Search

It is proposed a molecular tweezer, which makes possible to design a scaffold structure, allowing to trap organic molecules, regardless of the size of the guest. Weak carbon-carbon and carbon-hydrogens interactions stabilized the compound. The analysis carried out with the Grimme’s dispersion correction allow to quantify the weak interaction between the molecular tweezer and the guest. These assumptions were confirmed into the framework of quantum theory of atoms in molecules, due to the existence of bond critical points connecting carbon and hydrogen tweezer’s atoms and carbon atom of the guest species. The density of states shows that it is possible to observe that the only contribution near the active regions is made by the guest species. The tweezer molecule is the perfect host for organic molecules, because there is no electronic interaction between them. The electronic properties of the guest species remains unchanged under the molecular tweezer interaction. [ABSTRACT FROM AUTHOR]

This study reports a novel conceptual framework that can be easily experimented to evaluate the effects of hydrodynamic boundary layer mass transfer, methylaluminoxane (MAO) anion design, and comonomer steric hindrance on metallocene-catalyzed ethylene polymerization. This approach was illustrated by conducting homo- and isomeric copolymerization of ethylene with 1-hexene and 4-methyl-1-pentene in the presence of bis( n -butylcyclopentadienyl) zirconium dichloride ( n BuCp) 2 ZrCl 2 , using (i) MAO anion 1 (unsupported [MAOCl 2 ] − ) and pseudo -homogeneous reference polymerization, and (ii) MAO anion 2 (supported Si−O−[MAOCl 2 ] − ) and in-situ heterogeneous polymerization. The measured polymer morphology, catalyst productivity, molecular weight distribution, and inter-chain composition distribution were related to the locus of polymerization, comonomer effect, in-situ chain transfer process, and micromixing effect, respectively. The peak melting and crystallization temperatures and %crystallinity were mathematically correlated to the parameters of microstructural composition distributions, melt fractionation temperatures, and average lamellar thickness. These relations showed to be insightful. The comonomer-induced enchainment defects and the eventual partial disruption of the crystal lattice were successfully modeled using Flory and Gibbs–Thompson equations. The present methodology can also be applied to study ethylene−α-olefin copolymerization, performed using MAO-activated non-metallocene precatalysts. [ABSTRACT FROM AUTHOR]

Abstract: In this study, the density functional theory (DFT)-based Becke’s three parameter hybrid exchange functional and Lee–Yang–Parr correlation functional (B3LYP) and dispersion corrected B3LYP (B3LYP-D) calculations have been performed to predict the structure and stability of fullerene based endohedral sandwich complexes. The role of confinement on the structure of half-sandwich complex (i.e. (η5-Cp)Fe+ (Cp=cyclopentadienyl anion)) has also been probed using energy decomposition analysis. The standard redox potential has been calculated to understand the electrochemical property of newly designed endohedral sandwich complexes. Results have been compared with exohedral sandwich complexes and ferrocene. [Copyright &y& Elsevier]

In this contribution blend metallocene polypropylene (mPP) fibres modified with 4, 8, 12 wt.% of copolyamide A and 4, 8, 12 wt.% of compatibiliser L were prepared. Copolyamide A consists of 85 wt.% of &epsilon-caprolactam and 15 wt.% of nylon salt of adipic acid and diethylenetriamine. Compatibiliser L is low molecular isotactic polypropylene grafted with maleineanhydride. Thermal properties of the components, blends and modified mPP fibres were evaluated. Fibres modified with both copolyamide A and compatibilizer were successfully spun and drawn, with the opposite being the case for mPP fibres modified only with copolyamide A without compatibiliser L, which were not spun, confirming the positive role of compatibiliser L. The modified fibres conserve their thermal characteristics - melting temperature, melting enthalpy and entropy in the whole interval of both additive concentrations evaluated. The positive influence of the compatibiliser was found from the point of melting enthalpy and melting entropy. [ABSTRACT FROM AUTHOR]

By using molecular dynamic calculation, we obtained the most stable structures for metallocene catalyst and cocatalyst systems. And we found that by using the 3-dimensional factors and interaction energy of these catalyst-cocatalyst systems, we can estimate their activity. We obtained the stable structures of catalyst and external donor systems of various Ziegler-Natta catalyst systems by using a similar calculation method, and found that the 3-dimensional factors of these stable structures and interaction energy can be used for the estimation of activities of these catalyst systems. This calculation method is very simple and so can be used for the design of various catalyst systems. Also by the results of these analyses, we found that the activity of the polymerization catalyst increased with increasing distance between the catalyst-cocatalyst or catalyst-external donor. This suggests that the ease of monomer insertion and polymer detachment are important factors for the polymerization process. [ABSTRACT FROM AUTHOR]

