Natta Catalysis Research Articles

Structural analysis of transition metal beta-X substituent interactions. Toward the use of soft computing methods for catalyst modeling

Fuzzy logic and neural network techniques are used to classify intramolecular interactions between transition metals (M) and beta-X substituents in the following structural motif (LnMC(alpha)(A1)(A2)-C(beta)(B1)(B2)X). These interactions are relevant to the direct polymerization of functionalized olefins by Ziegler-Natta (ZN) catalysis. The efficiency and effectiveness of different soft computing techniques are compared. These methods give not only encouraging results with respect to general data mining issues but also insight into the factors that effect interactions between transition metals and beta-X substituents.

Silanolytic Chain Transfer in Ziegler−Natta Catalysis. Organotitanium-Mediated Formation of New Silapolyolefins and Polyolefin Architectures

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTSilanolytic Chain Transfer in Ziegler−Natta Catalysis. Organotitanium-Mediated Formation of New Silapolyolefins and Polyolefin ArchitecturesKwangmo Koo and Tobin J. MarksView Author Information Department of Chemistry, Northwestern University Evanston, Illinois 60208-3113 Cite this: J. Am. Chem. Soc. 1998, 120, 16, 4019–4020Publication Date (Web):April 11, 1998Publication History Received2 July 1997Published online11 April 1998Published inissue 1 April 1998https://doi.org/10.1021/ja972194xCopyright © 1998 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views596Altmetric-Citations64LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (94 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Alkyls,Catalysts,Copolymerization,Hydrocarbons,Polymers Get e-Alerts

Displacement of weakly coordinating anions from zirconocene alkyl cations by trialkyl phosphines: A model for olefin coordination in homogeneous Ziegler–Natta catalysis

Displacement of the anion H 3C–B(C 6F 5) 3 − from the zirconocene contact-ion pair [(C 5H 5) 2Zr(CH 3) +⋯( μ-H 3C)–B(C 6F 5) 3 −] by a series of phosphines and formation of a bisphosphine complex by uptake of a second phosphine ligand have been studied by NMR methods in C 6D 6 solutions. Evidence is presented that associated, outer-sphere ion pairs [(C 5H 5) 2Zr(CH 3)(PR 3)] + H 3C–B(C 6F 5) 3 − predominate at zirconocene concentrations up to ca. 2 mM; higher aggregates become apparent at higher zirconocene concentrations. Equilibrium constants for the formation of mono- and bis-PMe 3 complexes have been determined; the latter are found to be highly sensitive to steric perturbations. These data, together with density-functional estimates for reaction enthalpies, lead to the following qualitative conclusions with regard to olefin coordination in zirconocene-based polymerization catalysts: (i) The olefin substrate displaces the borate anion only from a small equilibrium fraction of the zirconocene alkyl cations present; (ii) the predominant fraction of the resulting zirconocene alkyl olefin cation remains in an outer-sphere association with the counteranion; (iii) mutual displacement of olefin and counteranion from the Zr center is slow compared to typical olefin insertions; (iv) uptake of a second olefin under formation of a cationic zirconocene alkyl diolefin complex is rather unlikely even at elevated olefin concentrations.

Structure and Stability of Aluminum Alkyl Cocatalysts in Ziegler−Natta Catalysis

The structure of organoaluminum(halide) complexes, used as cocatalysts in Ziegler−Natta catalysis, is investigated by experimental far-infrared spectroscopy and by quantum simulations. Experimental...

External Silane Donors in Ziegler–Natta Catalysis. A Three-Site Model Analysis of Effects on Catalyst Active Sites

A three-site model of the active sites in a MgCl2-supported Ziegler–Natta catalyst was used to simulate the effects of various external alkoxysilane donors on the catalyst active sites. The simulations correlated well with the experimental data, and it is evident that the polypropylene produced by this catalyst system originates mainly from three types of active sites. One site is a highly isospecific enantiomorphic site producing boiling heptane insoluble polymer. Two sites are responsible for the major part of the polymers of the heptane soluble firaction: a moderately isospecific enantiomorphic site (the fluctuating site) and a symmetric Bernoullian syndiospecific site. All the alkoxysilanes tested, including monoalkoxysilanes, exhibited a stabilizing role, making fluctuating sites more isospecific. In addition, the silanes having more than one alkoxy group were capable of deactivating active Ti-sites, and the selectivity of deactivation depends mainly on the size of the hydrocarbon part.

External silane donors in Ziegler—Natta catalysis: a two-site model simulation of the effects of various alkoxysilane compounds

The results of experiments with a supported Ziegler-Natta catalyst were interpreted by a two-site model of the active sites, in order to enhance the understanding of multiple types of catalyst active sites and in particular the role of the external donor. The applicability of the two-site model is discussed and the stochastic parameters of the model are correlated with both the experimental and molecular modelling data. The two-site model satisfactorily explains the major part of the active sites of this highly isospecific catalyst and is a suitable tool for analysing the effects of the donors on catalyst active sites. Nevertheless, it does not completely describe the behaviour of Mg-supported Ziegler-Natta Ti catalyst. There is probably more than one kind of active site producing the part of polypropylene that is soluble in boiling heptane. This study also supports the earlier suggestion that some of the changes in the population of active sites can be explained only through the formation of new sites, and more specifically through the existence of fluctuating sites that can be stabilized by external donors. The stability of these new sites depends on both the structural and electronic properties of the external donors.

