1‐(4,6‐Diethoxy­pyrimidin‐2‐yl)‐2‐thioxo‐2,3‐dihydro­pyrido[2,3‐d]pyrimidin‐4(1H)‐one

In the title compound, C₁₅H₁₅N₅O₃S, two parallel inter­molecular N—H⋯S hydrogen bonds, forming an eight‐membered ring, link two mol­ecules into a dimer unit; these dimer units linked into a chain of edge‐fused rings by weak C—H⋯O hydrogen bonds.     

Opposing Phase‐Segregation and Hydrogen‐Bonding Forces in Supramolecular Polymers

Phase segregation between different macromolecules and specific weak interactions are the basis of molecular organization in many biological systems, which are held together by attractive hydrogen bonds (H‐bonds) and dissociated by phase segregation. We report significant changes in the association behavior of covalent H‐bonds by the phase of attached polymer chains. Depending on the aggregation state, we observed either intact H‐bonds despite segregation of the phases, or macrophase separation with a larger amount of H‐bonding dissociation.     

A Metallosupramolecular Shape‐Memory Polymer with Gradient Thermal Plasticity

Solid‐state plasticity by dynamic covalent bond exchange in a shape‐memory polymer network bestows a permanent shape reconfiguration ability. Spatio‐selective control of thermally induced plasticity may further extend the capabilities of materials into unexplored domains. However, this is difficult to achieve because of the lack of spatio‐control in typical polymer network synthesis. Metal–ligand interactions possess the high strength of covalent bonds while maintaining the dynamic reversibility of supramolecular bonds. Metallosupramolecular shape‐memory polymer networks were designed and prepared, which demonstrated solid‐state plasticity. The metallo‐coordination bonds within these networks permit facile tuning of the plasticity behavior across a wide temperature range, simply by changing the metal ion. By controlling the diffusion of two different metal ions during preparation of a polymer film, a plasticity behavior with a spatial gradient was achieved, providing a unique shape‐morphing versatility with potential in shape‐memory devices.     

CO2‐Cross‐Linked Frustrated Lewis Networks as Gas‐Regulated Dynamic Covalent Materials

The design of structurally dynamic molecular networks can offer strategies for fabricating stimuli‐responsive adaptive materials. Herein we first report a gas‐responsive dynamic gel system based on frustrated Lewis pair (FLP) chemistry. Two trefoil‐like molecules with bulky triphenylborane and triphenylphosphine groups are synthesized as complementary Lewis acid and base with trivalent sites. They can together bind CO₂ gas molecules and further form a cross‐linked network via the bonding interactions between FLPs and CO₂. Such CO₂‐bridged dative linkages are shown to be dynamic covalent bonds, which endow the frustrated Lewis network with adaptable behaviors and unprecedented gas‐regulated viscoelastic, mechanical, and self‐healing performance. This study is an initial attempt to apply the FLP concept in materials chemistry, but we believe that this strategy will open a promising future for gas‐sensitive smart materials.     

Doubly Dynamic Self‐Healing Materials Based on Oxime Click Chemistry and Boronic Acids

The dynamic covalent characteristics of oxime and boronate ester bonds have been explored. A small excess of a competing aldehyde under acidic conditions resulted in oxime polymer degradation from high molecular weights (30 kDa) to low molecular weight oligomers (2.2 kDa). The dynamic nature of oxime bonds imparts oxime cross‐linked hydrogels with self‐healing properties and the incorporation of phenyl boronic acid groups into the hydrogel network provides a platform for hydrogel functionalization. The addition of a polyphenol (tannic acid) proves a facile means to incorporate a second, dynamic covalent cross‐linking network through boronate ester formation which, owing to the increase in the degree of cross‐linking, is found to be nearly double the hydrogel strength (storage modulus increased from 4.6 to 8.5 kPa). Finally, the tannic acid cross‐linking network is selectively degraded returning the hydrogel storage modulus to its initial value and providing a means for the synthesis of materials with tunable mechanical properties.

     

N‐Substituted Dicyanomethylphenyl Radicals: Dynamic Covalent Properties and Formation of Stimuli‐Responsive Cyclophanes by Self‐Assembly

Dynamic covalent bonds and their chemistry have been of particular interest both from a fundamental and materials science aspect. Demonstrated herein is that triphenylamine (TPA) and carbazole (Cz), substituted with a dicyanomethyl radical, are useful motifs for dynamic covalent chemistry as they have the appropriate bond strength between monomer units as well as high stability and synthetic simplicity. TPA and Cz units substituted by two dicyanomethyl radicals formed macrocyclic oligomers classified as novel types of azacyclophanes, and in particular, the TPA‐based diradical gave a cyclic dimer in almost quantitative yield. The cyclic oligomers exhibited thermo‐ and mechanochromic behavior resulting from the generation of radical species by intermonomer C?C bond cleavage.     

