Influence of the particle size of the MgCl2 support on the performance of Ziegler catalysts in the polymerization of ethylene to ultra‐high molecular weight polyethylene and the resulting polymer properties

A highly systematic size series of Ziegler catalysts with similar porosities and surface textures are synthesized by varying the stirring speed during the MgCl₂ support synthesis. Besides the mean particle size, the only substantial difference observed between the various catalysts is the size and number of nodules per particle. Varying the mean diameter of the catalyst particles between 1.5 and 11.9 µm, leads to a pronounced impact on the activity in ultra‐high molecular weight polyethylene (UHMWPE) polymerization, while the M_w capabilities are only affected to a limited extend. In addition, it is observed that both the M_ws as the polymer bulk density (BD) increases during the course of the polymerization. This particularity allows to optimize the M_w and/or BD at a set polymer size, by tuning the catalyst particle size. This is particularly interesting in UHMWPE production, as control of the morphological and structural properties of the UHMWPE reactor powders are critical for efficient processing as well as the performance of the final product. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2679–2690     

Unusual long-range spin-spin coupling in fluorinated polyenes: a mechanistic analysis

Nuclear magnetic resonance (NMR) is a prospective means to realize quantum computers. The performance of a NMR quantum computer depends sensitively on the properties of the NMR-active molecule used, where one requirement is a large indirect spin-spin coupling over large distances. F-F spin-spin coupling constants (SSCCs) for fluorinated polyenes F-(CH==CH)n-F (n=1cdots, three dots, centered5) are >9 Hz across distances of more than 10 A. Analysis of the F,F spin-spin coupling mechanism with our recently developed decomposition of J into Orbital Contributions with the help of Orbital Currents and Partial Spin Polarization (J-OCOC-PSP=J-OC-PSP) method reveals that coupling is dominated by the spin-dipole (SD) term due to an interplay between the pi lone-pair orbitals at the F atoms and the pi(C2n) electron system. From our investigations we conclude that SD-dominated SSCCs should occur commonly in molecules with a contiguous pi-electron system between the two coupling nuclei and that a large SD coupling generally is the most prospective way to provide large long-range spin-spin coupling. Our results give guidelines for the design of suitable active molecules for NMR quantum computers.     

On the stereochemistry of the aldol-addition

NOE-experiments and STO-3G calculations suggest that E-enol borates exist in an U-conformation $\text{7a}$, whereas the Z-enol borates should prefer the extended conformation $\text{8b}$. These observations have been linked to the stereoselectivity of the aldol addition. The resulting predictions are in line with the stereoselectivity observed on addition of cyclohexenol borates to benzaldehyde.     

Preparation,Reactions, and Stereochemistry of 4-Tert-Butyl-1-Chlorophosphorinane 1-Sulfide and Derivatives

Abstract

Nucleophilic substitution at tetracoordinated phosphorus centers has been extensively investigated.¹ Transesterification of acyclic phosphinates proceeds with inversion of configuration,² whereas four-membered phosphinates react with retention.³ In previous work⁴ we suggested that transesterification in six-membered phosphinates occurs via an S_N2 mechanism. The title compound was synthesized as a model structure to study the stereochemical and mechanistic aspects of nucleophlic substitution at phosphorus in a six-membered ring. A mixture of diastereomers was produced and separated by column chromatography. Both the mixture and separate isomers were used for an investigation of substitution with methanol, methoxide ions, and phenoxide ions; the reactions proceeded with inversion. Transesterification reactions of the resulting thiophosphinates were studied and found to proceed with inversion. Reactions of these with phenyllithium, benzyllithium, lithium phenylacetylide, and the corresponding organomagnesium halides were also carried out. The stereochemical assignments for the title compound (cis and trans) and two derivatives have been firmly anchored by X-ray studies;⁵ assignments for others are tentative and based on spectroscopic measurements, including ¹H, ¹³C, and ³¹P NMR data.     

A variationally stable quasi-relativistic method: low-order approximation to the normalized elimination of the small component using an effective potential

A simple and variationally stable quasi-relativistic method based on a modified low-order (LO) approximation to the normalized elimination of the small component (NESC) method is presented. The modification of the original LO-NESC scheme implies the use of an energy-independent factor in the relativistic correction to the potential energy. This factor cuts off the potential energy at short distances from the nucleus and in this way restores the variational stability of LO-NESC. The new method, dubbed LO-NESC-effective potential (EP) was tested in calculations on one-, two- and many-electron atoms. The LO-NESC-EP can be easily implemented into the existing nonrelativistic quantum-chemical program codes because its Hamiltonian matrix can be expressed entirely in terms of the integrals appearing in a nonrelativistic calculation. Received: 1 April 2002 / Accepted: 23 June 2002 / Published online: 30 August 2002     

