Ziegler-Natta methods active centers

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. 

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). 

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. 

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. 

---