Preparation and properties of 1,2‐polybutadiene grafting with poly(1,3‐butadiene)‐block‐(dimethylsiloxane)

The development of silica‐filling elastomers with high mechanical performance and good processability is still a great challenge. In this study, we fabricated siloxane‐grafted atactic 1,2‐polybutadiene (1,2‐PB) rubber through grafting poly(1,3‐butadiene)‐block‐(dimethylsiloxane) (PB‐b‐PDMS) onto 1,2‐PB molecular chains by coordination polymerization using a molybdenum (Mo)‐based catalyst system. The PB‐b‐PDMS with active double bonds was synthesized by anionic polymerization. Fourier transform infrared analysis (FTIR), elementary analysis, and GPC‐MALLS‐viscometer analyses verified the incorporation of PB‐b‐PDMS and the grafting structure in the resulting polymer. Scanning electron microscope (SEM), bound rubber testing, and dynamic mechanical analysis demonstrated that the graft‐modification with PB‐b‐PDMS improved silica dispersity in the 1,2‐PB matrix because the incorporation of siloxane groups provided stronger interfacial interaction with silica. Meanwhile, the graft‐modified 1,2‐PB exhibited lower Mooney viscosity, higher tensile strength, and lower heat build‐up than unmodified 1,2‐PB. This concept provides novel inspiration for the preparation of advanced rubber with promoted silica compatibility and mechanical performance.     

Chemical Reduction and Dimerization of 1‐Chloro‐2,3,4,5‐tetraphenylborole

As neutral isoelectronic analogues of the elusive cyclopentadienyl cation, boroles have been of interest for their prospective applications as strong Lewis acids, chromophores, and electron acceptors. Recently our group discovered a π‐nucleophilic boryl anion based on the borole system. In an effort to extend borole chemistry, we now report the molecular structure of 1‐chloro‐2,3,4,5‐tetraphenylborole ( 1 ) and its corresponding borole dianion resulting from the two‐electron reduction of 1 with KC₈. The thermally induced dimerization of 1 yields an unprecedented boracyclohexadiene/borolene spiro‐bicyclic compound and the resulting dimer was fully characterized including a single‐crystal X‐ray analysis.     

Comb‐like polymers pendanted with elastin‐like peptides showing sharp and tunable thermoresponsiveness through dynamic covalent chemistry

Comb‐like polymers carrying two elastin‐like polypeptide (ELP) pendants in each repeat unit were synthesized. The densely attached peptide chains afford these polymers with sharp thermally induced phase transitions, and their lower critical solution temperature (LCST) can be varied with molecular weights, solution pH and salt concentrations. Through amino terminals in ELP pendants, oligoethylene glycol (OEG)‐based dendrons cored with aldehyde were attached to the polymers through dynamic covalent imines. By virtue of dynamic characteristics of these novel dendronized polymers, their LCSTs can be tuned significantly by dendron coverage to shift from that dominated by ELPs to that dominated by OEG dendrons. Furthermore, dendron coverage can be enhanced obviously by the thermally induced phase transitions or greatly by freezing the polymer aqueous solutions. The work provides a convenient methodology to improve thermoresponsiveness of ELPs through polymer topology and to switch their properties through dynamic covalent chemistry. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3379–3387     

“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.     

The nido‐Cage⋅⋅⋅π Bond: A Non‐covalent Interaction between Boron Clusters and Aromatic Rings and Its Applications

Non‐covalent interactions involving multicenter multielectron skeletons such as boron clusters are rare. Now, a non‐covalent interaction, the nido‐cage???π bond, is discovered based on the boron cluster C₂B₉H₁₂^? and an aromatic π system. The X‐ray diffraction studies indicate that the nido‐cage???π bonding presents parallel‐displaced or T‐shaped geometries. The contacting distance between cage and π ring varies with the type and the substituent of the aromatic ring. Theoretical calculations reveal that this nido‐cage???π bond shares a similar nature to the conventional anion???π or π???π bonds found in classical aromatic ring systems. This nido‐cage???π interaction induces variable photophysical properties such as aggregation‐induced emission and aggregation‐caused quenching in one molecule. This work offers an overall understanding towards the boron cluster‐based non‐covalent bond and opens a door to investigate its properties.     

