Ziegler catalysts kinetics

For so-called multi-site catalysts, Ziegler catalysts for example, kinetic constants might vary with the type of active sites, see [10], In such a case, one should summarize Eqn.(5.4-1) over all contributions of different active sites, or kp is interpreted as an average value of all contributing sites. 

Breslow (139) discovered a homogeneous system well suited for kinetic analysis. He realized that bis(cyclopentadienyl)titanium(IV) compounds, which are very soluble in aromatic hydrocarbons, could be used instead of titanium tetrachloride as the transition-metal compound together with aluminum alkyls to give Ziegler catalysts. Subsequent research on this and other systems with various alkyl groups has been conducted by Natta et al. (140, 141), Belov el at. (142-144), Patat (145), Patat and Sinn (146) Sinn et al. (119, 147), Shilov and co-workers (148-150), Chien and Hsieh (20), Adema (151), Clauss and Bestian (152), Henrici-Olive and Olive (153), and Reichert and Schoetter (154) and Fink (155). 

Fink, G., Herfert, P. and Montag, P., The Relationship Between Kinetics and Mechanism , in Ziegler Catalysts, Springer-Verlag, Berlin, 1995, pp. 159-179. 

Subsequent research on this and other systems with various alkyl groups was conducted by Natta (39), Belov et al. (40,41), Patat and Sinn (42), Shilov et al. (43, 44), Chien (45), Adema (46), Clauss and Bestian (47), Henrici-Olive and Olive (48), Reichert and Schoetter (49), and Fink et al. (50, 51). Investigations of kinetics and various other methods have helped to define the nature of the active centers of some homogeneous catalysts, to explain aging effects of solid Ziegler catalysts, to establish the mechanism of the interaction of the catalyst with olefins, and to provide quantitative evidence of some elementary steps (10). 

K. H. Reichert, and K. R. Meyer, Kinetics of Low Pressure Polymerization of Ethylene by Soluble Ziegler Catalysts, Makromol. Chem. 169, 163-176 (1973). 

Ziegler-Natta Catalysts Kinetics of Ziegler-Natta Polymerizations Practical Features of Ziegler-Natta Polymerizations Comparisons of Cis-1,4-Polydienes Metallocene Catalysts... 

Kinetic studies and the application of various analytical methods have helped to define the nature of the active center, to explain the aging effects of Ziegler catalysts, to establish the meehanism of interaction with olefins, and to obtain, evidence of elementary steps [21]. 

Fink, G. Herfert, N. Montag, P. The Relationship Between Kinetics and Mechanisms. In Ziegler Catalysts-, Fink, G., Miilhaupt, R., Brintzinger, H.-H., Eds. Springer Berlin, 1995 p 159. 

Tail, P.J.T. Zohuri, G.H. Kells, A.M. McKenzie, I.D. Kinetic studies in propene polymerization using magnesium dichloride supported 65. Ziegler-Natta catalysts. In Ziegler Catalysts. 

The first generation of catalysts based on titanium tetrachloride or different modifications of titanium trichloride and ethyl-, isobutyl-, isoprenyl-, or chloride containing aluminum compounds, Ziegler catalysts, (Table 1) was characterized by low polymerization activity. Kinetic studies and applications of various methods have helped to define the nature of the active center . ... 

Most of the homogeneous Ziegler-type catalysts have been preferentially investigated in order to understand the elementary steps of the polymerization, which is simplified in soluble systems. Bis(cyclopentadienyl)titanium(IV) compounds, which are soluble in aromatic hydrocarbons could be used together with aluminum alkyls to give Ziegler-catalysts. As to the kinetics of polymerization and to side reactions of the catalyst components, this system is probably the best understood. It has not been used in a technical process because of the low activity and short life of systems that contain chloride (see Table 1). 

Fig. 30 Ethylene/propylene copolymerization (gas feed ratio 0.25) with the Ziegler-catalyst system TiCLi/AfEts/extemal donor supported on MgCl2 (0.2 MPa, 50°C) video microscopic snap-shots and kinetic evaluation of six particles...

