“Self-Lockable” Liquid Crystalline Diels–Alder Dynamic Network Actuators with Room Temperature Programmability and Solution Reprocessability

Novel main-chain liquid crystalline Diels—Alder dynamic networks (LCDANs) were prepared that exhibit unprecedented ease for actuator programming and reprocessing compared to existing liquid crystalline network (LCN) systems. Following cooling from 125 °C, LCDANs are deformed with aligned mesogens self-locked at room temperature by slowly formed Diels–Alder (DA) bonds, which allows for the formation of solid 3D actuators capable of reversible shape change, and strip walker and wheel-capable light-driven locomotion upon either thermally or optically induced order–disorder phase transition. Any actuator can readily be erased at 125 °C and reprogrammed into a new one under ambient conditions. Moreover, LCDANs can be processed directly from melt (for example, fiber drawing) and from solution (for example, casting tubular actuators), which cannot be achieved with LCNs using exchangeable covalent bonds. The combined attributes of LCDANs offer significant progress toward developing easily programmable/processable LCN actuators.     

Cyclic Alkyne Approach to Heteroatom‐Containing Polycyclic Aromatic Hydrocarbon Scaffolds

We report a modular synthetic strategy for accessing heteroatom‐containing polycyclic aromatic hydrocarbons (PAHs). Our approach relies on the controlled generation of transient heterocyclic alkynes and arynes. The strained intermediates undergo in situ trapping with readily accessible oxadiazinones. Four sequential pericyclic reactions occur, namely two Diels–Alder/retro‐Diels–Alder sequences, which can be performed in a stepwise or one‐pot fashion to assemble four new carbon–carbon (C?C) bonds. These studies underscore how the use of heterocyclic strained intermediates can be harnessed for the preparation of new organic materials.     

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     

Liquid‐Crystalline Soft Actuators with Switchable Thermal Reprogrammability

Thermal reprogrammability is essential for new‐generation large dry soft actuators, but the realization sacrifices the favored actuation performance. The contradiction between thermal reprogrammability and stability hampers efforts to design high‐performance soft actuators to be robust and thermally adaptable. Now, a strategy has been developed that relies on repeatedly switching on/off thermal reprogrammability in liquid‐crystalline elastomer (LCE) actuators to resolve this problem. By post‐synthesis swelling, a latent siloxane exchange reaction can be induced in the common siloxane LCEs (switching on), enabling reprogramming into on‐demand 3D‐shaped actuators; by switching off the dynamic network by heating, actuation stability is guaranteed even at high temperature (180 °C). Using partially black‐ink‐patterned LCEs, selectively switching off reprogrammability allows integration of completely different actuation modes in one monolithic actuator for more delicate and elaborate tasks.     

Preparation of liquid crystalline polymer networks containing a cinnamyl group in the main chain with tunable thermal actuation behavior

A series of liquid crystalline copolyesters (LCPs) with different concentrations of a photocrosslinking moiety have been synthesized by random polycondensation with 4,4′‐bis(6‐hydroxyhexyloxy)biphenyl, 2‐phenylsuccinic acid, and 4‐(6‐hydroxyhexyloxy)cinnamic acid (6HCA). Multifunctional monodomain liquid crystal networks (LCNs) with considerable and tunable actuation behavior are obtained by postphotocrosslinking. The influence of the photocrosslinking moiety on the phase transition behavior of the LCP and actuation behavior of the LCN has been investigated. The results suggest that incorporating 6HCA suppresses the smectic phase of the LCP and decreases the nematic‐isotropic phase transition temperature. Moreover, the starting actuation temperature of the LCN decreases from 55 to 40 °C as the 6HCA reached 50%. In addition, the actuation force and storage modulus of the LCN actuators are enhanced by incorporating a high concentration of 6HCA. A 1.64 MPa contractile force can be achieved, and it can lift burdens 1300 times heavier than its weight when 50% 6HCA is incorporated into the LCP. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 904–911     

Dendronized polymers via Diels–Alder “click” reaction

A modular approach toward the synthesis of polymers containing dendron groups as side chains is developed using the Diels–Alder “click” reaction. For this purpose, a styrene‐based polymer appended with anthracene groups as reactive side chains was synthesized. First through third‐generation polyester dendrons containing furan‐protected maleimide groups at their focal point were synthesized. Facile, reagent‐free, thermal Diels–Alder cycloaddition between the anthracene‐containing polymer and latent‐reactive dendrons leads to quantitative functionalization of the polymer chains to afford dendronized polymers. The efficiency of this functionalization step was monitored using ¹H and ¹³C NMR spectroscopy and FTIR and UV–vis spectrometry. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 410–416, 2010     

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.     