On the Electrochemistry and Spectroelectrochemistry of Small Model Star‐Shaped Compounds: 1,3,5‐Triaryl‐1‐Methoxybenzenes and 2,4,6‐Triaryl‐1,3,5‐Trimethoxybenzenes

Model structures of 1,3,5‐triarylbenzenes with a substituted benzene core linked to thienyl or 3,4‐ethylenedioxythienyl (EDOT) terminal groups are studied by electrochemical and in situ ESR/UV/Vis/NIR spectroelectrochemical techniques. Oxidative polymerization of the monomers results in C? C coupling of the thiophene moieties in the 5‐position, forming dimeric structures with bithiophene linkers as the first step. Both the doubly charged protonated dimer and the new dimer formed after proton release are studied in detail for 2,4,6‐tris[2‐(3,4‐ethylenedioxythienyl)]‐1‐methoxybenzene. Quite high stability of the doubly charged σ dimer formed on oxidation with unusual redox behavior at the electrode is observed. Density functional calculations of the molecular structure as well as spectroscopic and electronic properties of charged states in 1,3,5‐triarylbenzene derivatives in the monomeric, dimeric, and oligomeric form are presented. The complex spectroelectrochemical response of a thin solid film formed on the electrode surface upon potentiodynamic polymerization indicates the existence of different charge states of oligomeric structures within the solid matrix.     

Covalently Binding of Bovine Serum Albumin to Unsaturated Poly(Globalide‐Co‐ε‐Caprolactone) Nanoparticles by Thiol‐Ene Reactions

When nanoparticles (NPs) are introduced to a biological fluid, different proteins (and other biomolecules) rapidly get adsorbed onto their surface, forming a protein corona capable of giving to the NPs a new “identity” and determine their biological fate. Protein–nanoparticle conjugation can be used in order to promote specific interactions between living systems and nanocarriers. Non‐covalent conjugates are less stable and more susceptible to desorption in biological media, which makes the development of engineered nanoparticle surfaces by covalent attachment an interesting topic. In this work, the surface of poly(globalide‐co‐ε‐caprolactone) (PGlCL) nanoparticles containing double bonds in the main polymer chain is covalently functionalized with bovine serum albumin (BSA) by thiol‐ene chemistry, producing conjugates which are resistant to dissociation. The successful formation of the covalent conjugates is confirmed by flow cytometry (FC) and fluorescence correlation spectroscopy (FCS). Transmission electron microscopy (TEM) allows the visualization of the conjugate formation, and the presence of a protein layer surrounding the NPs can be observed. After conjugation with BSA, NPs present reduced cell uptake by HeLa and macrophage RAW264.7 cells, in comparison to uncoated NP. These results demonstrate that it is possible to produce stable conjugates by covalently binding BSA to PGlCL NP through thiol‐ene reaction.     

Specific Intermolecular Interactions in the Supramolecular Structure of 5‐Hydroxy‐6‐Methyluracil: A DFT Study of the Hydrogen‐bonded Dimers

Studying the self‐assembly of uracil derivatives has great importance for biochemistry and nanotechnology. For example, modification of the sorbent surfaces by 5‐hydroxy‐6‐methyluracil (HMU ) enhances their adsorption activity. It is assumed that these changes are caused by the self‐assembly of the network‐like supramolecular associates of the uracil derivative on the sorbent surface. In the present work, the relative stabilities of 15 hydrogen‐bonded dimers HMU have been studied by the TPSSh /TZVP density functional theory method and the strengths of the noncovalent interactions analyzed in terms of the reduced density gradient and natural bond orbital approaches. It was found that the symmetric dimer stabilized by two intermolecular hydrogen bonds N1 –H∙∙∙O–C2 (dimer 1‐1) is the most stable. This suggests that the self‐assembly of HMU should occur through the intermediate formation of the dimer 1‐1. The results may be useful for understanding the processes of self‐assembly of the uracil derivatives and the rationalized design of the uracil‐based supramolecular structures with specific properties.     

