From the N‐Heterocyclic Carbene‐Catalyzed Conjugate Addition of Alcohols to the Controlled Polymerization of (Meth)acrylates

Among various N‐heterocyclic carbenes (NHCs) tested, only 1,3‐bis(tert‐butyl)imidazol‐2‐ylidene (NHC^tBu) proved to selectively promote the catalytic conjugate addition of alcohols onto (meth)acrylate substrates. This rather rare example of NHC‐catalyzed 1,4‐addition of alcohols was investigated as a simple means to trigger the polymerization of both methyl methacrylate and methyl acrylate (MMA and MA, respectively). Well‐defined α‐alkoxy poly(methyl (meth)acrylate) (PM(M)A) chains, the molar masses of which could be controlled by the initial [(meth)acrylate]₀/[ROH]₀ molar ratio, were ultimately obtained in N,N‐dimethylformamide at 25 °C. A hydroxyl‐terminated poly(ethylene oxide) (PEO‐OH) macro‐initiator was also employed to directly access PEO‐b‐PMMA amphiphilic block copolymers. Investigations into the reaction mechanism by DFT calculations revealed the occurrence of two competitive concerted pathways, involving either the activation of the alcohol or that of the monomer by NHC^tBu.     

Cyclodimerization versus Polymerization of Methyl Methacrylate Induced by N‐Heterocyclic Carbenes: A Combined Experimental and Theoretical Study

The activation behavior of two N‐heterocyclic carbenes (NHCs), namely, 1,3‐bis(isopropyl)imidazol‐2‐ylidene(NHCiPr) and 1,3‐bis(tert‐butyl) imidazol‐2‐ylidene (NHCtBu), as organic nucleophiles in the reaction with methyl methacrylate (MMA) is described. NHCtBu allows the polymerization of MMA in DMF at room temperature and in toluene at 50 °C, whereas NHCiPr reacts with two molecules of MMA, forming an unprecedented imidazolium–enolate cyclodimer (NHCiPr/MMA=1:2). It is proposed that the reaction mechanism occurs by initial 1,4‐nucleophilic addition of NHCiPr to MMA, generating a zwitterionic enolate 2 , followed by addition of 2 to a second MMA molecule, forming a linear imidazolium–enolate 3 (NHCiPr/MMA=1:2). Proton transfer, generating intermediate 5 , followed by cyclization and release of methanol yielded the aforementioned zwitterionic cyclodimer 1:2 adduct 7 , the molecular structure of which has been established by NMR spectroscopy, X‐ray diffraction, and mass spectrometry. This unexpected difference between NHCtBu and NHCiPr in the reaction with MMA (polymerization and cyclodimerization, respectively) can be rationalized by using DFT calculations. In particular, the nature of the NHC strongly influences the cyclodimerization pathway, the cyclization of 5 and the release of methanol are the discriminating step and limiting step, respectively. In the case of NHCtBu, both steps are strongly disfavoured compared with that of NHCiPr (energetic difference of around 14 and 9 kcal mol^?1, respectively), preventing the cyclization mechanism from a kinetic viewpoint. Moreover, addition of a third molecule of MMA in the polymerization pathway results in a lower activation barrier than that of the limiting step in the cyclodimerization pathway (difference of around 14 kcal mol^?1), in agreement with the formation of polymethyl methacrylate (PMMA) by using NHCtBu as nucleophile.     

Zeta and <Emphasis Type="Italic">L</Emphasis>-functions of finite quotients of apartments and buildings

In this paper, we study relations between Langlands L-functions and zeta functions of geodesic walks and galleries for finite quotients of the apartments of G =PGL₃ and PGSp₄ over a nonarchimedean local field with q elements in its residue field. They give rise to an identity (Theorem 5.3) which can be regarded as a generalization of Ihara’s theorem for finite quotients of the Bruhat–Tits trees. This identity is shown to agree with the q = 1 version of the analogous identities for finite quotients of the building of G established in [KL14, KLW10, FLW13], verifying the philosophy of the field with one element by Tits. A new identity for finite quotients of the building of PGSp₄ involving the standard L-function (Theorem 6.3), complementing the one in [FLW13] which involves the spin L-function, is also obtained.     

Quality evaluation of Rhizoma Belamcandae (Belamcanda chinensis (L.) DC.) by using high-performance liquid chromatography coupled with diode array detector and mass spectrometry

A high-performance liquid chromatography coupled with diode array detector and mass spectrometry (HPLC-DAD-MS) method was developed to evaluate the quality of Rhizoma Belamcandae (Belamcanda chinensis (L.) DC.) through establishing chromatographic fingerprint and simultaneous determination of seven phenolic compounds. The analysis was achieved on an Alltima C(18) analytical column (250 mm x 4.6 mm i.d. 5 microm) using linear gradient elution of acetonitrile-0.1% trifluoroacetic acid. The correlation coefficients of similarity were determined from the HPLC fingerprints, and they shared a close similarity. By using an online APCI-MS/MS, twenty phenols were identified. In addition, seven of these phenols including mangiferin, 7-O-methylmangiferin, tectoridin, resveratrol, tectorigenin, irigenin and irisflorentin were quantified by the validated HPLC-DAD method. These phenols are considered to be major constituents in Rhizoma Belamcandae, and are generally regarded as the index for quality assessment of this herb. This developed method by having a combination of chromatographic fingerprint and quantification analysis could be applied to the quality control of Rhizoma Belamcandae.     

