Zirconocene/methylalumoxane catalysts

This situation changed when a new breed of homogeneous catalysts, based on metallocenes and methylalumoxane (MAO) as co-catalyst, which are 10-100 times more active than common heterogeneous ones, found great industrial and scientific interest [15,16]. The metallocene and the MAO, as well as the active complex, are soluble in hydrocarbons. Using these catalysts it became possible to tailor the microstructures of the polymers by tuning the ligands. Table 1 reviews the efficiency of the zirconocene/methylalumoxane catalysts. 

Figure 4.13 DSC determined melting point data for polyethylene prepared with singlesite zirconocene/methylalumoxane catalyst [41].

Work in the application of metallocene-based catalysis to olefin polymers has become a research topic of growing interest in recent years. A great number of symmetrie and chiral zirconocenes have been synthesized to give totally different structures of isotactic, syndiotactic, atactic or block polymers. The isotactic sequence length of polypropylene is influenced by the nature of the ligands of the metallocene. New ring or bridge substituted metallocene/methylalumoxane catalysts for the olefin polymerization are described. 

Figures 4.12, 4.13 and 4.14 illustrate analytical data that quantify the important structural differences between polyethylene prepared with a high activity Ti/Mg Ziegler-type catalyst and the polyethylene prepared with a zirconocene/methylalumoxane single-site catalyst.

Alt, H. G. Zenk, R. C2-symmetric bis(fluorenyl) complexes Four complex models as potential catalysts for the isospecific polymerization of propylene. J. Organomet. Chem. 1996, 512, 51-60. Chen, Y.-X. Rausch, M. D. Chien, J. C. W. C2y- and C2-Symmetric an5a-bis(fluorenyl)zirconocene catalysts Synthesis and a-olefin polymerization catalysis. Macromolecules 1995, 28, 5399-5404. Rieger, B. Stereospecific propene polymerization with rac-[l,2-bis(ti -(9-fluorenyl))-l-phenylethane] zirconium dichloride/methylalumoxane. Polym. Bull. (Berlin) 1994,32,41 6. 

Rossi, A. Odian, G Zhang, J. End groups in 1-butene polymerization via methylalumoxane and zirconocene catalyst. Macromolecules 1995, 28, 1739-1749. 

Figure 4 Mechanism of the polymerization of olefins by zirconocenes. Step 1 The cocatalyst (MAO methylalumoxane) converst the catalyst after complexation into the active species that has a free coordination position for the monomer and stabilizes the latter. Step 2 The monomer (alkene) is allocated to the complex. Step 3 Insertion of the alkene into the zirconium alkyl bond and provision of a new free coordination position. Step 4 Repetition of Step 3 in a very short period of time (about 2000 propene molecules per catalyst molecule per second), thus rendering a polymer chain.

With ansa(chiral) titanocenes, zirconocenes, and hafnocenes in combination with methylalumoxane (MAO) it is possible to obtain highly isotactic polypropene [366-374]. When changing the symmetry of the complex, different structures of the polypropene are yielded. The activity of these hydrocarbon soluble catalysts are extremely high. 

Thus, we demonstrate that norbomene-ethylene copolymers can be synthesized using comparatively simple nickel phosphorylide complexes. Although these copolymerization catalysts are less active than the systems based on zirconocene and methylalumoxane, they do not require any cocatalyst (e.g. alumoxane, which is an expensive chemical). Moreover, the lower oxophility of nickel in the ylide catalysts allows ethylene to be copolymerized with functionalized norbomenes. 

The role of methylalumoxane (MAO) as a cocatalyst to activate zirconocene compounds such as Cp ZrCl to create a single-site ethylene polymerization catalyst is similar, in some respects, to the role simple aluminum alkyls (AlRj) play in activating Ziegler catalysts. For example, MAO acts as an alkylating agent to form the initial Zr-carbon bond (Zr-CH ) necessary to initiate the polymerization process. However, experimental evidence obtained by a variety of methods clearly has shown that the MAO reacts with the zirconium center to form a zirconium cation of the type [CpjZr-CHS] in which the zirconium is not reduced to a lower oxidation state, but remains as a d° metal and Zr(IV) oxidation state. The MAO, therefore, forms an anion moiety to complete the ion pair necessary to create the active species, as illustrated in Equation 4.1. 

EtAlCl or EtAlcyEt AlCl as cocatalyst. The melting point data of the four examples shown above suggest that the polyethylene with a homogeneous branching distribution is consistent with a single-site polymerization catalyst similar to catalysts discovered by Kaminsky in the late 1970s based on a zirconocene compound and methylalumoxane as cocatalyst. 

---