Homogeneous catalyst Ziegler-Natta polymerization

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. 

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. 

Largely out of the need to understand the mechanistic details of Ziegler-Natta polymerization in order to take rational steps to improve the performance of Ti-based heterogeneous catalysts, attention has turned to the properties of Group 4 (Ti, Zr, Hf) metallocenes as homogeneous polymerization catalysts.15-17 As noted above, homogeneous catalysts offer the chemist precise knowledge of the nature of the catalytic site, and they also allow the properties and performance of the catalyst to be tailored to meet requirements. 

An ESI mass spectrometer coupled online to a microreactor was used to intercept the catalytically active cationic intermediates of the Ziegler-Natta polymerization of ethylene with the homogeneous catalyst system [Cp2Zr(Me)Cl]-MAO (MAO = methylaluminoxane). For the first time these intermediates were studied directly in the solution and their catalytic activity proved.60... 

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. 

Recently developed Ziegler-Natta polymerizations utilize zirconium complexes that are soluble in the reaction solvents typically used, and so they are homogeneous catalysts. Reactions that use these soluble catalysts are called coordination polymerizations. 

In borderline cases (e. g., clusters, supported catalysts, catalysts for Ziegler-Natta polymerizations) we have defined reactions to be homogeneous when the catalyst passes a detectable catalyst cycle or parts thereof. 

The kinetics are much simpler in homogeneous metallocene-based catalyst systems, especially in base-free cationic catalyzed polymerization systems, than those in heterogeneous systems. The polymerizations with homogeneous metallocene catalysts are no doubt the best systems for kinetic study of Ziegler-Natta polymerization. The a-olefin polymerization with these catalysts also offers a good opportunity to study the durability and deactivation of the catalysts, since the polymerization systems remain homogeneous over a considerable long reaction period [50]. 

In coordination polymerization, monomer forms an adduct with a transition-metal complex, and further monomer is then successively inserted between metal and carbon. Termination occurs when the metal complex splits off from the polymer or the chain is broken intentionally by hydrogenolysis. Since the initiator is restored to its original form, the process is catalytic. The most important industrial processes are Ziegler-Natta polymerizations of a-olefins and employ solid catalysts. Most catalysts for coordination polymerization are hydride complexes of transition metals. An important example is the Shell Higher Olefin Process (SHOP) for homogeneous oligomerization of ethene with a complex nickel catalyst. The molecular-weight distribution is a Schulz-Flory distribution. The rate is first order in the catalyst metal. 

In Ziegler-Natta polymerizations, the reaction systems are more often heterogeneous than homogeneous. While the relatively few polymerizations that are homogeneous behave in a manner generally similar to ionic polymerizations, described in Chapter 8, the heterogeneous systems usually exhibit complicated behavior, as can be seen from some typical kinetic rate-time profiles of Ziegler-Natta poymerizations. Types (a)-(f) in Fig. 9.5. The shapes of these profiles may be characteristic of particular catalysts or catalyst-monomer systems and may be considered to consist of three periods, viz., an acceleration period, a stationary period, and a decay period. Some catalyst systems, however, show all three types. 

Electrophiles do not have to be cations tris(pentafluorophenyl)borane B(C6Fs)3 and related Lewis acids are very efficient hydride and alkyl anion abstractors and have found important applications as co-catalysts in the homogeneous Ziegler-Natta polymerization of olefins.The quest for better activators with potential... 

As reported by Santos and Metzger [50], Ziegler-Natta polymerization can be carried out in a microflow system coupled directly to the ESI source of a Q-TOF mass spectrometer (Figure 14.12). In the first micromixer, catalyst (Cp2ZrCl2-MAO) and monomer solutions are mixed continuously to initiate the polymerization. The polymerization occurs in the microtube reactor. The solution thus obtained is introduced into the second micromixer M2, where the polymerization is quenched by acetonitrile. The quenched solution is fed directly into the ESI source. The transient cationic species can be characterized by mass spectrometry. This is the first case where an alkyl zirconium cation intermediate in the homogeneous Ziegler-Natta polymerization of ethylene is detected directly. 

Keywords metallocene catalyst, Ziegler-Natta catalyst, olefin polymerization, polyolefins, homogeneous catalysts, supported catalysts, stereoregularity, molecular weight distribution (MWD), chemical composition distribution, Unipol , Novolen , stereoselectivity, single site catalyst, multiple site catalyst, gas phase process, slurry process, homopolymerization, copolymerization. 

Keaton, R. J. Jayaratne, K. C. Fettinger, J. C. Sita, L. R. Structural characterization of zirconium cations derived from a living Ziegler-Natta polymerization system New insights regarding propagation and termination pathways for homogeneous catalysts. J. Am. Chem. Soc. 2000, 122, 12909-12910. 

Scheme 5.34 Proposed mechanism of Ziegler-Natta polymerization of C2H4 using the homogenous catalyst CpiZrClj/MAO.

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