Reversible Addition Reaction Research Articles

Reprocessed, shape-memory and self-healing robust epoxy resin by hindered urea bond

Given the fossil fuel energy crisis and serious environmental pollution in recent years, one way to alleviate the problem is to introduce dynamic covalent chemistry into thermosetting resins to give the material processable and recyclable properties, creating a new type of eco-friendly material. Hindered urea bonds can undergo reversible exchange reactions at high temperatures to decompose into amine and isocyanate. This reversible addition reaction is currently a prominent research focus in the development of reprocessable thermosets. In this work, we synthesize a novel curing agent with hindered urea bonds without any catalysts. Then characterize the resulting epoxy vitrimers based on hindered urea bond (HUG-EP) using FT-IR and 1H NMR, confirming the successful introduction of hindered urea bonds into the epoxy resin. A variety of tests are carried out, such as those to assess thermal stability, swelling, shape-memory, re-processability and tensile strength. The results demonstrate that HUG-EP can rapidly self-healed within 20 min at 160 °C, with the tensile strength was maintained at 87.15 % of the original during the welding experiments. Furthermore, it can also be reprocessed by hot-pressing several times at 160 °C. Thus, the introduction of hindered urea bonds into the crosslinked network proves to be an effective and practicable approach for producing thermosetting resins with reprocessing, self-healing, and shape memory capabilities.

Small molecule activation based on novel heavier group 13/15 interelement compounds, λ3,λ3-phosphanylalumanes

Abstract As a new entry of heteroatom–heteroatom bond species, λ3,λ3-phosphanylalumanes, having unperturbed P–Al single-bond species, are expected to show high reactivity due to the separation of Lewis acid/base moieties, which can be extended to develop novel small molecule activation reactions using single-bond and saturated compounds. We have designed novel λ3,λ3-phosphanylalumanes, which have all-carbon protecting groups on the λ3-P and λ3-Al moieties, respectively. This account highlights the synthesis and availability of phosphanylalumane derivatives. We demonstrate the addition reactions of phosphanylalumanes toward alkynes to give unique unsaturated C2-vicinal P/Al-based frustrated Lewis pairs and the resulting alkyne-adducts are found to undergo transformation into unique ring compounds. Furthermore, reversible addition reactions of a λ3,λ3-phosphanylalumane toward alkenes will also be described together with the results obtained with other small molecules.

A Fast and Reversible Responsive Bionic Transmembrane Nanochannel for Dynamic Single-Cell Quantification of Glutathione.

Biological channels can rapidly and continuously modulate ion transport behaviors in response to external stimuli, which play essential roles in manipulating physiological and pathological processes in cells. Here, to mimic the biological channels, a bionic nanochannel is developed by synergizing a cationic silicon-substituted rhodamine (SiRh) with a glass nanopipette for transmembrane single-cell quantification. Taking the fast and reversible nucleophilic addition reaction between glutathione (GSH) and SiRh, the bionic nanochannel shows a fast and reversible response to GSH, with its inner-surface charges changing between positive and negative charges, leading to a distinct and reversible switch in ionic current rectification (ICR). With the bionic nanochannel, spatiotemporal-resolved operation is performed to quantify endogenous GSH in a single cell, allowing for monitoring of intracellular GSH fluctuation in tumor cells upon photodynamic therapy and ferroptosis. Our results demonstrate that it is a feasible tool for in situ quantification of the endogenous GSH in single cells, which may be adapted to addressing other endogenous biomolecules in single cells by usage of other stimuli-responsive probes.

A smart and visible way to switch the aromaticity of silicon(IV) phthalocyanines.

Unlike traditional methods of modifying phthalocyanines (Pcs), we herein report a smart and visible way to switch the aromaticity of silicon(IV) phthalocyanines via a reversible nucleophilic addition reaction of the Pc skeleton induced by alkalis and acids, leading to an interesting allochroism phenomenon and the switching of photosensitive activities.

