Synthesis of polybenzoxazine precursors using thiols: Simultaneous thiol–ene and ring‐opening reactions

A new class of polybenzoxazine precursors of combined molecular structure of benzoxazine in the open and ring form has been developed. Thermally curable benzoxazine networks were obtained by simultaneous photoinduced thiol‐ene and Catalytic Opening of the Lateral Benzoxazine Rings by Thiols (COLBERT). The thiol‐ene reactions were initiated by photolysis of a free radical photoinitiator, 2, 2‐dimethoxy‐2‐phenyl acetophenone (DMPA), and the competing COLBERT reaction was triggered by thiol compound, 1,2‐ethanedithiol, present in the reaction mixture. The extent of reactions was evaluated by conducting experiments both under UV irradiation and in dark using model benzoxazines. The precursor soft films (pre‐P(B‐ala‐DTE)) were prepared by irradiating solutions of diallyl functional benzoxazine (B‐ala), 1,2‐ethanedithiol and DMPA. The obtained pre‐P(B‐ala‐DTE) films were then cured through thermally activated ring opening reaction of remaining benzoxazine groups yielding a more rigid and tough film. Thermal and mechanical properties of the films were investigated by DSC, DMA and TGA and compared with a typical polybenzoxazine, P(B‐ala). © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Synthesis, UV–Vis spectra, and Hammett correlation of some novel bis(dihydropyrrolo[3,4-c]pyrazoles)

Abstract  

Twenty-nine novel bis(dihydropyrrolo[3,4-c]pyrazole) derivatives were obtained by reacting C,N-phenyl-substituted nitrilimines generated in situ from corresponding hydrazonyl chlorides with bismaleimides. The structures were elucidated by physical and spectroscopic methods [m.p., R _f, infrared (IR), ¹H nuclear magnetic resonance (NMR), ¹³C NMR, correlation spectroscopy (COSY), heteronuclear correlation (HETCOR), nuclear Overhauser effect (NOE), and high-resolution mass spectrometry (HRMS)]. Also, Hammett correlation graphs were obtained between Hammett constants σ _p and λ _max values of bis(pyrrolopyrazoles) carrying p-substituted phenyl-ring-bearing electron-withdrawing and electron-donating groups and discussed in terms of substituent effects.     

A silver nanoparticles-catalyzed efficient three-component synthesis of polysubstituted 4H-chromenylphosphonates and their antioxidant activity

Abstract

A simple and efficient protocol for one-pot three-component synthesis of structurally diverse dialkyl 2-amino-3-cyano-4H-chromen-4-ylphosphonates from the condensation of salicylaldehydes, malononitrile, and trialkyl phosphite using silver nanoparticles as catalyst in ethanol at reflux has been developed. Selected new compounds were evaluated for their antioxidant activity by free radical screening using 2,2-diphenyl-1-picrylhydrazyl (DPPH) method. Among the compounds employed, 5a, 5?b, and 5?h showed promising DPPH radical scavenging activities compared to the experimental standards at low concentration (100?µg/mL) after 24?h incubation.     

Similarity solutions for creeping flow and heat transfer in second grade fluids

The two-dimensional equations of motions for the slowly flowing and heat transfer in second grade fluid are written in cartesian coordinates neglecting the inertial terms. When the inertia terms are simply omitted from the equations of motions the resulting solutions are valid approximately for Re?1. This fact can also be deduced from the dimensionless form of the momentum and energy equations. By employing Lie group analysis, the symmetries of the equations are calculated. The Lie algebra consist of four finite parameter and one infinite parameter Lie group transformations, one being the scaling symmetry and the others being translations. Two different types of solutions are found using the symmetries. Using translations in x and y coordinates, an exponential type of exact solution is presented. For the scaling symmetry, the outcoming ordinary differential equations are more involved and only a series type of approximate solution is presented. Finally, some boundary value problems are discussed.     

Polyacrylamide cryogels by photoinitiated free radical polymerization

A novel method for the preparation of poly(acrylamide) cryogels by photoinitiated polymerization of monomeric precursors was described. A series of poly(acrylamide) cryogels were easily prepared by irradiating aqueous solutions containing acrylamide and N,N′‐methylene(bis)acrylamide as monomer and cross‐linker, respectively, in the presence of 1‐[4‐(2‐hydroxyethoxy)phenyl]‐2‐hydroxy‐2‐methyl‐1‐propane‐1‐one (Irgacure 2959) as water‐soluble photoinitiator with the help of freezing–thawing procedures. Photolysis was conducted at ?13 °C isothermally through specially designed cryostat‐integrated Rayonet merry‐go‐round photoreactor. On comparing the described photochemical method with the conventional redox counter part, the polymerization is initiated, and gelation proceeds only on external stimulation by light. This way, concomitant hydrogel formation usually observed with the redox process as a result of premature polymerization during the cooling process was prevented. The obtained cryogels exhibited superfast swelling behavior. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Collective temporal coherence for subthreshold signal encoding on a stochastic small-world Hodgkin-Huxley neuronal network

We study the collective temporal coherence of a small-world network of coupled stochastic Hodgkin-Huxley neurons. Previous reports have shown that network coherence in response to a subthreshold periodic stimulus, thus subthreshold signal encoding, is maximal for a specific range of the fraction of randomly added shortcuts relative to all possible shortcuts, p, added to an initially locally connected network. We investigated this behavior further as a function of channel noise, stimulus frequency and coupling strength. We show that temporal coherence peaks when the frequency of the external stimulus matches that of the intrinsic subthreshold oscillations. We also find that large values of the channel noise, corresponding to small cell sizes, increases coherence for optimal values of the stimulus frequency and the topology parameter p. For smaller values of the channel noise, thus larger cell sizes, network coherence becomes insensitive to these parameters. Finally, the degree of coupling between neurons in the network modulates the sensitivity of coherence to topology, such that for stronger coupling the peak coherence is achieved with fewer added short cuts.     

Photo‐Induced Cross‐Linking of Divinyl Ethers by Using Diphenyliodonium Salts With Highly Nucleophilic Counter Anions in the Presence of Zinc Halides

This study describes the use of diphenyliodonium salts with highly nucleophilic counter anions to photoinitiate the cationic cross‐linking of divinyl ethers. Both direct and indirect initiating modes are used. In the direct acting system, only a diphenyliodonium salt with a highly nucleophilic counter anion and a zinc halide are employed as initiator and activator, respectively. In the indirect systems, in addition to direct system components, photosensitive additives such as anthrecene, perylene, 2,2‐dimethoxy‐2‐phenyl acetophenone, benzophenone, and thioxanthone, which absorb the energy of the incident light and activate the iodonium salt, are used to initiate polymerization. All systems employed in this study initiated quite vigorous polymerizations.