Some thoughts about the stability and reliability of commonly used exchange–correlation functionals – coverage of dynamic and nondynamic correlation effects

 The self-interaction error (SIE) of commonly used density functional theory (DFT) exchange functionals mimics long-range (nondynamic) pair correlation effects in an unspecified way. Slater exchange suffers from a larger SIE and, therefore, covers more nondynamic correlation effects than Becke exchange, which is the reason why exchange–correlation (XC) functionals based on Slater exchange lead to stabler restricted DFT solutions than those based on Becke exchange. However, the stability of an XC functional does not guarantee higher accuracy. On the contrary, if system-specific nondynamic correlation effects have to be introduced via the form of the wave function, these will be suppressed by nondynamic correlation effects already covered by the exchange functional. Hybrid functionals suffer less from the SIE and, therefore, cover a smaller number of nondynamic electron correlation effects. Accordingly, they are better suited when nondynamic electron correlation has to be introduced by the form of the wave function. It is shown that, for example, broken-symmetry unrestricted DFT calculations are more accurate when carried out with B3LYP than BLYP contrary to claims made in the literature. Received: 8 November 2001 / Accepted: 30 January 2002 / Published online: 8 April 2002     

Alkalimetalltetraethinylozincate und ‐cadmate AI2M(C2H)4 (AI = Na — Cs,M = Zn,Cd): Synthese,Kristallstruktur und spektroskopische Eigenschaften

Alkali Metal Tetraethinylozincates and ‐cadmates A^I₂M(C₂H)₄ (A^I = Na — Cs, M = Zn, Cd): Synthesis, Crystal Structures, and Spectroscopic Properties By reaction of A^IC₂H (A^I = Na — Cs) with divalent zinc and cadmium salts in liquid ammonia the alkali metal tetraethinylozincates and ‐cadmates A^I₂M(C₂H)₄ (M = Zn, Cd) were accessible as polycrystalline powders. While Na₂M(C₂H)₄ is amorphous to X‐rays and the crystal structure of Cs₂Zn(C₂H)₄ could not be solved up to now, the remaining compounds are isotypic to the already known crystal structures of the potassium compounds, as was deduced from powder diffraction with X‐rays and synchrotron radiation. They crystallise in the tetragonal space group I4₁a, contain [M(C₂H)₄]^2— tetrahedra and show structural relationships to the scheelit and anatas structure types. Raman spectroscopic investigations confirm the existence of tetrahedral fragments with C‐C triple bonds in the alkali as well as in the amorphous alkaline earth metal compounds A^IIM(C₂H)₄ (A^II = Mg — Ba, M = Zn, Cd).     

Synthesis and Stereochemical Studies of Derivatives of 1,4-Dimethyl-2-phosphabicyclo[2.2.1]heptane

Abstract

1,4-Dimethyl-2-phenyl-2-phosphabicyclo[2.2.1]heptane 2-oxide 1 was prepared by the reaction of 2,5-dimethyl-1,5-hexadiene with PhPCl₂-AlCl₃: stereo-assignments of the exo and endo isomers were established by ¹³C NMR spectroscopy (using lanthanide shift reagents) and by x-ray crystal structures. The isomers of 1 were separately reduced (phenylsilane) to give the phosphine derivative; in turn the phosphines were thermally equilibrated at 190°C to give a predominance (70%) of the exo-phenyl isomer.     

Liquid/solid interface of ultrathin ionic liquid films: [C1C1Im][Tf2N] and [C8C1Im][Tf2N] on Au(111)

Ultrathin films of two imidazolium-based ionic liquids (IL), [C(1)C(1)Im][Tf(2)N] (= 1,3-dimethylimidazolium bis(trifluoromethyl)imide) and [C(8)C(1)Im][Tf(2)N] (= 1-methyl-3-octylimidazolium bis(trifluoromethyl)imide) were prepared on a Au(111) single-crystal surface by physical vapor deposition in ultrahigh vacuum. The adsorption behavior, orientation, and growth were monitored via angle-resolved X-ray photoelectron spectroscopy (ARXPS). Coverage-dependent chemical shifts of the IL-derived core levels indicate that for both ILs the first layer is formed from anions and cations directly in contact with the Au surface in a checkerboard arrangement and that for [C(8)C(1)Im][Tf(2)N] a reorientation of the alkyl chain with increasing coverage is found. For both ILs, geometry models of the first adsorption layer are proposed. For higher coverages, both ILs grow in a layer-by-layer fashion up to thicknesses of at least 9 nm (>10 ML). Moreover, beam damage effects are discussed, which are mainly related to the decomposition of [Tf(2)N](-) anions directly adsorbed at the gold surface.