A Diels‐Alder/retro Diels‐Alder strategy to synthesize polymers bearing maleimide side chains

Polymers containing thiol‐reactive maleimide groups on their side chains have been synthesized by utilization of a novel methacrylate monomer containing a masked maleimide. Diels‐Alder reaction between furan and maleimide was adapted for the protection of the reactive maleimide double bond prior to polymerization. AIBN initiated free radical polymerization was utilized for synthesis of copolymers containing masked maleimide groups. No unmasking of the maleimide group was evident under the polymerization conditions. The maleimide groups in the side chain of the polymers were unmasked into their reactive form by utilization of retro Diels‐Alder reaction. This cycloreversion was monitored by thermo gravimetric analysis (TGA), differential scanning calorimetry (DSC), and ¹H and ¹³C NMR spectroscopy. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4545–4551, 2007     

Design of cross-linked semicrystalline poly(ε-caprolactone)-based networks with one-way and two-way shape-memory properties through Diels-Alder reactions

Cross‐linked poly(ε‐caprolactone) (PCL)‐based polyesterurethane (PUR) systems have been synthesized through Diels–Alder reactions by reactive extrusion. The Diels–Alder and retro‐Diels–Alder reactions proved to be useful for enhancing the molecular motion of PCL‐based systems, and therefore their crystallization ability, in the design of cross‐linked semicrystalline polymers with one‐way and two‐way shape‐memory properties. Successive reactions between α,ω‐diol PCL (PCL₂), furfuryl alcohol, and methylene diphenyl 4,4′‐diisocyanate straightforwardly afforded the α,ω‐furfuryl PCL‐based PUR systems, and subsequent Diels–Alder reactions with N,N‐phenylenedimaleimide afforded the thermoreversible cycloadducts. The cross‐linking density could be modulated by partially replacing PCL‐diol with PCL‐tetraol. Interestingly, the resulting PUR systems proved to be semicrystalline cross‐linked polymers, the melting temperature of which (close to 45 °C) represented the switching temperature for their shape‐memory properties. Qualitative and quantitative measurements demonstrated that these PUR systems exhibited one‐way and two‐way shape‐memory properties depending on their cross‐linking density.     

Manganese(I)‐Catalyzed C−H Activation/Diels–Alder/retro‐Diels–Alder Domino Alkyne Annulation featuring Transformable Pyridines

Complexity‐increasing Domino reactions comprising C?H allenylation, a Diels–Alder reaction, and a retro‐Diels–Alder reaction were realized by a versatile catalyst derived from earth‐abundant, non‐toxic manganese. The C?H activation/Diels–Alder/retro‐Diels–Alder alkyne annulation sequence provided step‐economical access to valuable indolone alkaloid derivatives through a facile organometallic C?H activation manifold with transformable pyridines.     

Crosslinked epoxy materials exhibiting thermal remendablility and removability from multifunctional maleimide and furan compounds

Crosslinked polymeric materials, which exhibit thermal remendability and removability through Diels–Alder (DA) and retro‐DA reactions, were obtained from using multifunctional maleimide and furan compounds as monomers. The synthesized monomers possess low melting points and good solubility in organo solvents to show excellent processing properties. The performance of DA and retro‐DA reactions were demonstrated with DSC and FTIR measurements. High performance of thermal remendablility and removability of the crosslinked materials were observed with SEM and solvent tests. These materials were applicable in advanced encapsulants and structural materials. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 905–913, 2006     

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

In 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     

A tailor‐made polymethacrylate bearing a reactive diene in reversible diels–alder reaction

A tailor‐made polymethacrylate bearing a pendant furfuryl group was prepared by atom transfer radical polymerization (ATRP), an important method of recent advances in controlled radical polymerization. It was otherwise difficult to prepare via conventional radical polymerization, because of several side reactions involving the reactive diene functionality of the furfuryl group. Successful Diels–Alder (DA) chemistry was carried out using this reactive furfuryl group of the tailor‐made polymer as diene and a bismaleimide as a dienophile. Interestingly, the resultant material was observed to be thermoreversible as evidenced by FT‐IR and DSC studies. This example of application of a tailor‐made polymer having controlled molecular architecture and with reactive diene functionality in DA chemistry will open new possibilities to prepare newer tailor‐made reversible materials. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4441–4449, 2007     

