Ziegler propagation centers

Two-component systems are obtained by the interaction of transition metal compounds of groups IV-VIII of the periodic system with or-ganometallic compounds of groups I-III elements (Ziegler-Natta catalysts). An essential feature of the formation of the propagation centers in these catalysts is the alkylation of the transition metal ions by an organo-metallic cocatalyst. 

Despite the difference in composition of various olefin polymerization catalysts the problems of the mechanism of their action have much in common. The difference between one-component and traditional Ziegler-Natta two-component catalysts seems to exist only at the stage of genesis of the propagation centers, while the mechanism of the formation of a polymer chain on the propagation center formed has many common basic features for all the catalytic systems based on transition metal compounds. 

In theory, any asymmetric olefmic, as well as acrylic, styre-nic, and vinylic, monomer can have tactic chain microstructures. However, it depends on the coordination power of catalysts. Free-radical polymerization processes do not have any coordination power and thus produce atactic polymers. Ionic polymerization processes have certain levels of coordination power, contributed by counterions. The counterion at the vicinity of a propagating center can coordinate the insertion of monomer molecules. Unfortunately, this coordination power is weak and yields a low degree of tacticity. As a result of the equilibrium between ion pairs and free ions, individual chains can have some tactic segments generated from the former with others atactic from the latter. In comparison, Ziegler-Natta catalysts possess strong coordination power for tactic polymer production. 

The coordination polymerization of ethylene and a-olefins with Ziegler-Natta catalysts involves, in general, many elementary reactions, such as initiation (formation of active centers), chain propagation, chain transfers and chain terminations. The length of growing polyolefin chains is limited by the chain-terminating processes, as schematically represented (for ethylene) by 21,49 51)... 

While this review discloses the kinetic and stereochemical features of soluble Ziegler-Natta catalysts, we have little information on the structure of the active center. The steric environments of active centers must be very important in determining the monomer reactivity, regiospecificity and stereospecificity of soluble catalyst. The influence of ligands such as the aluminum components on the rates of chain propagation and chain-terminating steps should be correlated to the electronic structure of... 

The active center I, which first initiates and then propagates the chain, can be a free radical, an anion, a cation or a transition-metal based initiator (Ziegler-Natta systems or metathesis reactions). 

Novel data on the composition of active centers of Ziegler-Natta catalysts and on the mechanism of propagation and chain transfer reactions are reviewed. These data are derived from the following trends in the study of the mechanism of catalytic polymerization a) determination of the number of active centers (mainly with the use of radioactive CO as a tag) b) analysis of the microstructure of polymers with the use of C-NMR c) analysis of specific features of highly active supported catalysts d) quantum-chemical calculation of the electronic structure of active centers and their reactions. 

The two-component catalytic systems used for olefin polymerization (Ziegler-Natta catalysts) are combinations of a compound of a IV-VIII group transition metal (catalyst) and an organometallic compound of a I-III group non-transition element (cocatalyst) An active center (AC) of polymerization in these systems is a compound (at the surface in the case of solid catalysts) which contains a transition metal-alkyl bond into which monomer insertion occurs during the propagation reaction. In the case of two-component catalysts an AC is formed by alkylation of a transition metal compound with the cocatalyst, for example ... 

Bukatov, G.D Zakharov, V.A. Propylene Ziegler-Natta polymerization numbers and propagation rate constants for stereospecific and non-stereospecific centers. Macromol. Chem. Phys. 2001, 202, 2003-2009. 

Ziegler-Natta polymerizations have the characteristics of living polymerization with regard to catalyst active sites but not individual propagating chains. Thus the propagating chains have lifetimes of seconds or minutes at most, while active sites have lifetimes of the order of hours or days. Each active site produces many polymer molecules. The termination of a polymer chain growing at an active center may occur by various reactions, as shown below with propylene as an example. 

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). 

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