rDA Reaction Research Articles

Sequence‐Controlled Polymers with Furan‐Protected Maleimide as a Latent Monomer

AbstractHerein, 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.

Thermoresponsive crosslinked isocyanate‐free polyurethanes by Diels‐Alder polymerization

ABSTRACTThis new study is a continuation of our previous work on thermocleavable nonisocyanate polyurethanes (NIPUs), but it is focused on crosslinked networks. Two systems are studied: the first system involves a dicyclocarbonate adduct with a PPO‐bicyclocarbonate and a triamine as crosslinker. The second system involves a tetracyclocarbonate DA adduct as crosslinker with the same PPO‐bicyclocarbonate and a difunctional amine. Firstly, Diels‐Alder adducts are synthesized and characterized. Then they are copolymerized to yield two types of cleavable polymer networks. The thermal behavior of synthesized polymers is fully characterized. Finally, by SEC, it was demonstrated that the obtained NIPU polymer chains are sliced up by rDA reaction. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44408.

Quantitative Analysis of Step-Growth Polymers by Size Exclusion Chromatography.

We report an advanced analysis protocol that allows to quantitatively study the course of step-growth reactions by size exclusion chromatography on the example of the depolymerization of a Diels-Alder polymer based on a furane/maleimide couple at elevated temperatures. Frequently occurring issues of molar mass calibrations and overlap of monomer with solvent signals are addressed for determining reliable molar masses. Thereby, even kinetic parameters (e.g., rate coefficients) can be derived that otherwise would require performing additional spectroscopic experiments. Our results confirm first-order behavior of the rDA reaction with an activation energy of 33 kJ mol-1.

A new synthetic strategy to 2,3-diallyl-1,4-quinones via one-pot double Claisen rearrangement and retro Diels–Alder reaction

A simple and an efficient synthetic strategy to 2,3-diallyl-1,4-quinone derivatives via a highly reliable and 100% atom economic reactions such as Diels–Alder (DA) reaction, Claisen rearrangement (CR), and retro Diels–Alder (rDA) reaction as key steps is reported.

Synthesis, curing process and thermal reversible mechanism of UV curable polyurethane based on Diels-Alder structure

Abstract A novel polyurethane resin containing acrylate function and Diels-Alder (DA) structure was synthesized, with potential thermal reversibility and UV curable properties. The UV curing process was monitored by Fourier transform infrared spectroscopy (FTIR), which indicated that the DA structure was stable during curing process and the final acrylate C C conversion is 82.5%. The cured film exhibited good repeatedly thermal reversibility. The retro-Diels-Alder (rDA) reaction of the cured film occurred at temperatures higher than 85.5 °C and was obviously accelerated by increasing temperature, which was confirmed by the results of FTIR, differential scanning calorimeter (DSC) and dynamic thermomechanical analysis (DMA). The cured film displayed acceptable thermal stability up to 278.3 °C according to the thermal gravimetric analysis (TGA) results. Finally, a cured film with excellent physical properties was obtained.

Reactivity descriptor for the retro Diels–Alder reaction of partially saturated 2-pyrones: DFT study on substituents and solvent effects

​The retro-Diels–Alder (rDA) reaction of partially saturated 2-pyrones were studied using density functional theory (DFT) calculations in polar and non-polar solvents, and fundamental descriptors were proposed to understand the electronic and solvent effect.

On the mechanism of retro-Diels–Alder reaction of partially saturated 2-pyrones to produce biorenewable chemicals

Partially saturated 2-pyrone molecules undergo ring-opening and decarboxylation via retro-Diels–Alder (rDA) reaction.

Analysis of Chaetoconvosins a and B Using Electrospray Ionization Quadrupole Time-of-Flight Mass Spectrometry

Two novel cytochalasins, chaetoconvosins A and B, isolated from Chaetomium convolutum were investigated using electrospray quadrupole time-of-flight tandem mass spectrometry (ESI-QTOF-MS/MS) in positive-ion mode. The main product ions in the high mass range for chaetoconvosins A and B are formed with the loss of H 2 O, CO, or both. The neutral loss of CH 3 NCO can be detected for chaetoconvosin B. However, their fragmentation patterns producing product ions in the low mass range vary greatly. It was found that hydroxyl or H atom linked to C1 is the key leading to the fragmentation differences. McLafferty-type rearrangement (producing the characteristic product ion at m/z 155), four-centered H rearrangement (generating m/z 193), and RDA reaction (producing m/z 203) are directly or indirectly related to the groups linked to C1. The identity of major product ions was supported by a D-labeling experiment.

New insights into the cross-linking and degradation mechanism of Diels-Alder hydrogels.

