No Evidence Found for Diels–Alder Reaction Products in Soybean Oil Oxidized at the Frying Temperature by NMR Study

It has been generally accepted that the Diels–Alder reaction mechanism is one of the major reaction mechanisms to produce dimers and polymers during heating process of vegetable oil. Soybean oil oxidized at 180 °C for 24 h with 1.45 surface area-to-volume ratio showed 36 % polymer peak area in gel permeation chromatogram. However, the NMR DEPT (Distortionless Enhancement by Polarization Transfer) 135 spectrum did not show any signals of possible Diels–Alder products. A fraction separated from the oxidized soybean oil by column chromatography contained 98 % polymers, but again, showed no signals of proposed Diels–Alder products in the DEPT 135 spectrum. Methyl oleate and triolein without a diene required for the Diels–Alder reaction produced 27 and 63 % of total polymers, respectively, under the same condition. This indicates that the polymers must be produced by reactions other than the Diels–Alder reaction for these oils. This study shows that the Diels–Alder reaction is not the major reaction to produce polymers during oxidation of soybean oil, within the DEPT 135 spectroscopy sensitivity level, about 5 mol %.     

Diels–Alder‐based crosslinked self‐healing polyurethane/urea from polymeric methylene diphenyl diisocyanate

Crosslinked self‐healing polyurethane/urea based on a Diels–Alder reaction (C‐PMPU–DA) was synthesized from a multiple‐furan monomer and a commercial bismaleimide. The multiple‐furan monomer (PMPU–furan) was obtained from a functionalized prepolymer (polymeric MDI: PBA‐2000 = 2:1) by furfuryl amine. The structures of both the PMPU–furan and C‐PMPU–DA were characterized by attenuated total reflectance (ATR)–Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and ¹H‐NMR. The Diels–Alder bonds enabled C‐PMPU–DA thermal reversibility, which was investigated by ATR–FTIR spectroscopy, ¹H‐NMR, gel–solution–gel experiments, and viscosity tests. Meanwhile, the self‐healing properties of C‐PMPU–DA were also investigated by the recovery of the mechanical properties. The results showed that C‐PMPU–DA exhibited good thermal reversibility and self‐healing properties. C‐PMPU–DA exhibited thermosetting properties at room temperature, although it exhibited thermoplastic properties at higher temperatures and may find applications in self‐healing materials, recyclable materials, or removable materials. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40234.     

Thermoreversible crosslinked thermoplastic starch

A novel furan‐modified thermoreversible crosslinked thermoplastic starch was synthesized. The crosslinking mechanism was based on a thermoreversible furan/maleimide adduct formed by the Diels ? Alder reaction. Thermoplastic starch (TPS) was first modified by forming a urethane linkage between the product of the reaction of furfuryl alcohol with 4,4‐methylene diphenyldiisocyanate and a starch hydroxyl group. Crosslinking was then achieved by the addition of 1,1′‐(methylene‐di‐4,1‐phenylene) bismaleimide to the furan‐grafted starch in solution (dimethylsulfoxide) and in the molten state during conventional extrusion. The materials were characterized by Fourier transform infrared and ¹H NMR spectroscopy. The thermoreversible reaction was assessed by viscosity measurement and ¹H NMR, for the system in dimethylsulfoxide, and by solubility tests, hot‐pressing experiments and dynamic mechanical analysis measurements for the extruded product. The crosslinked product showed a higher storage modulus and water resistance than its non‐crosslinked counterpart, opening novel opportunities to widen the range of TPS applications. © 2015 Society of Chemical Industry     

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

Novel 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.     

Recyclable biobased materials based on Diels–Alder cycloaddition

Poly(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.     

