Ziegler-Natta olefin polymerization

Ziegler-Natta polymerization Stereospecific polymerization of olefines using a Ziegler catalyst. See titanium(IIl) chloride. 

Ziegler-Natta polymerization is used extensively for the polymerization of simple olefins (such as ethylene, propene, and 1-butene) and is the focus of much academic attention, as even small improvements to a commercial process operated on... 

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. 

Protonation of the TMM complexes with [PhNMe2H][B(C6Fs)4] in chlorobenzene at —10 °C provided cationic methallyl complexes which are thermally robust in solution at elevated temperatures as determined by NMR spectroscopy. In contrast, addition of BfCgFsls to the neutral TMM precursors provided zwitterionic allyl complexes (Scheme 98). Surprisingly, it was found that neither the cationic nor the zwitterionic complexes are active initiators for the Ziegler-Natta polymerization of ethylene and a-olefins. °°... 

The mechanistic similarity between Ziegler-Natta polymerization of olefins and the alkene cyclization reactions described above suggested that early transition metal catalysts would be effective catalysts for the coupling of... 

In the propagation process of Ziegler-Natta polymerization, the insertion of olefin into a metal-carbon bond is the most important basic step, but many questions concerning to this process remained unanswered for a long time. 

Polymerization reactions of olefins and dienes cannot be treated here in detail. Knowledge of the early steps which occur on nickel, as in oligomerization reactions, help explain the course of polymerization reactions and particularly their stereospecific character, as in Ziegler-Natta polymerization. 

An example is provided by the structures of Group IV metallacycles, LL MCl2, where ligands L and L are cyclopentadienyl based and M is Ti or Zr. As a class, these compounds act as catalyst precursors in homogenous (Ziegler-Natta) polymerization of olefins, e.g. 

The involvement of the a-elimination reaction in this cycle has been in question following experiments on cyclopentadienyl cobalt complexes, where evidence for olefin insertion for Ziegler-Natta polymerization catalysis has been obtained by labelling experiments, using C2H4 with a deuterated cobalt complex (70) ... 

A patent by A. W. Anderson et al. (Du Pont) disclosed the first transition metal catalyzed polymerization of a cyclic olefin in 1955 [1]. Subsequent to the discovery of Ziegler-Natta polymerization [2] norbornene was found to polymerize in the presence of the catalyst systems TiCl /EtMgBr. Eleuterio [3 a] and Truett et al. [3 b] obtained polynorbornene by using transition metal catalysts based on Mo and Ti, respectively. IR-studies and ozonolysis of the polymer revealed the presence of carbon-carbon double bonds, indicating that polymerization had occured by unexpected ring opening Eq. 1 [3]. 

Switching from olefin metathesis to Ziegler Natta polymerization is of interest in order to prepare block copolymers and to establish the relationship between these two related modes of olefin polymerization. Model studies for this purpose included the conversion of titancyclobutanes 10 and 7a into titanium alkyl compounds Eq. (30a and b) by the addition of 1 equivalent of ethanol [43],... 

Theoretically, it is possible for the process of olefin coordination and insertion to continue as in Ziegler-Natta polymerization (Chapter 52) but with palladium the metal is expelled from the molecule by a p-hydride elimination reaction and the product is an alkene. For the whole process to be catalytic, a palladium(O) complex must be regenerated from the palladium(ll) product of P-hydride elimination. This occurs in the presence of base which removes HX from the palladium(II) species. 

By far the most important industrial coordination polymerization processes are Ziegler-Natta polymerizations of 1-olefins [107-110], most notably the production of high-density polyethene [111] and stereo-specific olefin polymers and copolymers [108], However, these processes employ solid catalysts, and the complex kinetics on their surfaces have no place in a book on homogeneous reactions. 

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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