Reverse Diels-Alder Reaction Research Articles

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.

Thermosensitive polylactic-acid-based networks

Polylactic-acid-based thermosensitive networks were prepared in a simple way through reversible Diels–Alder reaction. First, hydroxyl-telechelic PLA with well-controlled molar masses were synthesized by ring-opening polymerization of l-lactide using 1,4-butanediol/Sn(Oct)2 as catalytic system in a solvent-free process. The molar ratio of lactide/initiator was fixed to control the number average molecular weight (Mn). Before the synthesis of the network, a second precursor which is the Diels–Alder adduct was prepared apart for a better control of the Diels–Alder conversion. The thermoreversible networks were then obtained by simultaneous coupling of the hydroxyl-telechelic PLA, Diels–Alder adduct, multi-alcohols such as glycerol and xylitol, and a diisocyanate (H12MDI) through the alcoholysis reaction of isocyanate. The networks’ properties were characterized. The cross-linking and de-cross-linking thermal dependence of PLA-based networks was shown by solubility tests at room and high temperatures and by thermo-mechanical analysis. The effect of the node concentration and steric hindrance on the networks’ formation and properties was investigated. Low de-cross-linking temperatures were obtained for some materials.

Furan‐Modified Spherosilicates as Building Blocks for Self‐Healing Materials

AbstractOctafunctional spherosilicates were used to prepare self‐healing hybrid materials. The hydrosilation of the octakis(hydridodimethylsiloxy)‐substituted spherosilicate with furfuryl allyl ether generates an inorganic nano‐building‐block that is used to formulate various self‐healing hybrid materials based on a reversible Diels–Alder reaction. Curing with a molecular bismaleimide results in a hard, glassy but reversibly cross‐linkable hybrid material. The reversibility of the curing mechanism allows the preparation of films with a heated press, which also opens the possibility to process the materials by injection molding. Substitution of the molecular cross‐linker with an oligomeric poly(dimethylsiloxane) bismaleimide results in an elastomeric material. The kinetics of the Diels–Alder reaction upon cooling after a retro‐Diels–Alder reaction are mainly controlled by the mobility of the cross‐linker within the system.

Thermodynamically controlled Diels–Alder reaction of 12-N-methylcytisine: A DFT study

A DFT study was performed for the Diels–Alder traction of 12-N-methylcytisine with a number of dienophiles (in boiling toluene under atmospheric pressure), namely, N-phenylmaleimide, maleic anhydride, 2,4-benzoquinone, tetracyanoethylene and methyl methacrylate. It was shown that 12-N-methylcytisine selectively reacts with these dienophiles, only the reaction with N-phenylmaleimide (NPM) resulting in the formation of thermodynamically stable adducts, which is consistent with experimental data. This selectivity of 12-N-methylcytisine is attributable to the difference between the properties of the listed dienophiles, which is confirmed by the relative reactivity indices calculated within the framework of the frontier molecular orbital (FMO) and hard and soft (Lewis) acids and bases (HSAB) theories, the thermodynamic and activation parameters of the forward and retro-Diels–Alder reactions. According to analysis of the theoretical results, NPM is characterized by high chemical potential, hardness close to that of 12-N-methylcytisine, and commensurable heights of the activation barriers for the forward and reverse Diels–Alder reactions and also forms stable [4+2] adducts.

A rheological and spectroscopic study on the kinetics of self‐healing in a single‐component diels–alder copolymer and its underlying chemical reaction

ABSTRACTIn this work, pendant groups with both furan and maleimide moieties were incorporated into a polymethacrylate copolymer with lauryl methacrylate as comonomer to yield a one‐system Diels–Alder (DA) polymer. A combined Fourier transform infrared (FTIR) spectroscopy and rheological study was performed to quantify the extent of the reversible DA reaction and the resulting changes in mechanical properties of the polymer. The kinetics of the retro‐Diels–Alder (rDA) reaction was studied at different temperatures to determine an enthalpy of activation. Control polymers with only one functional moiety, that is, the furan or maleimide, were also synthesized to study the differences in viscoelastic behavior and the absence of self‐healing. Microscratch tests were performed to obtain information about the disappearance of well‐defined intentional surface scratches under different healing conditions. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1669–1675

Controlling Covalent Connection and Disconnection with Light

The on-going need for feature miniaturization and the growing complexity of structures for use in nanotechnology demand the precise and controlled formation of covalent bonds at the molecular level. Such control requires the use of external stimuli that offer outstanding spatial, temporal, as well as energetic resolution. Thus, photoaddressable switches are excellent candidates for creating a system that allows reversible photocontrol over covalent chemical connection and disconnection. Here we show that the formation of covalent bonds between two reagents and their scission in the resulting product can be controlled exclusively by illumination with differently colored light. A furyl-substituted photoswitchable diarylethene was shown to undergo a reversible Diels-Alder reaction with maleimide to afford the corresponding Diels-Alder adduct. Our system is potentially applicable in any field already relying on the benefits of reversible Diels-Alder reactions.