Thermo-healing and recyclable epoxy thermosets based on dynamic phenol-carbamate bonds

In recent years, the reversible addition reaction of isocyanates and active hydrogen compounds has aroused wide concern. The synthesis of a novel dynamic covalent system based on phenol-carbamate bonds (PCBs) and its application in the preparation of thermo-healing and recycled epoxy resins have become a research hotspot. Herein, a compound with a phenolic hydroxyl group was designed and synthesized, which was used to react with isocyanate to obtain the dynamic covalent system of PCBs. Based on PCBs, a cross-linked polyurethane curing agent and the epoxy thermosetting resin with recyclability and thermo-healing ability were prepared. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), stress relaxation analysis and tensile test were carried out on the synthesized epoxy resins based on PCBs (PCBs-EP). The results showed that the dynamic PCBs endow epoxy thermosets with outstanding reprocessability, thermo-healing, welding, recycling and shape memory properties. Based on the reversible dynamic phenol-carbamate bonds, PCBs-EP can be thermally healed within 2 h under 100 °C, and the thermo-healing effect is obvious. More surprisingly, the tensile strength of PCBs-EP reaches 95.89 (±1.06) MPa, and its tensile strength still maintains 74.13% of the original after healed and 70.6% of the original after welded. Therefore, the strategies for incorporating dynamic covalent PCBs into thermosetting epoxy resins provide a feasible strategy for preparing cross-linked epoxy thermosets with high thermo-healing efficiency and high mechanical strength in a cost-effective manner.

The formation mechanism of pinotin A in model wine: Experimental and theoretical investigation

Pinotin A is a crucial pyranoanthocyanin in aged red wine. The formation mechanism of pinotin A was investigated through experimental and theoretical approaches in model wine solution. Using UHPLC-DAD-QTOF, three intermediates were identified in the reaction solution of malvidin-3-O-glucoside and caffeic acid. The pH and oxidative conditions affected the reaction rate by modulating the accumulation of intermediates. Upon preparative HPLC and oxidation experiment, the transformation of intermediate 3 to intermediate 2 was observed through free radical oxidation, p-benzoquinone oxidation and autooxidation. However, the production of pinotin A was only achieved through autooxidation. A three-steps reaction mechanism of reversible addition reaction and decarboxylation, and autooxidation was proposed. The validity of the proposed mechanism was verified through quantum chemistry calculation. The negative variation of Gibbs free energy of the entire reaction was obtained, where the autooxidation step played a crucial role for the spontaneity of the whole reaction.

Effect of pH on the Properties of Hydrogels Cross-Linked via Dynamic Thia-Michael Addition Bonds.

Hydrogels cross-linked with dynamic covalent bonds exhibit time-dependent properties, making them an advantageous platform for applications ranging from biomaterials to self-healing networks. However, the relationship between the cross-link exchange kinetics, material properties, and stability of these platforms is not fully understood, especially upon addition of external stimuli. In this work, pH was used as a handle to manipulate cross-link exchange kinetics and control the resulting hydrogel mechanics and stability in a physiologically relevant window. Poly(ethylene glycol)-based hydrogels were cross-linked with a reversible thia-Michael addition reaction in aqueous buffer between pH 3 and pH 7. The rate constants of bond exchange and equilibrium constants were determined for each pH value, and these data were correlated with the resulting mechanical profiles of the bulk hydrogels. With increasing pH, both the forward and the reverse rate constants increased, while the equilibrium constant decreased. These changes led to faster stress relaxation and less stiff hydrogels at more basic pH values. The elevated pH values also led to an increased mass loss and a faster rate of release of an encapsulated model bovine serum albumin fluorescent protein. The connection between the kinetics, mechanics, and molecular release profiles provides important insight into the structure–property relationships of dynamic covalent hydrogels, and this system offers a promising platform for controlled release between physiologically relevant pH values.

Injectable adaptive self-healing hyaluronic acid/poly (γ-glutamic acid) hydrogel for cutaneous wound healing

The most significant challenge in designing wound dressings is to mimic the tissue microenvironment because of the pro-regenerative structural and functional properties of skin. Herein, we developed a type of bionic extracellular matrix (ECM) hydrogels based on thiol-modified poly (γ-glutamic acid) (γ-PGA-SH) and oxidized hyaluronic acid (HA-CHO). The rapid and reversible thiol-aldehyde addition reaction of thiols in γ-PGA-SH and aldehyde groups in HA-CHO provided hydrogels with a dynamic covalent network and endowed them with properties of adaptability and self-healing capability, which are conducive for initial wound coverage and for prolonging the lifespan of the dressing. Interestingly, these hydrogels also showed typical viscoelastic characteristics similar to those of natural ECM, degradation property in vitro and in vivo, and free radical scavenging capability. In addition, the gelation time, rheological behavior, mechanical property, porous structure, and degradation process of the hydrogels could be tuned by adjusting polymer content. Furthermore, the ECM-inspired hydrogels significantly enhanced the wound healing process in vivo in a full-thickness skin defect model compared to those by commercial dressing (Tegaderm™) by facilitating angiogenesis and promoting collagen deposition. The successful application of the multifunctional hydrogel as an antioxidant wound dressing for wound treatment significantly exhibited its great application potential for biomedical areas. Statement of significanceThe application of tissue engineering techniques to repair full-thickness skin wounds remains a great challenge in clinical trials. Among the recent approaches used for wound healing, in situ forming injectable hydrogels have gained much attention, and few of them have shown satisfactory overall performance, such as integration into the wound bed, biodegradability, immunocompatibility, vascularization, and recapitulation of the structure and function of skin. In the present study, we designed a simple and convenient in situ forming injectable adaptable self-healing hydrogels with biodegradability and antioxidative properties, which could substantially improve wound healing quality at an affordable cost. The hydrogel-based wound dressing is expected to solve the abovementioned problems and help in promoting cutaneous wound healing.

