Yury V. Kissin

Ethylene polymerization reactions with Ziegler–Natta catalysts. I. Ethylene polymerization kinetics and kinetic mechanism

Kinetics of ethylene homopolymerization reactions and ethylene/1-hexene copolymerization reactions using a supported Ziegler–Natta catalyst was carried out over a broad range of reaction conditions. The kinetic data were analyzed using a concept of multicenter catalysis with different centers that respond differently to changes in reaction parameters. The catalyst contains five types of active centers that differ in the molecular weights of material they produce and in their copolymerization ability. In ethylene homopolymerization reactions, each active center has a high reaction order with respect to ethylene concentration, close to the second order. In ethylene/α-olefin copolymerization reactions, the centers that have poor copolymerization ability retain this high reaction order, whereas the centers that have good copolymerization ability change the reaction order to the first order. Hydrogen depresses activity of each type of center in the homopolymerization reactions in a reversible manner; however, the centers that copolymerize ethylene and α-olefins well are not depressed if an α-olefin is present in the reaction medium. Introduction of an α-olefin significantly increases activity of those centers, which are effective in copolymerizing it with ethylene but does not affect the centers that copolymerize ethylene and α-olefins poorly. To explain these kinetic features, a new reaction scheme is proposed. It is based on a hypothesis that the Ti—C2H5 bond in active centers has low reactivity due to the equilibrium formation of a Ti—C2H5 species with the H atom in the methyl group β-agostically coordinated to the Ti atom in an active center.

Kinetics and mechanism of ethylene homopolymerization and copolymerization reactions with heterogeneous Ti-based Ziegler–Natta catalysts

A detailed kinetic analysis of ethylene homopolymerization reactions and its copolymerization reactions with 1-hexene with a supported Ti-based Ziegler–Natta catalyst (reactions in the absence and the presence of hydrogen) shows a number of distinct kinetic features which are interpreted as a manifestation of multi-site catalysis; the catalyst contains several types of polymerization centers which differ in stability and formation rates, the molecular weight of polymers they produce, and in their response to the presence of α-olefins and hydrogen. All these effects require introduction of a special kinetic mechanism which postulates an unusually low activity of growing polymer chains containing one ethylene unit, the Ti–C2H5 group, in the ethylene insertion reaction into the Ti–C bond. This peculiarity of the Ti–C2H5 group, which is probably caused by its β-agostic stabilization, predicts two kinetic/chemical features of ethylene polymerization reactions which have not been described yet, the deuterium effect on the homopolymer structure and the activation effect of α-olefins on chain initiation. Both effects were confirmed experimentally.

Ethylene polymerization reactions with Ziegler–Natta catalysts. III. Chain‐end structures and polymerization mechanism

Ethylene polymerization reactions with many Ziegler–Natta catalysts exhibit a number of features that differentiate them from polymerization reactions of α olefins: (1) a relatively low ethylene reactivity, (2) markedly higher polymerization rates in the presence of α olefins, (3) a high reaction order with respect to ethylene concentration, and (4) a strong reversible rate depression in the presence of hydrogen. A detailed kinetic analysis of ethylene polymerization reactions1 provided the basis for a new kinetic scheme that postulates the equilibrium formation of TiC2H5 species with the H atom in the methyl group β-agostically coordinated to the Ti atom in an active center. This mechanism predicts several new features of ethylene polymerization reactions, one being that chain initiation via insertion of any α-olefin molecule into the TiH bond should proceed with an increased probability compared to that via ethylene insertion into the same bond. As a result, a significant fraction of ethylene/α-olefin copolymer chains should contain α-olefin units as the starting units. This article provides experimental data supporting this prediction on the basis of both a detailed structural analysis of co-oligomers formed in ethylene/1-pentene and ethylene/4-methyl-1-pentene copolymerization reactions and a spectroscopic analysis of chain ends in the copolymers.

Ethylene polymerization reactions with Ziegler–Natta catalysts. II. Ethylene polymerization reactions in the presence of deuterium

Ethylene polymerization reactions with many Ziegler–Natta catalysts exhibit several features which differentiate them from polymerization reactions of α-olefins: a relatively low ethylene reactivity, higher polymerization rates in the presence of α-olefins, a high reaction order with respect to ethylene concentration, and strong reversible rate depression in the presence of hydrogen. A detailed kinetic analysis of ethylene polymerization reactions (see ref. 1) provided the basis for a new reaction scheme which explains all these features by postulating the equilibrium formation of a TiC2H5 species with the H atom in the methyl group β-agostically coordinated to the Ti atom in an active center. This mechanism predicts that the β-agostically stabilized TiC2H5 groups can decompose in the β-hydride elimination reaction with expulsion of ethylene and the formation of a TiH bond even in the absence of hydrogen in the reaction medium. If D2 is used as a chain transfer agent instead of H2, the mechanism predicts the formation of deuterated ethylene molecules, which copolymerize with protioethylene. To prove this prediction, several ethylene homopolymerization reactions were carried out with a supported Ziegler–Natta titanium-based catalyst in the presence of large amounts of D2. Analysis of gaseous reaction products and polymers confirmed the formation of several types of deuterated ethylene molecules and protio/deuterioethylene copolymers, respectively. In contrast, a metallocene catalyst, Cp2ZrCl2MAO, does not exhibit these kinetic features. In the presence of deuterium, it produces only DCH2CH2(CH2CH2)xCH2CH2D molecules.

‘Homogeneous’ interpretation of ethylene polymerization kinetics with supported ziegler-natta catalysts

Abstract Kinetic studies of ethylene gas-phase homopolymerization have been carried out using a supported Ti-based Ziegler-Natta catalyst in the presence and absence of hydrogen. The dependence of the gas-phase polymerization rate on ethylene pressure may be described by the empirical relationship R = k eff · P n E , with n = 1.5–1.6. This effect of ethylene pressure on the rate is completely reversible. The action of hydrogen is also reversible but decreases the ethylene polymerization rate. However, progressively higher hydrogen pressures are incapable of halting the polymerization reaction completely, but reduce the reaction rate by ca . one-half. A reaction scheme for ethylene homopolymerization has been developed which accounts for the observed kinetic behavior. The scheme includes two equilibrium reactions: the activation of potential polymerization centers by ethylene and the reversible poisoning of active centers by hydrogen.