aza-Michael Reaction Research Articles

4D Printing of Hygroscopic Liquid Crystal Elastomer Actuators.

Liquid crystal elastomers (LCEs) are broadly recognized as programmable actuating materials that are responsive to external stimuli, typically heat or light. Yet, soft LCEs that respond to changes in environmental humidity are not reported, except a few examples based on rigid liquid crystal networks with limited processing. Herein, a new class of highly deformable hygroscopic LCE actuators that can be prepared by versatile processing methods, including surface alignment as well as 3D printing is presented. The dimethylamino-functionalized LCE is prepared by the aza-Michael addition reaction between a reactive LC monomer and N,N'-dimethylethylenediamine as a chain extender, followed by photopolymerization. The humidity-responsive properties are introduced by activating one of the LCE surfaces with an acidic solution, which generates cations on the surface and provides asymmetric hydrophilicity to the LCE. The resulting humidity-responsive LCE undergoes programmed and reversible hygroscopic actuation, and its shape transformation can be directed by the cut angle with respect to a nematic director or by localizing activation regions in the LCE. Most importantly, various hygroscopic LCE actuators, including (porous) bilayers, a flower, a concentric square array, and a soft gripper, are successfully fabricated by using LC inks in UV-assisted direct-ink-writing-based 3D printing.

Tailoring the Surface Chemistry of Anion Exchange Membranes with Zwitterions: Toward Antifouling RED Membranes.

Fouling is a pressing issue for harvesting salinity gradient energy with reverse electrodialysis (RED). In this work, antifouling membranes were fabricated by surface modification of a commercial anion exchange membrane with zwitterionic layers. Either zwitterionic monomers or zwitterionic brushes were applied on the surface. Zwitterionic monomers were grafted to the surface by deposition of a polydopamine layer followed by an aza-Michael reaction with sulfobetaine. Zwitterionic brushes were grafted on the surface by deposition of polydopamine modified with a surface initiator for subsequent atom transfer radical polymerization to obtain polysulfobetaine. As expected, the zwitterionic layers did increase the membrane hydrophilicity. The antifouling behavior of the membranes in RED was evaluated using artificial river and seawater and sodium dodecylbenzenesulfonate as the model foulant. The zwitterionic monomers are effective in delaying the fouling onset, but the further build-up of the fouling layer is hardly affected, resulting in similar power density losses as for the unmodified membranes. Membranes modified with zwitterionic brushes show a high potential for application in RED as they not only delay the onset of fouling but they also slow down the growth of the fouling layer, thus retaining higher power density outputs.

Attenuation of the Reaction of Michael Acceptors with Biologically Important Nucleophiles.

β-Elimination of drugs tethered to macromolecular carbamates provides a platform for drug half-life extension. However, the macromolecular Michael acceptor products formed upon drug release can potentially react with biological amines and thiols and may raise concerns about safety. We desired to mitigate this possibility by developing linkers that have predictable rates of β-elimination but suppressed rates of nucleophilic addition to their Michael acceptor products. We prepared Michael acceptor products of β-eliminative linkers that contained a methyl group at the Cβ carbon or a gem-dimethyl group at the Cγ carbon and studied the kinetics of their reactions with the most prevalent biological nucleophiles-amine and thiol groups. Aza-Michael reactions with glycine are slowed about 20-fold by methylation of the β-carbon and 175-fold with a gem-dimethyl group at the γ-carbon. Likewise, addition of the glutathione thiol to γ-gem-dimethyl Michael acceptors was retarded 7-24-fold compared to parent unsubstituted linkers. It was estimated that in an in vivo environment of ∼0.5 mM macromolecular thiols or ∼20 mM macromolecular amines-as in plasma-the reaction half-life of a typical Michael acceptor with a γ-gem-dimethyl linker could exceed 3 years for thiols or 25 years for amines. We also prepared a large series of γ-gem-dimethyl β-eliminative linkers and showed excellent structure-activity relationships of elimination rates with corresponding unsubstituted parent linkers. Finally, we compared the first-generation unsubstituted and new gem-dimethyl β-eliminative linkers in a once-monthly drug delivery system of a 39 amino acid peptide. Both linkers provided the desired half-life extension of the peptide, but the Michael acceptor formed from the gem-dimethyl linker was much less reactive. We conclude that the γ-gem-dimethyl β-eliminative linkers provide high flexibility and greatly reduce potential reactions of Michael acceptor products with biologically important nucleophiles.

