Ethylene polymerization by novel Ziegler–Natta‐type catalysts obtained in situ by the oxidative addition of 8‐hydroxyquinoline‐based ligands to bis(1,5‐cyclooctadiene)nickel(0) and methylaluminoxane

Novel catalytic systems, prepared in situ by the oxidative addition of 8‐hydroxyquinoline ligands to bis(1,5‐cyclooctadiene)nickel(0) and activated by methylaluminoxane, were studied in ethylene polymerization. When 8‐hydroxyquinoline was employed, only oligomeric products were obtained. On the contrary, 5,7‐dinitro‐8‐hydroxyquinoline gave linear polyethylene (PE), but with a modest activity. For the catalyst based on 5‐nitro‐8‐hydroxyquinoline, the productivity was largely dependent on the content of free trimethylaluminum (TMA) present in the commercial aluminoxane. The progressive optimization of the TMA/oligomeric methylaluminoxane ratio increased the productivity, which reached 700 kg of PE/(mol of Ni × h), by an order of magnitude. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 200–206, 2006     

On Aging of Acid-Catalyzed Silica Sols—A Dynamic Light Scattering Study

Dynamic light scattering experiments in acid-catalyzed silica sols are discussed. It is shown that in spite of limited parameter accuracy and limited absolute knowledge of the particle sizes, the scattering data from various experiments are comparable with one another and give information about structural differences. Measurements at various angles indicate changes between non-spherical and nearly spherical particles and allow the estimation of gelling times before gelation.     

Lower bounds for faithful,preinjective modules

Let be the path algebra for some representation-infinite quiver over some field k. There exists a bound such that ^mI is faithful for all indecomposable injective -modules I and all , and such that there exists an indecomposable injective -module J satisfying that J is not faithful, denotes the Auslander-Reiten-translation. Let m() be the maximum of the taken over all possible orientations of the underlying graph . In this article we determine the bounds m() for representation-infinite quivers for which is a tree.     

Negative and positive secondary ion mass spectra of organic acids

Organic compounds can be ionized by sputtering the solid sample. The resulting negative and positive secondary ions provide mass spectra which characterize both the molecular weights and the structures of the compounds. Ionization occurs either by direct ejection of charged species from the solid into vacuum or by electron or proton transfer. The sputtered secondary ions dissociate unimolecularly to give fragment ions. These reactions are identical to those which occur when the secondary ions are independently generated by chemical ionization, selected by mass and dissociated in a high energy gas phase collision. The negative ion SIMS spectra show molecular ions (M^?.) or (M-H)^? ions as the dominant high mass species together with fragments due to decarboxylation, dehydration and losses of other simple molecules. Stronger acids show larger (M-H)^?/M^?.abundance ratios. The positive ion spectra are complementary and also useful in characterizing molecular structures. Attachment of cations to organic molecules (cationization) occurs much more readily than anion attachment and this makes negative SIMS spectra simpler than these positive ion counterparts.     

C 1s and Au 4f7/2 referenced XPS binding energy data obtained with different aluminium oxides, -hydroxides and -fluorides

A number of partially catalytically active aluminium compounds characterised by powder XRD have been investigated by XPS and XAES using a new method for static charge referencing [1, 2]. In detail, α-Al₂O₃, γ-Al₂O₃, boehmite γ-AlO(OH), bayerite α-Al(OH)₃, hydrargillite γ-Al(OH)₃, α-AlF₃, β-AlF₃, and AlF_2.3(OH)_0.7· H₂O and a hexa-fluoropropylene oxide (HFPO) modified γ-Al₂O₃ are examined. Well defined and chemically inert 20 nm gold particles are deposited as a nearly statistical distribution on the sample surface avoiding large coagulation effects. This procedure allows a determination of gold referenced XPS and XAES data sets. Binding energies (BE) of Al 2p, Al 2s, O 1s and F 1s photoelectron peaks as well as kinetic energies (KE) of Al KLL and F KLL Auger electron emission peaks are presented in relation to the Au 4f_7/2 BE reference. XPS and XAES data found in literature are, in most cases, C 1s referenced and scatter in a broad range. BE differences Δ between the C 1s charge reference BE and Au 4f_7/2 charge reference BE obtained with our samples are monitored by using the Al 2p orbital. These BE differences Δ clearly suggest that the chemical state of carbon observed in this study is not as uniform as required for reliable static charge referencing. Received: 24 June 1996 / Revised: 12 December 1996 / Accepted: 14 December 1996     

The application of size exclusion chromatography to the analysis of biopolymers

High performance size exclusion chromatography (HPSEC), employing microparticulate, hydrophilic, surface-modified silicas or hydrophilic, cross-linked, organic polymers and buffered eluents, is an effective means of separating native and denatured proteins of molecular weights in the 10 000–500 000 range. HPSEC can be further used for molecular weight identification, and the fractionation and isolation of proteins.