Die elektrochemische Reduktion von CSSe und CSe2 in Dimethylformamid: Heterocyclische 1,2-Dichalkogenolate und ihre Koordinationschemie

Electrochemical Reduction of CSSe and CSe₂ in Dimethylformamide: Heterocyclic 1,2-Dichalcogenolates and their Coordination Chemistry Starting from carbon diselenide or carbon selenidesulfide the electrochemical preparation (electrosynthesis) of heterocyclic dichalcogenolates C₃X₅^2? (X = Se: dsis; X = S/Se: C₃S_xSe_y^2?) is outlined. The 1,2-dichalcogenolate compounds were isolated and characterized as dibenzoyl derivatives. Bis- or tris-chelates of general type A_m[M(C₃X₅)_n] (with A = Bu₄N⁺, Ph₄As⁺; M = Zn^II, Pt^II, Pd^II, Ni^III, Cu^III, Au^III, In^III; X = Se, S/Se; m = 1, 2, 3; n = 2, 3, respectively) are available directly from methanolic solutions of the dibenzoylates after hydrolytic cleavage of the latter with sodium methanolate. In addition bis-chelates Bu₄N[Ni(C₃X₅)₂] (X = Se, S/Se) have been characterized by cyclovoltammetry and epr spectroscopy and compared with the corresponding all-sulfur ligand compound Bu₄N[Ni(dmit)₂] (X = S). Arguments are given for the fact that the allselenium ligand dsis (X = Se) yields the Cu^III or Ni^III chelate at once whereas with dmit using identical conditions the metal(II) compounds are formed.     

Synthesis and in vitro degradation of poly(N‐vinyl‐2‐pyrrolidone)‐based graft copolymers for biomedical applications

This work is devoted to the design of a novel family of hydrosoluble biomaterials: poly(N‐vinyl‐2‐pyrrolidone) (PVP)‐based graft copolymers. A synthesis route has been elaborated in which ω‐functionalized PVP is prepared via chain‐transfer radical polymerization, end‐group modified, and subsequently grafted onto a polyhydroxylated backbone, typically dextran or poly(vinyl alcohol). The resulting graft copolymer biomaterials are designed for use in various biomedical applications, particularly as materials with a stronger potential for plasma expansion than already existing products have. The graft copolymers are potentially degradable because the PVP grafts are connected to the polyol backbone via a hydrolytically labile carbonate or ester linkage. The degradation of the graft copolymers was performed in vitro over a period of 6 weeks. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3652–3661, 2002     

Influence of the aggregation state in solution on the supramolecular organization of adsorbed type I collagen layers

In the last years, adsorbed collagen was shown to form layers with a supramolecular organization depending on the substrate surface properties and on the preparation procedure. If the concentration of collagen and the duration of adsorption are sufficient, fibrillar collagen structures are formed, corresponding to assemblies of a few molecules. This occurs more readily on hydrophobic compared to hydrophilic surfaces. This study aims at understanding the origin of such fibrillar structures and in particular at determining whether they result from the deposition of fibrils formed in solution or from the building of assemblies at the interface. Therefore, type I collagen solutions with an increasing degree of aggregation were prepared, using the “neutral-start” approach, by ageing pH 5.8 solutions at 37 °C for 15 min, 2 or 7 days. The obtained solutions were used to investigate the influence of collagen aggregation in solution on the supramolecular organization of adsorbed collagen layers, which was characterized by X-ray photoelectron spectroscopy and atomic force microscopy. Polystyrene and plasma-oxidized polystyrene were chosen as substrates for the adsorption. The size and the density of collagen fibrils at the interface decreased upon increasing the degree of aggregation of collagen in solution. This is explained by a competitive adsorption process between monomers and aggregates of the solution, turning at the advantage of the monomers. More aggregated solutions, which are thus depleted in free monomers, behave like less concentrated solutions, i.e. lead to a lower adsorbed amount and less fibril formation at the interface. This study shows that the supramolecular fibrils observed in adsorbed collagen layers, especially on hydrophobic substrates, are not formed in the solution, prior to adsorption, but are built at the interface, through the assembly of free segments of adsorbed molecules.     

On the Effect of Organic Substitution of Silicon Alkoxides in Poly(Vinyl Acetate) Organic-Inorganic Composites

Abstract

Organic-inorganic composites (OICs) were prepared via the in-situ polymerization of an organically (phenyl) substituted trialkoxysilane, phenyltriethoxysilane (PhTEOS), in the presence of poly(vinyl acetate) (PVAc). The mechanical reinforcement above T _g previously observed in OICs of unfunctionalized organic polymers such as PVAc with acid catalyzed in-situ polymerized tetraalkoxysilane was not observed when the tetraalkoxysilane was replaced with PhTEOS. Although both systems are optically transparent and both exhibit a high degree of hydrogen bonding between the carbonyl of PVAc and the residual hydroxyls of the silicate, the polymerization of the alkoxide is different. The tetra-functional alkoxide polymerizes to form a load-supporting silicate network, leading to a high plateau in the tensile modulus above T _g, whereas the trifunctional alkoxide reacts to form primarily low molecular weight oligomers. These increase the T _g of the PVAc but do not provide mechanical reinforcement.