Synthesis of chalcone‐based fluorescent polymers: Diels‐Alder reaction of chalcones and their polymerization through ROMP

Ring opening metathesis polymerization (ROMP) was carried out on Diels‐Alder adducts formed from reactions between chalcones and cyclopentadiene. Most of the chalcones gave predominantly endo‐adducts and the exo‐adducts were obtained in good yields from reacting cyclopentadiene with furfurylidine acetone and N,N,diethylaminobenzylidine‐(4‐hydroxy)acetophenone. These exo‐adducts were subjected to ROMP using Grubbs catalyst, bis(tricyclohexylphosphine)benzylidinedichloride. The monomers and polymers were characterized using spectroscopic techniques like FT‐IR, ¹HNMR. The polymers were characterized using TGA, DSC, and GPC. The polymers were found to possess fluorescent properties and poly[2‐(4‐diethylamino)phenyl‐3,5‐divinylcyclopentyl](4‐hydroxyphenyl) methanone was found to have good emissive property at two wavelengths. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1521–1531, 2008     

Why can the activation volume of the cycloadduct decomposition in isopolar retro‐Diels–Alder reactions be negative?

Rate constants of the Diels–Alder cycloaddition reaction of anthracene with tetracyanoethylene, enthalpy of solution of reactants and adduct, enthalpy of the reaction in solution, enthalpy and entropy of activation of the forward and retro‐Diels–Alder reactions were determined in 14 solvents. Temperature and pressure effects on the rate of the decomposition of the adduct formed from 9‐chloroanthracene and tetracyanoethylene were studied. Since the electrostriction effect can be excluded from the consideration of the isopolar Diels–Alder reaction, negative values of the activation volume in the retro‐Diels–Alder reactions can be caused by the different possibilities of penetration of the solvent molecules to large steric branched structures of the transition states and adducts. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 42: 117–125, 2010     

Effect of the conversion degree and multiple healing on the healing efficiency of a thermally reversible self‐healing polymer

In this paper, the effect of the cross‐links' conversion degree on the healing efficiency of a thermally remendable polymer based on the Diels‐Alder (DA) reaction was studied quantitatively. By using the differential scanning calorimetry (DSC) results along with the Kissinger method, the conversion degree of the thermally reversible cross‐links was predicted as a function of time and temperature. For investigating the healing efficiencies at different conversion degrees, three‐point bending specimens were fabricated under certain curing conditions, which guaranteed the formation of both reversible and irreversible bonds. Afterward, specimens failed under three‐point bending test and healed up to certain conversion degrees. The results revealed on average 15%, 38%, and 83% recovery of flexural strength and 89%, 91%, and 93% recovery of flexural modulus at conversion degrees of 0.6, 0.8, and 1, respectively. Moreover, by repeating the damaging and healing procedure, it was shown that the synthesized polymer has the capability to be healed several times.     

Thermally amendable tailor‐made functional polymer by RAFT polymerization and “click reaction”

This investigation reports the preparation and characterization of thermally amendable functional polymer bearing furfuryl functionality via reversible‐addition fragmentation and chain transfer (RAFT) polymerization and Diels‐Alder (DA) reaction. In this case, furfuryl methacrylate (FMA) was polymerized using 4‐cyano‐4‐[(dodecylsulfanylthiocarbonyl)sulfanyl] pentanoic acid as RAFT reagent and 4,4′‐azobis(4‐cyanovaleric acid) as thermal initiator. ¹H NMR, ¹³C NMR, and matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry analysis showed that furfuryl group in poly(furfuryl methacrylate) (PFMA) was not affected during RAFT polymerization and the tailor‐made polymer had RAFT end group. The DA reaction was successfully carried out between the reactive furfuryl functionality of PFMA and different bismaleimides. The thermoreversible property of these DA polymers was characterized by FT‐IR and DSC analysis. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3365–3374     

