rDA Reaction Research Articles

Eco-friendly, self-repairing polymer materials based on reversible Diels-Alder chemistry

The recyclability and reuse of materials are essential for satisfying the current demand for polymers while alleviating the impact on the environment. Herein, a series of self-repairing polymeric materials were synthesized by exploiting the reversibility of the Diels-Alder (DA) reaction. The main polymer chains were composed of polyethylene glycol diglycidyl ether (PEGDGE) and polypropylene glycol diglycidyl ether (PPGDGE). The DA reaction kinetics and the mechanism of retro-Diels–Alder (rDA) reaction were studied with Fourier transform infrared (FTIR) spectroscopy. The cross-linking conversion rate exceeded 80% after 100 min and reached 99% after 50 h at 65 °C, which is remarkably higher and faster than previous studies. The polymeric material was shown to have excellent reusability and high-temperature self-repairing properties. In addition, the polymer was also capable of performing high-temperature shaping and solvent recovery shaping.

Mechanistic Insight into pH-Dependent Luminol Chemiluminescence in Aqueous Solution.

Luminol is one of the best known chemiluminescent cyclic hydrazides used in basic solution. Owing to the complexity of luminol oxidation, the mechanism of luminol chemiluminescence (CL), especially in aqueous solution, has not yet been fully elucidated. Recent theoretical computations have confirmed that luminol CL originates from the chemiexcitation of a 1,2-dioxane-3,6-dione dianion (CP2-). This seems to be inconsistent with the luminol oxidation in aqueous solutions, where only the decomposition of a monoanionic peroxyketal (L-OOH) is confirmed to yield CL. In this work, we theoretically investigated the complete decomposition of L-OOH and the pKa of key intermediates in aqueous solutions using (time-dependent) density functional theory. L-OOH first cyclizes to an endoperoxide monoanion (EP-). When pH < pKa (EP-), EP- directly decomposes by a retro-Diels-Alder (rDA) reaction to an aminophthalate monoanion (AP-) without CL activity. Moreover, when pH > pKa (EP-), EP- deprotonates to dianionic EP2-, which rapidly eliminates N2 to CP2-, inducing chemiexcitation. This conclusion is supported by the pKa (EP-) ≈ 8 estimated in this work, which is consistent with the pH-dependent profile of luminol CL observed in previous experiments. Thus, the pH-dependent CL determined by the acidity of endoperoxides may provide a theoretical basis to design new cyclic hydrazides with CL activity at different pH.

Regioselectivity of the 1,3-Dipolar Cycloaddition of Organic Azides to 7-Heteronorbornadienes. Synthesis of β-Substituted Furans/Pyrroles.

An efficient procedure for the preparation of β-substituted furans/pyrroles is presented. The methodology is based on the use of 7-oxa/azanorbornadienes as dipolarophiles in 1,3-dipolar cycloaddition with benzyl azide. The triazoline cycloadduct thus formed spontaneously decomposes via a retro-Diels-Alder (rDA) reaction to afford a β-substituted furan/pyrrole derivative and a stable triazole. The scope of this tandem 1,3-dipolar cycloaddition/rDA reaction was studied with thirteen 7-heteronorbornadienes. This study allowed a deep knowledge of the regioselectivity of the reaction, which can be tuned through the substituents of the heteronorbornadienic systems.

Multistimuli Responsive Reversible Cross-Linking-Decross-Linking of Concentrated Polymer Brushes.

Poly(furfuryl methacrylate) (PFMA) brushes were cross-linked using bismaleimide cross-linkers via the Diels-Alder (DA) reaction at 70 °C, generating cross-linked PFMA brushes (PFMA brush gels). The cross-linked PFMA brushes were decross-linked at 110 °C via the retro-Diels-Alder (rDA) reaction, offering the temperature-responsive reversible PFMA brush gels. The wettability of the brush was tunable by cross-linking and decross-linking. The use of a disulfide containing bismaleimide as a cross-linker gave the S-S bond at the cross-linking point. The S-S bond was cleaved upon thermal or photo stimulus and regenerated through oxidative stimulus, offering another reversible decross-linking/cross-linking pathway of the PFMA brush gel. The use of photo stimulus together with photomasks further offered patterned brushes with the cross-linked and decross-linked domains. The combination of the DA/rDA reactions and the reversible S-S bond cleavage provided multistimuli-responsive brush gels for switching the surface properties in unique manners. The reversible cross-linking, multiresponsiveness, access to patterned structures, and metal-free synthetic procedure are attractive features in the present approach for creating smart functional surfaces.

