Mechanistic study of thermal behavior and combustion performance of epoxy resins: I homopolymerized TGDDM

Tetraglycidyl 4,4′-diaminodiphenylmethane (TGDDM) undergoes homopolymerization on heating. Intramolecular reactions which compete with crosslinking favor the formation of cyclic structures with increasing thermal and fire resistance of the resin, whereas physical mechanical properties tend to decrease. The mechanism of thermal decomposition of TGDDM is studied by thermogravimetry, differential scanning calorimetry and thermal volatilization analysis with characterization of volatiles evolved and residue left. Thermal degradation of poly-(TGDDM) starts at 260°C with elimination of water from secondary alcoholic groups which is a typical pathway for epoxy resin degradation. Resulting unsaturations weaken bonds in the β-position and provoke the first chain breaking at allyl–amine and allyl–either bonds. With increasing temperature, saturated alkyl–ether bonds and alkyl carbon–carbon bonds are broken first, followed by the most stable alkyl–aryl bonds at T>365°C. The combustion performance of TGDDM is discussed on the basis of the thermal degradation behavior.     

ETUDE EN SPECTROMETRIE DE MASSE DE PHOSPHONATES ENYNIQUES ET DE β-CETOPHOSPHONATES-γ, δ-ETHYLENIQUES

The mass spectrometric study of acetylenic dialkylphosphonates (I) and of γ, δ-éthylenic-β-ketophosphonates (II) is characterised by rearrangement processes of the phosphorylated group and cleavage of the phosphorus-carbon bond. The fragmentation patterns of compounds II were identical to those found for I by loss of water.     

Synthesis of well‐designed polymers carrying saccharide moieties via RAFT miniemulsion polymerization

Two hydrophobic vinyl saccharide monomers based on D ‐glucose and D ‐fructose were polymerized by employing the reversible addition‐fragmentation transfer (RAFT) miniemulsion polymerization technique to prepare well‐designed glycopolymers. Three dithiobenzoate‐RAFT agents [S?C(Ph)S? R], 1‐phenylethyl dithiobenzoate (PED), 2‐phenylprop‐2‐yl dithiobenzoate (PPD), and 2‐cyanoprop‐2‐yl dithiobenzoate (CPD), were used to control the growth of polymer chains. The best results were obtained in the presence of the PPD‐RAFT agent and the formed polymers have polydispersity index's (PDI) lower than 1.15. Under adequate miniemulsion polymerization conditions, a glycopolymer with PDI of 1.1 and molecular weight of 5 × 10⁴ g/mol has been successfully synthesized in a short reaction time of 100 min. Furthermore, some block copolymers containing saccharide segment with butyl or methyl methacrylate were prepared. Copyright © 2007 John Wiley & Sons, Ltd.     

Theoretical analysis of the three-dimensional structure of tetrathiolato iron complexes

The three-dimensional structures of a number of [M(SR)(4)](n-) complexes, where M is a 3d transition metal and R is an alkyl or aryl group, have been analyzed using density functional theory (DFT). Special attention is paid to the Fe(II)/Fe(III) mimics of rubredoxin. The Fe(II) model complex [Fe(SCH(3))(4)](2-) has an equilibrium conformation with D2d symmetry. The DFT energy has been decomposed into contributions for ligand-ligand and metal-ligand interactions. The latter contribution is analyzed with the angular overlap model (AOM) and constitutes the dominant stereospecific interaction in the Fe(II) complex. The sulfur lone-pair electrons exert anisotropic pi interactions on the 3d(6) shell of Fe(II), which are controlled by the torsion angles, omega(i), for the rotations of the S(i)-C(beta) bonds around the Fe-S(i) axes. In contrast, the pi interactions acting on the high-spin 3d(5) shell of Fe(III) are isotropic. As a consequence, the stereochemistry of the Fe(III) complexes is determined by the Coulomb repulsions between the ligands and has S(4) symmetry. The electrostatic repulsions between the lone pairs of the sulfurs are an essential component of the ligand-ligand interaction. The lone-pair repulsions distort the 90 degree angle SFeS' angles (delta + delta(t)) and give rise to a correlation between delta and omega, which is confirmed by crystallographic data. Both the Fe(II) and Fe(III) complexes exhibit structural bistability due to the presence of low-lying equilibrium conformations with S(4) symmetry in which the complex can be trapped by the crystalline host.     

