Ziegler-Natta polymerization Metallocene catalysts

The successful polymerization of a, >-dienes via ADMET continually produces a small molecule, typically ethylene, and the removal of this small molecule drives the reaction. When Schrock s [W] and [Mo] alkylidenes (14) are used, care has to be taken in maintaining an inert atmosphere devoid of both moisture and air in order to avoid decomposition of the catalyst. For this reason, Schlenk line techniques such as those used to handle Ziegler-Natta or metallocene catalysts and high purity monomers are important. 

Transition metal polymerization catalysts have stimulated tremendous efforts in academic research resulting in numerous industrial applications. Ziegler-Natta and metallocene catalysts have been used for the synthesis of tailor-... 

Borstar A catalytic process for polymerizing ethylene. Use of two reactors, a loop reactor and a gas-phase reactor, allows better control of molecular weight distribution. The loop reactor operates under super-critical conditions to avoid bubble formation. Either Ziegler-Natta or metallocene catalysts can be used. The first commercial unit was installed in Porvoo, Finland, in 1995. 

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. 

Hungenberg, K. D., Kerth, J., Langhauser, F., Marczinke, B. and Schlund, R., Gas Phase Polymerization of a-Olefins with Ziegler-Natta and Metallocene Catalysts a Comparison , in Ziegler Catalysts, Springer-Verlag, Berlin, 1995, pp. 363-386. 

Borstar A catalytic process for polymerizing ethylene or propylene, subdivided into Borstar PE and Borstar PP. Use of two reactors — a loop reactor and a gas-phase reactor — allows better control of molecular weight distribution. The loop reactor operates under supercritical conditions to avoid bubble formation. Either Ziegler-Natta or metallocene catalysts can be used. The latest version, Borstar PE 2G, uses a single, multizone gas-phase reactor to make polymers that have bimodal molecular weight distributions. Developed by Borealis A/S. The first commercial unit, for polyethylene, was installed in Porvoo, Finland, in 1995. The first polypropylene plant was operated by Borealis in Schwechat, Austria, in 2000. In 2005, Borstar s total capacity for PE and PP was 1.3 million tons. 

Machado, F., Lima, E. L., Pinto, J. C., McKenna, T. F. (2011). An experimental study on the early stages of gas-phase olefin polymerizations using supported Ziegler—Natta and metallocene catalysts. Polymer Engineering and Science, 51(2), 302—310. 

These processes have been used successfully for many years to carry out gas-phase polymerization of polyolefins using the Ziegler-Natta catalysts. It was expected that replacement of the Ziegler-Natta by metallocene catalysts in fluidized-bed reactors will be rather straightforward. However, the adaptation of the more active metallocenes to existing commercial fluidized-bed reactors led to many operational problems. For... 

These deactivation equations are not easy to prove, but have been successful in describing several olefin polymerization processes with Ziegler-Natta and metallocene catalysts. 

All heterogeneous Ziegler-Natta and Phillips catalysts have two or more active-site types and many soluble Ziegler-Natta and metallocene catalysts may also show multiple-site behavior [36, 37]. In addition, several metallocene catalysts, when supported on organic and inorganic carriers, may behave like multiple-site catalysts even if they behaved as single-site catalysts in solution polymerization. Therefore, several of the catalysts used industrially for polyolefin manufacturing have in fact two or more active-site types. 

The preparation and structure determination of ferrocene marked the beginning of metallocene chemistry Metallocenes are organometallic compounds that bear cyclo pentadiemde ligands A large number are known even some m which uranium is the metal Metallocenes are not only stucturally interesting but many of them have useful applications as catalysts for industrial processes Zirconium based metallocenes for example are the most widely used catalysts for Ziegler-Natta polymerization of alkenes We 11 have more to say about them m Section 14 15... 

Metalorganic Catalysts for Synthesis and Polymerization Recent Results by Ziegler-Natta and Metallocene Investigations, Ed. W. Kaminsky, Springer, Berlin 1999. 

Some chiral initiators have structures such that alternate monomer placements occur with opposite faces of the monomer to yield the syndiotactic polymer. This is syndioselective polymerization proceeding with catalyst site control and is usually observed only with some homogeneous initiators, both traditional Ziegler-Natta and metallocene. 

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. 

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