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.

Aza-Michael Reaction as a Greener, Safer, and More Sustainable Approach to Biobased Polyurethane Thermosets

Polyurethanes (PUs) such as foams or coatings are generally obtained by polyaddition between polyols and toxic polyisocyanates. In the frame of green chemistry principles, which claims, for example...

Preparation of high-strength silicon-containing elastomer via coordination cross-linking reaction and its multiple recycling performance

<p indent=0mm>It is a critical challenge to prepare elastomers with both high mechanical properties and multiple recycling properties to date. Due to cumbersome and difficult to control, it is difficult to realize both high mechanical properties and industrial production with current methods reported in numerous literatures. Herein we demonstrate a new method for the preparation of high-strength silicon-containing elastomer based on the coordination crosslinking reaction between functionalized polysiloxane and rare earth salts (CeCl₃·7H₂O, EuCl₃·6H₂O and TbCl₃·6H₂O, respectively). The functionalized polysiloxane, named poly(dimethyl-methyl (<italic>N</italic>-nitrile ethyl aminopropyl)) siloxane (PANPMS), was syntheiszed by the aza-Michael reaction which having the molecular weight of about 100000 and containing 8% of nitrile groups. The crosslinking reaction occurs in the molding stage, making the preparation process is simple and easy to realize industrialization. These elastomers could achieve high tensile strength <sc>(2.58 MPa)</sc> and high elongation at break (500%). These data exceed the properties of EA 2900 Sealant of Dow Chemical Company which are <sc>2.1 MPa</sc> and 400%, respectively. What’s more, narrow-band red luminescence is observed in EuCl₃·6H₂O and green luminescence is observered in TbCl₃·6H₂O when these elastomers are exposure to ultraviolet light. This means that the high-strength elastomer could be used as luminescent materials. The coordination process was detected by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and vulcanization curves. The vulcanization curves show that the optimum vulcanization conditions is 90°C and <sc>25 min.</sc> The torque value in vulcanization curves increase with the content of rare earth salts which means that the vulcanization is closely related to rare earth salts. XPS analysis and FTIR show that the coordinations between nitrile group and rare earth ion take place. Therefore, a conclusion could be drawn that the coordination process plays a role of vulcanization and crosslinking. In addition, it can be noticed from the analysis that there are still a large number of nitrile and rare earth salts in the vulcanized elastomer not participating in coordination reaction. From scanning electron microscopy micrographs, it can be observed that all of the rare earth salts are evenly dispersed in functionalized polysiloxane to form a sea-island structure. This island structures indicate that a large number of rare earth salts are contained in the particales and cannot participate in the coordination-crosslinking process. This phenomenon is perfectly verified with XPS and FTIR data. To our surprise, we happened to find that the elastomer’s powder can be reprocessed into a new elastomer material when the elastomer is crushed. This means that this elastomer could be recycled. The tensile strength of the new elastomer can still reach up to <sc>1.48 MPa</sc> and the elongation at break is 228% after three times of recycling. This data exceeds the standard of 3165 Fast Tack RTV Adhesive Sealant of Dow Chemical Company which are <sc>0.9 MPa</sc> and 185%, respectively. This means that this kind of material can still be used as sealant after three times vulcanization. Further analysis show that there are a lot of excess rare earth ions and nitrile groups which are not coordinated in the elastomer. When the elastomer is crushed and reused, these excess nitrile group and rare earth ions can participate in the coordination process and form new crosslinks, achieving high mechanical properties and multiple recycling properties. If these elastomers are to be recycled for more than four times, it is necessary to optimize the molecular design to make up for the adverse effect of polysiloxane main chain fracture. This research work is expected to provide a reference for the development of materials with high mechanical properties and multiple recycling properties.

Surface functionalization of cellulose fibers via aza-Michael addition for CO2-assisted water remediation

The remediation of water that polluted by organic dyes and nanoparticles (NPs) remains a great challenge. However, second pollution of solid waste may produce if the adsorbents are not degradable or recyclable. The stimuli-responsive materials were regarded to be able to resolve this problem by reversible adsorption-desorption under stimuli. Here in this work, the surface of cellulose fibers was modified with CO2-responsive molecules through a simple esterification followed by an aza-Michael addition reaction. The chemical structure was confirmed with Fourier transform infrared spectra (FT-IR), X-ray photoelectron spectroscopy (XPS), x-ray diffractometer (XRD) and thermal analysis. The gas-responsiveness was demonstrated by monitoring of the pH, conductivity and surface zeta potential. Anionic dyes and negative charged NPs in water can be selectively and reversibly captured by the CO2-responsive surface on cellulose fibers, suggesting the inexpensive, readily available and smart cellulose fibers may find great potential in water remediation.

