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 ?
Elite resins produced with Dow's solution process with Insite metallocene catalyst technology range in densities from 0.
In earlier studies it is shown that polyethylene can be synthesized by using metallocene catalyst [7], In regard to catalyst activity in polymerization of ethylene, metallocene catalysts provide more flexibility than Ziegler-Natta catalysts, and are also in use for many industrial processes [8-11].
In addition, the Keltan ACE catalyst technology creates opportunities to develop products that are not attainable with classical Ziegler-Natta or state-of-the-art metallocene catalyst technologies, like high molecular weight high-5-vinyl-2-norbomene-EPDMs (refs.
The company added the SpectraSyn Elite mPAO base stocks are manufactured using a proprietary metallocene catalyst process and offer improved performance characteristics versus conventional PAO.
Clariant has obtained a patent for a method for dispersing pigments in a polymer matrix comprising the step of adding at least one dispersing auxiliary to the polymer matrix, wherein said at least one dispersing auxiliary is a polyolefin wax synthesized by means of a metallocene catalyst, and wherein the polyolefin wax is derived from ethylene, and wherein the polyolefin wax derived from ethylene is a homopolymer of ethylene or a copolymer of ethylene with one or more linear or branched olefins having 3-18 carbon atoms, wherein the polyolefin wax derived from ethylene has a dropping point of between 90 and 130[degrees]C, a melt viscosity at 140[degrees]C of between 10 and 10000 mPa x s, and a density at 20[degrees]C of between 0.
The propylene homopolymer composition and the propylene copolymer composition are obtained in separate stages using a single metallocene catalyst system comprising two different metallocene catalyst components.
The primary advantages metallocene catalyst technology holds over conventional Ziegler/Natta processes in polyolefins production are versatility and control.
With the use of a single metallocene catalyst and a single reactor, it is possible to tailor the crystalline distribution of ethylene copolymers.
Exxon, for its part, also owns a leading metallocene catalyst technology Exxpol, which is also licensed through Univation.
The Evolue, which we manufacture and sell, is high performance polyethylene manufactured using metallocene catalyst for highly functional sealant with impressive strength, airtightness, and heat seal-ability in addition to low odor and low fish-eye, and are therefore widely used as high-performance sealants in flexible packaging for foods and various daily necessities.
Al-Harthi et al produced polyolefin nanocomposites using an aluminum nitride nanofiller promoter with a metallocene catalyst.