Chemistry Reference
In-Depth Information
The amount of stereospecific placement, however, is usually not as great as is the isotactic placement
with heterogeneous catalysts.
Group I-III metal alkyl components are essential to the formation of active centers [
239
,
240
].
Blue complexes of di cyclopentadienyl titanium dichloride with aluminum diethyl chloride or with
aluminum ethyl dichloride were some of the early known soluble catalysts [
257
,
258
]:
Cl
R
Ti
Al
Cl
R
Many other soluble catalysts were developed over the last 30 or more years. Most of these
materials must be prepared and used at low temperatures [
259
]. The polymerization mechanism is
also believed to take place by an initial coordination of the monomers with the transition metals. This
is followed by a four-center insertion reaction of the monomer into the transition metal-carbon bond.
4.5.2.1 Steric Control with Homogeneous Catalysts
Stereospecific placement appears to result from steric interactions between the substituents on the
ultimate units of the growing chains and the incoming monomers [
239
]. NMR spectral evidence
shows that the double bond opening is also
, as with the heterogeneous catalysts [
240
,
258
-
262
].
The monomer-polymer interaction model, described above, requires sufficient steric hindrance at
the active centers to prevent rotation of the penultimate units. Such rotation would cause the
substituent to interfere with the incoming monomer. The fact that syndiotactic placement takes
place only at low temperatures and atactic placement at elevated ones supports this concept.
Following is an illustration of the structure of the catalyst and the active center [
258
]:
cis
Cl
Cl
R
Al
Al
R
V
Cl
Cl
where denotes a vacant site.
A proposed model for syndiotactic placement can be shown as follows [
234
]:
H
H
H
V
+
V
H
H
V
HH
H
V
s
-metal bonds [
239
,
240
]. NMR
spectroscopy supports the hypothesis [
243
]. Also, the model shows the vanadium to be penta-
coordinated, following a Zambelli and Allegra suggestion [
243
]. Hexacoordinated vanadium has
The above model shows the insertion of monomers on
