Chemistry Reference
In-Depth Information
were reviewed recently [
313
]. The new
catalysts for ethylene polymerization
can be divided into three
groups [
312
]:
1. Products from reactions of trivalent alkoxy chlorides of transition metals with certain halogen-free
organoaluminum compounds, e.g., triisobutylaluminum. Such catalysts are used without any support.
2. Products from reactions of magnesium compounds with titanium compounds. In these catalysts the
transition metals are attached chemically to the surfaces of solid magnesium compounds. The
reactions take place between the halogen atoms from titanium compounds and the hydroxyl groups
at the surfaces of magnesium hydroxide:
+
Mg
OH
TiX
n
Mg
TiX
n+1
O
+
HX
Titanium compounds bonded to the surface OH groups of Mg(OH)
2
are mainly inactive. The
active sites are the ones associated with the coordinated and unsaturated negative oxygen ions
[
317
]. Reactions with aluminum alkyls activate the catalysts [
312
]. For high efficiency, special
carriers must be used together with a correct balance of the reactants, and proper reaction
conditions. Some choice combinations are [
312
]: Mg(OH)Cl with TiCl
4
; MgCl
2
3Mg(OH)
2
with Ti(OR)
x
Cl
y
; MgSO
4
3Mg(OH)
2
with Ti(OR)
x
Cl
y
; and Mg(OH)
2
with TiCl
4
.
Highly active, unsupported catalysts form from reactions of magnesium alkoxides with tetra-
valent titanium chlorides. The same is true of reaction products of MgCl
2
or MgCl
2
-electron donor
adducts, like MgCl
2
6C
2
H
5
OH, with tetravalent titanium compounds.
3. Products from reactions of organisilanols with chromium trioxide are also very active catalysts.
The silylchromate which forms is deposited on a silica support and activated with alkyl aluminum
compounds [
312
]:
O
CrO
3
Si
Cr
Si
+ silica
O
O
2
Si
OH
O
3
3
3
AlR
3
suported sillyl chromate
caralyst
has
been also very much improved. Titanium compounds are also supported on some carriers and then
activated by reactions with aluminum alkyls [
341
]. The patent literature describes a variety of
inorganic supports [
342
]. The most common ones, however, are based on MgCl
2
treated with various
Lewis bases, like ethyl benzoate. One such catalytic system is described as being prepared by treating
a complex, TiCl
3
3C
5
H
5
N, with diethyl aluminum chloride in the presence of highly dispersed
MgCl
2
. The product of that reaction is then combined with triethylaluminum [
355
]. The reaction
between an electron donor, a Lewis base, and MgCl
2
is a two step exothermic reaction [
343
]. The first
one is a rapid adsorption to the inorganic surface and the second one, a slower one, is formation of the
complex. The most commonly used Lewis bases are ethyl benzoate, di-
The activity and performance of coordination
catalysts for the polymerization of propylene
n
-butyl phthalate, and methyl-
p
-toluenate. Amines, like 2,2,6,6-tetramethylpiperidine, and some phenols are also used.
A common practice is to ball mill the Lewis base with the support material first. The transition
metal component is then added and the mixture is milled some more or thoroughly mixed. This Lewis
base is called the
Lewis base. This is followed by addition of Group I-III metal-alkyl
component with an additional Lewis base. The base that is added the second time may be the same
or a different one from one used in the original milling. In either case it is called the
internal
Lewis
base. It is not uncommon to use an ester as the internal base and an organosilane compound, like
phenyltriethoxysilane, as the external one.
external
