Ziegler-Natta catalysis mechanism

The MWD of polyolefins constitutes a property of fundamental importance, affects rheological and applicative properties of the polymers, and allows better understanding of Ziegler-Natta catalysis mechanism. 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

Coose P, Ziegler-Natta Catalysis I Mechanism of Polymerization of a-Olefins with Ziegler-Natta Catalyst, J Catal, 3, 80 (1964)... 

A big step is also being made by metallocene research to understand the mechanism and the nature of the Ziegler-Natta catalysis. 

The dramatic advantages achieved in the Ziegler-Natta catalysis thanks to the introduction of magnesium chloride as a fundamental component of the catalyst system are proved by the successful industrial results obtained during the recent years in both applications and process engineering. However, this technological progress was not followed by a satisfactory scientific elucidation of structure and reaction mechanism of the new systems. 

Cossee and Arlman (13,14) were among the first to propose a comprehensive mechanism for Ziegler-Natta catalysis and supported their proposals with molecular orbital calculations. The Cossee-Arlman proposal involves a "migratory alkyl transfer" (15) and, with some refinements, remains the most widely cited mechanism for Ziegler-Natta catalysis. A summary is presented below. (For more details, see references 5,12,16 and 17.)... 

In this section, we shall work through a few different approaches to the molecular mechanics modeling of Ziegler-Natta catalysis. Other approaches used to model this system are listed in Tables 1 and 2. 

The discovery that group IV metallocenes can be activated by methylaluminox-ane (MAO) for olefin polymerization has stimulated a renaissance in Ziegler-Natta catalysis [63]. The subsequent synthesis of well-defined metallocene catalysts has provided the opportunity to study the mechanism of the initiation, propagation, and termination steps of Ziegler-Natta polymerization reactions. Along with the advent of cationic palladium catalysts for the copolymerization of olefins and carbon monoxide [64, 65], these well-defined systems have provided extraordinary opportunities in the field of enantioselective polymerization. 

According to the Cossee mechanism, the two key steps in Ziegler—Natta polymerizations are monomer coordination and migratory insertion into the metal-polymer chain bond (Scheme 10). In analogy to heterogeneous Ziegler—Natta catalysis, a first-order reaction rate with respect to monomer concentration is generally assumed also for metallocene-based catalysts, that is. 

Cossee P (1964) Ziegler-Natta catalysis. 1. Mechanism of polymerization of alpha-olelins with Ziegler-Natta catalysts. J Catal 3 80-88... 

P. Cossee, Ziegler-Natta catalysis L Mechanism of polymerization of -olefins with Ziegler-Natta catalysts. J. CataL 3, 80-88 (1964)... 

H. Sinn and W. Kaminsky, Ziegler-Natta catalysis,/4t/v. Organomet. Chem. 18, 99(1980). V.A. Zakharov, G. D. Bukatov, Y. 1. Yermakov, On the mechanism of olefin polymerization... 

Some monomers can only be polymerized by specific polymerization methods while some others can be polymerized by many methods. For example, styrene can be polymerized by fi-ee-radical, anionic or Ziegler-Natta methods, while vinyl chloride can be polymerized only by free-radical methods. Also, each polymerization method has specific attributes, which are reflected in the properties of the pol3uners. Thus, for example, polyethylene prepared by the free-radical polymerization of ethylene has a branched structure and has low mechanical strength. This is known as low-density polyethylene (LDPE). In contrast, polyethylene prepared by the Ziegler-Natta catalysis of ethylene has a linear structure and has higher mechanical strength. This is known as high-density polyethylene (HDPE). Thus, the choice of the polymerization method is dictated not only by the nature of the monomer but also by the properties that are required in the polymer. 

Although the exact mechanism of the polymerization of olefins by Ziegler-Natta catalysis is not delineated with the precision that would satisfy every one, it is possible to write a reasonable mechanism [1,2]. The first of these is a monometallic mechanism. The following mechanism is for a TiCl3/AlEt3 combination. We have seen above that titanium tetrachloride has been initially used in the polymerization of ethylene. It has been subsequently found that TiCls is better than TiCLj. 

As said before, an important contribution to lanthanide chemistry could be that of clarifying, through differences and analogies with other transition metals, the mechanism of Ziegler-Natta catalysis and the place of f elements in this class of catalysts (30). 

The mechanism of Ziegler-Natta catalysis is quite complex. A key step in the chain growth involves an alkyl-titanium bond and coordination of the monomer to the metal. The coordinated monomer then inserts into the carbon-titanium bond, and the process continues. 

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