Synthesis of a Bis‐Urea Dimer and Its Effects on the Physical Properties of an Amphiphilic Tris‐Urea Supramolecular Hydrogel

The successful development of stiff supramolecular gels is an important goal toward their practical application. One approach to stiffen supramolecular gels is to introduce covalent cross‐links. The bis‐urea dimer 2 , having a structure similar to that of the low‐molecular‐weight gelator 1 , was synthesized. Supramolecular hydrogels were formed from mixtures of 1 and 2 in appropriate ratios, with 2 acting as a covalent cross‐linker to connect the fibrous aggregates formed by the self‐assembly of 1 . The introduction of these covalent cross‐links greatly influenced the dynamic viscoelasticity of the supramolecular hydrogels. In the supramolecular hydrogel of 1 mixed with 5 % 2 , the storage modulus was 1.35 times higher than that of the supramolecular hydrogel of 1 alone, and the crossover strain was extended from 5 % to over 20 %. The supramolecular hydrogel of 1 and 2 was free‐standing and supported 13 times its own weight.     

Comparing Ullmann Coupling on Noble Metal Surfaces: On‐Surface Polymerization of 1,3,6,8‐Tetrabromopyrene on Cu(111) and Au(111)

The on‐surface polymerization of 1,3,6,8‐tetrabromopyrene (Br₄Py) on Cu(111) and Au(111) surfaces under ultrahigh vacuum conditions was investigated by a combination of scanning tunneling microscopy (STM), X‐ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. Deposition of Br₄Py on Cu(111) held at 300 K resulted in a spontaneous debromination reaction, generating the formation of a branched coordination polymer network stabilized by C?Cu?C bonds. After annealing at 473 K, the C?Cu?C bonds were converted to covalent C?C bonds, leading to the formation of a covalently linked molecular network of short oligomers. In contrast, highly ordered self‐assembled two‐dimensional (2D) patterns stabilized by both Br?Br halogen and Br?H hydrogen bonds were observed upon deposition of Br₄Py on Au(111) held at 300 K. Subsequent annealing of the sample at 473 K led to a dissociation of the C?Br bonds and the formation of disordered metal‐coordinated molecular networks. Further annealing at 573 K resulted in the formation of covalently linked disordered networks. Importantly, we found that the chosen substrate not only plays an important role as catalyst for the Ullmann reaction, but also influences the formation of different types of intermolecular bonds and thus, determines the final polymer network morphology. DFT calculations further support our experimental findings obtained by STM and XPS and add complementary information on the reaction pathway of Br₄Py on the different substrates.     

Synthesis of Optically Active 2‐Amino‐1,3,4‐oxadiazoles and their Hybrid Peptides

Synthesis of 2‐amino‐1,3,4‐oxadiazole derivatives of N^α‐Cbz(benzyloxycarbonyl)/Boc‐protected amino/peptide acids under sonication is described. The conditions involved in the present protocol are simple, mild, and racemization free. The utility of 2‐amino group in the substituted oxadiazoles for the incorporation of peptide and ureido bonds to obtain hybrid peptidomimetics is also delineated. The 2‐amino‐1,3,4‐oxadiazole 3b was obtained as a single crystal, and its molecular structure has been confirmed through X‐ray crystallographic study.     

{2,2′‐[1,1′‐(4‐Aza­heptane‐1,7‐diyl­dinitrilo)diethyl­idyne]­diphenolato}­copper(II)

The quinquedentate ligand 2,2′‐[1,1′‐(4‐aza­heptane‐1,7‐diyl­dinitrilo)diethyl­idyne]diphenol in the title compound, [Cu(C₂₂H₂₇N₃O₂)], furnishes an N₃O₂ donor set, which results in a distorted square‐pyramidal coordination; the two O and two imine N atoms lie in the basal plane, while the secondary amine N atom of the ligand occupies the axial position. The axial Cu—N bond is 0.33 Å longer than the average of the equatorial bonds, and the O atoms are trans. The symmetry of the mol­ecule is lowered by the twist–boat and chair conformations adopted by the two CuNN chelate rings. The complex contains two intra­molecular C—H⋯O inter­actions, and two mol­ecules of the complex are linked into a dimer by means of moderate N—H⋯O hydrogen bonds. Spectroscopic evidence supports the presence of hydrogen bonds.     

Molecular Pom Poms from Self‐Assembling α,γ‐Cyclic Peptides

The hierarchical self‐assembly properties of a dimer‐forming cyclic peptide that bears a nicotinic acid moiety to form molecular pom‐pom‐like structures are described. This dimeric assembly self organizes into spherical structures that can encapsulate small organic molecules owing to its porosity and it can also facilitate metal deposition on its surface directed by the pyridine moiety.