Multicomponent Diene‐Transmissive Diels–Alder Sequences Featuring Aminodendralenes

1‐Aminodecalins were prepared from acyclic precursors by combining the powerful twofold diene‐transmissive Diels–Alder chemistry of [3]dendralenes with the simplicity of enamine formation. On mixing at ambient temperature, a simple dienal condenses with a primary or secondary amine to generate the enamine, a 1‐amino‐[3]dendralene in situ, and this participates as a double diene in a sequence of two Diels–Alder events with separate dienophiles. Overall, four C?C bonds and one C?N bond are formed. Mechanistic insights into these reactions are provided by means of density functional theory calculations.     

Collective Total Syntheses of Atisane‐Type Diterpenes and Atisine‐Type Diterpenoid Alkaloids: (±)‐Spiramilactone B, (±)‐Spiraminol, (±)‐Dihydroajaconine,and (±)‐Spiramines C and D

The first total syntheses of the architecturally complex atisane‐type diterpenes and biogenetically related atisine‐type diterpenoid alkaloids (±)‐spiramilactone B, (±)‐spiraminol, (±)‐dihydroajaconine, and (±)‐spiramines C and D are reported. Highlights of the synthesis include a late‐stage biomimetic transformation of spiramilactone B, a facile formal lactone migration from the pentacyclic skeleton of spiramilactone E, a highly efficient and diastereoselective 1,7‐enyne cycloisomerization to construct the functionalized tetracyclic atisane skeleton, and a tandem retro‐Diels–Alder/intramolecular Diels–Alder sequence to achieve the tricyclo[6.2.2.0] ring system.     

Collective Total Syntheses of Atisane‐Type Diterpenes and Atisine‐Type Diterpenoid Alkaloids: (±)‐Spiramilactone B, (±)‐Spiraminol, (±)‐Dihydroajaconine,and (±)‐Spiramines C and D

The first total syntheses of the architecturally complex atisane‐type diterpenes and biogenetically related atisine‐type diterpenoid alkaloids (±)‐spiramilactone B, (±)‐spiraminol, (±)‐dihydroajaconine, and (±)‐spiramines C and D are reported. Highlights of the synthesis include a late‐stage biomimetic transformation of spiramilactone B, a facile formal lactone migration from the pentacyclic skeleton of spiramilactone E, a highly efficient and diastereoselective 1,7‐enyne cycloisomerization to construct the functionalized tetracyclic atisane skeleton, and a tandem retro‐Diels–Alder/intramolecular Diels–Alder sequence to achieve the tricyclo[6.2.2.0] ring system.     

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.     

Synthesis of a poly(vinyl ether) containing a benzocyclobutene moiety and its reaction with dienophiles

1‐Benzocyclobutenyl vinyl ether (1) was easily prepared by the elimination reaction of hydrogen bromide from 1‐benzocyclobutenyl 1‐bromoethyl ether obtained by 1‐bromobenzocyclobutene and ethylene glycol via two steps in a good yield. Cationic polymerizations of 1 was carried out at −78°C for 2 h in toluene in the presence of BF₃OEt₂ as an initiator to give quantitatively the corresponding polymers (2) as white solids. As a model reaction of the polymer reaction of 2 with dienophiles, the Diels–Alder reactions of 1‐methoxybenzocyclobutene with maleic anhydride (MA) in toluene at 100–140°C for 3 h were carried out to obtain the corresponding Diels–Alder adduct quantitatively at 140°C. The polymer reactions of 2 with MA and N‐phenylmaleimide (MI) in toluene were carried out to yield the corresponding Diels–Alder adduct polymers in good yields. The degree of introduction of the dienophile could be controlled by temperature, and the unreacted benzocyclobutene moiety could further react with another benzocyclobutene moiety or dienophile. The properties (solubilities, T_g, and temperature of 10% weight loss) of the polymers obtained from the polymer reaction were quite different from those of 2. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 59–67, 1999     

