Metallocene Ziegler-Natta polymerization

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... 

APAOs has limited their utility in a number of applications. The broad MWD produces poor machining and spraying, and the low cohesive strength causes bond failures at temperatures well below the softening point when minimal stress is applied. To address these deficiencies, metallocene-polymerized materials have been developed [17,18]. These materials have much narrower MWDs than Ziegler-Natta polymerized materials and a more uniform comonomer distribution (see Table 3). Materials available commercially to date are better suited to compete with conventional EVA and EnBA polymers, against which their potential benefits have yet to be realized in practice. 

The positive charge on the transition metal in XXVII is a consequence of the tetravalent oxidation state of the transition metal in Cp2TiCl2. The active sites in traditional Ziegler-Natta polymerizations may be neutral because the transition metal is trivalent in those initiators (Secs. 8-4e, 8-4h-l). The group 3 metallocene initiators have neutral metal centers because those metals are trivalent. 

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. 

Ziegler-Natta Catalysts Kinetics of Ziegler-Natta Polymerizations Practical Features of Ziegler-Natta Polymerizations Comparisons of Cis-1,4-Polydienes Metallocene Catalysts... 

Resurgent interest in Ziegler-Natta polymerization was triggered by Kaminsky s report of the high catalytic activity of Cp2ZrX2 and, to a lesser extent, the Hf counterpart coupled with methylaluminoxane (MAO) in a-olefin polymerizations [4]. There is evidence that the cationic complexes play an active role stereoregulation, isotactic or syndiotactic, became possible with the advent of structurally tailored metallocenes (Fig. 12) [27]. 

Ziegler-Natta polymerization of alkenes is an important industrial process for the manufacture of polyolefins. Although it originally involved the use of the triethylaluminum-TiCft complex as the catalysts, many other transition metal complexes and /-block compounds (lanthanides) also catalyze the polymerization of alkenes. Group IV metallocenes exhibit particularly outstanding properties. 

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]. 

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. 

Coordination polymerization involves the use of transition metal catalysts. Examples are Ziegler-Natta polymerization by Tl/Al systems, metallocene polymerization with Ti, Zr, Hf catalysts, or metathesis polymerization with W, Mo, Re metals. Synthesis of functional polymers by organometallic catalysts are particularly difficult because transition metds are not only l ed by protic functionality, they are often poisoned by heteroatoms (e.g. N, O). 

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. 

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