Polyether–maleimide‐based crosslinked self‐healing polyurethane with Diels–Alder bonds

The Diels–Alder (DA) reaction is particularly desirable for the preparation of heat‐stimuli self‐healing polymeric materials because of its thermal reversibility, high yield, and minimal side reactions. Some attempts were conducted to synthesize polyether–maleimide‐based crosslinked self‐healing polyurethane with DA bonds (C‐PEMIPU–DA) through the reactions of the prepolymer (polymeric MDI/PBA‐1000) functionalized by furfuryl amine and polyether–maleimide without benzene in this study. The structures of intermediates and C‐PEMIPU–DA were first confirmed by ¹H‐NMR, Fourier transform infrared spectroscopy, and differential scanning calorimetry. Next, the thermal reversibility and the self‐healing performance of C‐PEMIPU–DA were studied by ¹H‐NMR, polarizing optical microscopy, tensile testing, and a sol–gel process. The results show that C‐PEMIPU–DA exhibited interesting properties of thermal reversibility and self‐healing. The polymers could be applied to self‐healing materials or recyclable materials in the fields of the repair of composite structures and aging parts because of their thermosetting properties at room temperature and thermoplasticity at higher temperatures. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41944.     

Preparation and anticorrosive properties of hybrid coatings based on epoxy‐silica hybrid materials

A series of sol‐gel derived organic–inorganic hybrid coatings consisting of organic epoxy resin and inorganic silica were successfully synthesized through sol‐gel approach by using 3‐glycidoxypropyl‐trimethoxysilane as coupling agent. Transparent organic–inorganic hybrid sol‐gel coatings with different contents of silica were always achieved. The hybrid sol‐gel coatings with low silica loading on cold‐rolled steel coupons were found much superior improvement in anticorrosion efficiently. The as‐synthesized hybrid sol‐gel materials were characterized by Fourier‐transformation infrared spectroscopy, ²⁹Si‐nuclear magnetic resonance spectroscopy and transmission electron microscopy. Effects of the material composition of epoxy resins along with hybrid materials on the thermal stability, Viscoelasticity properties and surface morphology were also studied, respectively. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Multifunctional,robust, and self-healable polyurethane coatings cross-linked by Diels–Alder reaction of calix[4] arenes containing furan groups

Self-healing coatings can restore their performance after inevitable damage and are promising in the industry owing to their longer service life and lower repair cost. The use of Diels–Alder reaction bonds is well-known to generate thermally self-healing coatings. In this study, a series of cross-linked self-healing polyurethane (PU) coatings were synthesized through Diels–Alder reactions of p-tert-butyl calix[4]arene bearing furan groups (C4A-FA) as a new chain extender and a bismaleimide (BMI) polyether amine as a Diels–Alder cross-linking agent. Adding C4A-FA to PUs improves their mechanical properties, thermal stability, and self-healing ability. Additionally, these modifications can result in the formation of composite networks with PU that exhibit thermoreversibility and self-healing properties. These changes have led to PUs containing modified calix[4]arene (C4A-FA) having better properties compared with unmodified calix[4]arene PUs. The properties of prepared coatings were evaluated by Fourier transform infrared spectroscopy, scanning electron microscope, differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), and tensile tests compared with a typical PU sample. The tensile and TGA results show an improvement in the thermal and mechanical properties of the polymers by increasing the C4A-FA content. By replacing 15% of butandiol (BDO) with C4A-FA in PU pure, the tensile strength increased from 1.69 to 5.14 MPa. Furthermore, adding diels–alder (DA) bonds enhanced the tensile strength to 10.49 MPa for PU-C4A15-DA. According to DSC results, a broad endothermic peak from nearly 80–140°C confirmed the retro-DA reactions in the synthesized thermoreversible samples. The healing efficiency of the PU-C4A15-DA sample was obtained at 92.5% (measured by tensile test), which is the highest value among cross-linked self-healing PUs reported in the literature.     

Polyurethane adhesives containing Diels–Alder‐based thermoreversible bonds

A facile two‐step process is developed to obtain polyester urethanes that contain various concentrations of thermoreversible Diels–Alder adduct–based bonds for the development of adhesives. Besides linear systems thermoreversible networks have been included in the study. The reactions are verified using IR‐spectroscopy and gel permeation chromatography analysis. The material properties are characterized with solvent exposure tests, dynamic scanning calorimetry, microindentation and rheology and the potential for application as adhesives is tested with an elcometer. Material properties have been found highly tunable for the system, and high adhesive strengths (off‐scale) are found for polyester urethanes that are doped with an intermediate level Diels–Alder functional groups. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44972.     