Reactivity of 4‐Phenyl‐1,2,4‐triazoline‐3,5‐dione and Diethylazocarboxylate in [4+2]‐Cycloaddition and Ene Reactions: Solvent,Temperature, and High‐Pressure Influence on the Reaction Rate

We have studied the solvent, temperature, and pressure influences on the reaction rates of cyclic and acyclic N=N bonds in the Diels–Alder and ene reactions. The transfer from N‐phenylmaleimide ( 9 ) to a structural analogue, 4‐phenyl‐1,2,4‐triazoline‐3,5‐dione ( 2 ), is accompanied by the rate increase in five to six orders of magnitude in the Diels–Alder reactions with cyclopentadiene ( 4 ) and 9,10‐dimethylanthracene ( 5 ), whereas the transfer from dimethyl fumarate ( 10 ) to diethyl azodicarboxylate ( 1 ) increases only in one to two orders of magnitude. The ratio of the reaction rate constants ( 2 + 4 )/( 1 + 4 ) is very large (5.2 × 10⁷) and almost the same (5.3 × 10⁷) as in the ene reactions with tetramethylethylene ( 7 ), ( 2 + 7 )/( 1 + 7 ). It has been observed that the N=N bond in reagent 2 has strong electrophilic, and its N–N moiety in the transition state has nucleophilic properties, which results from the analysis of the solvation enthalpy transfer of reagents, activated complex, and adduct in the Diels–Alder reaction of 2 with anthracene 22 .     

Healable network polymers bearing flexible poly(lauryl methacrylate) chains via thermo‐reversible furan‐maleimide diels–alder reaction

A new ATRP initiator containing two furyl rings, namely, bis(furan‐2‐ylmethyl) 2‐bromopentanedioate was synthesized starting from commercially available l ‐glutamic acid as a precursor. Well‐defined bisfuryl‐terminated poly(lauryl methacrylate) macromonomers with molecular weight and dispersity in the range 5000–12,000 g mol^?1 and 1.30–1.37, respectively, were synthesized employing the initiator by atom transfer radical polymerization (ATRP). Independently, 1,1′,1″‐(nitrilotris(ethane‐2,1‐diyl))tris(1H‐pyrrole‐2,5‐dione) was synthesized as a tris‐maleimide counterpart for furan‐maleimide click reaction. Thermo‐reversible network polymer bearing flexible poly(lauryl methacrylate; (PLMA) chains was obtained by furan‐maleimide Diels–Alder click reaction of bisfuryl‐terminated PLMA with 1,1′,1″‐(nitrilotris(ethane‐2,1‐diyl))tris(1H‐pyrrole‐2,5‐dione). The prepared network polymer showed retro‐Diels–Alder reaction in the temperature range 110–170 °C as determined from DSC analysis. The presence of low Tg (–40 °C) PLMA chains induced chain mobility to the network structure which led to the complete scratch healing of the coating at 60 °C in five days due to furan‐maleimide adduct formation. The storage modulus of the network polymer was found to be 3.7 × 10⁴ Pa at the constant angular frequency of 5 rad/sec and strain of 0.5%. The regular reversal of storage (G ′) and loss modulus (G ″) was observed with repeated heating (40 to 110 °C) and cooling cycles (110 to 40 °C) at constant angular frequency and strain. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 2700–2712     

Palladium‐catalyzed three‐component coupling polycondensation of aromatic diiodide,aromatic bis(boronic acid propanediol ester), and norbornadiene: A new route for poly(p‐phenylene vinylene)

A novel synthetic method for soluble precursor polymers of poly(p‐phenylene vinylene) (PPV) derivatives by the palladium‐catalyzed three‐component coupling polycondensation of aromatic diiodides, aromatic bis(boronic acid) derivatives, and norbornadiene is described. For example, the polymerization of 1,4‐diiodo‐2,5‐dioctyloxybenzene, benzene‐1,4‐bis(boronic acid propanediol ester), and norbornadiene at 100 °C for 3 days provided a polymer consisting of the three monomer units in a 97% yield (number‐average molecular weight = 3100, weight‐average molecular weight/number‐average molecular weight = 1.37). A derivative of PPV was produced smoothly by the retro Diels–Alder reaction of the polymer both in a dodecyloxybenzene solution and in a film at 200 °C in vacuo. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3403–3410, 2005     

