An NMR study of the species produced by the reactions of dialkyl magnesium and HCl with calcined silica and the polymerisation performance of Ziegler-Natta catalysts produced from this support material

The effect of calcinations on the silica surface groups and thereby on the activity of Ziegler-Natta catalysts in ethylene homopolymerisation has been studied. Silica was calcined at different temperatures and treated with MgR₂ and HCl. Silica surface groups were identified by using ¹H MAS NMR and ¹³C and ²⁹Si CP MAS NMR techniques. Magnesium, titanium and chlorine were measured by elemental analysis. Ziegler-Natta catalysts were prepared from these supports and subsequently used in ethylene homopolymerisation. Maximum activity was obtained with the catalyst based on 590 °C calcined silica. The results indicate that MgR₂ reacts with siloxane-groups (Si-O-Si) in the 300 °C calcined silica, leaving the hydrogen-bonded hydroxyl-groups unreacted. Low activity Si-O-Ti(Cl)₂-O-Si species are formed after reacting with TiCl₄. The higher activity in the catalyst based on 590 °C calcined silica can be explained by the formation of -Si(R)-O-Si-O-TiCl₃ groups, originating from the siloxane bridges which cannot form in 300 °C calcined silica. Other explanations for the higher activity are a higher Mg/Ti ratio or small amounts of crystal water formed in the 590 °C calcined silica.     

Adiabatic potentials for a bridged three-site electron-transfer system

Born—Oppenheimer potential surfaces for a bridged three-site electron-transfer system are calculated. The adiabatic potentials depend upon both the vibrational sum and difference coordinates. Along the vibrational difference coordinate, the ground-state surface has single-minimun form when the through-bridge electronic coupling is strong, and double-minimum form for weak through-bridge coupling. The results are compared to those previously reported for two-site models.     

Immobilizing a Fluorescent Dye Offers Potential to Investigate the Glass/Resin Interface

Silane coupling agents are commonly applied to glass fibers to promote fiber/resin adhesion and enhance durability in composite parts. In this study, a coupling agent multilayer on glass was doped with trace levels of the dimethylaminonitrostilbene (DMANS) fluorophore. The fluorophore was immobilized on the glass surface by tethering the molecule to a triethoxy silane coupling agent, creating the DMANS/silane coupling agent molecule (DMSCA). DMSCA was then diluted with commonly used coupling agents and grafted to a glass microscope coverslip to create a model composite interface. A 53-nm blue shift in fluorescence from the immobilized DMSCA can be followed during cure of an epoxy resin overlayer, giving this technique potential to monitor the properties of the fiber/resin interface during composite processing. Contact angle measurements on these coupling agent layers were similar in the presence or absence of the DMSCA molecule, suggesting that trace levels of the fluorescent probe did not affect the structure of the layer. The immobilized DMSCA molecule behaved similarly to the DMANS precursor in solution. Both showed longer wavelength fluorescence in more polar environments. Copyright 2000 Academic Press.     

(Z)-3-p-tolylsulfinylacrylonitrile as a chiral dienophile: diels-alder reactions with furan and acyclic dienes

The behavior of (Z)-3-p-tolylsulfinylacrylonitrile (1) as a chiral dienophile has been evaluated from its reactions with furan and acyclic dienes. Electrostatic interactions of the cyano group with the sulfinyl one restrict the conformational mobility around the C-S bond, thus controlling the pi-facial selectivity, which is almost complete in all cases, the approach of the diene from the less-hindered face of the dienophile (that bearing the lone electron pair) in the predominant rotamer being the favored one. The regioselectivity is also completely controlled by the cyano group. Additionally, the reactivity of compound 1 as well as its endo-selectivity are both higher than those observed for the corresponding (Z)-3-sulfinylacrylates, thus proving the potential of sulfinylnitriles as chiral dienophiles.     

Electrochemical modification of steel by platinum nanoparticles

The aim of this study was to synthesize low-concentration catalysts with a highly developed surface based on nanodispersed platinum deposited onto the steel surface electrochemically modified with the use of an ionic liquid. Sintered fibers of austenitic steel without pretreatment (steel 1) and etched with hydrochloric acid to remove surface oxides (steel 2) were used as substrates. 1-Butyl-4-methylimidazolium acetate [BMIM][Ac] ionic liquid was used. Variation of the current intensity and anodic treatment time leads to the formation of different structures at the steel surface. For steel 2, optimal conditions leading to self-organization, namely, formation of hexagonal structures, have been selected. It has been demonstrated that formation of nanostructures at the steel surface can occur without the participation of fluoride ions. Low-concentration (Pt/steel 2) catalysts with a uniform distribution of platinum nanoparticles over the surface were prepared via galvanostatic deposition from an aqueous solution of H₂PtCl₆.