Biobased poly(2,5‐furandimethylene succinate‐co‐butylene succinate) crosslinked by reversible Diels–Alder reaction

ABSTRACTWe synthesized biobased poly(2,5‐furandimethylene succinate‐co‐butylene succinate) [P(FS‐co‐BS)] copolymers by polycondensation of 2,5‐bis(hydroxymethyl)furan, 1,4‐butanediol, and succinic acid. These copolymers could be crosslinked to form network polymers by means of a reversible Diels–Alder reaction with bis‐maleimide. The thermal properties, mechanical properties, and healing abilities of the P(FS‐co‐BS)s and the network polymers were investigated. The mechanical properties of the network polymers depended on the comonomer composition of the P(FS‐co‐BS)s and the maleimide/furan ratio in the network polymers. Some of the copolymers exhibited healing ability at room temperature, and their healing efficiency was enhanced by solvent or heat. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 216–222

Dextran‐Based Self‐Healing Hydrogels Formed by Reversible Diels–Alder Reaction under Physiological Conditions

A dextran-based self-healing hydrogel is prepared by reversible Diels-Alder reaction under physiological conditions. Cytocompatible fulvene-modified dextran as main polymer chains and dichloromaleic-acid-modified poly(ethylene glycol) as cross-linkers are used. Both macro- and microscopic observation as well as the rheological recovery test confirm the self-healing property of the dextran-l-poly(ethylene glycol) hydrogels ("l" means "linked-by"). In addition, scanning electrochemical microscopy is used to qualitatively and quantitatively in situ track the self-healing process of the hydrogel for the first time. It is found that the longitudinal depth of scratch on hydrogel surface almost completely healed at 37 °C after 7 h. This work represents a facile approach for fabrication of polysaccharide self-healing hydrogel, which can be potentially used in several biomedical fields.

An efficient reverse Diels–Alder approach for the synthesis of N-alkyl bismaleimides

Abstract Bismaleimides are useful precursors for Diels–Alder reactions, Michael additions, and thiol–maleimide based conjugation for the synthesis of materials and polymers. Use of bismaleimide cross linkers for generating polymers, bioconjugate molecules, and useful imaging molecules is an active area of research. An efficient and practical synthetic protocol for N-alkyl bis-maleimide cross linkers starting from furan protected maleimide employing a reverse Diels–Alder reaction is reported.

Approach to produce rigid closed-cell polyimide foams

A new chemical approach to prepare rigid closed-cell polyimide foams (PIFs) has been developed by thermal foaming of nadimide end-capped imide oligomers (NAIOs) obtained by thermally treating an in situ polymerization of monomeric reactant (PMR)-type poly(amide ester) resin solution. The PMR poly(amide ester) was synthesized by the reaction of diethyl ester of 2,3,3′,4′-biphenyltetracarboxylic dianhydride and aromatic diamines using monoethyl ester of cis-5-norbornene-endo-2,3-dicarboxylic acid as reactive end-capping agent in ethyl alcohol. The thermal foaming properties of the NAIOs were investigated. It was found that thermal foaming of the NAIOs was attributed to the blowing of cyclopentadiene, which was evolved from thermal decomposition of the nadimide end-caps in the imide oligomer by the reverse Diels–Alder reaction during cross-linking of the bisnadimide oligomers. The resultant rigid PIFs showed uniform cellular structures with high closed-cell content ( Cc) as well as high thermal stability and mechanical properties. The PIF with a density of 100 kg m−3 and a Cc of 89% exhibited compression strength at 10% compressive deformation of 1.34 MPa and outstanding thermal stabilities, making it a desirable candidate as high temperature structural foam materials for applications in aviation and spacecraft industries.

