Photoinitiated Free Radical Polymerization Using Benzoxazines as Hydrogen Donors

Summary: Free radical polymerization of methyl methacrylate (MMA) is initiated upon irradiation at λ > 350 nm in CH₂Cl₂ that contains benzoxazine (P‐a) and one of the following photosensitizers: benzophenone (BP), thioxanthone (TX), 2‐chlorothioxanthone (CTX), 2‐isopropyl thioxanthone (ITX), and camphorquinone (CQ). The postulated mechanism is based on the intermolecular reaction of the excited photosensitizer with the tertiary amino moiety of the ground state P‐a and a subsequent hydrogen abstraction reaction. The resulting aminoalkyl radicals initiate the polymerization. The incorporation of P‐a groups into polymers is demonstrated by spectroscopic methods. The possibility of deep curing using the described photoinitiating system followed by the thermal ring opening of the incorporated P‐a groups is also demonstrated.

Schematic of the photoinitiated free radical polymerization using benzoxazines as hydrogen donors.     

Synthesis and characterization of sulfone containing main chain oligobenzoxazine precursors

The preparation of soluble and processable polybenzoxazine precursors capable of forming high performance networks is an important field of research with a broad spectrum of application. This study demonstrates an approach that utilizes aromatic sulfonediamine, bisphenol‐A, and formaldehyde in Mannich‐type polycondesation to prepare polybenzoxazine precursor. The structure of the oligomeric precursor (M_n = 2600) was confirmed by FTIR and ¹H NMR spectral analysis. The precursor contained both sulfone and benzoxazine ring structures in the backbone. It was shown that small amount of ring‐opened phenolic groups were also present. Thermally activated self‐curing behavior of precursor in the absence of catalyst was studied by differential scanning calorimetry. Thermal properties of the cured polymers were also investigated by thermo gravimetric analysis. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

A two-step four-component queuing cascade involving a Heck coupling, π-allylpalladium trapping and Diels-Alder reaction

Palladium-catalyzed cross-coupling of bicyclopropylidene (1) with iodoethene (11) in the presence of a secondary amine 12 provides allylidenecyclopropanes 13 which undergo immediate Diels-Alder reactions upon addition of dienophiles 14-18 to provide 8-(1′-aminoethyl)-substituted spiro[2.5]oct-7-ene derivatives 23a-26a in 29-66% yield. The same one-pot, two-step queuing cascade can be carried out with other iodoalkenes including cyclic ones and with cyclic dienophiles such as N-arylmaleinamides 19-22 and N-phenyl-triazolinedione 37 to furnish highly substituted spiro[2.5]oct-4-enes and spirocyclopropanated heterobicycles 47a-49a, 41a-46a (17-50%). Spirocyclopropanated heterobicycles such as 55, 56 (25 and 38% yield, respectively) can also be obtained by an inter-intra-intra-intermolecular version of this queuing cascade involving 1-hydroxyethyl- and 1-aminoethyl-substituted iodoethenes 53, 54.     

Ethynyl-bridged porphyrin-corrole dyads and triads: Synthesis,properties and DFT calculations

Ethynyl-bridged porphyrin-corrole dyads and triads were synthesized by using Pd(0) mediated coupling reactions and their structures were characterized by NMR, FT-IR, UV/Vis and fluorescence techniques. Besides spectroscopic techniques, computational studies at B3LYP/6-311G(d,p) level of DFT were also used to elucidate the minimum energy geometries and the molecular orbital characteristics of the new dyads and triads. DFT calculations pointed out the presence of charge separated donor-acceptor property between macrocycles of dyads and triads, and the emission studies indicated an excited state interaction between macrocycles, and energy transfer from the porphyrin to the corrole unit.     

Folic‐Acid‐Modified Conducting Polymer: Electrochemical Detection of the Cell Attachment

Here, postfunctionalization and bioapplication of a π‐conjugated polymer named 4‐[4H‐dithieno(3,2‐b:2′,3′‐d)pyrrol‐4‐yl]aniline (DTP‐aryl‐NH₂) are reported, which is successfully synthesized via electropolymerization onto the glassy carbon electrode. Folic acid (FA) is used to modify the amino functional polymer via N‐(3‐dimethylaminopropyl)‐N′‐ethylcarbodiimide hydrochloride/N‐hydroxysuccinimide chemistry for the further steps. The selective adhesion of folate receptor positive cells on the surface is followed by the electrochemical methods. Cyclic voltammetry and electrochemical impedance spectroscopy have been used to characterize stepwise modification of the electroactive surface. After optimization studies such as scan rate during the polymer deposition, FA amount for the efficient surface targeting, incubation time with the cells etc., analytical characterization is carried out. The surface morphologies at each step are imaged by using fluorescence microscopy.

     

Numerical analysis of progressive damage in nonwoven fibrous networks under tension

Understanding a mechanical behaviour of polymer-based nonwoven materials that include large-strain deformation and damage can help to evaluate a response of nonwoven fibrous networks to various loading conditions. Here, a nonwoven felt made by thermal bonding of polypropylene fibres was used as a model system. Its deformation and damage behaviour was analysed by means of experimental assessment of damage evolution based on single-fibre failure and finite-element simulations. Tensile tests of nonwoven fabrics were carried out to characterise their damage behaviour under in-plane mechanical loading. It was found that progressive failure of fibres led to localization of damage initiation and propagation, ultimately resulting in failure of the nonwoven felt. To obtain the criteria that control the onset and propagation of damage in these materials, tensile tests on single fibres, extracted from the felt with bond points attached to their ends, were performed. A finite-element model was developed to study damage initiation and propagation in nonwovens. In the model, structural randomness of a nonwoven fibrous network was implemented by means of direct introduction of fibres according to the orientation distribution function. The evolution of damage in the network was controlled by a single-fibre failure criterion obtained experimentally. The proposed numerical model not only captured the macroscopic response of the felt successfully but also reproduced the underlying mechanisms involved in deformation and damage of nonwovens.     

Fully Developed Forced Convection Heat Transfer in a Porous Channel with Asymmetric Heat Flux Boundary Conditions

An analytical study is performed on steady, laminar, and fully developed forced convection heat transfer in a parallel plate channel with asymmetric uniform heat flux boundary conditions. The channel is filled with a saturated porous medium, and the lower and upper walls are subjected to different uniform heat fluxes. The dimensionless form of the Darcy–Brinkman momentum equation is solved to determine the dimensionless velocity profile, while the dimensionless energy equation is solved to obtain temperature profile for a hydrodynamically and thermally fully developed flow in the channel. Nusselt numbers for the lower and upper walls and an overall Nusselt number are defined. Analytical expressions for determination of the Nusselt numbers and critical heat flux ratio, at which singularities are observed for individual Nusselt numbers, are obtained. Based on the values of critical heat flux ratio and Darcy number, a diagram is provided to determine the direction of heat transfer between the lower or upper walls while the fluid is flowing in the channel.     

Mirror duality in a Joyce manifold

Previously the two of the authors defined a notion of dual Calabi-Yau manifolds in a G₂ manifold, and described a process to obtain them. Here we apply this process to a compact G₂ manifold, constructed by Joyce, and as a result we obtain a pair of Borcea-Voisin Calabi-Yau manifolds, which are known to be mirror duals of each other.