Metallocenes are positively charged metal ions, most commonly Titanium or
Zirconium, sandwiched between two negatively charged cyclopentadienyl rings (see
fig). Their big advantage over the Ziegler-Natta systems is that they catalyse
the reaction of olefins through only one reactive site. Due to this “single
site” reaction, the polymerization continues in a far more controllable fashion,
leading to polymers with narrow ranges of molecular weight and, more
importantly, predictable and desirable properties.