3D Printing of Recyclable Elastomers with Controllable Degradation and Adhesive Properties

Photopolymerization-based 3D printing is now a well-established manufacturing technique for a wide range of applications, as it allows the customized fabrication of objects with high resolution, tunable mechanical properties, and various functionalities. However, after printing, the crosslinked polymeric networks cannot be easily degraded and recycled, raising economic and environmental concerns. To date, only a few reprocessable or partially recyclable 3D printing resins have been reported. These resins are based on complex formulations or specific chemical structures. In addition to their high production costs and their limited scope of applicability, none of these materials can be completely depolymerized after photopolymerization 3D printing. Here, a general strategy is proposed to design degradable and recyclable photopolymers for 3D printing by taking advantage of the reversible aza-Michael addition reaction. With this approach, complete degradation of printed objects can be achieved under ambient conditions with tunable kinetics. The depolymerized products can be recycled and directly used for re-printing without the need of adding extra resins. Furthermore, using dopamine as a representative monomer, the possibility of incorporating additional chemical functions into the printed materials, such as adhesive properties, is demonstrated.

Attrition-Enhanced Deracemization and Absolute Asymmetric Synthesis of Flavanones from Prochiral Precursors

Seven racemic 5,7-dimethoxyflavanones afforded conglomerate crystals upon recrystallization from a solvent. Three methodologies were investigated to achieve asymmetric transformation based on dynamic crystallization of the chiral conglomerate system. The first was chiral symmetry breaking of racemic flavanones by attrition-enhanced deracemization. Continuous suspension of racemic flavanones in a small amount of propanol in the presence of a base (1,8-diazabicyclo[5.4.0]­undec-7-ene (DBU)) and glass beads promoted chiral symmetry breaking and converted the flavanones to crystals of (+)- or (−)-enantiomers with 78 to 99% ee. The second method involved cyclization of the intermediate aldol product to give optically active flavanone with 90% ee involving a reversible oxa-Michael addition reaction with attrition-enhanced deracemization. The third was a reaction starting from prochiral 2-hydroxy-4,6-dimethoxyacetophenone and 2-naphthaldehyde under basic conditions, which gave the corresponding flavanone in 89% ee.

Unique two-way free-standing thermo- and photo-responsive shape memory azobenzene-containing polyurethane liquid crystal network

In this work, azobenzene-containing polyurethane liquid crystal networks (PULCN(AZO)s) were synthesized using a one-pot strategy to demonstrate excellent two-way free-standing thermo-/photo-responsive shape memory effects. Based on the step-growth nature of hydroxyls and isocyanates, the architectures of the networks were adjusted by controlling the stoichiometries of chemical materials. A uniform monodomain sample was prepared by external stress relaxation via a reversible addition reaction of a dynamic carbamate bond. Two independent transition temperatures assigned to glass transition temperature/melting temperature and liquid crystal phase transition temperature were employed to thermally trigger triple shape memory effects and two-way autonomous actuation. In addition, taking advantage of the trans-cis photoisomerization of azobenzene, the programmed network showed a reversible bending and unbending shape change upon irradiation by visible light at 450 and 550 nm, respectively. Coupling the autonomously thermo-induced contraction/extension actuation and reversible photo-induced bending/unbending behaviors of PULCN(AZO)s in one system will expand their potential applications in emerging multifunctional devices.

Preferential Control of Forward Reaction Kinetics in Hydrogels Crosslinked with Reversible Conjugate Additions

Molecular substitutions were used to demonstrate preferential control over the kinetic rate constants in a poly(ethylene glycol)-based hydrogel with two different reversible thia-conjugate addition reactions. A strong electron-withdrawing nitrile group on the conjugate acceptor showed a 20-fold increase in the forward rate constant over a neutral withdrawing group, whereas the reverse rate constant only increased 6-fold. Rheometry experiments demonstrated that the hydrogel plateau modulus was primarily dictated by reaction equilibrium, whereas the stress relaxation characteristics of the hydrogel were dominated by the reverse rate constant. Furthermore, dynamic crosslinking allowed the hydrogel to rapidly and spontaneously self-heal. These results indicate that decoupling the kinetic rate constants of bond exchange allow systematic control over dynamic covalent hydrogel bulk properties, such as their adaptability, stress relaxation ability, and self-healing properties.

