Benzophenone‐functionalized,starlike polystyrenes as organic supports for a tridentate bis(imino)pyridinyliron/trimethylaluminum catalytic system for ethylene polymerization

Starlike polystyrenes composed of a microgel core and arms terminated with benzophenone groups were used as organic supports for a tridentate bis(imino)pyridinyliron catalyst for ethylene polymerization in the presence of trimethylaluminum as a cocatalyst. The microgels were synthesized by the atom transfer radical polymerization of styrene initiated by 4‐(1‐bromoethyl)‐benzophenone, with divinylbenzene as the crosslinker. The bromine polystyrene chain ends prevented the ethylene polymerization reaction and had to be removed. This was readily achieved with Cu⁰ together with dodecanethiol as a transfer agent. When used as supports in the presence of trimethylaluminum and 2,6‐bis[1‐2,6(diisopropylphenyl)imino]ethylpyridynyl iron, these bromine‐free, functionalized polystyrene stars enabled the production of polyethylene beads of a spherical morphology and high bulk density with a catalytic activity similar to that under homogeneous reaction conditions. Moreover, the molar mass distribution of the polyethylene was narrow, suggesting limited transfer to trimethylaluminum. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6997–7007, 2006     

Heck reactions of aryl halides with alk‐1‐en‐3‐ol derivatives catalysed by a tetraphosphine–palladium complex

cis,cis,cis‐1,2,3,4‐Tetrakis(diphenylphosphinomethyl)cyclopentane–[PdCl(C₃H₅)]₂ efficiently catalyses the Heck reaction of alk‐1‐en‐3‐ol with a variety of aryl halides. In the presence of hex‐1‐en‐3‐ol or oct‐1‐en‐3‐ol, the β‐arylated carbonyl compounds were selectively obtained. Turnover numbers up to 84 000 can be obtained for this reaction. Linalool and 2‐methylbut‐3‐en‐2‐ol led regio‐ and stereoselectively to the corresponding (E)‐1‐arylalk‐1‐en‐3‐ol derivatives. A minor electronic effect of the substituents of the aryl bromide was observed. Quite similar reaction rates were generally observed in the presence of activated aryl bromides such as bromoacetophenone and deactivated aryl bromides such as bromoanisole, indicating that, with these alkenols and this catalyst, the oxidative addition of aryl bromides to palladium is not the rate‐limiting step. It should be noted that this reaction also proceeds with sterically very congested aryl bromides such as 9‐bromoanthracene or 2,4,6‐triisopropylbromobenzene or with a vinyl bromide. On the other hand, low yields were obtained with aryl chlorides. Copyright © 2006 John Wiley & Sons, Ltd.     

Study of a plane‐free jet exhausting from a channel by vortex‐in‐cell method

A two‐dimensional simulation of a plane jet exhausting from a channel has been performed using the vortex in cell algorithm in the Reynolds number range of 100–900. The vorticity is generated on the wall of the entrance channel whose length has been fixed in order to obtain a fully developed velocity profile at the entry of the jet. The transient behaviour of the velocity field starting from rest has been observed until reaching a quasi steady regime. The mean value of the velocity field is compared with the results of a finite volume computation on the same mesh. The velocity fluctuations obtained using this method are analysed. Their effect on the mean flow is estimated to be smaller than the viscous effect. Copyright © 2006 John Wiley & Sons, Ltd.     

Preparation of a xanthate‐terminated dextran by click chemistry: Application to the synthesis of polysaccharide‐coated nanoparticles via surfactant‐free ab initio emulsion polymerization of vinyl acetate

Stable monodisperse poly(vinyl acetate) (PVAc) submicronic latex particles were synthesized by ab initio batch emulsion polymerization using a dextran derivative from renewable resource as an efficient steric stabilizer. The dextranend‐functionalized by a xanthate moiety was synthesized by Huisgen's 1,3‐dipolar cycloaddition (click chemistry). It was applied as a macromolecular RAFT (reversible addition fragmentation chain transfer) agent in surfactant‐free emulsion polymerization of vinyl acetate to form in situ an amphiphilic block copolymer able to efficiently stabilize the latex particles. The method afforded the preparation of high solids content (27%) latices coated by dextran. Both the kinetic study and the molar mass analyses confirmed the involvement of the dithiocarbonate group in the emulsion polymerization process. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2845–2857, 2008     

Selection between proportional and stratified hazards models based on expected log-likelihood

The problem of selecting between semi-parametric and proportional hazards models is considered. We propose to make this choice based on the expectation of the log-likelihood (ELL) which can be estimated by the likelihood cross-validation (LCV) criterion. The criterion is used to choose an estimator in families of semi-parametric estimators defined by the penalized likelihood. A simulation study shows that the ELL criterion performs nearly as well in this problem as the optimal Kullback–Leibler criterion in term of Kullback–Leibler distance and that LCV performs reasonably well. The approach is applied to a model of age-specific risk of dementia as a function of sex and educational level from the data of a large cohort study.     

Electron Counting in Carbaalane Clusters with Cubic Aluminum Core

Extended Hückel Theory (EHT) and Density Functional Theory (DFT) calculations on hexacapped cubic aluminum clusters of the type [(AlH)₈(μ₄-CH)_6−n (μ₄-H) _n ]^+/−q (n ≤ 6) indicate that their favored number of skeletal electron pairs (SEP), 12 SEPs for n ≥ 3, 14 SEPs for n = 2, 16 SEPs for n = 1 and 18 SEPs for n = 0, depends on the relative number of the two types of capping ligands: one-orbital ligands such as H and conical three-orbital CH units. Although only 16- and 18-SEP species have been isolated so far, DFT calculations indicate that 14-SEP clusters of the type [(AlR′)₈(μ₄-CR)₄(μ₄-X)₂]²⁺ (X = one-orbital ligand) should be synthesized. Computed 12-SEP models were not found to be energy minima, but this should not preclude the possibility for stabilizing such species, for example with bulky substituents. Changing the CR capping ligands into AlR units in the above series is expected to limit their electron counts to 12 SEPs. The incorporation of a main-group atom in the middle of the cube does not change the favored cluster electron count. However, calculations indicate that only 12-SEP species are likely to be synthesized. Dedicated to Professor Günter Schmid on the occasion of his 70th birthday