Chemistry Reference
In-Depth Information
Table 6.5
Comparison of isotactic and syndiotactic polypropylenes
Typical
Density at 25
C P(
C)
Isomer
Crystal structure
M
w
M
n
0.92-0.43 g/cm
3
Isotactic
Monoclinic
171-186
220-700 K
38-160 K
0.943 g/cm
3
Triclinic
Hexagonal
0.89-0.91 g/cm
3
Syndiotactic
Orthorhombic
138
No isotactic fractions formed. This led to development of many effective soluble catalysts. The catalyst
components and the conditions for their preparation are quite important in maintaining control over
syndiotactic placement. For the most effective soluble catalyst the ratio of AIR
2
X to the vanadium
compound has to be maintained between 3 and 10 [
66
]. The organic portion of the organoaluminum
compound can be either methyl, ethyl, isobutyl, neopentyl, phenyl, or methylstyryl [
9
,
67
]. In addition
to VCl
4
and to vanadium tri-acetylacetonate [
66
], various other vanadates can be used, like [VO
(OR)
x
Cl
3
x
], where
x ¼
1, 2, or 3 [
65
]. The exact nature of the vanadium compound, however, is very
important to the resultant steric arrangement of the product. For instance, VCl
4
combined with Al
(C
2
H
5
)
2
F forms a heterogeneous catalyst that yields the isotactic isomer [
65
]. Vanadium tri-
acetylacetonate, on the other hand, upon reacting with Al(C
2
H
5
)
2
F forms a soluble catalyst that yields
the syndiotactic isomer [
66
]. Addition of certain electron donors increases the amount of syndiotactic
placement. These are anisole, furan, diethyl ether, cycloheptanone, ethyl acetate, and thiophene [
67
].
The optimum results are obtained when an anisole to vanadium ratio is 1:1. Also, the highest amount of
syndiotactic polymer is obtained when the soluble catalysts are prepared and used at low temperatures.
Even at low temperatures, however, like
78
C, the amount of syndiotacticity that can be obtained
with a specific catalyst decreases with time [
65
,
66
,
68
]. This indicates a deterioration of the
syndiotactic placing sites. On the other hand, polymerization of propylene with soluble vanadium
tri-acetylacetonate-Al(C
2
C
5
)
2
Cl system was reported to be a “living” type polymerization [
69
]. The
product has a narrowmolecular weight distribution (
1.05-1.20). A kinetic study indicates an
absence of chain transferring and termination at temperatures below
M
w
/
M
n
¼
65
C.
More recent catalysts for syndiotactic polypropylene are complexes, like
-propyl(cyclopen-
tadienyl-1-fluorenyl)hafnium dichloride with methyl aluminoxane [
70
]. Another, similar catalyst is
i
i
3
-fluorenyl)zirconium dichloride with methyl aluminoxane. These
catalysts yield polymers that are high in syndiotactic material (the zirconium-based compound yields
86% of racemic pentads) [
70
,
71
]. Commercial production of syndiotactic polypropylene is in
the early stages. What catalytic system is used, however, is not disclosed at this time. Some of the
properties of the two isomers, isotactic and syndiotactic polypropylenes, are compared in Table
6.5
.
The molecular weights of syndiotactic polypropylenes can vary from a number average molecular
weight of 25,000-60,000, depending upon reaction conditions [
70
]. Also, in isotactic polypropylene
there is less than one double bond per 1,000 carbon atoms [
72
]. A typical
5
-cyclopentadienyl-
-propyl(
Z
Z
M
w
/
M
n
¼
5-12.
6.3 Polyisobutylene
The original commercial methods for preparing high molecular weight polyisobutylene by cationic
polymerization in good yields were reported in 1940. The reaction was carried out at
80
Cin
a diluent with BF
3
catalysis [
72
]. This developed into current commercial practices of polymerizing
isobutylene at
40 to
100
C, using liquid ethylene or methyl chloride as a diluent [
73
,
74
]. Even at
these low temperatures the reaction is quite violent. Methods were developed, therefore, to dissipate
80 to