Reaction of selenobenzophenones with olefins. Formation of 1H-2-benzoselenopyrans. Reaction of 1H-2-benzoselenopyrans with diazoalkanes

Selenobenzophenone reacts as a diene with dimethyl acetylenedicarboxylate (DMAD) to lead to dimethyl 1H-1-diphenylmethyl-1-phenyl-2-benzoselenopyran-3,4-dicarboxylate (5c) in moderate yield; the initial [4 + 2] cycloaddition is followed by the addition of another 1 mol equiv of selenobenzophenone. The reaction might proceed through carbene insertion of the primary cycloadduct. On the other hand, 4,4'-dimethoxyselenobenzophenone combines as a diene with DMAD furnishing dimethyl 1H-1-p-methoxyphenyl-6-methoxy-2-benzoselenopyran-3,4-dicarboxylate (4a). The reaction of benzoselenopyran derivative (4) with diaryldiazomethanes afforded another type of carbene insertion product.     

Recherches sur la formation et la transformation des esters LXXIII Sur la préparation d'acides ω-[aryl (ou aralcoyl ou alcoyl) carbamylamino]-alcoylphosphoriques et sur leur vitesse de scission à divers pH

N-Aryl(or aralkyl or alkyl)carbamoyl-aminoalkyl phosphoric monoesters have been prepared by the reaction of isocyanates R′-N?C?O on the corresponding amino-alkylphosphoric monoesters H₂N? R? OPO₃H₂ in the presence of 2 equivalents of NaOH. The rates of scission of the phosphoric monoester group and occasionally of the phenylcarbamoylamino group of these monoesters have been studied at 100°, in solutions 0.1M in ester and at various pH.     

A rapid and selective HPLC‐UV method for the quantitation of efavirenz in plasma from patients on concurrent HIV/AIDS and tuberculosis treatments

Owing to heterogeneity in therapeutic response, efavirenz is of research and clinical interest. There is a need to quantitate it using noncostly and selective methods. A method for efavirenz quantitation in plasma containing HIV and tuberculosis drugs was developed. Chromatographic separation was carried out using a C₁₈ column. The mobile phase consisted of 0.1% formic acid and acetonitrile, and was pumped at a flow rate of 0.3 mL/min. Efavirenz and ritonavir (internal standard) were monitored at 247 nm. Plasma proteins were precipitated by centrifugation. The analysis time was 6 min. The response was linear (r = 0.9997). The accuracy ranged between 98 and 115% (intraday) and between 99 and 117% (interday). The precision ranged from 1.670 to 4.087% (intraday) and from 3.447 to 13.347% (interday). Recovery ranged from 98 to 132%. Stability ranged between 99 and 123%. The selectivity was proven by analysis of drugs used for the management of HIV/AIDS and tuberculosis. Plasma sample analysis showed an efavirenz retention time of 5.57 min and a peak plasma concentration of 2.4 µg/mL occurring at 2 h. This method is rapid and selective, and thus suitable for monitoring efavirenz in patients with HIV/AIDS alone or co‐infected with tuberculosis in a less resourced setting. Copyright © 2013 John Wiley & Sons, Ltd.     

Coexistence and harvesting control policy in a food chain model with mutual defense of prey

A model is proposed to understand the dynamics in a food chain (one predator‐two prey). Unlike many approaches, we consider mutualism (for defense against predators) between the two groups of prey. We investigate the conditions for coexistence and exclusion. Unlike Elettreby's (2009) results, we show that prey can coexist in the absence of predators (as expected since there is no competition between prey). We also show the existence of Hopf bifurcation and limit cycle in the model, and numerically present bifurcation diagrams in terms of mutualism and harvesting. When the harvest is practiced for profit making, we provide the threshold effort value that determines the profitability of the harvest. We show that there is zero profit when the constant effort is applied. Below (resp. above) , there will always be gain (resp. loss). In the case of gain, we provide the optimal effort and optimal steady states that produce maximum profit and ensure coexistence. Recommendations for resource managers As a result of our investigation, we bring the following to the attention of management:

• 1. In the absence of predators, different groups of prey can coexist if they mutually help each other (no competition among them).
• 2. There is a maximal effort to invest in order to gain profit from the harvest. Above , the investment will result in a loss.
• 3. In the case of profit from harvest, policy makers should recommend the optimal effort to be applied and the optimal stock to harvest. This will guarantee maximum profit while ensuring sustainability of all species.

     

Reactions of Selenobenzophenones with Olefins

Reactions of selenobenzophenones with methyl propiolate afforded two types of cycloadducts regioselectively. The reaction with tetracyanoethylene gave selenophene derivatives. Mechanisms of these reactions were discussed.     

Time-resolved torsional relaxation of spider draglines by an optical technique

The sensitivity of the torsional pendulum demonstrates the self-shape-memory effect in different types of spider draglines. Here we report the time-resolved noncovalent bonds recovery in the protein structure. The torsional dynamics of such multilevel structure governed by reversible interactions are described in the frame of a nested model. Measurement of three different relaxation times confirms the existence of three energy storage levels in such two protein spidroin systems. Torsion opens the way to further investigations towards unraveling the tiny torque effects in biological molecules.