Chemistry Reference
In-Depth Information
Ziegler-Natta catalysts [
226
] are products from reactions of metal alkyls or hydrides of Groups I
through III of the Periodic Table with metals salts or complexes of Groups IV through VIII. Not all
compounds, however, that fit this broad definitions are actually useful catalysts. In fact, they range
from very active to useless ones. Also, among them can be found materials that initiate polymeriza-
tion of some monomers by free-radical, cationic, or anionic mechanisms and not by a coordinated
anionic. Nevertheless, the number of active Ziegler-Natta catalysts is still large. The catalysts that are
based on metals with large numbers of d-electrons (mostly Group VIII), are effective in
polymerizations of conjugated dienes. They don't appear to work too well, however, with
-olefins.
On the other hand, metals of Groups IV, V, or VI with fewer d-electrons are useful in polymerizing
both, olefins and conjugated dienes. Also, all catalysts [
163
] that polymerize propylene also poly-
merize ethylene. Yet, the converse is not always true.
The Ziegler-Natta catalysts can be sub-divided into two groups: (1) heterogeneous insoluble
catalysts, and (2) homogeneous, or soluble ones. At times, however, it is difficult to distinguish
between the two. For instance, it may be hard to determine whether a particular catalyst is truly in
solution or merely in a form of a very fine colloidal suspension (and in fact heterogeneous).
a
4.5.1 Heterogeneous Ziegler-Natta Catalysts
These catalysts form when a soluble metal alkyl like triethylaluminum or diethyl aluminum chloride
is combined with a metal salt, like titanium chloride, in a medium of an inert hydrocarbon diluent.
The transition metal is reduced during the formation of the catalyst.
The following chemical scheme illustrates the reactions that are believed to take place between
aluminum alkyls and transition metal halides [
227
,
228
]. Titanium chloride is used as an example:
TiCl
4
þ
AlR
3
!
TiCl
3
R
þ
AlR
2
Cl
R
TiCl
4
þ
AlR
2
Cl
!
TiCl
3
þ
AlRCl
2
þ
TiCl
4
þ
2AlR
3
!
TiCl
2
R
2
þ
2AlR
2
Cl
R
TiCl
2
R
2
!
TiCl
2
R
þ
TiCl
3
þ
AlR
3
!
TiCl
2
R
þ
AlR
2
Cl
R
TiCl
2
R
!
TiCl
2
þ
TiCl
2
R
þ
AlR
3
!
TiClR
2
þ
AlR
2
Cl
The radicals that form in the above shown reactions probably undergo combinations or other
reactions of radicals, or perhaps react with solvents and decay. The reduction of the tetravalent
titanium is unlikely to be complete due to the heterogeneous nature of the catalyst. Better catalytic
activity results when TiCl
3
is used directly in place of TiCl
4
. Many catalysts, however, are prepared
with TiCl
4
. In addition, TiCl
3
exists in four different crystalline forms, referred to as
a
b
g
d
,
,
, and
.Of
b
g
d
a
a
these, the
,
, and
forms yield highly stereospecific polymers from
-olefins. The
-form, however,
yields polymers that are high in atactic material.
The ratio of the transition metal compounds to those of the compounds from metals in Group I to
III can affect polymerization rates. They can also affect the molecular weights, and the steric
arrangement of the products. Also, additives, like Lewis bases, amines, or other electron donors
