3-M) via DA bonds. Upon being heated to the retro-DA reaction temperature, the covalent adaptable composite network exhibited low viscosity due to de-crosslinking, enabling melt processing and well dispersion of high-concentration fillers. During the subsequent cooling, the composite network was regained because of DA reaction, benefiting increase of the composite’s strength. Consequently, the contradictory requirements for molecular weight of matrix polymer under the circumstances, which had better to be as low as possible during the manufacturing but as high as possible in the ultimate composites, were skillfully satisfied.

Recyclable Epoxy Resin via Simultaneous Dual Permanent/Reversible Crosslinking Based on Diels–Alder Chemistry

AbstractTaking advantage of the reversible Diels–Alder (DA) reaction, a simple strategy to obtain recyclable epoxy resins is presented. For this purpose, blends of furan‐functionalized and nonfunctionalized epoxy resin are prepared. After the addition of diamine and bismaleimide, blends are heated at 150 °C for 5 min, where the permanent amine/epoxy reaction has taken place and upon cooling to room temperature the reversible DA reaction has happened, giving rise to a dual permanent/nonpermanent network. Both reactions are confirmed by Fourier‐transform infrared (FTIR) and13C‐crosspolarization, magic‐angle spinning (CP/MAS) nuclear magnetic resonance (NMR). Modulated differential scanning calorimetry (MDSC) shows that the epoxy/amine and bismaleimide/amine curing reaction take place, after the DA reaction, simultaneously with the retroDA reaction and before the bismaleimide homopolymerization. Therefore, under the appropriate curing conditions, the Michael's addition and the bismaleimide homopolymerization do not avoid the formation of a hybrid network, as stated in other reports. The reversibility of the DA reaction in three consecutive cycles is confirmed by DSC. Finally, the dual‐cured sample is reprocessed three times without significant loss of mechanical properties.

Thermally Remendable Polyurethane Network Cross-Linked via Reversible Diels-Alder Reaction.

We prepared a series of thermally remendable and recyclable polyurethanes crosslinked via reversible furan-maleimide Diels–Alder reaction based on TDI end-caped branched Voranol 3138 terminated with difurfurylamine and 4,4′-bis(maleimido)diphenylmethane (BMI). We showed that Young modulus strongly depends on BMI content (from 8 to 250 MPa) that allows us to obtain materials of different elasticity as simple as varying BMI content. The ability of DA and retro-DA reactions between furan and maleimide to reversibly bind material components was investigated by NMR spectroscopy, differential scanning calorimetry, and recycle testing. All polymers obtained demonstrated high strengths and could be recovering without significant loss in mechanical properties for at least five reprocessing cycles.

Diels-Alder Polymer Networks with Temperature-Reversible Cross-Linking-Induced Emission.

A novel synthetic strategy gives reversible cross‐linked polymeric materials with tunable fluorescence properties. Dimaleimide‐substituted tetraphenylethene (TPE‐2MI), which is non‐emissive owing to the photo‐induced electron transfer (PET) between maleimide (MI) and tetraphenylethene (TPE) groups, was used to cross‐link random copolymers of methyl (MM), decyl (DM) or lauryl (LM) methacrylate with furfuryl methacrylate (FM). The mixture of copolymer and TPE‐2MI in DMF showed reversible fluorescence with “on/off” behavior depending on the Diels–Alder (DA)/retro‐DA process, which is easily adjusted by temperature. At high temperatures, the retro‐DA reaction is dominant, and the fluorescence is quenched by the photo‐induced electron transfer (PET) mechanism. In contrast, at low temperatures, the emission recovers as the DA reaction takes over. A transparent PMFM/TPE‐2MI polymer film was prepared which shows an accurate response to the external temperature and exhibited tunable fluorescent “turn on/off” behavior. These results suggest the possible application in areas including information security and transmission. An example of invisible/visible writing is given.