Retro-Diels–Alder Reaction on Surface: Generating Energy-Prohibited Structures in Bulk Film Condition through Surface-Adsorbing Neutralization Effect

Retro-Diels–Alder (rDA) reactions on surfaces were few reported in comparison with the fruitful results of rDA in liquids, gas phase, or bulk films. On the other hand, the important effects of iner...

Self‐healing semi‐IPN materials from epoxy resin by solvent‐free furan–maleimide Diels–Alder polymerization

ABSTRACTNovel self‐healing Diels–Alder (DA) polymer and the corresponding semi‐interpenetrated polymer networks (semi‐IPNs) were synthesized and characterized. Initially, a furan‐functionalized resin (FFR) was synthesized through the ring‐opening reaction of a conventional epoxy resin [diglycidyl ether bisphenol A (DGEBA)] with furfuryl alcohol as a bio‐based compound. Subsequently, semi‐IPNs with different compositions were obtained through the blending of DGEBA, FFR, 4,4′‐diaminodiphenylmethane, and 1,1′‐(methylenedi‐1,4‐phenylene) bismaleimide in the molten state by following a predetermined time–temperature program. Fourier transform infrared and nuclear magnetic resonance analyses confirmed the successful synthesis of the materials. Thermoreversibility via retro‐DA (rDA) reaction was evidenced by differential scanning calorimetry (DSC) and sol–gel transition tests. Repeated DSC cycle was successfully performed thrice on the DA polyadduct which corroborated repeatability of the DA/rDA association/dissociation. Self‐healing and mechanical properties were preliminarily evaluated by scanning electronic microscopy and flexural testing analyses, respectively. The self‐healing efficiencies were around 80 and 95% for semi‐IPN and DA polyadduct, respectively, based on flexural strength. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48015.

Organocatalytic aza-Michael Reaction to 3-Vinyl-1,2,4-triazines as a Valuable Bifunctional Platform.

An unprecedented catalytic aza-Michael addition to substituted 3-vinyl-1,2,4-triazines, as original bifunctional platforms for the domino conjugate addition inverse-electron-demand hetero-Diels-Alder/retro-Diels-Alder ( ihDA/ rDA) reaction, was achieved using the highly acidic triflimide as an organocatalyst. Based on the use of alkoxyamine nucleophiles, this sequence not only highlights a rare example of the catalytic aza-Michael reaction to alkenylazaarenes but also proves to be useful for the elaboration of an array of biorelevant tetrahydro-[1,6]-naphthyridines.

Recyclable biobased materials based on Diels–Alder cycloaddition

ABSTRACTPoly(ethylene glycol) diglycidyl ether‐furfurylamine (PGFA) containing pendant furan was synthesized, and a series of crosslinked materials with thermally reversible capacity were synthesized through a furan/maleimide Diels–Alder (DA) reaction between PGFA and bismaleimide (BMI). The kinetics of the PGFA/BMI DA reaction were studied by Fourier transform infrared spectroscopy (FTIR). The reaction conversion rate, the reaction rate constant, and the energy of the DA reaction at different temperatures were calculated. In addition, the retro Diels–Alder (rDA) reaction was studied via 1H‐NMR, differential scanning calorimetry, and in situ FTIR. The occurrence of the retro DA reaction has been characterized clearly. Finally, the mechanical properties of the materials were obtained by dynamic mechanical and tensile tests. The storage modulus decreased obviously when the temperature reached over 90 °C, which proved that the materials were thermally reversible at high temperature. By changing the proportion of the crosslinking agent BMI, the best‐performing materials were obtained, and the properties of the materials were basically unchanged after recycling. Thus we have obtained an excellent reusable material. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47352.

