Chemistry Reference
In-Depth Information
stereospecificity of the process was enhanced by the introduction of a third com-
ponent comprising electron-donors, such as amines, esters, and ethers. Major
innovations were achieved with the introduction of supported catalyst systems.
The supports are primarily magnesium compounds, such as anhydrous MgCl
2
,
which is isomorphous with TiCl
3
(i.e., exchange of the cations does not affect the
crystal form). Catalyst production is a complicated process involving mechanical
treatment of mixtures of TiCl
4
with MgCl
2
and Lewis base electron donors.
Electron donors have a plurality of functions, including improvement of the
stereoregularity of polymers of 1-olefins. Esters of aromatic acids (e.g., dibutyl
phthalate) and alkoxysilanes enjoy the greatest industrial application as electron
donors at the time of writing. The functions and mechanisms whereby electron
donors act remain to be clarified.
Ti atoms in the catalyst are located on exposed sites in the MgCl
2
crystals.
About 10% of the Ti atoms are active polymerization sites in these supported
catalysts. The fraction of active sites and the propagation rate are both much
higher in MgCl
2
-supported catalysts. These catalysts are used as slurries in
hydrocarbon diluents. Productivity exceeds 2000 kg of isotactic polypropylene
per gram of Ti. The low residual metal level in the polymer eliminates the
need for catalyst removal processes. Catalyst particles are broken up by the
growing polymer in the early stages of polymerization, producing an expanding
reaction locus in which fragments of the initial catalyst granule are embedded
in polymer.
Supported catalysts are also used in gas phase olefin polymerizations. The
most widely used of these processes employ fluidized bed reactors, described in
more detail in Section 12.4.2.5. Various catalysts are used, including bis(tripheny-
silyl) chromate supported on high surface area silica gel and reduced with diethyl-
aluminum ethoxide. This catalyst produces polyethylene with a broad molecular
weight distribution. Heterogeneous catalysts with appropriate structures allow
control of the granular shape of the product polyolefins (and hence their packing
density and transport properties) because the polymer replicates the shape of the
catalyst. The monomer is able to reach all the active sites on and in the catalyst
particle, when the latter has a porous structure composed of primary crystals of
the right size. As a result, the initial granule of catalyst expands during the poly-
merization reaction and is always enveloped in a skin of polymer.
Important copolymerizations with Ziegler
Natta catalysts are between hydro-
carbon monomers. An example is the reaction of ethylene, propylene, and a non-
conjugated diene, such as 5-ethylidene-2-norbornene, to produce the so-called
EPDM (ethylene-propylene-diene monomer) elastomers. These products have lit-
tle or no crystallinity, depending on the ethylene content, and can be vulcanized
with sulfur systems (Section 1.3.3), because of the residual unsaturation provided
by the unconjugated diene comonomer. Copolymerization of ethylene with low
levels of 1-olefins, principally butene, hexene, and octene, produces polyethylene
with controlled short branch contents. These are the LLDPEs, which dominate the
packaging film market.
