Kinetics heterogeneous Ziegler-Natta polymerization

The kinetics of Ziegler-Natta polymerization, like the mechanism of the reaction, are complex. The relatively few polymerizations that are homogeneous behave in a manner generally similar to noncoordination ionic polymerizations (Chapter 8). However, as has been pointed out earlier, the heterogeneity of the reaction system is more of a rule in Ziegler-Natta polymerization than an exception. The heterogeneous systems usually exhibit complicated behavior [2,5,7], as can be seen from some typical kinetic rate-time profiles, types (a)-(f) in Fig. 9.6. Such profiles are significant in... 

The kinetics of Ziegler-Natta polymerization are complicated by the heterogeneous nature of the reaction and so will only be considered in simple outline here. The rate of polymerization is given by... 

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). 

Adsorption phenomena are important in most Ziegler-Natta polymerizations, and this requires treatment by heterogeneous kinetics [Bohm, 1978 Boor, 1979 Chien and Ang, 1987 Chien and Hu, 1987 Cooper, 1976 Keii, 1972 Tait and Watkins, 1989]. The exact form of the resulting kinetic expressions differs depending on the specific adsorption... 

Compared to conventional heterogeneous Ziegler-Natta systems in which a variety of active centers with different structures and activities usually coexist, homogeneous metallocene-based catalysts give very uniform catalyt-ically active sites which possess controlled, well-defined ligand environments [37]. Consequently, the polymerization processes in homogeneous systems are often more simple, and kinetic and mechanistic analyses for these systems are greatly simplified [38]. 

The kinetics are much simpler in homogeneous metallocene-based catalyst systems, especially in base-free cationic catalyzed polymerization systems, than those in heterogeneous systems. The polymerizations with homogeneous metallocene catalysts are no doubt the best systems for kinetic study of Ziegler-Natta polymerization. The a-olefin polymerization with these catalysts also offers a good opportunity to study the durability and deactivation of the catalysts, since the polymerization systems remain homogeneous over a considerable long reaction period [50]. 

Kinetics of Olefin Polymerization and Active Sites of Heterogeneous Ziegler-Natta Catalysts... 

There is vast experimental and theoretical evidence supporting the existence of multiple site types on heterogeneous Ziegler-Natta catalysts. From a mathematical modelling point of view, each site type has distinct polymerization kinetic constants and consequently produces polymer chains with different average properties. 

Very many kinetic models have been proposed for both heterogeneous and homogeneous Ziegler-Natta polymerization (c.f. 2,8,18) but for the purposes of this paper only the recent models proposed by Tait et al (26,29f30), Yermakov et al (31) and Bohm (39) will be considered. These three kinetic treatments have the common features that chain propagation is treated as a two-stage reaction, monomer... 

Kinetic model of Bohm. A very comprehensive reaction model has been proposed by Bohm (39) idiich is claimed to explain, at least in a qualitative manner, the knovm kinetic phenomena for both heterogeneous and homogeneous Ziegler-Natta polymerization reactions. This model has the following characteristics. 

Keii, T. Heterogeneous Kinetics Theory of Ziegler—Natta—Kaminsky Polymerization Springer Berlin, 2004 Vol. 77. 

Much effort has been devoted during the last 30 years toward understanding the mechanisms operative in the coordination catalysis of ethylene and a-olefin polymerization using Ziegler-Natta systems (metal halide and aluminum alkyl, sometimes with Lewis base modifiers). Aspects of the complex heterogeneous reactions have been elucidated (jL- ) but the intimate mechanistic detail - for example the role of inhibitors and promoters, kinetics and thermodynamics of chain growth, modes of chain transfer and termination - comes primarily from studies of homogeneous catalysts ... 

Catalysts and Kinetics. Hundreds of variants and combinations of catalysts, cocatalysts, catalyst pretreatments, and reaction conditions have been discovered and described, mostly in the patent literature (28). It is now generally agreed that most coordination polymerizations are heterogeneous, but that some are clearly homogenous. The basic characteristic that distinguishes all Ziegler/Natta-type stereoregular polymerization catalysts is that... 

Ziegler-Natta catalyst systems being mostly heterogeneous in nature, adsorption reactions are most likely to occur in such polymerizations and feature in their kinetic schemes (Erich and Mark, 1956). A number of kinetic schemes have thus been proposed based on the assumption that the polymerization centers are formed by the adsorption of metal alkyl species on to the surface of a crystalline transition metal halide and that chain propagation occurs between the adsorbed metal alkyl and monomer. In this regard the Rideal rate law and the Langmuir-Hinshelwood rate law for adsorption and reaction on solids assume importance see Problem 9.4). 

Heterogeneous Kinetics Theory of Ziegler-Natta-Kaminsky Polymerization... 

Keii T (2004) Heterogeneous kinetics theory of Ziegler-Natta-Kaminsky polymerization, vol 77, Springer series in chemical physics. Springer, Berlin... 

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