Thermally reversible polymer networks for scratch resistance and scratch healing in automotive clear coats

This study investigated the application of a thermally reversible polymer network fabricated using Diels-Alder (DA)/retro-DA (rDA) reactions for use as a scratch-healing automotive clear coat. For this purpose, a new scratch-healing poly(urethane acrylate) network containing a DA adduct unit (DA-CL) was prepared, and its material properties and scratch-healing performance were compared to the properties of a commercial clear coat system (C-CL). The thermally reversible crosslinking and de-crosslinking reactions among the DA-CL, via DA and rDA reactions, were systematically evaluated using in-situ oscillatory rheology coupled with FT-IR spectroscopy. The material properties of the DA-CL and C-CL materials, including the thermal stability, thermal transitions, hardness, and scratch resistance, were measured using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), nanoindentation, and nanoscratch test methods. The scratch-healing performance of the DA-CL was quantitatively characterized and compared to the performance of the C-CL using a nanoscratch tester equipped with an optical microscope (OM) and an atomic force microscope (AFM). The DA-CL polymer network exhibited superior scratch healing and scratch resistance compared to the C-CL polymer network. These data indicated that the DA self-healing polymer network is potentially useful as a scratch-healing clear coat for the automotive industry.

A Car–Parrinello Molecular Dynamics Simulation Study of the Retro Diels–Alder Reaction for Partially Saturated 2-Pyrones in Water

2-Pyrones have been identified as potential platform molecules derived from biomass to produce high-value chemicals. Theoretical studies utilizing density functional theory (DFT) simulations have shown the partially saturated form of these molecules undergoing a retro Diels–Alder (rDA) reaction in both vapor and solvent conditions. Developing an understanding of the effect of solvent media is crucial to improve the processing of these biomass-derived compounds. In this context, DFT simulations had shown limited access due to either implicit solvent or static explicit solvent environment applied in the model, which did not account for the contribution from the dynamics. Herein, Car–Parrinello molecular dynamics (CPMD) simulations, in conjunction with metadynamics, were performed to understand the effect of dynamics of the solvent on the rDA reaction of partially saturated 2-pyrones. For this, compounds containing both electron-donating and -withdrawing groups as substituent were studied. It was observed th...

Comparison of thermally actuated retro-diels-alder release groups for nanoparticle based nucleic acid delivery

The present study explores alternate pericyclic chemistries for tethering amine-terminal biomolecules onto silver nanoparticles. Employing the versatile tool of the retro-Diels-Alder (rDA) reaction, three thermally-labile cycloadducts are constructed that cleave at variable temperature ranges. While the reaction between furan and maleimide has widely been reported, the current study also evaluates the reverse reaction kinetics between thiophene-maleimide, and pyrrole-maleimide cycloadducts. Density Functional Theorem (DFT) calculations used to model and plan the experiments, predict energy barriers for the thiophene-maleimide reverse reaction to be greatest, and the pyrrole-maleimide barriers the lowest. Based on the computational analyses, it is projected that the cycloreversion rate would occur slowest with the thiophene, followed by furan, and finally pyrrole would yield the promptest release. These thermally-responsive linkers, characterized by Electrospray Ionization Mass Spectrometry, 1H and 13C NMR, are thiol-linked to silver nanoparticles and conjugate single stranded siRNA mimics with 5′ fluorescein tag. Second harmonic generation spectroscopy (SHG) and fluorescence spectroscopy are used to measure release and rate of release. The SHG decay constants and fluorescence release profiles obtained for the three rDA reactions confirm the trends obtained from the DFT computations.

Multiple-responsive shape memory polyacrylonitrile/graphene nanocomposites with rapid self-healing and recycling properties.

It still remains a great challenge to endow polymer materials with multiple superior material properties by precise molecular design. Herein, we report a Diels–Alder (DA) based crosslinked polyacrylonitrile/graphene nanocomposite (PAN-DA/GR), which has multiple-responsive properties of shape memory, self-healing, and reprocessing in addition to enhanced mechanical properties. The graphene sheets, which are well dispersed in the DA-based crosslinked PAN network, can act as intrinsic localized thermal sources by converting the absorbed external IR/microwave energies into heat, to trigger the glass transition for elasticity-based shape memory properties and retro-DA (rDA) reactions for healing. The incorporation of Diels–Alder bonds also gives the material solid state plasticity through topological network rearrangement, thus leading to versatile shape adaptability. Moreover, both regional shape control and targeted self-healing of the nanocomposites can be simply achieved by IR laser irradiation. Besides, the incorporation of a small amount of graphene can significantly improve the mechanical strength with respect to the DA-based crosslinked PAN. Both DSC and in situ variable temperature 13C solid-state NMR experiments were used to monitor the reversible DA reactions.