Stable Pyrrole‐Linked Bioconjugates through Tetrazine‐Triggered Azanorbornadiene Fragmentation

An azanorbornadiene bromovinyl sulfone reagent for cysteine‐selective bioconjugation has been developed. Subsequent reaction with dipyridyl tetrazine leads to bond cleavage and formation of a pyrrole‐linked conjugate. The latter involves ligation of the tetrazine to the azanorbornadiene‐tagged protein through inverse electron demand Diels–Alder cycloaddition with subsequent double retro‐Diels–Alder reactions to form a stable pyrrole linkage. The sequence of site‐selective bioconjugation followed by bioorthogonal bond cleavage was efficiently employed for the labelling of three different proteins. This method benefits from easy preparation of these reagents, selectivity for cysteine, and stability after reaction with a commercial tetrazine, which has potential for the routine preparation of protein conjugates for chemical biology studies.     

Regioselective Synthesis of Polycyclic and Heptagon‐embedded Aromatic Compounds through a Versatile π‐Extension of Aryl Halides

A versatile π‐extension reaction was developed based on the three‐component cross‐coupling of aryl halides, 2‐haloarylcarboxylic acids, and norbornadiene. The transformation is driven by the direction and subsequent decarboxylation of the carboxyl group, while norbornadiene serves as an ortho ‐C−H activator and ethylene synthon via a retro‐Diels–Alder reaction. Comprehensive DFT calculations were performed to account for the catalytic intermediates.     

Synthesis and luminescence of polyphosphazenes with carbazolyl side chains

Two new blue‐light‐emitting polyphosphazenes ( 1 and 2 ) containing carbazolyl groups as side chains were synthesized from a highly reactive macromolecular intermediate by a nucleophilic substitution reaction. Molecular structural characterization for the polymers was presented by ¹H NMR, IR, and ultraviolet–visible spectra, gel permeation chromatography, and differential scanning calorimetry. The polymers exhibited excellent solubility in common organic solvents and were thermally stable. A fluorescence analysis of the two materials in tetrahydrofuran showed a strong blue light emitting. The quantum yields of the polyphosphazenes were 0.55 for 1 and 0.64 for 2 , relative to quinoline (in 0.1 N H₂SO₄). An electroluminescent diode was fabricated, and a bright blue light was observed; the maximum external quantum efficiency was about 0.026% at an applied forward voltage of 23 V. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3428–3433, 2001     

Improvement of thermal stability of sulfonated polyphosphazenes by introducing a self‐crosslinkable group

We prepared sulfonated polyphosphazenes having various aryloxy substituents, and studied their thermal stabilities and membrane properties. Sulfonated polyphosphazenes were synthesized by the reaction of polydichlorophosphazene with sodium aryloxides and subsequent sulfonation with fuming sulfuric acid. With increasing the degree of sulfonation, the polymers showed higher proton conductivity, but suffered more from swelling in an aqueous solution. We introduced a hydroxymethylphenoxy group onto the phosphazene backbone as a self‐crosslinkable group by reaction of poly(dichlorophosphazene) and a sodium salt of 4‐hydroxymethylphenol. When a film of a sulfonated polymer having a methylol group was heated at 80 °C under vacuum for 1 h, it became insoluble in NMP, indicating the formation of a network structure. We investigated the crosslinking reaction of the polymers by DSC and FTIR. The crosslinking reaction proceeded only in the sulfonated polymers. Because the sulfonated polymers provide acidic protons, the methylol groups became more electrophilic and reacted with neighboring aromatic rings. A condensation reaction between themselves could also occur. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5850–5858, 2008     

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.     

