Mechanism of activation of a hafnium pyridyl-amide olefin polymerization catalyst: ligand modification by monomer

We have investigated the olefin polymerization mechanism of hafnium catalysts supported by a pyridyl-amide ligand with an ortho-metalated naphthyl group. Ethylene-alpha-olefin copolymers from these catalysts have broad molecular weight distributions that can be fit to a bimodal distribution. We propose a unique mechanism to explain this behavior involving monomer modification of the catalyst, which generates multiple catalyst species when multiple monomers are present. More specifically, we present evidence that the hafnium alkyl cation initially undergoes monomer insertion into the Hf-naphthyl bond, which permanently modifies the ligand to generate new highly active olefin polymerization catalysts. Under ethylene/octene copolymerization conditions, a plurality of new catalysts is formed in relative proportion to the respective monomer concentrations. Due to the asymmetry of the metal complex, two "ethylene-inserted" and eight "octene-inserted" isomers are possible, but it is a useful approximation to consider only one of each in the polymerization behavior. Consequently, gel permeation chromatography data for the polymers can be fit to a bimodal distribution having a continuous shift from a predominantly low molecular weight fraction to predominantly higher molecular weight fraction as [octene]/[ethylene] is increased. Theoretical calculations show that such insertions into the Hf-aryl bond have lower barriers than corresponding insertions into the Hf-alkyl bond. The driving forces for this insertion into the Hf-aryl bond include elimination of an eclipsing H-H interaction and formation of a stabilizing Hf-arene interaction. These new "monomer-inserted catalysts" have no beta-agostic interaction, very weak olefin binding, and olefin-insertion transition states which differ on the two sides by more than 4 kcal/mol. Thus, the barrier to site epimerization is very low and high polymerization rates are possible even when the chain wags prior to every insertion. Experimental evidence for aryl-insertion products is obtained from reactions of ethylene (13C2H4 NMR studies) or 4-methyl-1-pentene (4M1P) using relatively low monomer/catalyst ratios. Quantitative generation of monomer-inserted products is complicated by slow initiation kinetics followed by fast polymerization kinetics. However, NMR evidence for reaction with 13C2H4 was observed in situ at low temperature, and the attachment of monomer to ligand was confirmed by GC/MS and 13C NMR after quenching. Furthermore, a 4M1P-appended ligand was isolated from a polymerization reaction (50:1 monomer:catalyst) by column chromatography followed by multiple recrystallizations. One isomer was characterized by X-ray crystallography, which unequivocally shows a 4-methylpentyl substituent at the 2-position of the naphthyl, consistent with 1,2-insertion into the Hf-aryl bond. NMR suggests a second diastereomer (not isolated) is formed from a 1,2-insertion of opposite stereoselectivity.     

Applying a hybrid DEA model to evaluate the influence of marketing activities to operational efficiency on Taiwan’s international tourist hotels

This study uses the hybrid DEA model, in which the proportionate inputs are evaluated with a radial measure and the non-proportionate inputs are evaluated with a non-radial measure in order to evaluate the impact of marketing expense on the efficiency of Taiwan's international tourist hotels. We further introduce the hybrid DEA into the technology gap measurement to evaluate the technology gaps of marketing expense utilization between different operating types. The empirical results indicate that the inefficiency caused from excess marketing expense is the main reason for lower efficiency in many tourist hotels, especially to chain-operated hotels and hotels that cater to domestic clients. Through the hybrid technology gap measurement, we find that the utilization of marketing resources is more effective for independent, city type hotels, as well as those that cater to international clients.     

Synthesis of highly ordered mesoporous alumina thin films and their framework crystallization to γ-alumina phase

Here we report the preparation of highly ordered mesoporous alumina films existing both as P6(3)/mmc and Fm-3m mesostructures by using triblock copolymer Pluronic P123 as the structure-directing agent. 2D grazing-incidence small-angle X-ray scattering (GI-SAXS) completely proves the existence of two different mesopore structures (i.e., [001]-oriented P6(3)/mmc and [111]-oriented Fm-3m symmetries). After calcination at 1000 °C, the amorphous alumina framework is successfully converted to γ-alumina crystals. During the crystallization process, large uniaxial shrinkage occurs along the direction perpendicular to the substrate with the retention of horizontal mesoscale periodicity, thereby resulting in formation of partially vertical mesoporosity in the film. Through detailed electron microscopic study, we discuss the formation mechanism for the vertical mesoporosity upon calcination. The obtained mesoporous γ-alumina film shows high thermal stability up to 1000 °C, which is highly useful in wide research areas such as catalyst supports and separators.     