Fire behavior of novel imidized norbornene-containing poly(ethylene terephthalate) copolymers: Influence of retro-Diels-Alder reaction at high temperature

Retro-Diels-Alder reaction (rDA) has been widely used as a common modification approach for expanding functional applications of polymers. Especially, it has become an effective method of current cross-linking chemistry. With the goal of designing a cross-linkable PET-based copolyester toward flame retardancy and anti-dripping during combustion, dimethyl 5-(1,3-dioxo-3a,4,7,7a-tetrahydro-1H-4,7-methanoisoindol-2(3H)-yl) isophthalate (DMTMI) was incorporated as the functional co-monomer into PET chains via random transesterification polycondensation. Thermogravimetry-differential scanning calorimetry (TG-DSC) and rheological studies demonstrated the existence of cross-linking behavior at high temperature. By controlling the co-monomer content, expected flame retardancy was obtained, as illustrated by the results from the limiting oxygen index (LOI) and cone calorimetry. Furthermore, LOI values increased firstly and then decreased with the content of DMTMI increased, which was related to the existence of rDA reaction, and further proposed that a competitive relationship between the release of inflammable cyclopentadiene during rDA reaction and cross-linking behavior occurred in combustion. The hypothesis was verified through pyrolysis-gas chromatograph-mass spectrometry (Py-GC-MS).

Influence of material properties on scratch-healing performance of polyacrylate-graft-polyurethane network that undergo thermally reversible crosslinking

Scratch-healing poly (methyl methacrylate)-co-[poly (methyl metharyleate)-graft-(oligo-caprolactone)] urethane networks containing a Diels Alder (DA) adduct unit (GCPNp-DAs) were successfully synthesized and shown to be capable of undergoing thermally reversible crosslinking. The synthesized polymers were coated on steel substrates to investigate the influence of their material properties on their scratch-healing performance. The reversible formation of crosslinked and de-crosslinked structures of the GCPNp-DAs at DA and retro-DA (rDA) reaction temperatures was demonstrated using FT-IR spectroscopy, differential scanning calorimetry (DSC), oscillatory rheology, and nanoindentation. The scratch-resistance and healing performances of the GCPNp-DA coatings were evaluated quantitatively using a scratch test machine equipped with an optical microscope (OM) and an atomic force microscope (AFM). These results were found to be greatly influenced by the material properties of the coatings such as the elastic modulus, indentation hardness (HIT), crosslinking density (vc), and thermal transition temperature as well as by whether the deforming load that produced the scratches was increased in a progressive (gradual) or step-wise manner.

Dynamically Cross-linked Elastomer Hybrids with Light-Induced Rapid and Efficient Self-Healing Ability and Reprogrammable Shape Memory Behavior.

Pristine carbon nanotubes (CNTs) were activated to exhibit Diels-Alder (DA) reactivity in a polymer matrix, which was modified with monomers containing furan groups. The DA-active polymer matrix was transferred into a dynamic reversible cross-linked inorganic-organic network via a Diels-Alder reaction with CNTs, where pristine CNTs were used as dienophile chemicals and furan-modified SBS acted as the macromolecular diene. In this system, the mechanical properties as well as resilience and solvent resistance were greatly improved even with the presence of only 1 wt % CNTs. Meanwhile, the hybrids retained recyclability and exhibited some smart behaviors, including self-healing and reprogrammable shape memory properties. Furthermore, due to the photothermal effect of CNTs, a retro-Diels-Alder (rDA) reaction was activated under laser irradiation, and healing of a crack on the hybrid surface was demonstrated in approximately 10 s with almost complete recovery of the mechanical properties. Such fast and efficient self-healing performance provides a new concept in designing self-healing nanocomposites with tunable structures and mechanical properties. Furthermore, the DA and rDA reactions could be combined to reprogram the shape memory behavior under laser irradiation or thermal treatment, wherein the temporary shape of the sample could be transferred to a permanent shape via the rDA reaction at high temperature.

Design and Synthesis of Spirocycles

In this account we disclose a variety of simple strategies demonstrated in our laboratory in the last two decades for the assembly of diverse and intricate molecular frameworks containing spiro linkage(s) by using various metathesis protocols such as ring‐closing metathesis (RCM), ring‐opening cross‐metathesis (ROCM), ring‐closing enyne metathesis (RCEM) and ring‐rearrangement metathesis (RRM). Also, cycloaddition reactions ([2+2], [4+2] and [2+2+2]) and other key processes such as Claisen rearrangement (CR), Grignard addition, Fischer indolization (FI), Suzuki–Miyaura (SM) cross‐coupling and the retro‐Diels–Alder (rDA) reaction have been used as key steps to construct spirocycles.