Abstract: A series of all-metal dinuclear alkaline-earth metallocenes, M2(η5-E5)2 (M=Be, Mg, Ca; E=Sb, Bi), have been calculated by the density functional theory (DFT). Natural Bonding Orbital (NBO) analysis indicates that the metal–metal bonds of the title compounds are all single bonds with each metal in its +1 oxidation state, and the bonding between the metal and the all-metal ligand is mainly ionic. For both M2(η5-Sb5)2 and M2(η5-Bi5)2, the metal–ligand bonds are very strong and the lighter alkaline-earth metal has stronger metal–ligand bonding. By comparing the dissociation energies of metal–metal bond and metal–ligand bond for each M2(η5-E5)2, the single metal–metal bond is much weaker than the metal–ligand bond with the same metals. The Mg–Mg bond is the strongest in the M2(η5-Sb5)2 and M2(η5-Bi5)2 species. Nucleus-independent chemical shifts (NICS) values suggest the planar exhibits characteristics of aromaticity in these M2(η5-E5)2 species. All the NICS values decrease in the order of Be2(η5-E5)2 >Mg2(η5-E5)2 >Ca2(η5-E5)2 except for the NICS (1.0) values of the M2(η5-Bi5)2 (M=Be, Mg, Ca) species. The absolute values of NICS (0.0), NICS (0.5) and NICS (1.0) for M2(η5-Sb5)2 are larger than those of the corresponding M2(η5-Bi5)2. [ABSTRACT FROM AUTHOR]

Abstract: In this study, the rheological characteristics of metallocene linear low density polyethylene (mLLDPE) were analyzed. Cyclohexane was used to prepare the dilute solutions of mLLDPE. The concentrations used in this study ranged from 1000 to 4500ppm with 1000ppm increments. The test temperatures ranged between 293 and 323K at 10K increments. The shear rate range was from 0.8 and 600s−1. Rheological measurements were performed on a rotational rheometer model AR-G2 with parallel plate geometry. The solution rheology was affected by shear rate, polymer concentration, and testing temperature. Different models were used to describe the rheological behavior of the mLLDPE dilute solutions. A model that accounted for concentration was successful in describing the rheological behavior as compared to models that were developed for polymer melt, which do not take the polymer concentration into account. [Copyright &y& Elsevier]

Copyright of Polimery is the property of Industrial Chemistry Research Institute and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The band structures of metallocene-based polymers in which the nearest metalloecenes were directly linked by a C-C bond have been determined by employing the tight-binding crystal orbital (CO) method. In particular, we determined the band structures of metallocene-based alternate copolymers whose metal sequences in a unit cell were Fe-Co, Fe-Fe-Co-Co, Fe-Ni, Ru-Co and Os-Co. The band structures of alternate copolymers of Fe-Co and Fe-Ni possessed a partially occupied (PO) band whose crystal orbitals were extremely localized at the 3d orbitals of cobalt or nickel atom. Alternate copolymers of Ru-Co and Os-Co had the same tendency as that of band structure of Fe-Co. Therefore, it was clear that the band structures of the metallocene-based alternate copolymers were considered to be a superposition of the polymetallocenylenes. Furthermore, we examined the effect of inserting the bridging part of boron-, phosphorus-, and sulfur bridges between the nearest ferrocenes upon the band structures of ferrocene-based polymers. The electronic structure of the boron-bridged polymer was similar to that of the silicon-bridged polymer whose band structure was determined in our previous study. On the other hand, the phosphorus- and sulfur-bridged polymers yielded two degenerated HO bands whose COs were localized at P or S 2p orbitals. [ABSTRACT FROM AUTHOR]

The present study is aimed at investigating the molecular structure, crystallinity, and morphology of polyethylene (PE) and polypropylene (PP) blends by using Raman mapping, scanning electron microscopy (SEM), wide angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC). In this study, three kinds of PEs. high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), and metallocene polyethylene (MEPE) were used. MEPE is one of the LLDPEs but its density is very low and its melt flow index (MFI) is very high. Blends of each PE and PP with a PP content ranging from 20 to 80 wt % with an increment of 20 wt % were prepared. Raman mapping images and SEM images show that the 80/20 blends of HDPE/PP and LLDPE/PP have similar dispersibility behavior and that only the 80/20 blend of MEPE/PP shows a different behavior in this respect. For the 20/80 blends, the differences are not so large. For the Raman mapping, the intensity ratio of the two bands at 1128 and 974cm-1 was used. The former is due to a symmetric C-C stretching mode of all-trans --(CH2)n- groups of PE while the latter is assigned to a CH3 rocking mode of PP. MEPE/PP blends yield quite different X-ray diffraction patterns compared to HDPE/PP and LLDPE/PP blends; the MEPE/PP blends show that with increasing MEPE content the crystalline size of PP become smaller. DSC curves of MEPE/PP show that the peak area changes linearly with the blending ratio and that the crystallization temperature does not change for any blend. These results mean that the density and MFI of PE influences the blend properties. [ABSTRACT FROM AUTHOR]