Silicone Polymers—Celebrating 80 Years of the Direct Process

Silicone Polymers—Celebrating 80 Years of the Direct Process

Four-armed functional siloxane enables ratiometric unconventional fluorescence for the detection of ONOO−

Abstract Unconventional luminogens have been extensively exploited due to their environmentally friendly, fine biocompatibility, and less toxicity. Peroxynitrite (ONOO−) represents one of the most significant reactive oxygen species (ROS) in live samples. However, unconventional fluorescence has not been employed to detect ONOO− yet. In this report, unconventional fluorescence red-shift was first applied to monitor the level changes of ONOO− in live systems. A novel four-armed siloxane (Si-B1) was synthesized via a “one-pot” and noncatalytic aza-Michael reaction. Upon reaction of Si-B1 with ONOO−, the hydrolysis of the ester group changed the aggregation state of Si-B1, thus, induced a fluorescence red-shift fluorescent response, which provided a selective and sensitive ratiometric approach for the recognize of ONOO−. Moreover, the applications of Si-B1 for imaging the ONOO− were performed in living cells and zebrafish. Therefore, this report carved out a new way to fabricate the unconventional fluorescent probes for the recognize of ONOO−.

Enantioselective Total Synthesis of (+)-Stephadiamine through Bioinspired Aza-Benzilic Acid Type Rearrangement.

We report the first enantioselective total syntheses of the hasubanan alkaloid (-)-metaphanine and the norhasubanan alkaloid (+)-stephadiamine. Key features of these syntheses include diastereoselective oxidative phenolic coupling reaction and subsequent regioselective intramolecular aza-Michael reaction, which efficiently construct the hasubanan skeleton with the all-carbon quaternary stereogenic center at C13. Based on our hypothesis regarding the biosynthetic pathway of (+)-stephadiamine, we found that (-)-metaphanine is easily converted to (+)-stephadiamine via aza-benzilic acid type rearrangement reaction.

Reactivity of 5-phenyl-3-[(2-chloroquinolin-3-yl)methylene] furan-2(3H)-one towards hydrazine and benzylamine: A comparative study

The reactivity of a 2-chloroquinolinylfuranone derivative 3 was investigated against two nitrogen nucleophilic reagents namely, hydrazine and benzylamine. It was found that the regioselectivity of the reaction products was mainly dependent on the nature of nucleophiles, reaction conditions, and solvent used. Therefore, hydrazinolysis of 3 afforded the corresponding acid hydrazide and pyridazinone derivatives depending on the reaction conditions. Otherwise, benzylamine reacted with 3 at different reaction aspects to provide N-benzylamide, N-benzylpyrrolone, and 2-benzylaminoquinolinyl-N-benzylpyrrolone derivatives. The chemical structures of all synthesized compounds were substantiated from their analytical as well as spectroscopic data. Based on the charge density calculations and computational chemical study which excluded the aza-Michael addition reaction and confirm the higher electron-deficiency of lactone carbonyl group than C2-quinoline position, it can be concluded that the C2-furanone becomes more susceptible to attack by the nucleophilic reagent, hydrazine and benzylamine.

Morphology Control and Metallization of Porous Polymers Synthesized by Michael Addition Reactions of a Multi-Functional Acrylamide with a Diamine.

Porous polymers have been synthesized by an aza-Michael addition reaction of a multi-functional acrylamide, N,N′,N″,N‴-tetraacryloyltriethylenetetramine (AM4), and hexamethylene diamine (HDA) in H2O without catalyst. Reaction conditions, such as monomer concentration and reaction temperature, affected the morphology of the resulting porous structures. Connected spheres, co-continuous monolithic structures and/or isolated holes were observed on the surface of the porous polymers. These structures were formed by polymerization-induced phase separation via spinodal decomposition or highly internal phase separation. The obtained porous polymers were soft and flexible and not breakable by compression. The porous polymers adsorbed various solvents. An AM4-HDA porous polymer could be plated by Ni using an electroless plating process via catalyzation by palladium (II) acetylacetonate following reduction of Ni ions in a plating solution. The intermediate Pd-catalyzed porous polymer promoted the Suzuki-Miyaura cross coupling reaction of 4-bromoanisole and phenylboronic acid.