Synthesis of Self-Healing Bio-Based Tannic Acid-Based Methacrylates By Thermoreversible Diels–Alder Reaction

This investigation reports the effective use of the Diels–Alder (DA) click reaction in the preparation of self-healing bio-based dendritic methacrylates having reactive furfuryl functionality. Bio-based methacrylates were synthesized by modifying tannic acid with glycidyl methacrylate and furfuryl functionality was introduced by atom transfer radical polymerization with varied amount of furfuryl methacrylate monomer. The thermoreversible network was successfully achieved by DA and retro-DA reaction between the furfuryl groups and a bifunctional maleimide crosslinker, bismaleimide. This process was studied by nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and solvent exposure tests. Differential scanning calorimetry analysis was used to determine the endothermic retro-DA reaction in the DA adduct. The self-healing property of the above crosslinked material was demonstrated by monitoring the repair of a scratch in the polymer film upon heating and cooling. This was analyzed by scanning electron microscopy. POLYM. ENG. SCI., 60:140–150, 2020. © 2019 Society of Plastics Engineers     

Effect of the plasticizer on the self-healing properties of a polymer coating based on the thermoreversible Diels–Alder reaction

A self-healing polymer for coating application was developed based on thermoreversible Diels–Alder (DA) reactions. The polymer network is formed by reacting a mixture of trifunctional and difunctional furanized resins (3F and 2F) with a bismaleimide (2M). The DA reaction occurs at temperature above 50 °C whilst retro-DA reaction occurs at about 120 °C. FTIR spectra were taken in order to monitor the reaction progress, and the thermal reversibility of the reaction was proved by DSC and DMA tests. A significant improvement of both the mechanical properties and the self-healing behaviour was achieved by introduction of small amount of a suitable plasticizer like benzyl alcohol. Self-healing properties of the plasticized polymer resulted in complete scratch recovery after retro-DA and DA reaction, whilst tensile testing reveals a 48% restoring of the pristine mechanical strength.     

Alder Ene functionalization of polypropylene through reactive extrusion

A commercial isotactic polypropylene was degraded to increase its terminal vinylidene group concentration, and it was subsequently functionalized with maleic anhydride through the Alder Ene reaction at temperatures above 200°C in a co‐rotating twin screw extruder. Characterization of the maleated product by Fourier transform infrared spectroscopy, ¹H NMR, differential scanning calorimetry, and gel permeation chromatography showed the anhydride group to be terminally attached, and the degree of functionalization was determined by infrared analysis. Increased temperature and maleic anhydride concentration, as well as improved mixing in the extruder, were found to improve the extent of the reaction. The catalytic contribution of Lewis acid species was evaluated, and ruthenium chloride was found to increase the extent of the reaction by 16% in comparison with stannous chloride as a catalyst in the Alder Ene reaction. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 503–516, 1999     

基于壳聚糖/银离子的可逆温敏水凝胶的制备与性能

以壳聚糖(CS)和Ag NO₃为主要原料,通过CS与Ag⁺的超分子络合作用制备了一系列可逆温敏CS-Ag水凝胶,采用FTIR、XRD、XPS、SEM和EDS对CS-Ag水凝胶的结构和化学状态进行了表征。结果表明,通过同时调节Ag NO₃用量和体系p H可制备具有相同凝胶温度的CS-Ag水凝胶。当CS用量为0.60 g、Ag NO₃用量为0.19 g、体系总体积为21 m L、保持水凝胶p H=4.24时,制备的CS-Ag-19水凝胶凝胶温度为50℃,具有最佳的热可逆性能,至少可循环相变5次,具有室温避光保存5个月的稳定性,并表现出对大肠杆菌良好的抗菌性能。CS-Ag-19的溶胶-凝胶转换是由CS的氨基在不同温度下的超分子相互作用控制,H⁺和Ag⁺在温度、浓度变化的驱动下,对游离氨基的竞争反应(质子化和络合作用)之间的平衡关系使水凝胶具有温敏可逆性。CS-Ag-19水凝胶的可注射和温敏凝胶性能使其具有作为水凝胶抗菌敷料的应用潜力。     