Exchange bias and blocking temperature in Co/FeMn/CuNi trilayers

Aspects of exchange bias between antiferromagnets and ferromagnets remain unclear despite recent research. An outstanding issue is the relationship between exchange bias and enhanced coercivity in the ferromagnetic layer. This Letter reports the unexpected finding that a substantial exchange bias can be generated between an antiferromagnet (FeMn) with a higher ordering temperature than that of the ferromagnet (CuNi). We interpret the result in terms of a temperature-dependent competition between interfacial exchange and antiferromagnet anisotropy energies. Crossover of these energies during cooling is responsible for the onset of exchange bias at the blocking temperature.     

A numerical investigation of aerofoil boundary-layer manipulator profile for cruise-flight conditions

A computational study has been conducted to determine the variation of device drag with profile shape and angle of attack for aerofoil boundary-layer manipulators (LEBUs) operating at high subsonic Mach numbers. Calculations have been made at a free-stream Mach number of 0.80 for both symmetrical and asymmetrical NACA-00xx and 44xx series devices including an inverted cambered NACA-4409 profile. The LEBUs considered were located in a turbulent boundary-layer at a mid-chord heighth equivalent to 0.67 from the wall. The present investigations sought to confirm suggestions based upon experimental observations that there may be some advantage in replacing a symmetrical device by an inverted asymmetrical profile form.The computations were performed using an unstructured adaptive-mesh 3D Navier-Stokes code incorporating a Lam and Bremhorst low-Reynolds number two-equationk — turbulence model. The calculated flow field around a NACA-0009 aerofoil at zero angle of attack was initially verified against experimental interferometric data.The calculated device drag coefficient at zero incidence was 0.026 for the NACA-0009, similar to that measured in experiments. However predicted drag for an inverted NACA-4409 was 0.085, this being considerably higher than anticipated. The results suggested that a slightly positive angle of attack may help minimise device drag but neither profile is appropriate for use in transonic conditions. Improved results may be obtained from inverted flat-topped profiles designed to minimise losses associated with localised shocks.     

Application of holographic interferometry for evaluation of 3D Navier-Stokes calculations

Holographic interferometry is a non-intrusive optical measuring technique which can potentially capture instantaneous information about a complete three-dimensional flow field onto a single holographic image. However subsequently extracting the flow data so that it can be used to validate CFD results, is for most practical wind tunnel applications not yet possible.An approach applied in this paper, is to compare experimental interferograms with equivalent images obtained by post-processing the CFD solution. We have investigated transonic flow in a wind tunnel environment and computationally reconstructed both 2D image-plane interferograms and full 3D holographic interferograms. These comparisons have subsequently helped us to evaluate and improve our 3D Reynolds Averaged Navier-Stokes calculations.The main part of this study was conducted by the authors at Cambridge University Engineering Department where the work was funded by RAE Farnborough under contract 2029/233 XR Aero and monitored by Dr. B. Williams and Dr. P. Smith     

Measuring non-convex metafrontier efficiency in international tourist hotels

The non-convex metafrontier model was introduced by O’Donnell et al (2008) with a brief sketch and interpreted by Tiedemann et al (2011) through a graphical instruction. However, the specific function was not present in former studies. In order to strengthen the non-parametric metafrontier approach, this study proposes the specific modelling function and calculational operation for the non-convex metafrontier model and applies the developed model to investigate the technology gaps between the four operating types of Taiwan’s international tourist hotels. The empirical findings show that management contract technology has achieved the potential best practice within the four groups and that there exists a significant gap between the potential best practice and present performance in the domestic, franchise, and membership chain technologies.     

An improved branch and bound algorithm for a strongly correlated unbounded knapsack problem

An unbounded knapsack problem (KP) was investigated that describes the loading of items into a knapsack with a finite capacity, W, so as to maximize the total value of the loaded items. There were n types of an infinite number of items, each type with a distinct weight and value. Exact branch and bound algorithms have been successfully applied previously to the unbounded KP, even when n and W were very large; however, the algorithms are not adequate when the weight and the value of the items are strongly correlated. An improved branching strategy is proposed that is less sensitive to such a correlation; it can therefore be used for both strongly correlated and uncorrelated problems.     

Fragile-to-strong liquid transition in deeply supercooled confined water

Confining water in lab synthesized nanoporous silica matrices MCM-41-S with pore diameters of 18 and 14 A, we have been able to study the molecular dynamics of water in deeply supercooled states, down to 200 K. Using quasielastic neutron scattering and analyzing the data with the relaxing cage model, we determined the temperature variation of the average translational relaxation time and its Q-dependence. We find a clear evidence of an abrupt change of the relaxation time behavior at T approximately equal to 225 K, which we interpreted as the predicted fragile-to-strong liquid-liquid transition.