Kinetics and mechanisms of non-isothermal polymerization of N,N′-bismaleimide-4,4′-diphenylmethane with cyanuric acid

Mechanisms and kinetics of non-isothermal polymerization of N,N′-bismaleimide-4,4′-diphenylmethane (BMI) with cyanuric acid (CA) were investigated. Competitive free radical polymerization and aza-Michael addition reaction mechanisms involved in the BMI/CA reaction system were characterized by using DSC, 1H-NMR, electron spin resonance (ESR) and gel permeation chromatography (GPC). Free radical polymerization mechanism predominated in the BMI/CA reaction system. Both model-free and model-fitting methods were used to determine the isoconversional activation energy (Eα) and pre-exponential factor (Aα). The values of Eα and Aα for aza-Michael addition reaction evaluated at α = 0.1 were 86 kJ mol−1 and 2.42 × 1010 min−1, respectively, and the reaction model was f(α) = 1.73(1 - α)α0.11. By contrast, Eα gradually decreased with increasing α (decreased from 88 to 48 kJ mol−1) in the α range 0.15−0.9, which was attributed to the inherent multi-step kinetics in free radical polymerization.

Double asymmetric intramolecular aza-Michael reaction: a convenient strategy for the synthesis of quinolizidine alkaloids.

A new methodology to access the quinolizidine skeleton in an asymmetric fashion was devised. It involves two consecutive intramolecular aza-Michael reactions of sulfinyl amines bearing a bis-enone moiety, in turn generated by a monodirectional cross metathesis reaction. The sequence, which takes place with excellent yields and diastereocontrol, was applied to the total synthesis of alkaloids lasubine I and myrtine.

A bifunctional iminophosphorane squaramide catalyzed enantioselective synthesis of hydroquinazolines via intramolecular aza-Michael reaction to α,β-unsaturated esters.

An efficient synthesis of enantioenriched hydroquinazoline cores via a novel bifunctional iminophosphorane squaramide catalyzed intramolecular aza-Michael reaction of urea-linked α,β-unsaturated esters is described. The methodology exhibits a high degree of functional group tolerance around the forming hydroquinazoline aryl core and wide structural variance on the nucleophilic N atom of the urea moiety. Excellent yields (up to 99%) and high enantioselectivities (up to 97 : 3 er) using both aromatic and less acidic aliphatic ureas were realized. The potential industrial applicability of the transformation was demonstrated in a 20 mmol scale-up experiment using an adjusted catalyst loading of 2 mol%. The origin of enantioselectivity and reactivity enhancement provided by the squaramide motif has been uncovered computationally using density functional theory (DFT) calculations, combined with the activation strain model (ASM) and energy decomposition analysis (EDA).

Heterogeneous Aza-Michael Addition Reaction by the Copper-Based Metal–Organic Framework (CuBTC)

The copper benzene-1, 3, 5-tricarboxylate metal–organic framework (CuBTC) was found to be an effective heterogeneous catalyst for the aza-Michael addition reaction of the four types of amines to electron deficient alkenes at room temperature. The catalytic protocol showed high product yields and outstanding chemo selectivity. The cyclic amines (piperidine and pyrrolidine) and aliphatic amines (n-dibutylamine) provided aza-Michael addition with a high yield of product (⁓98%) within shorter reaction period (2 h) at room temperature under mild reaction conditions using CuBTC. However, it was observed that the aza-Michael reaction proceeded more slowly, giving 62% yield of product after 24 h in the case of aromatic amine (aniline) with n-butyl acrylate in the presence of CuBTC under identical reaction conditions. The catalyst could be reused four recycles without losing its initial catalytic activity and selectivity. XRD and SEM analysis further confirmed that the crystallinity of catalyst was retained during the reaction. A reaction mechanism is proposed for the aza-Michael addition reaction over heterogeneous CuBTC catalyst.

Zwitterionic Silicone Materials Derived from Aza-Michael Reaction of Amino-Functional PDMS with Acrylic Acid.