Synthesis and characterization of photopatternable epoxy hybrid materials for the fabrication of thick and thermally stable microstructures with a high aspect ratio

Photosensitive cycloaliphatic-epoxy oligosiloxane was synthesized using a nonhydrolytic sol–gel reaction for the fabrication of thick and thermally stable microstructures with high aspect ratios. Its formation was confirmed by ²⁹Si and ¹H nuclear magnetic resonance spectroscopy, small-angle neutron scattering, and Fourier transform infrared spectroscopy. Photocuring of cycloaliphatic-epoxy oligosiloxane resin resulted in a thermally stable epoxy hybrid material (epoxy hybrimer). Micropatterns with a high aspect ratio (>5), an excellent sidewall shape, and low shrinkage were fabricated directly from these materials using a simple photolithographic process. The fabricated micropattern sustained temperatures of up to 250°C. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effect of baking treatment and materials composition on the properties of bulky PMMA–silica hybrid sol–gel materials with low volume shrinkage

A series of bulky PMMA–silica hybrid sol–gel materials had been successfully prepared through the conventional HCl‐catalyzed sol–gel approach with 2‐hydroxyethyl methyl methacrylate (HEMA) as coupling agent under pumping pretreatment (i.e., exhaustive vacuum evacuation). In this work, the hydroxyl groups of HEMA monomers were first cohydrolyzed with various contents of tetraethyl orthosilicate (TEOS) to afford chemical bonding for the forming silica networks by removal of solvent and byproduct of sol–gel reactions through pumping pretreatment before gelation reactions. Subsequently, the resultant viscous solution was then copolymerized with methyl methacrylate (MMA) monomers at specific feeding ratios by using benzoyl peroxide (BPO) as free‐radical initiator. Eventually, transparent bulky organic–inorganic hybrid sol–gel materials loaded with different silica content were always achieved. The obtained bulky hybrid sol–gel materials were found to be transparent, crack‐free, and of relatively low volume shrinkages even in high silica content. The as‐prepared bulky hybrid sol–gel materials were then characterized through silicon element mapping studies of energy‐dispersive X‐ray (EDX) and transmission electron microscopy (TEM). Effect of heating process at 150°C for 5 h after polymerization and material composition on the thermal properties, mechanical strength, and optical clarity of a series of bulky PMMA–silica hybrid sol–gel materials was investigated and compared by thermogravimetric analysis (TGA), thermomechanical analysis (TMA), hardness test, dynamic mechanical analysis (DMA), and UV–vis transmission spectroscopy, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1151–1159, 2006     

Biobased thermosetting resins composed of terpene and bismaleimide

Prepolymers prepared by reactions of 1,1′‐(methylenedi‐4,1‐phenylene)bismaleimide (BMI) with myrcene (Myr) and limonene (Lim) in 1,3‐dimethyl‐2‐imidazolidinone (DMI) at 150°C were compressed at 250°C to produce crosslinked Myr/BMI [molar ratio = 2:2–2:5 (MB22–MB25)] and Lim/BMI [molar ratio = 1:1 (LB11)] resins. The ¹H‐NMR analysis of the model reaction products of Lim and Myr with N‐phenyl maleimide (PMI) in DMI at 150°C revealed that a Diels–Alder reaction for Myr/PMI and a vinyl copolymerization for Lim/PMI preferentially proceeded in addition to the occurrence of the ene reaction to some extent. The Fourier transform infrared data of the cured resins were consistent with the results of the model reactions. All of the cured resins, except for MB22, showed tan δ peak values and 10% weight loss temperatures that were higher than 330 and 440°C, respectively. The flexural strength and modulus values of the MBs were higher than those of LB11. Field emission scanning electron microscopy analysis revealed that MB22–MB24 were homogeneous, whereas some combined particles appeared in LB11. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Free‐radical copolymerization of [(4‐isopropyl phenyl) oxycarbonyl] methyl methacrylate with acrylonitrile and methyl methacrylate

