To the metallocene developed in the previous section, it is necessary to add a cocatalyst called MAO (methyl alumoxane), which is itself a polymer with metal atoms in its backbone:
Large amounts of MAO are required to cocatalyze metallocenes (MAO/metallocene ratios up to 1000 times by weight), although one of the most important topics of current research in the area of metallocene catalysis is the reduction of these amounts. The MAO has as a fundamental function in catalysis the replacement of the Cl atoms in the metallocene by methyl groups (Figure 3.14). However, it has other functions not directly associated with the polymerization mechanism, such as stabilization of the metallocene groups, preventing their degradation (by dimerization, for example).
Figure 3.14: alkylation of metallocene group by MAO
The replacement of the Chloros is due to the fact that, because of the strong stabilization of the positive charge of the metal by the two indenyl groups, the Chloro groups are weakly bound to the Zr and tend to fall off rather easily, being replaced by methyl groups coming from the MAO according to this scheme:
Finally, one of the methyl groups also detaches (Figure 3.15) generating a complex cation with the structure shown in the figure.
Figure 3.15: Formation of metallocene cation by desalting
For the purpose of simplifying the representation of the metallocene group in future analyses, a compact symbolic scheme for indenyl will be used from this point on, as shown in Figure 3.16.
Figure 3.16: Simplified representation form of metallocene