Chemistry Reference
In-Depth Information
-complex that in turn can undergo two possible
reactions. Insertion of the olefin into the Zr-C
a
bond may take place after rotation around Zr-C
a
that
moves C
b
out of plane. The
Front-side attack leads to the formation of a
p
-hydrogen can be transferred from the polymer chain to the olefin. This
leads to chain termination and formation of a vinyl-terminated polymer and an ethyl-zirconocene that
can start a new polymer chain [
296
].
The backside attack on the other hand allows insertion of the olefin without rotation around Zr-C
a
bond. The front-side insertion is accompanied by chain migration from one side to the other whereas
backside attack does not involve inversion at the metal center.
Lohrenz et al. [
296
] concluded that insertion into the metal-polymer bond takes place exclusively
from the backside. That means that no inversion takes place. They also feel that in propylene
polymerization two orientations occur. In the first step the polymer chain points to the larger ligand
side and the propylene methyl group points away from the large ligand and the polymer chain. The
next step is governed by a stronger interaction of propylene with the polymer chain than with the
ligand [
296
].
An analysis of molecular mechanics using model metallocene complexes, as possible intermediates
for propylene polymerization was also reported by Guerra et al. [
298
]. The two coordination positions
available for the monomer and the growing chain are diastereotopic. The conclusion was that the
energy difference between the corresponding diastereoisomeric pre-insertion intermediates appear to
be relevant for the model complexes [
298
]. It was also concluded [
391
] that energy differences can be
related to an increased probability of a back-skip of the growing chain toward the outward coordina-
tion position after the monomer insertion and prior to the coordination of a new olefin molecule.
Busico et al., on the other hand, came to a conclusion [
299
] that the stereoregularity of polypro-
pylene produced with C2-symmetric group 4
b
metallocene catalysts is a result of the interplay of
two competing reactions. These are: isotactic monomer polyinsertion and a side process of
epimerization of the polymer chain at its active end. That makes this class of homogenous catalysts
different from the typical Ziegler-Natta catalyst, because with these catalysts enantioselectivity and
stereoselectivity are not necessarily coincidental [
96
].
Zang et al. reported [
300
] that they achieved highly efficient, rapid, and reversible chain transfer
reactions between active transition-metal based propagating centers with catalysts derived from
{Cp*Hf (Me)[N(Et)C(Me)N(Et)]} [B(C
6
F
5
)
4
] (Cp*
ansa-
5
-C
5
Me
5
) or {Cp*Hf (Me)[N(Et)C(Me)N
¼ Z
(Et)]} [B(C
6
F
5
)
3
Me]
N
Hf
[B(C
6
F
5
)
4
]
N
with multiple equivalents of dialkylzinc (ZnR
2
) acting as “surrogate” chain-growth sites. This was
done to achieve
-
non conjugated dienes. It is claimed by these investigators that these living coordinated chain transfer
processes provide a work-around solution to the “one chain per metal” cap on product yield. In
addition, they are claimed to provide access to practical volumes of a variety of unique new classes of
precision polyolefins of tunable molecular weights and very narrow polydispersity (
living
coordinative chain-transfer polymerization of ethylene with
a
-olefins, and
a
,
o
1.1).
A Japanese patent issued to Watanabe and Okamoto [
301
] describes preparation and illustrates an
iron containing catalyst for polyethylene preparation. It is shown here as an illustration:
M
w
/
M
n
