Chemistry Reference
In-Depth Information
4.5.5 Effect of Lewis Bases
Many investigationswere carried out to understand the roleLewis bases play in affecting stereospecificity
of the Ziegler-Natta catalysts. One of them, for instance, included studies with optically active bases to
determine the reactions with the active sites [
252
]. Nevertheless, the effect of Lewis bases like tertiary
amines is not fully understood. Evidence, gathered to date, suggests that when they react with
aluminum alkyl halides
,
[
306
] they increase the number of stereospecific sites by assisting in the
stabilization and/or dispersal of the catalyst on the support. They appear to alter the identities of the
attached ligands. This might be due to increasing the steric bulk at these sites. The Lewis bases are also
given credit for reducing the reactivity of the less stereospecific sites. Because both internal and
external Lewis bases are used, the enhancement of stereospecificity by the external bases is attributed
to displacing the internal base and/or augmenting its effect [
307
].
4.5.6 Terminations in Coordination Polymerizations
The terminations usually occur by chain transferring, either by an internal hydride transfer [
261
]:
X
X
X
X
+
H
R
n
catalyst site
R
catalyst site
n
or by a transfer to a monomer:
X
X
X
X
X
X
+
+
R
R
n
n
Terminal unsaturation in the products was confirmed by infrared spectroscopy. Hydrogen
terminates the reactions and it is often added to control the molecular weights of the products.
Protonic acids do the same thing:
XX
XX
+
H (A)
+
H
2
(HA)
R
R
n
4.5.7 Reduced Transition Metal Catalysts on Support
The catalysts that belong to this group are efficient in homopolymerizations of ethylene. They can
also be used in some copolymerizations of ethylene with
-olefins. Such catalysts are prepared from
TiO
2
,V
2
O
5
,Nb
2
O
5
,Ta
2
O
5
, CrO
3
,Cr
2
O
3
, and MoO
3
.WO
3
, NiO, and CoO. The supports are charcoal,
silica, alumina, zinc oxide, or alumina-silica. These catalysts require higher temperatures and
pressures for activity than do the typical Ziegler-Natta catalysts. The temperatures range from 140
to 230
C and pressures from 420 to 1,000 psi.
One of the original disclosures of such catalyst preparation was as follows [
308
]. A transition
metal nitrate solution is used to saturate a charcoal bed, previously leached with nitric acid.
The charcoal bed is then heated to temperatures high enough to decompose the nitrate to the oxide.
a
