Ziegler active centers

Most Kaminsky catalysts contain only one type of active center. They produce ethylene—a-olefin copolymers with uniform compositional distributions and quite narrow MWDs which, at their limit, can be characterized by M.Jratios of about 2.0 and MFR of about 15. These features of the catalysts determine their first appHcations in the specialty resin area, to be used in the synthesis of either uniformly branched VLDPE resins or completely amorphous PE plastomers. Kaminsky catalysts have been gradually replacing Ziegler catalysts in the manufacture of certain commodity LLDPE products. They also faciUtate the copolymerization of ethylene with cycHc dienes such as cyclopentene and norhornene (33,34). These copolymers are compositionaHy uniform and can be used as LLDPE resins with special properties. Ethylene—norhornene copolymers are resistant to chemicals and heat, have high glass transitions, and very high transparency which makes them suitable for polymer optical fibers (34). 

Mejzlik, J., Lesna, M. and Kratochvila, J. Determination of the Number of Active Centers in Ziegler-Natta Polymerizations of Olefins. Vol. 81, pp. 83 — 120. 

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)... 

Radioactive, 4C0 has been used to determine the number of active centers in Ziegler-Natta catalysts for the polymerization of ethylene and a-olefins 31 33>. However, the reliability of this method for active center determination is a matter of discussion 105,106). The argument arises from an uncertainty in the mechanism of the reaction of CO with a metal-polymer bond in the active center. Some model systems have been employed to investigate the mechanism. 

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). 

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). 

A classical catalyst for metathesis reactions 1 reminiscent of a polymerization Ziegler catalyst it is essentially a combination of a transition metal halide (WCU. MoCl ) and an alkyl metal derivative (AIR). SnK. etc). It is noteworthy that a reduction step occurs during the constitution of the active center, since an elTicieni metathesis of terminal olefins has been achieved under electro-catalytic conditions [40]. (CI4 being the active carbenic entities in... 

In Ziegler-Natta catalysts, quantitative information about the number of active centers is very important. Together with kinetic data and polymer microstructural and stereochemical analyses, they contribute to the formulation of the most likely reaction mechanism and to the understanding of the roles the different catalyst system components play. With the advent of supported catalysts, this information has become... 

Attempts to determine the number of active centers in Ti complex/MgCl2 catalysts have been made using the chemical or radiochemical and kinetic methods commonly employed in Ziegler-Natta catalysis. 

It is known that in propylene polymerization, both with conventional and supported Ziegler-Natta catalysts, at least two types of active centers can be distinguished. Such species can be associated with the so-called isotactic and atactic polymeric fractions, which have different configurations and may be separated by simple extraction with boiling heptane. Based on the 13C NMR analysis of the microstructure of the atactic and isotactic fractions, Inoue 1451 has recently proposed a two site model. At one site the stereospecific polymerization proceeds according to the Bernouillian model, and at the other it proceeds according to the enantiomorphic site model. However, it is understood that a two site model is an oversimplification. As a matter of fact, the crude polypropylene can usually be separated into several fractions having different tacticity 51>. 

For the supported catalysts for propylene polymerization, the basic question has been to establish whether or not the donor participates in the formation of active centers. The role of the aluminum alkyl, on the other hand, is still subject to debate even with regard to non-supported Ziegler-Natta catalysts. 

Recently, Doi152) speculated on the presence of two types of bimetallic active centers, based on 13C NMR analysis of the structure and stereochemistry of polypropylene fractions obtained with different Ziegler-Natta catalyst systems (see Fig. 44). Site A produces highly isotactic polypropylene, site B atactic polypropylene consisting of isotactic and syndiotactic stereoblocks. The formation of the latter fraction would be due to the reversible migration of the aluminum alkyl, made... 

With regard to the active centers, which are generally believed to consist of a plurality of species with different structure, stereospecificity, reactivity and stability, neither composition nor structure could be experimentally determined. This question, still unanswered 30 years after the discovery of the Ziegler-Natta catalysis and more complex now with the supported systems, is still dealt with in terms of more or less likely and contrasting hypothetical models. 

Determination of the Number of Active Centers in Ziegler-Natta Polymerizations of Olefins1... 

The methods developed for the determination of the number of active centers are classified into three main categories, based on (i) labelling of the macromolecules, (ii) consumption data of an effective catalyst poison, and (iii) other principles. The methods are characterized and their advantages, shortcomings and pitfalls are specified. Examples of procedures used to obtain the number of active centers employing various methods are presented. The importance of the data obtainedfor understanding Ziegler-Natta polymerizations of olefins is outlined. 

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