ABSTRACT
Sodium polyacrylate (NaPA) in dilute aqueous solution at an ionic strength of [NaNO3] = 0.01M establishes a rich phase behavior in the presence of low amounts of silver cations, which were introduced at a few millimoles or less by replacing the corresponding amount of Na+ cations. Beyond an extremely low level of Ag+ cations, anionic PA chains aggregate. By increasing the concentration of Ag+, the aggregates become denser and keep on growing without limit. Once a certain range of [Ag+] is reached, the instantaneously formed dense aggregates remain stable. Irradiation of the PA aggregate solutions with UV-light induces formation of silver nanoparticles (Ag-Nps). Based on a combination of UV-vis spectroscopy, light scattering, transmission electron microscopy, and small angle neutron scattering, the mechanism of this NaPA assisted formation of Ag-Nps is studied. One focus of the study is lying on the effect of the two different solution states of dense aggregates, corresponding to the unstable growing AgPA aggregates and to the stable AgPA aggregates and another focus is aiming at the characterisation of the morphology of the generated hybrid particles composed of Ag-Nps and hosting PA chains.
ABSTRACT
Dilute solutions of sodium poly(styrene sulfonate) (NaPSS) in the presence of Al3+, Ca2+, and Ba2+ were analysed by means of isothermal titration calorimetry (ITC) in order to investigate the heat effect of bond formation between those cations and the anionic SO3- residues of NaPSS. The selection of the cations was guided by the solution behavior of the corresponding PSS salts from a preceding study [M. Hansch et al., J. Chem. Phys. 148(1), 014901 (2018)], where bonds between Ba2+ and anionic PSS showed an increasing solubility with decreasing temperature and Al3+ exhibited the inverse trend. Unlike to Al3+ and Ba2+, Ca2+ is expected to behave as a purely electrostatically interacting bivalent cation and was thus included in the present study. Results from ITC satisfactorily succeeded to explain the temperature-dependent solution behavior of the salts with Al3+ and Ba2+ and confirmed the non-specific behavior of Ca2+. Additional ITC experiments with salts of Ca2+ and Ba2+ and sodium poly(acrylate) complemented the results on PSS by data from a chemically different polyanion. Availability of these joint sets of polyanion-cation combinations not only offers the chance to identify common features and subtle differences in the solution behavior of polyelectrolytes in the presence of multi-valent cations but also points to a new class of responsive materials.
ABSTRACT
The dilute solution behavior of sodium poly(styrene sulfonate) is studied in the presence of trivalent Al3+ and bivalent Ba2+ cations at various levels of excess NaCl. The study evaluates the phase behavior and the morphology of the polyelectrolyte chains with increasing extent of decoration with the Al3+ and Ba2+ cations and analyses the effect of temperature on these decorated chains. The phase behavior is presented in the form of the cation concentration versus the respective poly(styrene sulfonate) concentration, recorded at the onset of precipitation. Whereas poly(styrene sulfonate) with Al3+ exhibits a linear phase boundary, denoted as the "threshold line," which increases with increasing poly(styrene sulfonate) concentration, Ba2+ cations show a threshold line which is independent of the poly(styrene sulfonate) concentration. An additional re-entrant phase, at considerably higher cation content than those of the threshold lines, is observed with Al3+ cations but not with Ba2+ cations. The threshold line and the re-entrant phase boundary form parts of the liquid-liquid phase boundary observed at the limit of low polymer concentration. The dimensions of the polyelectrolyte chains shrink considerably while approaching the respective threshold lines on increase of the Al3+ and Ba2+ cation content. However, subtle differences occur between the morphological transformation induced by Al3+ and Ba2+. Most strikingly, coils decorated with Al3+ respond very differently to temperature variations than coils decorated with Ba2+ do. As the temperature increases, the poly(styrene sulfonate) chains decrease their size in the presence of Al3+ cations but increase in size in the presence of Ba2+ cations.
ABSTRACT
Chiral nonracemic thiomorpholines have been synthesized in four to six steps from limonene or achiral alkenes using alpha-methylbenzylamine to control absolute stereochemistry. These aminosulfides have been used to generate sulfur ylides, which have been applied in the asymmetric epoxidation of aldehydes as easily recoverable catalysts. Excellent yields (up to 98 %), enantioselectivities (up to 97:3 e.r.), and diastereoselectivities (>or=98:2 trans/cis) were achieved in these epoxidations and the sulfides were easily recovered in high yield (up to 97 %) by simple acid/base extraction.
ABSTRACT
The aldol-Tishchenko reaction of ketone aldols as enol equivalents has been developed as an efficient strategy to furnish differentiated 1,3-anti-diol monoesters in one step. The thermodynamically unstable ketone aldols undergo a facile retro-aldolization to yield a presumed zirconium enolate in situ, which then undergoes the aldol-Tishchenko reaction in typically high yields and with complete 1,3-anti diastereocontrol. Evaluation of a broad range of metal alkoxides as catalysts and optimization of the reaction protocol led to a modified zirconium alkoxide catalyst with attenuated Lewis acidity and dichloromethane as solvent, which resulted in suppression of the undesired acyl migration to a large extent. Various ketone aldols have been prepared and subjected to the general process, giving rise to a broad range of differently substituted 1,3-anti-diol monoesters, which may be hydrolyzed to the corresponding 1,3-anti-diols.