Cyclopentadienylvanadium complexes (CpV), in combination with triphenylcarbenium tetrakis (pentafluorophenyl) borate (IO) and trialkylaluminum, have been reported to be active catalysts for polymerization of butadiene. Polybutadiene obtained by using them is suitable as graft rubber for High-Impact Polystyrene (HIPS) because of its unique microstructure (cis: -90%, vinyl: -10%). The concentration of active centers and polymerization rate were determined by using a continuous polymerization plant with CpVCl3-Ph3C[B(C6F5)4]- Et3Al-H2O as a catalyst system and H2 as a chain transfer agent and by controlling H2 concentration by gas-liquid equilibrium. The efficiency of catalyst was evaluated to be 20–30%. The number average molecular weight (Me) was found to have first-order dependence on monomer concentration (Ma), concentration of active center ([C∗]), and the reciprocal of H2 concentration ([H2]). A polymerization simulator of this system was constructed using these results. [ABSTRACT FROM AUTHOR]

It is demonstrated that modern optical methods allow for a very fast study of the behaviour of supported metallocene catalysts. By videomicroscopy several polymeric supports functionalised with cyclopentadiene units or poly(ethylene oxide) and poly(propylene oxide) chains and loaded with a metallocene were simultaneously studied. In this example it is shown that this method allows for a direct comparison of different catalysts under identical conditions in a single experiment. In addition by this experiment more detailed information on the behaviour of the supports can be obtained by studying the growth kinetics of single grains. As a second tool, laser scanning confocal fluorescence microscopy (LSCFM) was applied. Using dye-labelled supports LSCFM can be used as a very rapid tool to study the fragmentation of supports. This is demonstrated by 3D images showing the distribution of different support fragments in the polyethylene product particles. [ABSTRACT FROM AUTHOR]

The objective of this paper is to provide a better understanding of how thermal relaxations in linear low density polyethylenes are related to the crack initiation energy of rotomoulded parts. Trials were carried out on two Ziegler–Natta catalysed linear low density polyethylenes (LLDPE) and two metallocene catalysed linear low-density polyethylenes (mLLDPE). Instrumented impact tests and dynamic mechanical thermal analysis (DMTA) were carried out on each material at a wide range of temperatures. The frequency of impact at each test temperature was determined, and the DMTA results were shifted to a corresponding frequency. A correlation can be seen between changes in loss modulus and crack initiation energy with temperature. When comparing the crack initiation energy of the samples and the tan δ values, it can be seen that the β transition affects the way in which the polymer behaves under impact. A method for predicting impact performance over a wide temperature range is proposed. [ABSTRACT FROM AUTHOR]

Studies the chemical heterogeneity of metallocene-catalysed copolymers of ethylene and higher 1-olefins using crystallization analysis fractionation and size exclusion chromatography coupled to Fourier transform infrared spectroscopy. Linear decrease in melting and crystallization temperature from differential scanning calorimetry with increasing amount of comonomer; Significance of comonomer concentration in chemical heterogeneity analysis.

Cites findings of a study in which copolymerization of ethylene and 4-vinylcyclohexene (VCH) were carried out by various metallocenes in the presence of Al(iBu)[SUB3]/[Ph[SUB3]C][B(C[SUB6]F[SUB5])[SUB4]]. Process used to insert VCH into the copolymer; Results of calculation of copolymerization parameters according to Kelen-Tudos; Reasons behind relatively low reactivity of VCH.

Reports that trialkylsilanols modify and also improve the catalytic performance in ethylene polymerization when added to zirconocene/methyl-aluminoxane (MAO) systems. Role of MAO with respect to the metallocene complex; Homopolymerization of ethylene in the presence of MAO; Characterization of polymers.

