Polymerization with metallocenes is one of the most dynamic areas in the polymer world, as it is the most novel process for making vinyl polymers since the invention of the Ziegler-Natta processes.
The new catalysts enable polyolefins to achieve levels of performance never before obtained. For example, it is possible to synthesize extremely high molecular weight linear polymers (6 to 8 million) and thus replace more expensive engineering thermoplastics such as Kevlar. Furthermore, with metallocene catalysts it is possible to achieve elastomeric polyolefins in a single reaction unit. The level of control of the tacticity of the products is very high, including the possibility of achieving the various forms in a single reactor.
In summary, it is possible to say that a metallocene is a complex formed by a positively charged metal ion sandwiched between two (negatively charged) anions of the cyclopentadienyl family.
The cyclopentadienyl anion is the result of the stabilization of a cyclopentadiene molecule, a closed ring of 5 carbon atoms, as shown in Figure 3.8:
Figure 3.8: 
It can be seen from the color differentiation that one of the carbon atoms is bonded to two hydrogen atoms, while the other four carbons are bonded to only one hydrogen atom. The two hydrogens mentioned first are bonded to a carbon with relatively less electron availability (it is bonded to the adjacent carbons in the ring by single bonds) and therefore greedy for them. This greediness results in the attraction of the shared with the hydrogens and, consequently, in the acidic nature of the latter.
The acidic hydrogens have weak bonds with the carbon, and one of the H's is easily detached leaving the electron of the covalent bond in the ring:
Figure 3.9: cyclopentadienyl anion by dehydrogenation of cyclopentadiene.
The cyclopentadienyl anion presents a particularity, as shown in Figure 3.9: the pair of electrons incorporated into the ring after the exit of the hydrogenion is added to the 4 electrons of the second bonds of the olefinic pairs in the ring to reach 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 pentadienyls, 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 more charges is involved, such as a Zr+4, more anions are required to balance the charges of the set resulting from adding the metal cation to the two cyclopentadienyls. Usually, two chloride anions are added on top of the Zr+4, each with a negative charge, to form the first of the metallocenes capable of being employed as catalysts for vinyl polymerizations, called zirconocene or zirconocene (more specifically bis-chlorocyrconocene, Figure 3.11):
Figure 3.11: bis-chlorocyrconocene
Zirconocenes present important differences with ferrocenes, starting from the presence of chlorine atoms. These are relatively bulky and have very little space to be located between the two cyclopentadienyl rings which, to make room, are located in an inclined position (not parallel) opening like a clam (Figure 3.12). In general, whenever the sandwich is formed with metal cations with charges greater than 2, the cyclopentadienyl rings are tilted. A schematic is included to indicate this opening:
Figure 3.12: by the volume of the Cl atoms.
At this point, it is convenient to introduce the concept that, for reasons that will become obvious later, it is of great importance and convenience to employ modifications to the cyclopentadienyl rings such as in the structure that follows:
Figure 3.13: ethylene bridge and aromatic rings in the metallocene group
The differences between this structure and that of bis-chlorocyrconocenes are:
Each cyclopentadienyl ring has been added an aromatic group with six carbon atoms, indicated in red in Figure 3.13. The resulting group, formed by the fusion, with 2 shared carbons, of the five-carbon aromatic ring with the 6-carbon aromatic ring is called the indenyl group. In addition, an ethylene bridge, indicated in blue in the last figure, appears between the two replaced cyclopentadienyls.
These two modifications are fundamental to the catalytic activity of metallocene in obtaining isotactic polymers. The bulky indenyl groups, pointing in opposite directions as shown in the figure above, guide the monomer molecules approaching the metal and define the orientation of the pendant groups of the monomer. Thus, they allow these to arrange themselves in such a way as to generate an isotactic product. In turn, the ethylene bridge keeps the indenyl groups in the position shown, preventing them from rotating and being placed in a relative spatial arrangement that does not allow isotacticity of the polymer.