Chemistry Reference
In-Depth Information
polypropylene, which is the logical consequence of a faster ligand rotation; however, depending on the
use conditions (in particular, on the nature of the cocatalyst and the polarity of the solvent),
the polymerization products may also contain appreciable amounts of a fairly isotactic fraction.
The peculiar microstructure of this fraction, with isotactic blocks of the same relative configuration
spanned by short atactic ones, rules out the possibility that the latter are due to an active species in
meso-like conformation and point rather to a conformationally “locked” racemic-like species
with restricted ring mobility. The hypothesis of a stereorigidity induced by the proximity to a counter
anion, which would play the role of the inter-annular bridge in the racemic-bis(indenyl)
ansa-metallocenes, was tested by computer modeling and found viable.
Preparation of elastomeric polypropylenes was also reported by Chien et al. [
64
]. Two metallocene
catalysts of different stereospecificities were used. The isospecific catalyst precursors were either
rac
-ethylene bis-(1-
Z
5
-indenyl)zirconium dichloride or
rac
-dimethylsilylene bis(1-
Z
5
-indenyl)zirco-
5
-fluorenyl)zirconium dichloride.
The precursors were activated with triphenyl carbenium tetrakis(pentafluorophenyl)borate and
triisobutylaluminum. The resultant catalysts exhibit very high activity, yielding products that range
from tough plastomers to weak elastomers [
64
].
nium dichloride. The unspecific one was ethylene bis(9-
Z
6.2.1 Manufacturing Techniques
The earliest commercial methods used slurry polymerizations with liquid hydrocarbon diluents, like
hexane or heptane. These diluents carried the propylene and the catalyst. Small amounts of hydrogen
were fed into the reaction mixtures to control molecular weights. The catalyst system consisted of a
deep purple or violet-colored TiCl
3
reacted with diethyl aluminum chloride. The TiCl
3
was often
prepared by reduction of TiCl
4
with an aluminum powder. These reactions were carried out in stirred
autoclaves at temperatures below 90
C and at pressures sufficient to maintain a liquid phase.
The concentration of propylene in the reaction mixtures ranged between 10 and 20%. The products
formed in discrete particles and were removed at 20-40% concentrations of solids. Unreacted
monomer was withdrawn from the product mixtures and reused. The catalysts were deactivated
and dissolved out of the products with alcohol containing some HCl, or removed by steam extraction.
This was followed by extraction of the amorphous fractions with hot liquid hydrocarbons.
Later bulk polymerization processes were developed where liquid propylene was either used as the
only diluent in a loop reactor or permitted to boil out to remove the heat of reaction. The second was
done in stirred vessels with vapor space at the top. More recently, gas-phase polymerizations of
propylene were introduced. The technology is similar to the gas-phase technology in ethylene
polymerizations [
15
] described in Sect.
6.1
.
6.2.2 Syndiotactic Polypropylene
Isotactic polypropylene receivedmost attention because it is commerciallymore desirable. Nevertheless,
syndiotactic polypropylene, though less crystalline, has greater clarity, elasticity, and impact resis-
tance. It melts, however, at lower temperature. This isomer was originally prepared with both,
heterogeneous, titanium-based catalysts and soluble, vanadium-based ones. The heterogeneous
catalysts gave very low yields of the syndiotactic fractions. In fact, original samples contained only
a few percent of the desired material, almost an impurity. The yield of syndiotactic polypropylene
increased with a decrease in polymerization temperature, but still remained low [
65
].
Highly syndiotactic polypropylene was prepared byNatta et al. [
38
] with homogeneous catalysts formed
from VCl
4
or from vanadium tri-acetylacetonate, aluminum dialkyl halide, and anisole at