High surface area of porous silicon drives desorption of intact molecules

The surface structure of porous silicon used in desorption/ionization on porous silicon (DIOS) mass analysis is known to play a primary role in the desorption/ionization (D/I) process. In this study, mass spectrometry and scanning electron microscopy (SEM) are used to examine the correlation between intact ion generation with surface ablation and surface morphology. The DIOS process is found to be highly laser energy dependent and correlates directly with the appearance of surface ions (Si(n)(+) and OSiH(+)). A threshold laser energy for DIOS is observed (10 mJ/cm(2)), which supports that DIOS is driven by surface restructuring and is not a strictly thermal process. In addition, three DIOS regimes are observed that correspond to surface restructuring and melting. These results suggest that higher surface area silicon substrates may enhance DIOS performance. A recent example that fits into this mechanism is the surface of silicon nanowires, which has a high surface energy and concomitantly requires lower laser energy for analyte desorption.     

Novel 99mTc labeled σ receptor ligand as a potential tumor imaging agent

A novel ^99mTc labeled complex, [N-[2-((2-oxo-2-(4-(3-phenylpropyl)piperazin-1-yl)ethyl) (2-mercaptoethyl)amino)acetyl]-2-aminoethanethiolato]Technetium(V) oxide (PPPE-MAMA’-^99mTcO) ([ ^99m Tc]-2) has been designed and prepared based on the integrated approach. The corresponding rhenium complex (PPPE-MAMA’-ReO)(Re-2) has been prepared and characterized. In vitro competition binding assays show moderate affinity of Re-2 towards σ₁ and σ₂ receptors with K _i values of 8.67 ± 0.07 and 5.71 ± 1.88 μmol, respectively. Planar images obtained at 0.5 h, 4 h, 20 h after i.v. injection indicate the accumulation of [ ^99m Tc]-2 in MCF-7 human breast tumor bearing mice at 20 h. Furthermore, the accumulation of [ ^99m Tc]-2 has been inhibited at 20 h after co-injection of [ ^99m Tc]-2 plus haloperidol (1 mg/kg). Biodistribution studies of [ ^99m Tc]-2 display an in vivo tumor uptake of 0.14% ± 0.01% ID/g at 24 h post i.v. injection with a tumor/muscle ratio of 6.02 ± 0.87. The above results suggest that [ ^99m Tc]-2, derived from a previously published lead compound, retains certain tumor uptake and affinity for σ receptors. [ ^99m Tc]-2 may be used as a basis for further structural modifications to develop tumor imaging agents with high affinity for σ receptors.     

Chameleon Metals: Autonomous Nano-Texturing and Composition Inversion on Liquid Metals Surfaces

Studies on passivating oxides on liquid metals are challenging, in part, due to plasticity, entropic, and technological limitations. In alloys, compositional complexity in the passivating oxide(s) and underlying metal interface exacerbates these challenges. This nanoscale complexity, however, offers an opportunity to engineer the surface of the liquid metal under felicitous choice of processing conditions. We inferred that difference in reactivity, coupled with inherent interface ordering, presages exploitable order and selectivity to autonomously present compositionally biased oxides on the surface of these metals. Besides compositional differences, sequential release of biased (enriched) components, via fractal-like paths, allows for patterned layered surface structures. We, therefore, present a simple thermal-oxidative compositional inversion (TOCI) method to introduce fractal-like structures on the surface of these metals in a controlled (tier, composition, and structure) manner by exploiting underlying stochastic fracturing process. Using a ternary alloy, a three-tiered (in structure and composition) surface structure is demonstrated.     

Structure and Shape Memory Properties of Polyurethane Copolymers Having Urethane Chains as Soft Segments

Shape memory polyurethanes are usually fabricated with low-molecular weight polyols through a two-step copolymerization, which often results in difficulty attaining both desired shape memory switch temperature and optimal thermomechanical properties. Here we present a series of shape memory polyurethane copolymers having urethane chains as soft segments. The structure and shape memory properties of copolymers were investigated with differential scanning calorimetry, dynamic mechanical analysis, small angle x-ray scattering, and thermomechanical tests. Increasing the length of the urethane soft segments enhanced phase separation, while it brought little change to the glass transition temperature (T _g). Based on the urethane soft segments, some rigid chain extenders could be readily introduced into the backbone of copolymers, resulting in better phase separation. All polyurethane copolymers exhibited more than 90% of shape recovery. The shape recovery of the materials was proved to be inversely proportional to the fraction of hard phase and directly proportional to the stability of hard domains. The copolymers containing longer soft and hard segments and rigid chain extenders exhibited higher deformation stress and thus larger recovery stress. The copolymerization employing urethane chains as soft segments can greatly expand flexibility for molecular design and favor the optimization of shape memory properties.