The present investigation has been achieved in accordance with the Diels–Alder reaction (1,4 cycloaddition) to produce a new halogenated bicyclic adduct. ortho‐Bromoallylbenzoate is a new dienophile that was prepared in a pure form, and its structure was confirmed. The Diels–Alder syntheses of hexachlorocyclopentadiene and the new dienophile were studied to determine the optimum condensation reaction conditions under a temperature range of 90–160°C, reaction times of 1–8 h, and molar diene/dienophile ratios from 1:1 to 5:1 as a consequence. The optimum conditions reached were a temperature of 140°C, an initial diene/dienophile molar ratio of 3:1, and a duration time of 6 h. The maximum stoichometric yield under these optimum conditions (82.5%) was obtained. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2331–2338, 2003     

Preparation of inorganic–organic hybrid proton exchange membrane with chemically bound hydroxyethane diphosphonic acid

Proton‐conductive inorganic–organic hybrid intermediate‐temperature membranes were prepared from 3‐glycidoxypropyltrimethoxysilane (GPTMS) and 1‐hydroxyethane‐1,1‐diphosphonic acid (HEDPA) by sol–gel process. To prevent the leaching out of phosphonic acid, triethylamine was used as catalyst to promote the reaction of HEDPA and GPTMS to immobilize phosphonic acid groups. Fourier transform infrared spectra revealed that phosphonic acid groups of HEDPA were chemically bounded to organosiloxane network as a result of the reaction of P? OH of HEDPA and epoxy ring of GPTMS. TG‐DSC results indicated that the hybrid membranes were thermally stable up to 250°C. The proton conductivity of the hybrid membranes increased with temperature from 30 to 130°C. The proton conductivity of hybrid membrane with the molar ratio of GPTMS/HEDPA = 2/1 can reach up to 1.0 × 10^?3 S/cm under anhydrous condition at 130°C, which reveals that this membrane is a promising proton exchange membrane for intermediate‐temperature proton exchange membrane fuel cell. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Polymerization of acrylated epoxidized soybean oil with phenol furfural resins via repeated forward and retro diels–alder reactions

In this work, the Diels–Alder reaction between the acrylate groups of acrylated epoxidized soybean oil and the furan rings of p‐tertiary butyl phenol furfural resin (TBPF) is described. The reaction was carried out at 110°C in presence of FeCl₃ catalyst, and tough polymers were obtained in 1 h. Surprisingly, samples that were heated and cooled 5, 10, and 20 times to 140°C and room temperature had better mechanical properties than samples that were kept at 140°C for the same total duration. This unexpected behavior is attributed to a series of forward and retro Diels–Alder reactions between the functional groups. To prove this hypothesis, a model reaction between TBPF and n‐butyl acrylate was studied. At 100°C, ¹H‐nuclear magnetic resonance signals of the furan ring protons disappeared, only to reappear at 140°C. Thermogravimetric analysis of the adduct showed a weight loss at 140–150°C, which was in quantitative agreement with the amount of butyl acrylate. Infrared analysis showed that furan rings were not completely consumed by extended heating at 110°C. After five heating and cooling cycles of much shorter duration at 140°C, the furan absorption in the infrared disappeared. The storage modulus of acrylated epoxidized soybean oil‐TBPF samples after 20 heating cycles was 1.15 GPa. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Preparation,solubility and thermal behaviour of new bismaleimides containing silicone linkages

A series of new bismaleimides containing silicone linkages have been prepared via Diels–Alder reaction of bismaleimides containing silicone and bisfurans containing silicone; their molecular structures have been characterized by FTIR, NMR and elemental analysis. The solubility of the prepared bismaleimides was tested in six types of solvent of different boiling point and polarity, and their curing temperatures were determined by DSC. The thermal-oxidative stability of the cured networks was investigated by TGA and their glass transition temperatures were measured by DSC, revealing that the bismaleimides are soluble in low boiling point solvents and their curing temperatures are in the range 206–285°C. The DSC and TGA traces of cured networks show that the glass transition temperatures are in the range 287–331°C and that these resins are stable up to 353–384°C. © 1999 Society of Chemical Industry