To the metallocene described 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 cocatalize metallocenes (MAO/metallocene ratios of up to 1,000 times by weight), although one of the most important current research topics in the field of metallocene catalysis is the reduction of these amounts. The fundamental role of MAO in catalysis is to replace Cl atoms in the metallocene with methyl groups (Figure 3.14). However, it also performs other functions not directly associated with the polymerization mechanism, such as stabilizing the metallocene groups by preventing their degradation (through dimerization, for example).
Figure 3.14: Alkylation of the metallocene group using MAO
The replacement of the chlorine atoms occurs because the two indenyl groups strongly stabilize the metal's positive charges, causing the chlorine atoms to be weakly bound to Zr and tend to detach relatively easily, being replaced by methyl groups derived from MAO according to the following scheme:
Finally, one of the methyl groups is also cleaved off (Figure 3.15), forming a complex cation with the structure shown in the figure.
Figure 3.15: Formation of the metallocene cation by dealkylation
To simplify the representation of the metallocene group in future analyses, a compact symbolic notation for indenyl will be used from this point forward, as shown in Figure 3.16.
Figure 3.16: Simplified representation of a metallocene