Chemistry Reference
In-Depth Information
O
Al
O
Al
O
O
Zr
CH
2
Al
Zr
CH
2
Al
O
O
CH
2
polymer
H
CH
polymer
+ CH
2
CH
2
CH
2
CH
2
11-24
O
A
l
O
Al
O
O
Zr
CH
2
Al
Zr
CH
2
Al
O
O
H
CH
CH
2
CH
2
polymer
CH
2
CH
2
CH
2
polymer
(11-68)
β
The growing polymer chain is terminated usually by
-hydride elimination
(
Eq. 11-63
), yielding vinylidene (
CH
2
) end groups.
When propylene and higher olefins are polymerized the configuration of the poly-
mer is controlled by the catalyst structure. Catalyst 11-25 contains two equivalent Cl
atoms, because the molecule is symmetrical about the Zr-indenyl bonds. Recall that in
the active catalyst one Cl will be replaced by the growing polymer and the other by the
incoming monomer. The polymer chain could occupy either of the two Cl positions,
while the monomer could coordinate at the other position. As a result, both positions
are equivalent and this metallocene produces isotactic polypropylene. In structure
11-26, the two Cl sites are not equivalent. At one position, the monomer would find
the same environment as in 11-25, but the second site is less sterically hindered and the
configuration of the inserting monomer is not controlled. As a result, this metallocene
would produce a mixture of isotactic and atactic polypropylene.
Q
Cl
Z
r
Cl
Cl
Z
r
Cl
11-25
11-26
