Ziegler-type metallocene system

However, one should also be aware of possibilities that elements of carbocationic processes may directly influence Ziegler-Natta catalytic chemistry. For instance, polymers containing vinylidene end groups of the type CH2=CMe-polymer are often formed during Ziegler-Natta processes and might recoordinate to the metal cation in an fashion, as in I and J for monocyclopentadienyl and metallocene systems, respectively. intermediates of these types were to... 

The active site in single-site metallocene catalysts is tetrahedrally coordinated while the active sites in heterogenous Ziegler type systems are octahedrally coordinated in the TiCh or MgCl2 lattice. This difference leads to a greater openness of the active site, facilitating incorporation of higher alkenes in the production of LLDPE. 

Brookhart and coworkers [1] have recently developed Ni(II) and Pd(II) bis-imine based catalysts of the type (ArN=C(R)-C(R)=NAr)M-CH3+ (la of Figure 1) that are promising alternatives to both Ziegler-Natta systems and metallocene catalysts for olefin polymerization. Traditionally, such late metal catalysts are found to produce dimers or extremely low molecular weight oligomers due to the favorability of the P-elimination chain termination process [2],... 

It is important to note that the tendency of a monomer towards polymerization and therefore also towards copolymerization is strongly dependent on the nature of the growing chain end. In radical copolymerization the composition of the copolymer obtained from its given monomer feed is independent of the initiating system for a particular monomer pair, but for anionic or cationic initiation this is normally not the case. One sometimes observes quite different compositions of copolymer depending on the nature of the initiator and especially on the type of counterion. A dependence of the behavior of the copolymerization on the used catalyst is often observed with Ziegler-Natta or metallocene catalysts. 

There are relatively many different catalysts that are usually used for specific chemical reactions. Types include Ziegler-Natta Catalyst (Z-N), metallocene, and others including their combinations. These systems are available and used worldwide from different companies.73... 

The versatility of polymerization resides not only in the different types of polymerization reactions and types of reactants that can be polymerized, but also in variations allowed by step-growth synthesis, copolymerization, and stereospecific polymerization. Chain polymerization is the most important kind of copolymerization process and is considered separately in Chapter 7, while Chapter 9 describes the stereochemistry of polymerization with emphasis on the synthesis of polymers with stereoregular structures by the appropriate choice of polymerization conditions, including the more recent metallocene-based Ziegler-Natta systems. Synthetic approaches to starburst and hyperbranched polymers which promise to open up new applications in the future are considered in an earlier chapter dealing with step-growth polymerization. 

This type of -alkyl elimination is recognized as an important chain transfer step in Ziegler-Natta and metallocene polymerization catalysis [161]. When it occurs the polymer chain terminates in a C=C bond and in certain systems can undergo insertion and get back into the polymer growth cycle. 

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