Facilitated transport membranes by incorporating self-exfoliated covalent organic nanosheets for CO2/CH4 separation

Two-dimensional (2D) covalent organic nanosheets (CONs) present versatile 2D materials, which hold great promise in mixed matrix membranes. The facile synthesis of CONs for the fabrication of mixed matrix membranes represents the development trend. Herein, mixed matrix membranes with facilitated transport ability were prepared based on self-exfoliated covalent organic nanosheets (CONs) and poly(ether-block-amide) (Pebax® MH 1657) matrix. The self-exfoliated TpDT CONs were facilely synthesized via Schiff base condensation reaction between 1,3,5-Triformylphloroglucinol (Tp) and 3, 5-diamino-1,2,4-triazole (DT). Abundant amine groups on TpDT CONs skeleton served as facilitated transport carriers to promote the transport of CO2 by reversible nucleophilic addition reactions. And the highly ordered channels of TpDT CONs allowed the rapid transportation of CO2 molecules to reach the facilitated transport carriers. Moreover, the incorporated TpDT CONs disturbed the packing of the Pebax chains by the intervention effects and increased the chain mobility. The facilitated transport carriers from TpDT CONs and the optimized structure endowed the as-prepared membranes with elevated permeance and selectivity. Specially, the Pebax-TpDT/PVDF membranes with 1 wt% TpDT CONs achieved an optimum CO2/CH4 separation performance with a separation factor of 36.0 and a CO2 permeance of 142.3 GPU, which were increased by 68% and 131%, respectively, compared with the pristine Pebax membrane.

Reprocessable Aliphatic Polydithiourethanes Based on the Reversible Addition Reaction of Diisothiocyanates and Dithiols

Aliphatic polydithiourethanes (PDTUs) were successfully synthesized from diisothiocyanates and dithiols having different values of the carbon atom number. The thermal properties and hydrogen-bondin...

Efficient visible-light photocatalytic degradation of imidacloprid and acetamiprid using a modified carbon nitride/tungstophosphoric acid composite induced by a nucleophilic addition reaction

This study proposed a novel method to combine modified carbon nitride (MCN) and tungstophosphoric acid (HPW). First, carbon nitride (CN), containing more uncondensed amino and carbonyl groups, was prepared by thermal condensation from urea at a low temperature. Then, in the presence of formic acid, a reversible nucleophilic addition reaction was performed between the carbonyl groups of formaldehyde (or CN) and amino groups of CN. Subsequently, during the HPW impregnation, the reverse reaction occurred, which increased the HPW loading. The results of the XPS and FTIR analyses verified the occurrence of a nucleophilic addition reaction during the formaldehyde treatment, as well as the formation of hydroxyl functional groups in the intermediate products. After impregnation with HPW, the disappearance of the hydroxyl groups and the recovery of the carbonyl groups were observed in the XPS C1s spectra. The as-prepared MCN450/HPW, obtained by modifying CN450 (CN calcined at 450 °C) with formaldehyde and loading with HPW, exhibited excellent visible-light (λ > 400 nm) photocatalytic degradation of imidacloprid and acetamiprid. The degradation rate constant (0.70 h−1) of imidacloprid was 6.4 times that of CN450, while the degradation rate of acetamiprid by MCN450/HPW was 11 times that of CN450. The results of the active species capture experiments showed that OH and h+ were the main active species in the visible-light photocatalytic degradation process.

The Road Travelled: After Main‐Group Elements as Transition Metals

AbstractSince the latter quarter of the twentieth century, main group chemistry has undergone significant advances. Power's timely review in 2010 highlighted the inherent differences between the lighter and heavier main group elements, and that the heavier analogues resemble transition metals as shown by their reactivity towards small molecules. In this concept article, we present an overview of the last 10 years since Power's seminal review, and the progress made for catalytic application. This examines the use of low oxidation state and/or low coordinate group 13 and 14 complexes towards small molecule activation (oxidative addition step in a redox based cycle) and how ligand design plays a crucial role in influencing subsequent reactivity. The challenge in these redox based catalytic cycles still centres on the main group complexes’ ability to undergo reductive elimination, however considerable progress in this field has been reported via reversible oxidative addition reactions. Within the last 5 years the first examples of well‐defined low valent main group catalysts have begun to emerge, representing a bright future ahead for main group chemistry.

