Chemistry Reference
In-Depth Information
Table 6.3
Polypropylenes prepared by Natta [
37
]
Transition metal halide
Metal alkyl halide
% Crystallinity
TiCl
3
(
b
)
Al(C
2
H
5
)
3
40-50
TiCl
3
(
a
,
g
,or
d
)
Al(C
2
H
5
)
3
96-98
TiCl
3
(
a
,
g
,or
d
)
Al(C
2
H
5
)
2
CI
96-98
TiCl
3
(
a
,
g
,or
d
)
Be(C
2
H
5
)
2
94-96
TiCl
3
(
a
,
g
,or
d
)
Mg(C
2
H
5
)
2
78-85
TiCl
3
(
a
,
g
,or
d
)
Zn(C
2
H
5
)
2
30-40
VCl
3
Al(C
2
H
5
)
3
73
TiCl
2
Al(C
2
H
5
)
3
75
Table 6.4
Effect of addition of Lewis bases on the amount of crystalline fraction in
polypropylene
Transition metal halides
Aluminum alkyl
Lewis base
% Crystallinity
TiCl
3
2Al(C
2
H
5
)Br
2
Pyridine
>
98.5
TiCl
3
2Al(C
2
H
5
)Cl
2
N(C
2
H
5
)
3
95
TiCl
3
2Al(C
2
H
5
)Cl
2
NH(C
2
H
5
)
2
93
N
+
(C
4
H
9
)
4
I
TiCl
3
2Al(C
2
H
5
)Br
2
>
99
N
+
(C
4
H
9
)
4
Br
TiCl
3
2Al(C
2
H
5
)Cl
2
96
a
From Natta et al. [
38
]
and impedes processing. Table
6.3
lists some of the catalysts and the amounts of crystallinity in polymers
that were reported by Natta et al. [
37
]. To avoid costly purification of isotactic polypropylene,
three-component catalyst systems were developed. Some of the original ones appear to have been
reported by Natta, himself, who found that addition of Lewis bases enhances the quantity of the
crystalline material. Table
6.4
shows the effects of addition of Lewis bases on the amount of crystallinity,
reported by Natta et al. [
38
].
Many other three-component systems were developed since [
39
-
43
]. Also, development of more
active catalysts [
44
,
45
] eliminates a need to remove them from the finished product [
15
]. The first
improvement in catalyst productivity came from treating TiCl
3
(formed from TiCl
4
and Al(C
2
H
5
)Cl
2
)
with aliphatic ethers resulting in yields of 520 g of polymer for each gram of Ti [
46
]. Further
improvement was achieved by supporting TiCl
3
on MgCl
2
or by producing a supported catalyst by
reacting TiCl
4
with Mg(OC
2
H
5
) or with other magnesium compounds. This raised the productivity to
over 3,000 g of polymer for every gram of Ti [
46
]. The products, however, contained low percentages
of
N
0
-tetramethyl
ethylenediamine in solid component and ethyl benzoate in solution raised the isotactic content to
93% with a productivity of 2,500 g of polymer per gram of Ti [
41
]. Claims are made today for much
greater catalyst activity. It was reported, for instance, that catalyst efficiencies of 40 kg of polymer per
1 g of Ti can be achieved. Such yields require proper choice of catalysts and control over polymeri-
zation conditions. The isotactic fractions in the products are reported to range from 95 to 97%
[
47
-
49
].
In a catalyst systemTiCl
3
/MgCl
2
/C
6
H
5
COOC
2
H
5
/Al(C
2
H
5
)
3
, the high activity was initially attributed
to higher propagation rates rather than to an increase in the concentration of the active sites [
50
].
The higher activity of these catalysts, however, was shown instead to be due to higher numbers of
active centers and only slightly higher values of
N
0
,
the isotactic isomer
(20-40%). Addition of a Lewis base like
N
,
N
,
K
P
[
51
]. Subsequent trends inmodifications of supported
Ziegler-Natta catalysts consisted of using sterically hindered amines [
52
-
54
]. For
instance,
