metallocene catalyst[mə¦tal·ə‚sēn ′kad·ə‚list]
A transition-metal atom sandwiched between ring structures having a well-defined single catalytic site and well-understood molecular structure used to produce uniform polyolefins with unique structures and physical properties.
In the early 1980s, W. Kaminsky discovered that an appropriate co-catalyst activated metallocene compounds of group 4 metals, that is, titanium, zirconium, and hafnium, for alpha-olefin polymerization, attracting industrial interest. This observation led to the synthesis of a great number of metallocene compounds for the production of polymers already made industrially, such as polyethylene and polypropylene, and new materials. Polymers produced with metallocene catalysts represent a small fraction of the entire polyolefin market, but experts agree that such a fraction will increase rapidly in the future.
The simplest metallocene precursor has the formula Cp2MX2, where M is one of the group 4 metals (mainly Zr and Ti) and X are halogen atoms (mainly chlorine, Cl). The latter are known as mobile ligands because during polymerization they are substituted or removed. A typical co-catalyst, in the absence of which the activity is very low, is methylaluminoxane (MAO), an oligomeric compound described by the formula (CH3AlO)n, whose structure is not yet fully understood. MAO plays several roles: it alkylates the metallocene precursor by replacing chlorine atoms with methyl groups; it produces the catalytic active ion pair Cp2MCH3+/MAO-, where the cationic moiety is considered responsible for polymerization and MAO- acts as weakly coordinating anion.
The simplest metallocene structures are easily modified by replacing the Cp ligands with other variously substituted derivatives. In this way, a great number of catalysts with different steric and electronic properties are generated. The catalysts contain two C5 ring derivatives, always lying on tilted planes, which can be bridged or unbridged. Some examples are shown in the illustration, where the influence of the metallocene structure on the microstructure of the polymer product is also shown.
Because activity, stereospecificity, regiospecificity, and relative reactivity toward different monomers depend on the catalysts' characteristics, the metallocene systems offer the advantage of controlling the product through modifications of their chemical structure.