Chemistry Reference
In-Depth Information
These catalysts also polymerize ethylene and propylene in the presence of methyl-aluminoxane
[
293
]. The
-1/MAO in ethylene
polymerization. In propylene polymerization, however, the racemic one is active and the meso is
inactive. Ethylene polymerization activity with
meso
-1/MAO analog was found to be more active than the
rac
-1/MAO increases as the temperature increases.
An inverse temperature effect is observed for propylene polymerization with the
rac
-1/MAO catalyst
system. The stereoregularity of polypropylene formed with this racemic catalyst is low [
293
]. This led
Han et al. to conclude [
293
] that the
rac
a
-olefin polymerization is influenced by changes in the structure
of
chiral metallocene catalysts such as variation of the transition metal, the steric and electronic
properties of
ansa
p
s
- and
-ligands, and the length and rigidity of the bridging groups. The activity and the
a
stereoselectivity of
-olefin polymerization reactions can be significantly affected by slight structural
variations of the bridging group in metallocene catalysts. The length of the bridging chain affects the
angle between the cyclopentadienes and the metal atom. The chiral metallocenes having
p
-ligands
bridged with one atom (carbon or silicon) or two carbon atoms have either C
2
(
racemic
)orC
8
(
meso
)
symmetry, if the
-ligand is enantiotropic [
293
].
The mechanism of monomer insertion and steric control in polymerizations of
ansa
-olefins by the
metallocene catalysts received considerable attention [
293
-
297
]. There is no consensus on the
mechanism of polymerization. Many studies of chain propagation tend to support the Cossee-Arleman
mechanism [
293
-
297
]. An example is work by Miyake et al. [
294
] who synthesized unsymmetrical
ansa
a
isomers. Both isomers coupled with
methylaluminoxane polymerize propylene in toluene to highly isotactic polymers of
-metallocenes and separated them into
threo
and
erythro
M
w
ΒΌ
105,000.
The isotactic placement is greater that 99.6% and the polymer melting point is 161
C.
t-Bu
t-Bu
Cl
M
Cl
Cl
M
Cl
Y
Y
R
R
threo- form
erythro-form
Based on experimental evidence obtained with the above catalysts, it was concluded by Miyake
et al., that the isotactic propylene polymerization with zirconium catalysts takes place by a regiose-
lective 1,2-insertion of the propylene monomer into the metal-polymer bond [
294
]. Monomer
insertion is believed to take place at two active sites on the metal center in an alternating manner.
In addition, it was shown [
295
] that the substituents on the cyclopentadiene rings determine the
conformation of the polymer chain end, and the fixed polymer chain end conformation in turn
determines the stereochemistry of olefin insertion in the transition state as a form of indirect steric
control.
With the above catalysts, however, because the stereochemistry of the two sites is different,
Miyake et al. [
294
], suggest that monomer insertion takes place at the same active site on the metal
center:
R
R
R
R
Zr
Zr
Cp
Ind
Cp
Ind
Zr
Cp
Ind
Zr
Cp
Ind