Relativistic DFT calculations recognized the regularity of geometric/electronic, electron-transfer, bonding and redox properties as well as their relationship for a series of low-valent uranium metallocenes while tuning the substituent of cyclopentadienyl ligands. • Uranium(II) metallocenes were theoretically designed via tuning substituent on Cp. • Reduction from [U(CpX) 3 ] to [U(CpX) 3 ]– slightly strengthens metal–ligand bond. • +III valence is assigned to U of [U(CpX) 3 ] but approximately + II for [U(CpX) 3 ]–. • Uranium-dominated redox mechanism is proposed. • Correlation is built between substituent and various properties of complex. Over a century of investigations on uranium complexes resulted in great progress in establishing their fundamental properties and developing applications. However, the in-depth understanding of uranium chemistry, particularly in record low + II valence, is still challenging due to rare crystallographically-identified U(II) complexes, extremely low stability and rigid synthetic conditions. Herein, to further expand experimental findings, various substituted-cyclopentadienyl uranium complexes, [U(CpX) 3 ] n (X = Me, H, Cl, SiMe 3 and 2SiMe 3 ; n = -1 and 0), have been explored using relativistic density functional theory. It is shown that the reduction from [U(CpX) 3 ] to [U(CpX) 3 ]– slightly strengthens the U-Cp bonding, while the great enhancement is found in previously reported uranium arene analogues. The + III oxidation state is assigned to the metal of [U(CpX) 3 ]; in contrast, it is approximately + II valence for [U(CpX) 3 ]– because of non-negligible electron density residing on the CpX ligands, whose amount positively correlates with the electron-withdrawing ability of X substituent. Calculated reduction potentials range from −3.02 to −2.74 V while considering solvation and spin–orbit coupling effects. The largest value of the chloride complex couples is attributed to the better balance of electronic and steric effects. [ABSTRACT FROM AUTHOR]

meso-Tris(4-ferrocenylphenyl)subporphyrin 1 and mesotriferrocenylsubporphyrin 2 were prepared. The structure of 2 was confirmed by X-ray analysis. Both molecules exhibit substantially red-shifted absorption spectra and almost complete fluorescence quenching, due to the influence of the electrondonating ferrocene substituents. Subporphyrin 1 exhibits a nonsplit oxidation wave for the oxidation of the ferrocene moieties, while subporphyrin 2 exhibits a split reversible oxidation wave with ΔE = 168mV to indicate effective electronic communication. [ABSTRACT FROM AUTHOR]

Kinetically stabilized 1,1′-bis(diphosphenyl)ferrocenes have been synthesized by taking advantage of extremely bulky substituents, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) and 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)-methyl]phenyl (Bbt) groups, and fully characterized by the spectroscopic and X-ray crystallographic analyses. The electrochemical behavior showed well-defined two reversible one-electron reduction couples. [ABSTRACT FROM AUTHOR]

The synthesis of bis[(3a,4,5,6,6a-η)-1,3,4,5,6-pentamethylcyclopenta[d]imidazo-2-thionoyl]iron(II) is reported. Structural determination of the unusual hetero-substituted metallocene by X-ray crystallographic analysis is described. Preliminary electrochemical studies reveal that oxidation and reduction potentials for the subject ferrocene lie midway between those of ferrocene and decamethylferrocene. [ABSTRACT FROM AUTHOR]

A polymer matrix isolation technique was examined as a potentially novel strategy for realizing optically switchable materials. Not only was a ferricinium ion ([Cp[sub2]Fe[subIII]][sup+]) produced, but also was produced an isomer of ferrocene (Cp[sub2]Fe) with high spin Fen in a poly(methyl methacrylate) (PMMA) matrix film containing CCl[sub4] by the photoillumination of ferrocene at low temperature. The high spin state could be maintained for an extended period when the sample was kept below ≈ 130 K. This is the first observation of a photoinduced spin transition for ferrocene. The strategy that we used to achieve the spin transition using optical stimuli may open up many possibilities for the development of new magneto-optical devices. [ABSTRACT FROM AUTHOR]

Studies the direct incorporation of primary amine into nonpolar polyolefins mediated by a metallocene IFZ catalyst. Information on the polymerization system; Details on the experiments; Results of the experiments.

Reports on the investment of Clariant GmbH to build the first commercial-scale plant. Production of waxes based on polypropylene, polyethylene, and copolymers via metallocene catalysis in the plant; Significance of the new plant for the increased production of Licocene.

Reports on ExxonMobil Chemical's launching of a metallocene ethylene elastomer (MEE) manufacturing facility in Baton Rouge, Louisiana. Plant to serve as the global supply point for the company's MEE products including Exact plastomers and Vistalon ethylene propylene diene monomer; Use of Exxpol metallocene catalyst technology and MEE technology to allow customer-specified adjustment in product formulation.

Features the Vistamaxx propylene-based specialty elastomers from ExxonMobil Chemical Co. Production of olefin products that possess properties and attributes beyond conventional metallocene technology; Control of the level and nature of propylene crystallinity in polymer; Inclusion of free-flowing elastomer pellets.