metallocene catalyst

metallocene catalyst

[mə¦tal·ə‚sēn ′kad·ə‚list]
(organic chemistry)
A molecular structure with a well-defined single catalytic site, consisting of an organometallic coordination compound in which one or two cyclopentadienyl rings (with or without substituents) are bonded to a central transition-metal atom; used to produce uniform polyolefins with unique structures and physical properties.

Metallocene catalyst

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.

References in periodicals archive ?
A metallocene catalyst with a 2,3,5-tri([C.sub.1]-[C.sub.2] alkyl) cyclopentadienyl ligand yields polymers with greater melt strength than that from catalysts without such a ligand.
Interest is reportedly growing in reactive extrusion of TPE and TPV compounds, driven by the relatively recent availability of EPDM rubbers in granular form, which are made by new gas-phase metallocene catalyst processes.
Plastomers are ethylene alpha-olefin copolymers made from the single site metallocene catalyst using a variety of polymerization processes.
Patent 6,943,225 (September 13, 2005), "Multinuclear Metallocene Catalyst," Min-Hyung Lee et al.
Recently, LLDPE has been made using higher percentages of the above-mentioned comonomers that are uniformly distributed along the chain using metallocene catalyst. Although many researchers have paid attention to LLDPEs made by Ziegler-Natta catalysts, modified LLDPE synthesized by metallocene catalysis is an interesting material to characterize.
This invention develops a metallocene catalyst system with improved control for the copolymerization of ethylene with small amounts of 1-hexene.
Nordel MG, the first EPDM to combine the gas phase process with Insite metallocene catalyst, is now available over a range of Mooney viscosities from 60 to 140 ML.
(P&G) of Cincinnati to develop and commercialize P&G's new metallocene catalyst technology for elastomeric polypropylene resins.
We have analyzed the dynamic viscoelasticity of two groups of HDPE samples: a series of commercial available short branched polyethylenes produced with conventional Ziegler-Natta catalysts systems and a new type of HDPE-s obtained via single-site metallocene catalyst. The monomodal metallocene polyethylenes were prepared following the method of Kaminsky et al.
The Vistamaxx product platform is derived from the company's Exxpol metallocene catalyst technology and process.
The development of metallocene catalyst technology has lead during recent years to the synthesis of a rich set of new polyolefins of differing structure.
In contrast, there are 488 metallocene catalyst patents, 58 patents on other single-site catalysts, and 137 U.S.