Jon Andreas Støvneng

Ethene homopolymerization and copolymerization with 1‐hexene for all methyl‐substituted (RnC5H5−n)2ZrCL2/MAO catalytic systems: Effects of split methyl substitution

Ethene homopolymerization and copolymerization with 1-hexene were catalyzed by methyl-substituted cyclopentadienyl (Cp) zirconium dichlorides, (Rn C5H5−n)2ZrCl2 (Rn = H, Me, 1,2-Me2, 1,3-Me2, 1,2,3-Me3, 1,2,4-Me3, Me4, or Me5), and methylaluminoxane. The polymers were characterized with Fourier transform infrared, nuclear magnetic resonance, gel permeation chromatography, and differential scanning calorimetry techniques. Generally, an increasing number of methyl substituents on the Cp ligand results in lower 1-hexene incorporation in the copolymer. The two catalysts with split methyl substitution (Rn = 1,3-Me2 and Rn = 1,2,4-Me3) show a higher comonomer response than their disubstituted and trisubstituted counterparts (Rn = 1,2-Me2 and Rn = 1,2,3-Me3). They even incorporate more 1-hexene than Rn = H and Rn = Me. These findings are qualitatively in agreement with the results of a theoretical study based on density functional calculations. The presence of comonomer does not influence the termination reactions after the insertion of ethene. There is more frequent termination after each hexene insertion with increasing comonomer incorporation except for the two catalysts with split methyl substituents. The termination probability per inserted comonomer is highest for the less substituted catalysts.

On the nonsingle-site character of Bis(2-dimethylsilyl-indenyl)zirconium(IV) dichloride/MAO and Bis(2-trimethylsilyl-indenyl)zirconium(IV) dichloride/MAO: polymerization characteristics and mechanistic implications.

Ethene polymerization with bis(2-dimethylsilyl-indenyl)zirconium(IV) dichloride (1)/MAO and bis(2-trimethylsilyl-indenyl)zirconium(IV) dichloride (2)/MAO and ethene-co-1-hexene polymerization with 1/MAO are presented. The end group analysis of homopolymers reveals a pronounced dependence of the termination rate on temperature changes. In combination with the high molecular weights obtained, these results are in accord with theoretical predictions. Gel permeation chromatography, Fourier transform infrared, and 13C NMR analyses of copolymerization products from 1/MAO as a function of comonomer concentration at two different temperature series denote its tendency to form inhomogeneous polymer blends. Thermal analysis and fractionation results of one such blend indicate an inhomogeneity in the enchainment process and the existence of multiple active sites of differing geometry. These indications are further supported by AMBER force field and density functional theory studies of the catalyst precursors and the active site of 1/MAO. For this system, delta-agostic interactions for the stabilization of the zirconium cation are favored over beta-agostic interactions, which, in contrast to the situation in studies on bis-Cp systems, is a sparsely populated species. The gap in activation enthalphies for beta-hydride transfer and elimination is marginalized for these bulky zirconocenes, and conceptually new mechanisms for the isomerization of the vinyl end groups are discussed. Further, unexpected activation of the silicon-hydrogen bond within the ligand framework is observed with an activation enthalpy as low as 14 kcal/mol.

Theoretical investigation of the interaction of oxygen with pure and K-doped NiTi shape memory surface alloys

Abstract Density functional based theories and experiments agree well on the value of the heat of formation of TiO2 on NiTi surfaces. However, experimental studies of polycrystalline NiTi surfaces tend to indicate that Ti atoms are always abundantly available at the surface. This theoretical study indicates that whether Ti atoms are dominantly available at the surface depends on the surface index. On NiTi(001) in the B2 phase, the surface can be Ti or Ni terminated, with equal probability, while on NiTi(110) in the B2 phase and NiTi(010) in the B19ʼ phase, Ti atoms are favored to be present at the surface.

Surface and bulk properties of chromium oxide: Implications for reduction by methane

A computational method for Cr2O3 and Cr3C2 has been established based on a systematic investigation of functionals, basis sets and corrections for dispersion, self-interaction and relativistic effects. The suggested method comprises of the PBE functional with Grimme’s dispersion correction, the TZ2P basis set with a frozen core of up to 2p for chromium and 1s for oxygen and carbon, and with the zeroth-order regular approximation for relativistic effects, and is in good agreement with experimental results for both bulk crystals and surface structures. Self-interactions have been corrected for by the DFT+U approach, but it still gives band gaps significantly different from the experimental band gap. We have also calculated the adsorption energy of methane on a chromium terminated (0001) Cr2O3 surface, and the significance of dispersion and self-interaction corrections for the adsorption of methane on Cr2O3 was found to be substantial.

Ethene Polymerization Catalyzed by Monoalkyl and Methyl Substituted Zirconocenes. Possible Effects of Reaction Barriers and Ligand-Metal Agostic Interactions

Ethene polymerization with the monoalkyl substituted zirconocenes (η5-C5H4R)2ZrCl2 (R=C n H2n+1 ,n=0-5,8,12) and the complete set of methyl substituted complexes (η5-C5H5-m Me m )2ZrCl2 (m=0-5) has been performed together with polymer characterization. Monoalkylation of the cyclopentadienyl (Cp) ligand shifts the distribution from vinyl towards trans-vinylene. For the methyl substituted complexes, we find a particularly high trans-vinylene content for pentamethyl cyclopentadienyl (Cp*). A kinetically controlled competition between termination and isomerization is used to rationalize the findings. Density-functional theory calculations show that an agostic interaction between the metal and the alkyl substituent on the Cp ligand may be important when the alkyl is ethyl or longer.

Is quantum tunnelling faster than the speed of light

The time taken for a single photon to tunnel through a potential energy barrier has been measured for the first time. Surprisingly, perhaps, the photon appears to tunnel through the barrier faster than an identical photon travelling through free space. However, closer scrutiny shows that this apparently superluminal velocity is not in conflict with the theory of relativity.