Synthesis of C60‐containing polyphosphazenes from a new reactive macromolecular intermediate: Polyphophazene azides

A novel synthetic strategy was developed to prepare polyphosphazenes containing C₆₀ moieties as side chains. Thus, a new reactive macromolecular intermediate, polyphosphazene azides ( P1 ), was obtained from poly(dichlorophosphazene) by the direct nucleophilic substitution reaction. Then the azide group in P1 reacted with C₆₀ molecules to afford the first example of C₆₀‐containing polyphosphazenes ( P2 and P3 ). The polymers are soluble in common organic solvents. Molecular structural characterization for the polymers was presented by ¹H NMR, ¹³C NMR, IR, ultraviolet–visible spectra, and gel permeation chromatography. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 194–199, 2004     

Preparation of a Porous polymer by a catalyst‐free diels‐alder reaction and its structural modification by post‐reaction

A microporous polymer is prepared by a catalyst‐free Diels–Alder reaction. A cyclopentadiene with both a diene and a dienophile functionality and a dienophilic maleimide are used for the Diels–Alder reaction. 1,3,5‐Tris(bromomethyl)‐2,4,6‐trimethylbenzene is reacted with sodium cyclopentadienide to produce the multicyclopentadiene‐functionalized monomer. A crosslinked polymer ( CDAP ) is obtained by the reaction of the cyclopentadiene monomer with N,N′‐1,4‐phenylenedimaleimide. The thermal dissociation of the cyclopentadiene dimeric unit and the subsequent Diels–Alder reaction with the maleimide group are investigated by the model reaction. We are able to restructure the crosslinked polymer network by taking advantage of the thermal reversibility of the Diels–Alder linkage. After the post thermal treatment, the BET surface area of the polymer ( CDAP‐T ) is greatly increased from 317 to 1038 m² g^?1. CDAP‐T is functionalized with pyrene by bromination with N‐bromosuccinimide and the subsequent substitution reaction with aminopyrene. The adsorption property of the pyrene‐functionalized polymer for an aromatic dye is investigated using malachite green. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3646–3653     

Synthesis of thermally degradable epoxy adhesives

We have developed a new strategy for the synthesis of epoxide‐containing polymers where the pendant reactive groups are connected to the main backbone via thermally labile oxonorbornene groups. The polymers were synthesized by radical 1,4‐polymerization of the appropriate bicyclic diene monomer. The produced polymers can be crosslinked in the presence of a diamine and de‐crosslinked by thermal treatment at 160 °C, which induces retro‐Diels–Alder reaction and cleaves pendant groups from the polymer backbone, as confirmed by differential scanning calorimetry. The potential for the utilization of this polymer as a thermally removable adhesive was demonstrated by a simple adhesion test. This method provides access to thermally cleavable epoxy networks that can be quickly and irreversibly disintegrated into nonvolatile components upon heating to a specified temperature. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4992–4997     

Efficient Method for the Synthesis of Functionalized Basic Maleimides

A three‐step procedure involving Diels–Alder condensation of maleic anhydride with furane, formation of N‐substituted imide upon reaction with appropriate diamine, and a final retro Diels–Alder regeneration of the maleic carbon–carbon double bond is proposed for an unequivocal synthesis of N‐substituted basic maleimides. The novel method is characterized by mild reaction conditions, easy work‐up, high yields, and no need for additional catalysis.     

Hydrogel synthesis by aqueous Diels‐Alder reaction between furan modified methacrylate and polyetheramine‐based bismaleimides

A simple method for preparing cross‐linked hydrogels in an aqueous medium is investigated using Diels‐Alder (DA) “click” reaction, without employing a catalyst. A polymeric diene is first synthesized by the functionalization of poly(2‐aminoethyl methacrylate) hydrochloride with furfural. Suited bisdienophiles are prepared by modification of Jeffamine^® ED of different molecular weights with maleic anhydride. Both precursors of the DA coupling are thoroughly characterized before their reactions. The ensuing hydrogels are analyzed in terms of their microstructure, swelling, and rheological behavior, as a function of the reaction conditions. The influence of the molecular weight of the cross‐linker and the furan‐to‐maleimide ratio on the final properties of the hydrogels were also investigated. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 699–708