Bio-Based Furan Polymers with Self-Healing Ability

We report the preparation of a furan polymer, poly(2,5-furandimethylene succinate) by means of a condensation reaction between bio-based monomers. A reversible Diels–Alder reaction between furan and maleimide groups allowed the formation of network polymers cross-linked by a bismaleimide. By controlling the amount of the bismaleimide, mechanical properties were varied widely. These network polymers healed well when their broken surfaces were activated by bismaleimide solutions or solvent. The polymers also displayed excellent self-healing ability without external stimulus. This polymer class offers a wide range of possibilities to produce materials from biomass that have both practical mechanical properties and healing ability. These materials have the potential to bring great benefits to our daily lives by enhancing the safety, performance, and lifetime of products.

A new method to make polymers with flexible main chains and photoelectric pendants for organic semiconductors

Two novel polymers (PFMA-Ma and PFMA-Mb) were synthesized through an efficient and simple method combining conventional free radical polymerization and a Diels–Alder reaction. Different molar percentage compositions of the functionalized pendants in the polymers and two different steric configurations: endo and exo in the D–A adduct structures are revealed in 1H NMR spectra. In addition, DSC curves demonstrate that the reverse Diels–Alder reactions take place at higher temperatures (between 120 °C and 150 °C). Moreover, both of the fluorescence intensities of the two polymer films (PFMA-Ma (x = 0.700) and PFMA-Mb (x = 0.609)) become weak and even almost disappear with the increase of the annealing temperature, indicating that thermal annealing induces π–π stacking and aggregation of the pendants in the polymers. Concurrently, as the molar ratios of pendant moieties in the polymers are increased, a characteristic peak of face-to-face stacking appears in the fluorescence spectra of PFMA-Ma and a slight red-shift is observed in the fluorescence spectra of PFMA-Mb. These phenomena suggest that high molar percentage of pendants is also beneficial to their π–π stacking and aggregation. This study may provide some insights into the stacking of pendants in polymers and its influencing factors, which will be useful when these polymer materials are applied in optoelectronic devices.

A thermally healable polyhedral oligomeric silsesquioxane (POSS) nanocomposite based on Diels–Alder chemistry

A novel, tough, transparent, self-healing POSS nanocomposite material was prepared through a thermally reversible Diels-Alder (DA) reaction between furan-end functionalized POSS and bismaleimide.

Synthesis and characterization of linear self-healing polyurethane based on thermally reversible Diels–Alder reaction

New linear polyurethane was synthesized via a Diels–Alder (DA) reaction between a polyurethane prepolymer end-capped with furan groups (MPF) and bismaleimide (BMI). The structure of the linear polyurethane with DA bonds was characterized by 1H NMR, ATI-FTIR, TGA and AFM. The introduction of DA bonds enables the polyurethane thermal reversibility, which was studied thoroughly by 1H NMR, DSC and GPC. Besides, the new linear polyurethane with DA bonds also exhibits self-healing properties under heat treatment, which was observed by polarized optical microscope. The self-healing efficiency determined by the recovery of breaking tensile strength after being damaged and healed can reach up to 80%.

Thermally Induced Nanoimprinting of Biodegradable Polycarbonates Using Dynamic Covalent Cross-Links

The introduction of reversible covalent bonds into polymeric systems afford robust, yet dynamic, materials that can respond to external stimuli. A series of aliphatic polycarbonate polymers were synthesized via ring-opening polymerization of furanyl and maleimido-bearing cyclic carbonate monomers. These side chains undergo thermally induced Diels-Alder reactions to afford cross-linked films. Because both the diene and dienophile were incorporated into the same polymer backbone, a protected maleimido group, in the form of the furan adduct, was used. Both the forward and reverse Diels-Alder reaction are triggered thermally, which allows the deprotection of the maleimido group and the subsequent reaction with the furanyl side chains to form cross-links. Random copolymers and poly(ethylene glycol) containing block copolymers were formed using diazabicyclo[5.4.0]undec-7-ene as the catalyst and a thiourea cocatalyst. The polymers form uniform films that can be cross-linked in the bulk state. To further illustrate the dynamic nature of the covalent bonds within the cross-linked films, a patterned silicon mold was used to transfer a series of nanoscale patterns using a thermal nanoimprint process.

Atomic force microscopy–based study of self-healing coatings based on reversible polymer network systems

A self-healing polymer system is created by incorporating reversible covalent bonds into an epoxy–amine-based network structure. The self-healing concept is based on the reversible Diels–Alder reaction between furan and maleimide functional groups. The thermal and mechanical properties of the reversible network structure are tailored in order to achieve good self-healing properties for the corrosion protection of metal surfaces. Atomic force microscopy is proposed as a technique to study the self-healing behavior of coatings. Local thermal analysis techniques are used to study the local thermomechanical behavior of the reversible network. Nanosized defects in the coatings are made by means of nanolithography. The actual self-healing behavior is studied by atomic force microscopy imaging before and after the heating steps. The healing capability of elastomeric and glassy model systems is compared.