Eight-armed poly(ethylene glycol) was functionalized with furyl and maleimide groups. The two macromonomers were cross-linked by Diels-Alder (DA) reactions and the degradation behavior of the formed hydrogels was investigated. UV spectroscopy showed that maleimide groups were subject to ring-opening hydrolysis above pH 5.5, with the reaction rate depending on the pH and temperature. As a result of this, the gelation kinetics and stiffness of DA hydrogels were dependent on the temperature and the pH of the cross-linking medium, as demonstrated by rheological experiments. The gel time varied between 87.8 min (pH 3.0, 37 °C) and 374.7 min (pH 7.4, 20 °C). Values between 420 Pa (pH 9.0, 37 °C) and 3327 Pa (pH 3.0, 37 °C) were measured for the absolute value of the complex shear modulus. Hydrogel swelling and degradation were influenced by the same parameters. With increasing pH and temperature the degradation time was reduced from 98 days (pH 7.4, 20 °C) to 2 days (pH 7.4, 50 °C); no degradation was observed at pH 3.0 and 5.5. Molecular modeling studies of the DA and retro-Diels-Alder (rDA) moieties revealed that hydrogel degradation occurred by rDA reaction followed by OH--catalyzed ring-opening hydrolysis of maleimide groups to unreactive maleamic acid derivatives.

Correlation between carbon–carbon bond length and the ease of retro Diels–Alder reaction

The bond length between C8-C9 in (1′ R,4′ S,4a′ R,8a′ S)-6′,7′-dimethyl-1′,4′,4a′,8a′-tetrahydrospiro [cyclopropane-1,9′-[1,4]methanonaphthalene]-5′,8′-dione is 1.571 (2) Å and between C7-C12 is 1.567 (2) Å which are longer than the corresponding bond length for saturated bicyclic systems (1.531-1.535 Å). This paper reports the correlation between bond length and the ease of retro Diels −Alder reaction. We examined the crystal structure of (1′R,4′S,4a′R,8a′S)-6′,7′-dimethyl-1′,4′,4a′,8a′-tetrahydrospiro[cyclopropane-1,9′-[1,4] methano- naphthalene]-5′,8′-dione and found that the bond length between atoms connecting diene and the dienophile is somewhat longer than the normal C–C bond length. The increased bond length is responsible for the facile rDA reaction of 5.

Self‐Healing Polyurethanes with Shape Recovery

Two new thermoresponsive self‐healing polyurethanes (1DA1T and 1.5DA1T) based on the Diels–Alder (DA) reaction between furan and maleimide moieties are developed that use the shape‐memory effect to bring crack faces into intimate contact such that healing can take place. Unlike other self‐healing polymers, these polymers do not require external forces to close cracks but rather they use the shape‐memory effect to autonomously close the crack. Both polyurethanes have a stable polymer structure and comparable mechanical properties to commercial epoxies. A differential scanning calorimeter is employed to check the glass transition temperature of the polymers as well as the DA and retro‐DA (rDA) reaction temperatures. These DA and rDA reactions are confirmed with variable‐temperature proton nuclear magnetic resonance. Healing efficiency is calculated using a measurement of the failure load from compact tension testing. The results show that the shape‐memory effect can replace external forces to close two crack surfaces and the DA reaction can be repeatedly employed to heal the cracks.

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

ChemInform Abstract: Recent Developments in the retro‐Diels—Alder Reaction

AbstractReview: 118 refs.

One-step Synthesis of PCL-Urethane Networks using a Crosslinking/de-crosslinking Agent

Thermally reversible cross-linked polycaprolactone-urethane (PCL-U) was prepared in one-step procedure. The PCL-U networks were synthesized from a di-isocyanate (4,4′-methylene bis(cyclohexylisocyanate) (HMDI) and hydroxyl bearing macromers and monomers: hydroxy-terminated PCL, glycerol and a di-alcohol Diels-Alder (DA) adduct. This adduct was used to introduce both diene and dienophile functions in the structure and also to protect the maleimide functions from polymerization. The thermoresponsive behavior of the material was characterized by commonly used methods such as solubility tests at different temperatures and differential scanning calorimetry analyses to highlight rDA reactions and also by rheological analysis. The effects of the cooling rate, the molar mass of polycaprolactone as well as the molar ratio [di-Isocyanate]/[PCL-diol] on the cross-linking/de-cross-linking temperatures were also analyzed. The reversible networks obtained have a self-healing behavior.

