On the lack of evidence for contact contributions to lanthanide-induced N.M.R. shifts (L.I.S.'s) in aromatic aldehydes: a rigorous error analysis and the isolation of diamagnetic (complexation) shifts.

The adoption of a rigorous error analysis of L.I. S.'s allows the non-pseudo-contact components of the Yb(fod)₃ induced shifts in benzaldehyde and thiophen-2-aldehyde to be isolated. They are largest at the corbonyl carbon and at the ortho and para carbons and appear to be entirely due to complexation (diamagnetic) shifts. Estimation of these with La (fod)₃ allows a precise analysis of the ¹H and ¹³CL.I.S.'s of these aldehydes $\text{without the need to invoke any contact shifts}$.     

The polarographic determination of traces of titanium-(IV) in the presence of n-benzoyl-n-phenylhydroxylamine

The polarographic behavior of the titanium(IV)-N-benzoyl-N-phenyl-hydroxylamine (BPHA) system in acidic medium and in water-ethanol mixtures has been studied. In (1+3) water-ethanol containing 2 M sulfuric acid and 0.05 M BPHA, titanium(IV) gives a single kinetically controlled wave. Titanium(IV) can be determined at concentrations as low as 5·10^-6M, in the presence of Fe(III), Cu(II), V(V), etc., but Cd(II), Sn(II and IV), As(V), U(VI) and Mo(VI) interfere.     

The effects of short‐term changes in environmental parameters on the release of biocides from antifouling coatings: cuprous oxide and tributyltin

Current and forthcoming UK and European legislation requires environmental risk assessment of antifouling paints. For assessments to be carried out successfully, the leaching rate of biocides from antifouling paints should be determined. Current methods for the measurement of biocide leach rate have been shown to be very susceptible to changes in parameters such as pH, salinity and temperature. Using apparatus designed to simulate environmental conditions the effect of short‐term changes in salinity, pH, temperature, suspended particulate matter and simulated vessel speed on cuprous oxide and tributyltin (TBT) leaching from self‐polishing antifouling paint was investigated. No effect on copper leaching was observed over a wide range of environmentally relevant conditions, whilst vessel speed was the only parameter found to influence TBT release rates significantly. It is suggested that the decrease observed may be due to the formation of a boundary layer which slows down the release of TBT from the paint surface. The environmental authenticity of this observation and its possible implications for the environmental risk assessment of TBT are discussed. Copyright © 1999 Crown Copyright.     

Propylene polymerization with catalysts containing divalent titanium

The behavior in propylene polymerization of divalent titanium compounds of type [η⁶-areneTiAl₂Cl₈], both as such and supported on activated MgCl₂, has been studied and compared to that of the simple catalyst MgCl₂/TiCl₄. Triethylaluminium was used as cocatalyst. The Ti–arene complexes were active both in the presence and in the absence of hydrogen, in contrast to earlier reports that divalent titanium species are active for ethylene but not for propylene polymerization. ¹³C-NMR analysis of low molecular weight polymer fractions indicated that the hydrogen activation effect observed for the MgCl₂-supported catalysts should be ascribed to reactivation of 2,1-inserted (“dormant”) sites via chain transfer, rather than to (re)generation of active trivalent Ti via oxidative addition of hydrogen to divalent species. Decay in activity during polymerization was observed with both catalysts, indicating that for MgCl₂/TiCl₄ catalysts decay is not necessarily due to overreduction of Ti to the divalent state during polymerization. In ethylene polymerization both catalysts exhibited an acceleration rather than a decay profile. It is suggested that the observed decay in activity during propylene polymerization may be due to the formation of clustered species that are too hindered for propylene but that allow ethylene polymerization. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 2645–2652, 1997     

A Novel Beryllium Thiolate Resulting from N−Si Bond Cleavage: Liberation of Ammonia in the Reaction of Be[N(SiMe3)2]2 with HSPh

A side reaction leads to the product. In the synthesis of [{Be(SPh)₂(py)(NH₃)}₂{[18]crown-6}] ( 1 , py=pyridine) from [Be{N(SiMe₃)₂}₂] and HSPh, the coordinated ammonia molecules (see the structure of 1 in the picture) are formed in a competing reaction between liberated hexamethyldisilazane and the thiol.     

Effect of 1‐hexene comonomer on polyethylene particle growth and copolymer chemical composition distribution

An investigation of the polymer particle growth characteristics and polymer molecular weight and composition distributions in ethylene homopolymerization and ethylene/1‐hexene copolymerization has been carried out with a catalyst comprising a zirconocene and methylaluminoxane immobilized on a silica support. The presence of 1‐hexene leads to higher productivity and easier fragmentation of the support during particle growth. Crystallization analysis fractionation and gel permeation chromatography analysis of ethylene/1‐hexene copolymers prepared at different polymerization times reveals a broadening of the chemical composition distribution with increasing polymerization time as a result of the gradual formation of a relatively high‐molecular‐weight, ethylene‐rich fraction. The results are indicative of significant monomer diffusion effects in both homopolymerization and copolymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2883–2890, 2006     

Structural characterisation of the silica and alumina Zener pinning phases in nanocrystalline CeO2 by 29Si and 27Al nuclear magnetic resonance

Nanocrystalline CeO₂ samples have been manufactured using sol-gel techniques, containing either 15 % silica or 10 % alumina by weight to restrict growth of the ceria nanocrystals during annealing by Zener pinning. ²⁹Si and ²⁷Al MAS NMR have been used to investigate the structure of these pinning phases over a range of annealing temperatures up to 1000 °C, and their effect on the CeO₂ morphology has been studied using electron microscopy. The silica pinning phase resulted in CeO₂ nanocrystals of average diameter 19 nm after annealing at 1000 °C, whereas the alumina pinned nanocrystals grew to 88 nm at the same temperature. The silica pinning phase was found to contain a significant amount of inherent disorder indicated by the presence of lower n Qⁿ species even after annealing at 1000 °C. The alumina phase was less successful at restricting the growth of the ceria nanocrystals, and tended to separate into larger agglomerations of amorphous alumina, which crystallised to a transition alumina phase at higher temperatures.