Polymer Basics - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 92

treated with diethyl aluminum chloride cocatalyst, it is known that the active
center is the product of reactions between the Ti species and the cocatalyst.
The ethyl groups from the diethyl aluminum chloride form the initial starting
ends of the polymer chains. The growth reaction is an insertion of the olefin
into the Ti - C bond in the active center. The active centers are coordinatively
unsaturated, and therefore easily poisoned by ligands such as CO, phosphines
and amines. However, other aspects of the polymerization mechanism and
structure of the active centers are less certain [3].
Another type of olefin polymerization catalyst system, often referred to as a
Phillips type catalyst, is based upon chromium trioxide, typically impregnated
on a solid support such as silica or alumina [4].
In the 1970's Kaminsky, Sinn, and others discovered that bis(cyclo-
pentadienyl)dimethyltitanium when mixed with trimethyl aluminum and
water provided a catalyst system capable of polymerizing ethylene [5]. The
titanium structure bears some resemblance to that of ferrocene. Ferrocene
was reported in 1951[6, 7] and the following year, the correct structure
reported [8]. A brief, interesting account of the early days of the research on
the structure proof of ferrocene has been written [9]. Because the Kaminsky
catalysts have the same “sandwich” structure of ferrocene, they are referred
to as metallocene catalysts. Just as the ferrocene ushered in a new era of
organometallic chemistry, the Kaminsky metallocene spurred a tremendous
amount of research in olefin polymerization catalysts.

CH 3
CH 3

Ti

Fe

Ferrocene Bis(cyclopentadienyl)dimethyltitanium

The research has varied the metal, the non-coordinating methyl ligands,
and the coordinating anionic pi systems. The combination of trimethyl
aluminum and water is considered to be an in-situ source of methyl
alumoxane (MAO), which can both methylate the metallocene and form a
metal cation. Common metallocene catalyst systems now use a zirconium
dichloride and MAO. MAO is often represented by the formula (CH 3 AlO) n
but this is an oversimplification and it is likely that MAO has a cage-like
structure [10] and that the ratio of methyl to aluminum is greater than 1.0.
The polymerization can be rationalized by the following sequence. The
zirconium dihalide is methylated by MAO and the MAO forms a zirconium
cation, with the negative charge being dispersed along the MAO framework.
MAO serves as a non-reactive anion. The zirconium cation is a strong elec-
trophile and coordinates with the olefin; ethylene in this example. In an inser-
tion step, the methyl group is transferred to the olefin and a propyl zirconium