Effects of methylaluminoxane immobilization on silica on the performance of zirconocene catalysts in propylene polymerization

Investigation of the characteristics and performance in propylene polymerization of silica‐immobilized methylaluminoxane (MAO), in combination with a moderately and a highly isospecific zirconocene catalyst, has revealed that a simple impregnation of silica with MAO at ambient temperature is insufficient to obtain uniform distribution of MAO throughout the support particle. Homogeneous Al distribution throughout the support, giving increased catalyst activity, was achieved by a more rigorous impregnation of silica with MAO at elevated temperatures. The highest catalyst activities were obtained by precontacting the MAO with the zirconocene to generate the activated species before immobilization on silica. Polymer particle morphology was strongly dependent on the characteristics of the silica used for catalyst immobilization. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2734‐2748, 2005     

Synthesis of polypeptide based rod-coil block copolymers

A bifunctional initiator was synthesized and used for a sequence of a nickel initiated polymerization of gamma-benzyl-L-glutamate-N-carboxy anhydride and atom transfer radical polymerization of methyl methacrylate yielding a rod-coil block copolymer.     

Electronic state and three-dimensional structure of Mn(III) active sites in manganese-containing aluminophosphate molecular sieve catalysts for the oxyfunctionalisation of alkanes

Combined in situ X-ray absorption spectroscopy measurements and quantum mechanical calculations yield a quantitative three-dimensional structure of the tetrahedrally coordinated Mn(III) active sites in MnAlPO catalysts.     

Metastable ion studies. XII—Molecular and fragment ion structures for isomeric C4H6O2 acids

Metastable peak characteristics, ionization and appearance energy data and isotopic labelling experiments have been applied to a study of the fragmentation behaviour of the molecular ions of the isomeric C₄H₆O₂C acids, cis and trans-crotonic acids, methacrylic acid, butenoic acid and cyclopropane carboxylic acid. Prior to the losses of H₂O and CH₃, all the metastable molecular ions rearrange to [cis-crotonic acid]⁺? ions. Loss of H₂O, which generates a composite metastable peak, is proposed to yield vinylketene and/or cyclobutenone molecular ions. Detailed mechanisms are presented for the isomerizations of the various molecular ions and for the above fragmentations. Ionized 3-butenoic and cyclopropane carboxylic acids display a major loss of CO from their metastable ions, a minor process in the other isomers. The metastable peaks consist of two components and these are ascribed to the formation of propen-1-ol and allyl alcohol as daughter ions. Some comparative data are presented for the isomeric C₅H₈O₂ acids, tiglic acid, angelic acid and senecioic acid.     

A Gauss-Kusmin theorem for optimal continued fractions

A Gauss-Kusmin theorem for the Optimal Continued Fraction (OCF) expansion is obtained. In order to do so, first a Gauss-Kusmin theorem is derived for the natural extension of the ergodic system underlying Hurwitz's Singular Continued Fraction (SCF) (and similarly for the continued fraction to the nearer integer (NICF)). Since the NICF, SCF and OCF are all examples of maximal -expansions, it follows from a result of Kraaikamp that the SCF and OCF are metrically isomorphic. This isomorphism is then used to carry over the results for the SCF to any other maximal -expansion, in particular to the OCF. Along the way, a Heilbronn-theorem is obtained for any maximal -expansion.

     

On Schweiger’s problems on fully subtractive algorithms

In his monograph [6] on multi-dimensional continued fractions, Schweiger has presented two conjectures on fully subtractive algorithms. We affirm one and refute another.     

Partially miscible polystyrene/polymethylphenylsiloxane blends for nanocomposites

The potential of polystyrene/polymethylphenylsiloxane (PS/PMPS) blends as a matrix for nanocomposites is investigated. It was proven by dynamic rheometry and conductivity measurements that PMPS effectively disperses carbon nanotubes, as was already known for polydimethylsiloxane (PDMS). The phase behaviour of PS/PMPS blends was investigated using differential scanning calorimetry or modulated temperature differential scanning calorimetry. The blends were found to exhibit partial miscibility, in contrast to the known immiscible behaviour of PS/PDMS blends. A miscibility window exists for PS/PMPS blends containing less than approximately 10 wt% PMPS.     

Effects of MgCl2 Crystallographic Structure on Active Centre Formation in Immobilized Single‐Centre and Ziegler–Natta Catalysts for Ethylene Polymerization

The ability of a MgCl₂ support to activate a transition metal catalyst has been found to depend both on the crystallographic structure of the support and on the nature of the catalyst. A high degree of crystallographic disorder can be very effective for the immobilization and activation of titanium and vanadium complexes, but is not necessarily effective for zirconocene activation. A highly disordered support prepared by the reaction of MgBu₂ with HCl gave high activity with TiCl₄ but low activity with (n‐PrCp)₂ZrCl₂. High polymerization activities with the zirconocene were only obtained with supports of type MgCl₂/AlR_n(OEt)_3−n prepared from the reaction of AlR₃ with MgCl₂ · 1.1EtOH. These supports are characterized by additional peaks in the X‐ray diffraction pattern, indicating the presence of a crystalline structure which is absent in the other supports and contains highly Lewis acidic sites able to generate the active metallocenium species.