Metallocene polymerization is one of the most dynamic areas in the world of polymers, as it is the most innovative process for manufacturing vinyl polymers since the development of the Ziegler-Natta processes.
The new catalysts enable polyolefins to achieve performance levels never before attained. For example, it is possible to synthesize linear polymers with extremely high molecular weights (6 to 8 million) and thus replace more expensive engineering thermoplastics, such as Kevlar. Furthermore, with metallocene catalysts, it is possible to produce elastomeric polyolefins in a single reaction unit. The level of control over the product’s tacticity is very high, including the ability to produce various forms in a single reactor.
In summary, it can be said that a metallocene is a complex consisting of a positively charged metal ion sandwiched between two (negatively charged) anions of the cyclopentadienyl family.
The cyclopentadienyl anion is formed by the stabilization of a cyclopentadiene molecule—a closed ring with 5 carbon atoms—as shown in Figure 3.8:
Figure 3.8: 
As can be seen from the color coding, one of the carbon atoms is bonded to two hydrogen atoms, while the other four carbons are each bonded to only one hydrogen atom. The two hydrogens mentioned first are bonded to a carbon atom with relatively fewer available electrons (it is bonded to the adjacent carbons in the ring by single bonds) and is therefore eager for them. This eagerness results in the attraction of the shared electrons toward the hydrogens and, consequently, the acidic nature of the latter.
Acidic hydrogens have weak bonds with carbon, and one of the H atoms is easily removed, leaving the electron from the covalent bond in the ring:
Figure 3.9: Cyclopentadienyl anion formed by the dehydrogenation of cyclopentadiene
The cyclopentadienyl anion has a distinctive feature, as shown in Figure 3.9: the pair of electrons incorporated into the ring after the hydrogen ion leaves combines with the 4 electrons from the second bonds of the olefinic pairs in the ring to form a group of 6 electrons. This set of electrons defines an aromatic ring and, therefore, a very high level of stability.
When a cation with two positive charges is added to a pair of pentadienyl groups, a charge-neutral sandwich is formed (Figure 3.10). For example, if the cation is Fe++, a metallocene group called ferrocene is formed.
Figure 3.10: Ferrocene
If a cation with a higher charge, such as Zr+4, is involved, more anions are required to balance the charges of the complex formed by adding the metal cation to the two cyclopentadienyl groups. Typically, two chloride anions are added to the Zr+4, each carrying a negative charge, to form the first of the metallocenes capable of serving as catalysts for vinyl polymerizations, known as zirconocene (more specifically, bis-chlorzirconocene; Figure 3.11):
Figure 3.11: bischlorocirconocene
Circonocenes differ significantly from ferrocenes due to the presence of chlorine atoms. These atoms are relatively bulky and leave very little space between the two cyclopentadienyl rings; to make room, the rings adopt a tilted (non-parallel) orientation, opening like a clam (Figure 3.12). In general, whenever a “sandwich” is formed with metal cations having charges greater than 2, the cyclopentadienyl rings tilt. A diagram is included to illustrate this opening:
Figure 3.12: by the volume of the Cl atoms
At this point, it is worth introducing the concept that, for reasons that will become clear later, it is of great importance and utility to introduce modifications to the cyclopentadienyl rings, as shown in the following structure:
Figure 3.13: Ethylene bridge and aromatic rings in the metallocene group
The differences between this structure and that of bis-chlorocirconocenes are:
Each cyclopentadienyl ring has been substituted with an aromatic group containing six carbon atoms, shown in red in Figure 3.13. The resulting group, formed by the fusion—with two shared carbon atoms—of the five-carbon aromatic ring with the six-carbon aromatic ring, is called an indenyl group. In addition, an ethylene bridge—shown in blue in the last figure—appears between the two substituted cyclopentadienyl groups.
These two modifications are essential for the catalytic activity of the metallocene in the production of isotactic polymers. The bulky indenyl groups, pointing in opposite directions as shown in the figure above, guide the monomer molecules as they approach the metal and determine the orientation of the monomer’s side groups. Thus, they allow the monomer groups to arrange themselves in a way that yields an isotactic product. In turn, the ethylene bridge holds the indenyl groups in the position shown, preventing them from rotating and adopting a relative spatial arrangement that would preclude the polymer’s isotacticity.