Active‐center equilibrium in Ziegler–Natta butadiene polymerization

The Ziegler–Natta‐catalyzed polymerization of 1,3‐butadiene was investigated at a low aluminum alkyl/cobalt (Al/Co) ratio using two different soluble catalyst systems: cobalt(II) octanoate/diethylaluminum chloride/water and cobalt(II) octanoate/methylaluminoxane/tert‐butyl chloride. When the active‐center concentration was determined by the number‐average molecular weight technique, it was found that the percentage of active cobalt depended on the Al/Co ratio. Subsequently, an equilibrium reaction was proposed to be Co + 2Al ? CoAl₂, where Co is cobalt(II) octanoate, Al is either of the aluminum alkyl‐activator species, and CoAl₂ is the active catalyst. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2256–2261, 2001     

Photoassisted degradation of dye pollutants in aqueous TiO2 dispersions under irradiation by visible light

The photoassisted degradation of two dye compounds, erythrosine and rhodamine B, has been investigated in an aqueous TiO₂ dispersion under irradiation by visible light. The compounds can be easily decomposed photochemically by visible light, which provides possibly a new approach for the treatment of dye pollutants in wastewater.     

Long chain heterocyclic dyes. Part one. Hydrophobic structures

A series of polymethine dyes and of their precursors, containing long and short alkyl chains, was prepared and their chemical and spectroscopic (uv, ¹H-nmr) properties were briefly discussed. The electronic absorption spectra of the dyes in aqueous medium and in presence of anionic or cationic surfactants evidenced aggregation phenomena. The best deaggregating effect was shown by the anionic surfactant when added above its critical micellar concentration.     

Photochemical reactions in the tropospheric aqueous phase and on particulate matter

This paper is a tutorial review in the field of atmospheric chemistry. It describes some recent developments in tropospheric photochemistry in the aqueous phase and on particulate matter. The main focus is regarding the transformation processes that photochemical reactions induce on organic compounds. The relevant reactions can take place both on the surface of dispersed particles and within liquid droplets (e.g. cloud, fog, mist, dew). Direct and sensitised photolysis and the photogeneration of radical species are the main processes involved. Direct photolysis can be very important in the transformation of particle-adsorbed compounds. The significance of direct photolysis depends on the substrate under consideration and on the colour of the particle: dark carbonaceous material shields light, therefore protecting the adsorbed molecules from photodegradation, while a much lower protection is afforded for the light-shaded mineral fraction of particulate. Particulate matter is also rich in photosensitisers (e.g. quinones and aromatic carbonyls), partially derived from PAH photodegradation. These compounds can induce degradation of other molecules upon radiation absorption. Interestingly, substrates such as methoxyphenols, major constituents of wood-smoke aerosol, can also enhance the degradation of some sensitisers. Photosensitised processes in the tropospheric aqueous phase have been much less studied: it will be interesting to assess the photochemical properties of Humic-Like Substances (HULIS) that are major components of liquid droplets. The main photochemical sources of reactive radical species in aqueous solution and on particulate matter are hydrogen peroxide, nitrate, nitrite, and Fe(iii) compounds and oxides. The photogeneration of hydroxyl radicals can be important in polluted areas, while their transfer from the gas phase and dark generation are usually prevailing on an average continental scale. The reactions involving hydroxyl radicals can induce very fast transformation of compounds reacting with (*)OH at a diffusion-controlled rate (10(10) M(-1) s(-1)), with time scales of an hour or less. The hydroxyl-induced reactivity in solution can be faster than in the gas phase, influencing the degradation kinetics of water-soluble compounds. Moreover, photochemical processes in fog and cloudwater can be important sources of secondary pollutants such as nitro-, nitroso-, and chloro-derivatives.     

An ecoepidemiological predator‐prey model with standard disease incidence

This investigation accounts for epidemics spreading among interacting populations. The infective disease spreads among the prey, of which only susceptibles reproduce, while infected prey do not grow, recover, reproduce nor compete for resources. The model is general enough to describe a large number of ecosystems, on land, in the air or in the water. The main results concern the boundedness of the trajectories, the analysis of local and global stability, system's persistency and a threshold property below which the infection disappears. A sufficiently strong disease in the prey may avoid predators extinction and its presence can destabilize an otherwise stable predator‐prey configuration. The occurrence of transcritical, saddle‐node and Hopf‐bifurcations is also shown. Copyright © 2008 John Wiley & Sons, Ltd.     

Kinetics and mechanism of oxidation of quinols by tris(1,10-phenanthroline) iron(III) complexes in aqueous acidic perchlorate media

The kinetics of oxidation of several substituted quinols by a series of Tris(1,10-phenanthroline)iron(III) complexes has been investigated with a stopped-flow technique at 6.0 and 20.0°C. The reactions were found to be first order on both reactants and independent of acidity. The second-order specific rate constants were strongly dependent on free energy of reaction. An interpretation of the mechanism in the light of Marcus theory has been developed. The first electron abstraction with semiquinone radical formation has been suggested as the rate-determining step, and on this basis, intrinsic parameters of the reactions have been derived. A good agreement was found between experimental and computed data.     

Modulation of chemoselectivity by protein additives. Remarkable effects in the oxidation of hyperforin

Protein additives have a dramatic effect on the H(2)O(2) oxidation of hyperforin, either protecting the enolized phloroglucinol core from oxidation (human albumin) or promoting (HRP and ovalbumin) reaction pathways derived from the intermediacy of the enollactone 4, a minor component of the oxidation mixture in the absence of protein additives. To rationalize the exquisite specificity of several steps and their mechanistic oddity, an organocatalytic effect is postulated. The use of protein additives allows a straightforward and multigram preparation of the enollactone 6, an interesting multifunctionalized scaffold for bioactivity induction and/or modulation.