Self-assembled elastomer nanocomposites utilizing C60 and poly(styrene-b-butadiene-b-styrene) via thermally reversible Diels-Alder reaction with self-healing and remolding abilities

In this article, C60 could be co-assembled into PB phase of the furan modified poly(styrene-b-butadiene-b-styrene) (SBS) via Diels-Alder (DA) reaction at the molecular level. The morphology of SBS could be effectively tailored by the incorporation of C60 and even small amount of C60 could lead to great change in the morphologies of SBS. Three types of morphologies of SBS could be observed as the content of C60 was increased from 0.1 to 2 wt%, which included honeycomb like, worm-like bi-continuous phase and their intermediate transition state. In the meantime, it was unexpectedly found that C60 could be used as effective crosslinking agents to build the network toward the furan modified SBS via DA reaction. Therefore, its solvent resistance, elastic resilience and tensile strength were greatly improved for the chemical crosslinking. Besides, due to the thermal reversible character for DA reaction, the hybrids based on SBS and C60 possessed the self-healing and remolding abilities via retro-Diels-Alder (rDA) reaction. All in all, C60 could provide the multiple functions toward the modification of SBS via DA reaction.

Kinetics of cross-linking and de-cross-linking of EPM rubber with thermoreversible Diels-Alder chemistry

The kinetics of the Diels-Alder (DA) cross-linking and retro-Diels-Alder (rDA) de-cross-linking reactions have been studied for furan-containing EPM rubber thermoversibly cross-linked with a bismaleimide. Cross-linking conversions obtained from spectroscopic methods deviate from rheological methods as the former take into consideration both the intermediate DA adduct and complete cross-links, while the latter only determine the formation and decomposition of complete cross-links. The thermoreversibility of the system was shown by a combination of equilibrium swelling and rheological measurements. Dynamic time sweeps of the rubber at different temperatures were used to determine the Arrhenius activation energies of 7.04 and 57.9kJ/mol for the DA and the rDA reactions, respectively. Ea,DA is relatively low and suggests a diffusion limitation while Ea,rDA correspond to values reported in literature. Both values were used to simulate cross-link density profiles as a function of time and temperature that correspond well with the experimental data.

Reaction of Aldehydes/Ketones with Electron-Deficient 1,3,5-Triazines Leading to Functionalized Pyrimidines as Diels-Alder/Retro-Diels-Alder Reaction Products: Reaction Development and Mechanistic Studies.

Catalytic inverse electron demand Diels-Alder (IEDDA) reactions of heterocyclic aza-dienes are rarely reported since highly reactive and electron-rich dienophiles are often found not compatible with strong acids such as Lewis acids. Herein, we disclose that TFA-catalyzed reactions of electron-deficient 1,3,5-triazines and electron-deficient aldehydes/ketones can take place. These reactions led to highly functionalized pyrimidines as products in fair to good yields. The reaction mechanism was carefully studied by the combination of experimental and computational studies. The reactions involve a cascade of stepwise inverse electron demand hetero-Diels-Alder (ihDA) reactions, followed by retro-Diels-Alder (rDA) reactions and elimination of water. An acid was required for both ihDA and rDA reactions. This mechanism was further verified by comparing the relative reactivity of aldehydes/ketones and their corresponding vinyl ethers in the current reaction system.

Sequence-Controlled Polymers with Furan-Protected Maleimide as a Latent Monomer.

Herein, a novel methodology for preparing sequence-controlled polymers is illustrated by using a latent monomer, furan protected maleimide (FMI). At 110 °C, FMI is deprotected by retro Diels-Alder (rDA) reaction, and the released MI is immediately involved in the cross-polymerization with styrene (St) to deliver heterosegments. At 40 °C the rDA reaction does not proceed, therefore homo-poly(styrene) segments are produced. By implementing programmable temperature changes during polymerization of St and FMI, "living" polymers with tailored a sequence are created. A ternary copolymerization produces complex sequences as designed. Alkynyl-functionalized FMI, used as a latent monomer, leads to the desirable placement of functional groups along the polymer chain. This latent-monomer-based strategy opens a new avenue for fabricating sequence-controlled polymers.