ZnO piezoelectric film resonator modified with multi-walled carbon nanotubes/polyethyleneimine bilayer for the detection of trace formaldehyde

We fabricated a ZnO piezoelectric film resonator modified with multi-walled carbon nanotubes/polyethyleneimine bilayer as the sensitive layer for the detection of trace gaseous formaldehyde. The resonator consists of a ZnO piezoelectric stack and an SiO2/W Bragg reflector. The multi-walled carbon nanotubes were self-assembled on the resonator surface using the n-octadecyl mercaptan monolayer and further modified with polyethyleneimine. The formaldehyde molecules are absorbed on the multi-walled carbon nanotubes/polyethyleneimine bilayer based on the reversible nucleophilic addition reaction between formaldehyde molecules and the amine functional groups on polyethyleneimine. The high working frequency (~ 3.1 GHz) of the resonator provided enough mass sensitivity to probe the ultra-small mass change of the sensitive biolayer. The downshift of resonant frequency was linear with the increase of formaldehyde concentration. The experimental results show that our proposed sensor can yield rapid, sensitive, reversible and repeatable responses to formaldehyde in the concentration range of 50–400 ppb at room temperature. The piezoelectric film resonator is a promising and feasible sensor for the indoor pollution monitoring.

Insight into the Mechanism of Reversible Ring-Opening of 1,3-Benzoxazine with Thiols.

The reversible ring-opening addition and fragmentation reaction of p-cresol-based N-phenylbenzoxazine with aliphatic and aromatic thiols was investigated in solvent-mediated and solvent-free reactions. Independently of the used thiol, N-phenylbenzoxazine and the thiols reacted to equilibrium with comparable amounts of reactants and products in aprotic solvent, whereas in protic solvent almost full conversions were reached. In contrast, thiol reactivity was a crucial factor in solvent-free reactions yielding fast and complete conversions for a more acidic thiol and balanced equilibrium concentrations in case of thiols with high pKa values. The strong influence of thiols with low pKa values emphasizes the relevance of the protonation step in the ring-opening reactions of 1,3-benzoxazines with thiols in absence of solvents where acidity predominates nucleophilicity. The reverse reactions, namely adduct dissociation and benzoxazine recovery, were successfully conducted at elevated temperatures and reduced pressure facilitated by the removal of the formed thiols yielding up to 95% recovered 1,3-benzoxazine. These results provide deeper understanding of the reversible ring-opening reaction mechanism of 1,3-benzoxazine with thiols.

Theoretical and Experimental Investigation of Thermodynamics and Kinetics of Thiol-Michael Addition Reactions: A Case Study of Reversible Fluorescent Probes for Glutathione Imaging in Single Cells.

Density functional theory (DFT) was applied to study the thermodynamics and kinetics of reversible thiol-Michael addition reactions. M06-2X/6-31G(d) with the SMD solvation model can reliably predict the Gibbs free energy changes (ΔG) of thiol-Michael addition reactions with an error of less than 1 kcal·mol(-1) compared with the experimental benchmarks. Taking advantage of this computational model, the first reversible reaction-based fluorescent probe was developed that can monitor the changes in glutathione levels in single living cells.

Highly sensitive formaldehyde sensors based on polyvinylamine modified polyacrylonitrile nanofibers

A novel, sensitive, selective, and reproductive formaldehyde sensor has been developed by coating polyvinylamine (PVAm) modified electrospun polyacrylonitrile (PAN) nanofibrous membranes on quartz crystal microbalance (QCM) for the first time. Benefiting from the abundant primary amine groups in PVAm, large specific surface area, high porosity and hierarchal structure of PAN nanofibers, and the strong interaction between PVAm molecule and formaldehyde, the as-prepared QCM sensors achieved an extremely low detection limit of 500 ppb in a remarkably short period of time (120 s). Moreover, the sensitivity of the fibrous membranes based sensors are 2.5 times higher than that of flat film-based ones when upon exposure to 500 ppb formaldehyde. Furthermore, the as-prepared QCM sensors possess excellent reproducibility, reversibility, and good selectivity by virtue of the reversible nucleophilic addition reaction between formaldehyde molecule and primary amine group in PVAm. Hence, such promising QCM sensors could not only potentially allow for monitoring gaseous formaldehyde, but also pave a way for designing and development of novel QCM sensing systems based on functionalized nanofibrous membranes.