Chemistry Reference
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cation is formed. Another ethylene complexes with the zirconium cation and
after the transfer of the propyl group, a new zirconium cation is formed. This
process repeats and after thousands of iterations polyethylene is formed.
Cl
CH
3
CH
3
+
MAO
+
Zr
Zr
Zr
Zr
+
Cl
CH
2
CH
2
CH
3
(CH
2
)
4
CH
3
+
+
Zr
Polyethylene
Zr
CH
2
CH
2
CH
3
In the case of the homopolymerization of ethylene, there is no tacticity,
but with propylene polymerizations the catalyst symmetry can influence
the tacticity. Bis-cyclopentadienyl zirconocenes are symmetric and give
atactic polypropylene. The isopropyl bridged cyclopentadienyl fluorenyl
zirconocene has a center of asymmetry at the metal (C
s
symmetry) and
provides syndiotactic polypropylene. The same catalyst, but with a t-butyl
substituent on the cyclopentadienyl ligand has C
2
symmetry and yields
isotactic polypropylene [11].
CH
3
CH
3
Zr
Syndiotactic polypropylene
+
CH
3
CH
3
Zr
Isotactic polypropylene
+
Because of the commercial significance of polyethylene and polypropy-
lene and to a lesser extent, polystyrene, and the ability to tune polymer
properties such as stereochemistry, molecular weight, and comonomer
incorporation, research on these organometallic catalyst systems continues
to this day. Metallocene catalyst systems are often based upon substituted
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