Coordination Bonds and Diels–Alder Bonds Dual Crosslinked Polymer Networks of Self‐healing Polyurethane

A dual crosslinked self‐healing polyurethane was prepared with robust mechanical properties through the dynamic reversible pyridine‐Fe³⁺ coordination bonds and Diels–Alder (DA) covalent bonds dual crosslinking strategy. Moreover, the mechanical properties and self‐healing ability of polyurethane can be tuned readily by different ratio of the coordination bonds and DA bonds. Under external load, the coordination bonds serve as sacrificial bonds are broken to dissipate energy, the DA bonds can keep the shape of sample. With the coordination bonds participation, the damaged samples can be healed under moderate heating treatment or with the aid of FeCl₃ solution. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2228–2234     

Benzocyclobutene in polymer synthesis. II. Solid state diels–alder polymerization utilizing an in situ generated diene and an alkyne

A novel class of aromatic imide AB-monomers with benzocyclobutene and an alkyne (primarily phenylethynyl group) as the reactive units have been prepared. The monomers have been utilized in thermally induced Diels–Alder polymerizations. The differential scanning calorimetric study of the AB-monomers provided two observations: (i) primary acetylene began its homopolymerization (202°C max.) before the electrocyclic ring opening of benzocyclobutene (270°C max.); (ii) the phenoxy group connecting between phenylacetylenyl group and the aromatic imide fragment suppressed polymerization in Diels–Alder fashion. Furthermore, thermoxidative stability evaluation on the cured samples (250°C for 8 h and then 350°C for another 8 h under N₂ atmosphere), carried out at 650°F (air) for 200 h, indicated the more rigid phenylethynyl phthalimide system was the most heat-resistant.     

Epoxy‐Functionalized Porous Organic Polymers via the Diels–Alder Cycloaddition Reaction for Atmospheric Water Capture

The synthesis of highly microporous, epoxy‐functionalized porous organic polymers (ep‐POPs) by a one‐pot, catalyst‐free Diels–Alder cycloaddition polymerization is reported. The high oxygen content of ep‐POPs offer efficient hydrogen‐bonding sites for water molecules, thus leading to high water‐uptake capacities up to 39.2–42.4 wt % under a wide temperature range of 5–45 °C, which covers the span of climatic conditions and manufacturing applications in which such materials might be used. Importantly, ep‐POPs demonstrated regeneration temperatures as low as 55 °C, as well as excellent water stability, recyclability, and high specific surface areas up to 852 m² g⁻¹.     

V‐shaped graft copolymers via triple click reactions: Diels–alder,copper‐catalyzed azide–alkyne cycloaddition,and nitroxide radical coupling

We report here a simple and universal synthetic pathway covering triple click reactions, Diels–Alder, copper‐catalyzed azide–alkyne cycloaddition (CuAAC), and nitroxide radical coupling (NRC), to prepare well‐defined graft copolymers with V‐shaped side chains. The Diels–Alder click reaction between the furan protected‐maleimide‐terminated poly(ethylene glycol) (PEG) and a trifunctional core ( 1 ) carrying an anthracene, alkyne, and bromide was carried out to yield the corresponding α‐alkyne‐ and α‐bromide‐terminated PEG (PEG‐alkyne/Br) in toluene at 110 °C. Subsequently, the polystyrene or polyoxanorbornene with pendant azide functionality as a main backbone is reacted with the PEG‐alkyne/Br and 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO)‐terminated poly(ε‐caprolactone) using the CuAAC and NRC reactions in a one‐pot fashion in N,N′‐dimethylformamide at room temperature to result in the target V‐shaped graft copolymers. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4667–4674     

Fourfold Diels–Alder Reaction of Tetraethynylsilane

A series of ethynylated silanes, including tetraethynylsilane, was treated with tetraphenylcyclopentadienone at 300 °C under microwave irradiation to give the aromatized Diels–Alder adducts as sterically encumbered mini‐dendrimers with up to 20 benzene rings. The sterically most congested adducts display red‐shifted emission through intramolecular π–π interactions in the excited state.