Evaluation of mono- and dipeptides as organocatalysts for enantioselective aldol reaction

Catalytic activities of (3S)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (THIQA)-based mono- and dipeptides and their l-proline analogs in asymmetric aldol reaction were investigated. THIQA-based dipeptides showed better enantioselectivity than proline analogs, whereas proline-based dipeptides gave higher yield in the aldol reaction of cyclohexanone with several aldehydes in dichloromethane at −10 °C in the presence of benzoic acid.     

Rewritable self-assembled long-period gratings in photonic bandgap fibers using microparticles

We report the demonstration of rewritable self-assembled long-period gratings in air-core photonic bandgap fibers. The long-period gratings are written by filling the air-core region of the fiber with a solution containing polystyrene microspheres. The microspheres are self-assembled into a periodic structure as the liquid inside the fiber evaporates, forming the long-period grating. The formed grating can then be easily erased for rewriting purposes by connecting the end of the fiber to a microfluidic pump through a microfluidic channel and washing out the microparticles with an appropriate liquid. The same microfluidic system can also be used to fine control the writing process of the gratings. This technique creates possibilities for writing various different filtering functions in air-core photonic bandgap fibers by varying the composition and/or the shape of the used microparticles. The presented technique is potentially applicable to solid-core photonic crystal fibers as well.     

Clean and efficient microwave-solvent-free conversion of homochiral amines, α-amino alcohols and α-amino acids to their chiral 2-substituted pyrrole derivatives

Efficient synthesis of 1,2-disubstituted homochiral pyrroles has been achieved by a two-component coupling of chloroenones and amine compounds on the surface of silica gel without any solvent under microwave irradiation.     

Developing teachers' models for assessing students' competence in mathematical modelling through lesson study

Applications and modelling have gained a prominent role in mathematics education reform documents and curricula. Thus, there is a growing need for studies focusing on the effective use of mathematical modelling in classrooms. Assessment is an integral part of using modelling activities in classrooms, since it allows teachers to identify and manage problems that arise in various stages of the modelling process. However, teachers’ difficulties in assessing student modelling work are a challenge to be considered when implementing modelling in the classroom. Thus, the purpose of this study was to investigate how teachers’ knowledge on generating assessment criteria for assessing student competence in mathematical modelling evolved through a professional development programme, which is based on a lesson study approach and modelling perspective. The data was collected with four teachers from two public high schools over a five-month period. The professional development programme included a cyclical process, with each cycle consisting of an introductory meeting, the implementation of a model-eliciting activity with students, and a follow-up meeting. The results showed that the professional development programme contributed to teachers’ knowledge for generating assessment criteria on the products, and the observable actions that affect the modelling cycle.     

Hyperbranched Polymers by Type II Photoinitiated Self‐Condensing Vinyl Polymerization

Type II photoinitiated self‐condensing vinyl polymerization for the preparation of hyperbranched polymers is explored using 2‐hydroxyethyl methacrylate (HEMA) or 2‐(dimethylamino)ethyl methacrylate (DMAEMA), and methyl methacrylate as hydrogen donating inimers and comonomer, respectively, in the presence of benzophenone and camphorquinone under UV and visible light. Upon irradiation at the corresponding wavelength, the excited photoinitiator abstracts hydrogen from HEMA or DMAEMA leading to the formation of initiating radicals. Depending on the concentration of inimers, type of the photoinitiator, and irradiation time, hyperbranched polymers with different branching densities and cross‐linked polymers are formed.

     

Quasi-phase-matched grating characterization using minimum-phase functions

Measurement of the second-harmonic power generated by a quasi-phase-matched (QPM) grating as a function of the frequency detuning parameter yields the Fourier transform (FT) magnitude of the complex nonlinear coefficient profile along the QPM device. This FT magnitude can be measured by tuning either the wavelength of the fundamental laser beam or the temperature of the QPM grating. However, the measured magnitude of the FT cannot be unambiguously inverted without the FT phase to uniquely recover the complex nonlinear coefficient profile of the QPM grating. As we demonstrate in this work, this ambiguity can be completely eliminated by placing a stronger and thinner nonlinear sample against the input or output of the QPM device of interest and measuring the detuning curve of this composite structure. By construction, the nonlinear profile of this assembly has a sharp peak due to the thinner sample, followed by the weaker, broader profile of the QPM grating, which essentially constitutes a minimum-phase function. Therefore, its FT phase can be calculated uniquely from its measured FT magnitude, for example by applying to the FT amplitude the logarithmic Hilbert transform or an iterative error-reduction algorithm. This processing then enables the full recovery of the complex nonlinear coefficient profile of the QPM device from its measured detuning curve. In this paper, we demonstrate with numerical examples that this powerful new technique can accurately recover the period, envelope, and chirp parameters of any QPM grating.     

Preparation of 1,2- and 1,4-dihydro derivatives of 6,7-dimethyl-2,3-diphenylquinoxaline <Emphasis Type="Italic">via</Emphasis> its dianion formed by reductive metallation

Treatment of 6,7-dimethyl-2,3-diphenylquinoxaline with sodium in tetrahydrofuran formed a monomeric dianion. The chemical behavior of this dianion was investigated by a variety of reagents. As the result, alkylation reactions gave 1,2-dihydro derivatives, while acylation reactions occured at 1,4-positions. Annulation of the pyrazine ring system was accomplished by treating the dianion with oligomethylene dichlorides, Cl(CH₂) _n Cl, n = 2–4. Correspondence: Seniz Kaban, Department of Chemistry, Yildiz Technical University, Davutpasa Campus, Esenler-Istanbul 34220, Turkey.     

Synthesis,characterization, and thermally activated curing of oligosiloxanes containing benzoxazine moieties in the main chain

Oligosiloxanes containing thermally curable benzoxazine units in the main chain have been synthesized. For this purpose, first the diallyl functional monomer is synthesized through the Mannich and ring closing reaction of 4,4′‐isopropylidenediphenol (bisphenol A), formaldehyde and allylamine. Hydrosilylation reaction of the resulting diallylic monomer (B‐ala) with 1,1,3,3‐tetramethyldisiloxane (TMDS) in the presence of Pt catalyst yields the corresponding poly(B‐ala‐tetramethyl disiloxane)s (PBTMDS) with the molecular weights in the range of 1800–4100 Da. The structures of the precursor diallyl monomer and the resulting polymers are confirmed by FT‐IR and ¹H NMR analysis. Curing behavior of both the monomer and polymers has also been studied by Differential Scanning Calorimetry (DSC). Flexible free standing transparent films of the oligosiloxanes are obtained by solvent casting from dichloromethane solution on Teflon plates. The films preserve shape and, to some extent, toughness after thermal curing between 100 and 180 °C. Thermal properties of the cured polymers are also investigated by thermogravimetric analysis (TGA). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 804–811, 2009     

Leaching of Celestite in Sodium Hydroxide Solutions and Kinetic Modelling

In this study, the dissolution kinetics of celestite in solutions of sodium hydroxide was investigated by batch process. The results showed that the parameters which had the greatest effect on the dissolution of celestite in sodium hydroxide solutions were reaction temperature, the concentration of sodium hydroxide and stirring speed. It was determined that the dissolution rate increased with increased stirring speed, sodium hydroxide concentration, reaction time and temperature and decreased with increasing particle size and solid-liquid ratio. The leaching process fitted the shrinking core model with diffusion through the product layer model as the rate-determining step. The activation energy of the dissolution of celestite was calculated as 62.24?kJ/mol. A semi-empirical kinetic model was obtained for dissolution of celestite in sodium hydroxide solution.