Fink G, Miilhaupt R, Brintzinger HH, eds. Ziegler Catalysts. Berlin Springer-Verlag, 1995. Kucera M. Mechanism and Kinetics of Addition Polymerizations. Amsterdam Elsevier, 1992. Carraher CE, Moore JA. Modification of Polymers. New York Plenum Press, 1983. 

In this work a mesoscale/macroscale level approach of the Borstar plant is attempted focusing on the study of average polymer properties, dynamic behaviour and control of process units. To describe the kinetic of ethylene-1-butene copolymerization in the plant a unified kinetic scheme for the three reactor units based on a two-site Ziegler catalyst is employed (Table 1). The symbol denotes the concentration of live copolymer chains of total length n ending in an i monomer unit, formed at the k catalyst active site. Pp and denote the concentrations of the activated vacant catalyst sites of... 

Table 1 Kinetic mechanism of olefin copolymerization over a Ziegler catalyst.

The polymerization of olefins, promoted by homogeneous Ziegler catalysts based on biscyclopentadienyltitanium(IV) or analogous compounds and aluminum alkyls, is accompanied by a series of other reactions that greatly complicate the kinetic interpretation of the polymerization process ... 

The resulting complex (complex II), octahedral and with a vacant coordination position, is considered to be a potentially active site. The kinetic behavior has been shown to be consistent with a simple Ziegler-Natta kinetic scheme involving a rapid bimolecular decomposition of the active centers [481]. In general, all these reactions were found to have polymerization rates that are first order with regard to the monomer and 0.5 order with regard to the catalyst or initiator and oxygen [458]. 

The polymerization of olefins, promoted by homogeneous Ziegler catalysts based on biscyclopentadienyltitaniumflV) or analogous compounds and aluminum alkyls, is accompanied by a series of other reactions such as alkylation, hydrogen transfer, and reduction that greatly complicate the kinetic interpretation of the polymerization. It was found that the polymerizatiOTi takes place primarily into a Ti-C bond where the titanium exists as the titaniumflV) alkyl cation formed by alkylation and dissociation (1) and (2) [8-11] ... 

Methods used to more completely characterize the kinetic behavior of high-activity Ziegler catalysts in terms of the number and behavior of each of these multi-site catalysts will be summarized below [67,68]. 

However, ethylene polymerization reactions in the presence of supported Ti-based Ziegler catalysts often exhibit behavior that is unusual compared to the polymerization reactions of a-olefins such as propylene, 1 -butene, etc. This behavior cannot be explained by the standard kinetic scheme shown above. 

A first simple kinetic scheme for the copolymerization of two monomers, and M2, with heterogeneous Ziegler catalysts can be presented as follows ... 

The use of water as a co-catalyst in Ziegler-type polymerizations was first introduced in 1962 (47). The reaction kinetics and crystallinity of the resulting polymers measured by x-ray scattering has been studied (48—51). 

Evaluation and Analysis of a Multisite Kinetic Model for Polymerization Initiated with Supported Ziegler—Natta Catalysts... 

Heterogeneous Ziegler-Natta catalysts used to polymerize olefins exhibit phenomena characteristic of active site heterogeneity (1- 5). Complex kinetic models which account for this likelihood have been developed and used only in simulation studies (6-7). 

The sterically unencumbered catalyst active site allows the copolymerization of a wide variety of olefins with ethylene. Conventional heterogeneous Ziegler/Natta catalysts as well as most metallocene catalysts are much more reactive to ethylene than higher olefins. With constrained geometry catalysts, a-olefins such as propylene, butene, hexene, and octene are readily incorporated in large amounts. The kinetic reactivity ratio, rl, is approximately... 

The summations extend from n = 2 to n. = oo.) Keii [Kinetics of Ziegler-Natta Polymerization, Kodansha, Tokyo, 1972] has noted that under steady-state reaction conditions, the number of polymer molecules with degree of polymerization n desorbing per unit catalyst surface area in unit time may be written as... 

Although Ziegler-type catalysts have been widely investigated for the homogeneous hydrogenation of polymers, their catalytic mechanism remains unknown. One possible reason for this may be the complexity of the coordination catalysis and the instability of the catalysts. Metallocene catalysts are highly sensitive to impurities, and consequently it is very difficult to obtain reproducible experimental data providing reliable kinetic and mechanistic information. 

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