Supramolecular zwitterionic silicones are synthesized by aza-Michael reaction between acrylic acid and amine-functional polydimethylsiloxanes. The in-depth characterization of this chemistry, applied for the first time to silicones, is investigated first with model alkylamines (hexylamine, 2-ethylhexylamine and N-propylethylenediamine), a model oligosiloxane (3-aminopropylmethyl bis(trimethylsiloxy)silane), and finally various amino-polysiloxanes. It is shown that after a first acid-base reaction resulting in ionic pairing, aza-Michael addition proceeds smoothly in mild conditions (50°C, 1-week reaction). Both monoadducts and di-adducts, together with residual amine, are observed by NMR. The supramolecular assembly of the thus-created zwitterionic moieties is highlighted by a concomitant increase in viscosity and phase separation, as observed by transmission electron microscopy, bringing an additional glass transition at -40°C assigned to highly polar ionic clusters. Below the stoichiometry in acrylic acid, all zwitterionic silicones follow the same classical behavior of nonentangled polymers according to the Rouse model, whereas upon introducing an excess of acrylic acid to amino groups, an enhancement of the elasticity is observed. Finally, silicone elastomers with solid-like behavior and elastomeric mechanical properties are obtained using a high molar mass polymer bearing bifunctional N-(2-aminoethyl)-3-aminopropyl units that favor a high degree of physical crosslinking.

Benefits of time-resolved nonuniform sampling in reaction monitoring: The case of aza-Michael addition of benzylamine and acrylamide.

Monitoring of chemical reactions is best carried out using methods that sample the test object at a rate greater than the time scale of the processes taking place. The recently proposed time-resolved nonuniform sampling (TR-NUS) method allows the use of two-dimensional (2D) nuclear magnetic resonance (NMR) spectra for this purpose and provides a time resolution equivalent to that achievable using one-dimensional spectra. Herein, we show that TR-NUS acquired data eliminates 2D spectral line disturbances and enables more accurate signal integration and kinetics conclusions. The considerations are exemplified with a seemingly simple aza-Michael reaction of benzylamine and acrylamide. Surprisingly, the product identification is possible only using 2D spectra, although credible monitoring requires TR-NUS.

Synthesis of Network Polymers by Means of Addition Reactions of Multifunctional-Amine and Poly(ethylene glycol) Diglycidyl Ether or Diacrylate Compounds.

Addition reactions of multi-functional amine, polyethylene imine (PEI) or diethylenetriamine (DETA), and poly(ethylene glycol) diglycidyl ether (PEGDE) or poly(ethylene glycol) diacrylate (PEGDA), have been investigated to obtain network polymers in H2O, dimethyl sulfoxide (DMSO), and ethanol (EtOH). Ring opening addition reaction of the multi-functional amine and PEGDE in H2O at room temperature or in DMSO at 90 °C using triphenylphosphine as a catalyst yielded gels. Aza-Michael addition reaction of the multi-functional amine and PEGDA in DMSO or EtOH at room temperature also yielded corresponding gels. Compression test of the gels obtained with PEI showed higher Young’s modulus than those with DETA. The reactions of the multi-functional amine and low molecular weight PEGDA in EtOH under the specific conditions yielded porous polymers induced by phase separation during the network formation. The morphology of the porous polymers could be controlled by the reaction conditions, especially monomer concentration and feed ratio of the multi-functional amine to PEGDA of the reaction system. The porous structure was formed by connected spheres or a co-continuous monolithic structure. The porous polymers were unbreakable by compression, and their Young’s modulus increased with the increase in the monomer concentration of the reaction systems. The porous polymers absorbed various solvents derived from high affinity between the polyethylene glycol units in the network structure and the solvents.

Exploring the acidic catalytic role of differently structured deep eutectic solvents in the aza-Michael addition of amines to 2-vinylpiridine

The search for innovative and green reaction media is a prominent topic in the recent research in chemistry. Deep Eutectic Solvents (DESs) are currently gaining relevance in this field thanks to their unique properties in terms of their “greenness” as well as in terms of their catalytic properties. In this work we developed an efficient protocol for the conjugate aza-Michael addition of amines to 2-vinylpyridine using 14 differently structured acid DESs as both reaction media and catalysts, so preventing the use of other acid additives. The results are influenced by the acidity of the components of the DESs, showing the best results with weak acids in the DESs components, with isolated yields up to 86%. The aza-Michael reaction is a widely used synthetic route for the C–N bond formation in organic synthesis. In particular, the addition of amines to 2-vinylpyridine is used for the realization of pharmaceutically relevant compounds with anticancer, antiarrhythmic, and analgesic properties. Comparing the results with the ones observed in literature for the same reaction, this procedure revealed to be a convenient and efficient method for this relevant transformation.