Synthesis of a Self-Healing Polymer Based on Reversible Diels–Alder Reaction: An Advanced Undergraduate Laboratory at the Interface of Organic Chemistry and Materials Science

This laboratory experiment exposes students to the chemistry of self-healing polymers based on a Diels–Alder reaction. Students accomplish a multistep synthesis of a monomer building block and then polymerize it to form a cross-linked polymer. The healing capability of the polymer is verified by differential scanning calorimetry (DSC) experiments. Furthermore, healing is demonstrated by damaging and heat-treating a thin polymer film.

Thermoreversible and remendable glass–polymer interface for fiber-reinforced composites

Adhesion of the reinforcement to the polymer matrix is essential for load transfer from the polymer matrix to the reinforcement material in fiber-reinforced composites. The reversible Diels–Alder reaction between a furan-functionalized epoxy-amine thermosetting matrix with a maleimide-functionalized glass fiber was used to impart remendability at the polymer–glass interface for potential application in glass fiber-reinforced composites. At room temperature the Diels–Alder adduct is formed spontaneously and above 90 °C the adduct breaks apart to reform the original furan and maleimide moieties. Healing of the interface was investigated with single fiber microdroplet pull-out testing. Following complete failure of this interface, significant healing was observed, with some specimens recovering over 100% of the initial properties. Healing efficiency was not affected by the distance of displacement, with an overall average of 41% healing efficiency. Up to five healing cycles were successfully achieved. It is expected that a glass fiber-reinforced composite of maleimide-sized glass within a furan-functionalized network will demonstrate extension of fatigue life.

Towards Development of a Self-Healing Composite using a Mendable Polymer and Resistive Heating

Mendomers are a group of polymers that are mendable upon heating. Specifically, cracks in these polymers have been shown to heal themselves when heated close to the glass transition temperature. The main mechanism behind the healing is the thermally reversible Diels—Alder reaction, where a dicyclopentadiene unit in the polymer backbone breaks apart into two cyclopentadiene terminal groups, which then reunite upon heating. The present study investigates the feasibility of using a mendomer as a matrix for re-mending composites reinforced with graphite fibers. The graphite fibers are used as electrical conductors to provide the necessary heat to the polymer. Specimens were prepared by spreading a monomer, called mendomer, powder on a graphite/epoxy laminate substrate and curing in a vacuum oven. Microcracks were introduced by bending the substrate coupon, and the latter was heated by applying electric currents. The healing behavior was confirmed by disappearance of microcracks that were observed with an optical microscope and a scanning electron microscope (SEM).

A Rational Protocol for the Synthesis of Arylated Bipyridine Ligands via a Cycloaddition Pathway

A generic design principle for the preparation of a variety of substituted phenyl-polypyridine ligands is described. These ligands are readily prepared by a regioselective [4+2] cycloaddition between electron-deficient dienes, such as 2,6-disubstituted-1,3,4-triazines, and ethynyl-arenes or ethynyl-alkanes. Exceptional reactivity is found with electron-rich dienophiles bearing ethynylgallate or ethynylphenyldibutylamino groups. Two regioisomers are formed, the meta being preferred due to favorable pi-pi interactions in the transition state, while the para isomers are formed in low yields in most cases. The use of tert-butylacetylene or N,N-dimethylamino-2-propyne, however, drives the reaction exclusively to the para isomer. Di-N,N-dibutylaminophenyl or isoquinoline ligands can also be produced in a single step by reverse Diels-Alder reactions. Cross-coupling reactions of iodo-substituted ligands or their platinum(II) complexes under Pd(0) catalysis gives branched ligands and complexes bearing paraffin chains, electron-donor or electron-acceptor groups. The use of a chloro-Pt(II) complex of an iodo-functionalized ligand allows both halogens to be replaced by ethynyl groups by using different catalysts. This methodology readily accommodates various functional groups and has been successfully extended to systems containing a variety of donor/acceptor frameworks. All ligands strongly absorb in the near-UV and luminesce in solution at rt with quantum yields ranging from 0 to 66%. Excited state lifetimes are in the nanosecond range and the solvent effects are in keeping with singlet excited states mixed with charge-transfer character. As deduced from spectroscopic and electrochemical studies, the di-n-butylamino derivatives are strong reductants in the excited state.