Organogelation behavior, thermal and mechanical properties of polymer network formed by the Diels–Alder reaction of furan- and maleimide-terminated four-arm star-shaped ε-caprolactone oligomers

Furan- and maleimide-terminated 4-arm star-shaped ε-caprolactone oligomers (F4CLO and M4CLO) were synthesized by the reactions of hydroxy-terminated 4-arm star-shaped ε-caprolactone oligomers (H4CLO) having the degree of polymerization per one oligocaprolactone chain, n = 5 or 10 with furfuryl isocyanate and 4-(maleimidophenyl)isocyanate, respectively. Solutions of F4CLO/M4CLO or F4CLO/4,4′-bismaleimidediphenylmethane (BMI) in some organic solvents changed to gels by the Diels–Alder (DA) reaction at 50 °C or room temperature, which came back to solutions by the retro-DA (rDA) reaction at 80 °C. The thermo-reversible sol–gel process due to the DA and rDA reactions was monitored by 1H NMR and GPC analyses. DSC and XRD analyses of polymer network formed by the DA reaction revealed that the network composed of the oligocaprolactone chain of n = 10 is semicrystalline, while that of n = 5 is amorphous. The former polymer network showed higher tensile strength and modulus than the latter.

ChemInform Abstract: Tandem inverse‐Electron‐Demand hetero‐/retro‐Diels—Alder Reactions for Aromatic Nitrogen Heterocycle Synthesis

AbstractReview: 85 refs.

Analysis of matrine-type alkaloids using ESI-QTOF

A series of six matrine-type alkaloids, including a pair of stereoisomers, was investigated using electrospray ionization quadrupole time-of-flight tandem mass spectrometry (ESI-QTOF-MS/MS) in positive-ion mode. Protonated molecules were fragmented by collision-induced dissociation (CID) and the products identified. The identity of the major product ions was also supported by deuterium-labeling experiments. It was found that sequential four-centered H rearrangements play a significant role in the fragmentation of the piperidine-ring; the RDA reaction dominates the dissociations for sophocarpine and sophocarpidine; and the McLafferty-type rearrangement occurs with neosophoramine. Interestingly, a pair of stereoisomers matrine and sophoridine, can be unambiguously distinguished using CID, and this characterization was supported by the DFT calculations of the potential energy profiles for protonated matrine and sophoridine before dissociation to the product ion at m/z 152.

Recent developments in the retro-Diels–Alder reaction

The retro-Diels–Alder reaction (rDA) has become an important tool for synthetic chemists in their quest towards the synthesis and design of novel scaffolds. The ability to carry out the rDA reaction under ambient conditions has made it quite attractive. In recent years the scope of the rDA in several areas has increased manifold. The present review covers the developments in this ever-expanding field over the last decade. The review will summarize various aspects of the rDA reaction and its applications to natural and non-natural products.

Tandem inverse-electron-demand hetero-/retro-Diels–Alder reactions for aromatic nitrogen heterocycle synthesis

The merged inverse-electron-demand hetero-Diels-Alder (ihDA)/retro-Diels-Alder (rDA) reaction sequence can be used to rapidly synthesise highly functionalised nitrogen heteroaromatics. The reaction offers many advantages: high atom economy, high levels of regioselectivity, it rarely requires a catalyst and, in some cases, can be performed in the absence of solvent. In this tutorial review we discuss the range of commonly used dienophiles and aza-dienes for this process whilst highlighting the reactivity trends, and illustrating their applications.

Thermally reversible Diels–Alder-based polymerization: an experimental and theoretical assessment

A pair of monomers capable of undergoing reversible polymerization—based on reversible Diels–Alder (DA) chemistry—as a function of the applied reaction temperature is presented. Specifically, the reaction of isophorone bis(sorbic carbamate), a difunctional diene, with 1,4-phenylenebis(methylene)bis((diethoxyphosphoryl)methanedithioformate), a difunctional dithioester, was studied in detail. Various factors, including the monomer concentration, the type of solvent, and the presence of a Lewis acid, that influence this step-growth polymerization were evaluated. The solvent type was found to have a significant effect on the DA reaction rate. Under the optimized conditions, which are 1.8 g mol−1 of each monomer in acetonitrile with 1.1 equivalents of zinc chloride at 50 °C for 4 h, a polymer with a peak molecular weight of 9600 g mol−1 (relative to poly(styrene) standards) was obtained. The resulting polymer was employed to investigate the correlation between time, temperature, and percentage of debonded monomers achieved during the retro DA (rDA) reaction. In addition, theoretical predictions of the rDA temperature were obtained via ab initio quantum chemical calculations. The monomeric diene and dienophile system was employed for the calculations of the equilibrium constants at various rDA reaction temperatures to correlate the percentage of bonded molecules with the applied temperature. It was calculated that 60% of the polymer becomes debonded at a temperature (Tqc) of around 220 °C, a result that agrees well with that obtained experimentally (Texp = 219 °C).