Transition Metal Complexation of σ Bonds

AbstractThe first σ complexes were found in the 1960s and 1970s, but they did not attract more than passing attention. Only now are we beginning to recognize their key role in the chemical reactions of σ bonds, and this has encouraged more detailed study. In contrast with the more familiar π‐donor complexes such as M(CH2CH2) and complexes like MNH3, in which the one pair of electrons on the N atom is bound to the metal atom, in a σ complex an XH group binds to the transition metal atom; the XH σ‐bonding electron pair acts as a 2e donor to give an (X‐H)‐M type complex. Dihydrogen complexes (X = H) are one important group of σ complexes. C‐H‐M complexes (X = R3C) with an agostic C‐H‐M interaction have not only been found in the ground state but also implicated in the transition states of many important organometallic transformations such as Ziegler–Natta catalysis and sigma bond metathesis. The importance of XH bond activation will encourage continued growth in this field.

Heterogeneous Ziegler‐Natta catalysts: On going progress for the polyolefin industry

AbstractA great deal of research and development efforts in Ziegler Natta catalysis has been put within the last ten years into the polyolefin industry. The exploitation of new catalysts, the introduction and consolidation of advanced polymerization process technologies and, finally, the development of a broader range and of a better tailoring of product grades, have made the polyolefin market an expression of a young, dynamic, technology‐driven industry. In this context heterogeneous catalysts have played a key role. Many drawbacks of the catalysts of the previous generation have been overcome. However some problems have still to be solved in order to both further improve catalyst performance and reduce the residual areas of empiricism in catalyst design and manufacture.

U.S. 4,665,114 Stabilizers

A better understanding of structure-property relations is necessary to design novel materials. In this study, we investigate the morphology and chemical structure of five commercial grades of propylene-based polymers in relation to the change in yield- stress as a function of strain-rate. Substantial emphasis has been laid on understanding the chain microstructure in the relation to chain dynamics in the amorphous phase. Heterogeneous Ziegler–Natta catalysis was used to prepare the samples with differing ratios of propylene and ethylene units. Various analytical techniques such as WAXS, SAXS, solution- and solid-state NMR were employed to characterize their structure. The results indicate a reduction in crystallinity, melting temperature, long-period and crystal thickness with increasing ethylene content. Solid-state NMR data reveal the presence of four components in these samples, which is an extension of the traditional three phase model found in most semi-crystalline polymers. The additional fourth phase is attributed to a rubber-like component that is primarily composed of chain segments rich in ethylene units and shows an increase in chain dynamics with increasing ethylene content in the samples. Mechanical experiments show that yield stress decreases with increase in the amount ethylene which can be correlated to the observed increase in chain dynamics in the amorphous phase.

Ethylene–styrene diblock copolymers: Synthesis by Ziegler–Natta catalysis and identification

AbstractA new way of directly synthesizing ethylene–styrene diblock copolymer by Ziegler–Natta catalysis was studied. It consists of using a soluble complex prepared by the reaction of with TiCl4 in the ratio r = [Li]/[Ti] = 2 to initiate the formation of the polyethylene block. The well‐defined linear diblock structure was established both by GPC of the polyethylene block and by 13C NMR and DSC analyses of the copolymer. Moreover, by using such a catalytic system, the efficiency was found to be 0.88 with respect to carbon‐titanium bond. This value is relatively high compared to those of conventional Ziegler‐Natta catalytic systems.

Template catalysis via non-transition metal complexes. New highly selective syntheses on phenol systems

Abstract

A SCF-Xα-SW molecular-orbital study of a possible reaction path for Ziegler–Natta catalysis

SCF-Xα-SW molecular-orbital calculations for the coordination complex [CH3TiCl4 (C2H4)]n = 0,−2, which is postulated to be an intermediate in Ziegler–Natta polymerization reactions, demonstrate that the energy gap between the lower lying Ti-alkyl orbital and the higher Ti (3dyz)-olefin (π*) orbital is sufficiently large (0.9 eV for n = 0 and 1.5 eV for n = −2) to make the Cossee mechanism for the propagation step seem unlikely. A series of calculations for CH3TiCl4(C2H4), where the alkyl and olefin groups are moved closer together in a sequence of concerted steps, suggests that this migration is a plausible mechanism for the polymerization reaction.

Ziegler–Natta catalysis: an alternative mechanism involving metallacycles

Semi-empirical molecular orbital calculations suggest a new catalytic sequence for Ziegler–Natta polymerization involving a metallacyclopentane intermediate.

Hydrodynamically induced crystallization of isotactic polystyrene

This report provides a brief preliminary account of the solution crystallization of isotactic polystyrene under the influence of velocity gradients. During crystallizations from 1,3,5-trimethylbenzene and cyclohexanol solutions, the polymer appears in a number of morphological forms which are composed of microfibrillar units. In isotactic polystyrene produced by heterogeneous Ziegler–Natta catalysis, similar microfibrillar units are observed. It is suggested, therefore, that these two apparently different processes have a common underlying mechanism in the formation of the microfibrils. The possible mechanisms of fibril formation are discussed.

Intrinsic viscosity and molecular weight of isotactic polypropylene

AbstractTwo samples of highly isotactic polypropylene prepared by a NATTA's catalysis, have been fractionated by a column fractionation technique.Fractions having M̄w/M̄n ratio very small (1.14 – 1.27) have been obtained and used for the determination of [η]−M̄n and [η]−M̄w relations. Because of the small values of M̄w/M̄n ratio, the [η]−M̄w relation may be assumed also valuable as [η]−M relation. This relation is for molecular weight range of 40,000–650,000, where [η] is measured in tetralin at 135°C. Moreover values of \documentclass{article}\pagestyle{empty}\begin{document}$\sqrt {\overline {r_z^2 } }$\end{document} and of second virial coefficients A2 are given in function of molecular weight.