Chemistry Reference
In-Depth Information
metallocene catalysts yield polypropylenes with different stereochemical struc-
tures, including isotactic, syndiotactic, atactic, and stereoblock configurations.
Metallocene catalysts are particularly able to polymerize a wide range of
monomers, including 1-olefins, various aromatics, cyclic olefins, nitriles, (meth)
acrylates, and silanes.
These catalyst systems consist of two components. The catalyst itself is a soluble
Group IVB transition metal (Ti, Zr, Hf) attached to two bulky
-carbocyclic ligands,
usually cyclopentadienyl (Cp), fluorenyl, indenyl, or their substituted structures. At
the time of writing the cocatalyst comprises oligomeric alumoxanes formed by
hydrolysis of trialkyl aluminum. Methylalumoxane (MAO), the preferred form, con-
tains some 6-20 Al(CH
3
) repeating units. Its structure is not known in detail, but it is
thought to consist of a mixture of linear and cyclic oligomers (11-23).
π
CH
3
CH
3
CH
3
Al
O
Al
O
Al
CH
3
CH
3
n
CH
3
CH
3
CH
3
Al
O
Al
O
Al
CH
3
CH
3
n
O
11-23
The MAO hydrolysis reaction can be represented generally as
CH
3
(11-67)
Al
nAl(CH
3
)
3
+ nH
2
O
+ 2n CH
4
O
n
The active species in MAO cocatalyzed metallocene polymerization is likely a
coordinatively unsaturated cationic complex, as in 11-24 (for a zirconium-based
metallocene), where substituents on the metal atoms have been omitted for clarity.
The Zr
a
O
a
Al bond withdraws electron density from the Zr atom. The mecha-
nism for ethylene polymerization is proposed to be as follows, where the mono-
mer forms a
π
-complex with the transition metal
[14]
:
