The Mechanism of
Ethylene Polymerization Using a Homogeneous Zirconium Catalyst
How do metal-based catalysts
orchestrate the formation of polyethylene?
As discussed in Session 2, Ziegler and
Natta discovered that metal-based catalysts, particularly
titanium/aluminum systems, would catalyze the polymerization of
ethylene to long, unbranched chains under mild conditions of
temperature and pressure. Recall that the role of a catalyst is
to provide an alternative pathway for a chemical reaction, which
allows the reaction to proceed under milder conditions. In the
case of metal-based catalysis, the metal center normally serves
as a site where reagent molecules come together and react. Since
the catalyst is not consumed in the reaction, it is usually
responsible for many molecular "turnovers".
The Ziegler-Natta Ti/Al catalysts are
heterogeneous, which means that they are insoluble under the
conditions of the polymerization reaction. This, in turn, makes
them difficult to study because most of the spectroscopic
characterization tools of chemistry (NMR, IR, UV-Vis, etc.) are most easily applied
to homogeneous or soluble systems. For this and other reasons,
chemists have designed and synthesized homogeneous (soluble)
catalysts for ethylene polymerization. These have provided a much
better molecular-level understanding of the mechanism of ethylene
polymerization. In addition, since the soluble catalysts are
generally organometallic complexes, they can be readily modified
by chemists, and the effects of the modifications on catalytic
activity and polymer structure can be evaluated.
The class of soluble catalysts that have
been most extensively studied are generated by reacting (h5-cyclopentadienyl)2Zr(Cl)2complexes with aluminum
alkyl reagents like Al(R)(Cl)2
. ("R" represents
an alkyl group like methyl.) As shown in Scheme 2, the aluminum
reagent alkylates the zirconium center (i.e., the R-
group replaces a Cl- ligand on zirconium) and then
removes the remaining Cl-, generating [(h5-cyclopentadienyl)2Zr(R)]+ (1). It is this
highly reactive ion that serves as the actual catalyst for
ethylene polymerization. Note that [(h5-cyclopentadienyl)2Zr(R)]+ (1) has an open
coordination site on the Zr center, indicated by a square (o ) in the structure
drawing. The h5-cyclopentadienyl group (often abbreviated
"cp") is a common ligand in organometallic chemistry.
As an anion, it is isoelectronic with benzene (six p
-electrons) and it uses
its p-electron cloud to coordinate to a metal center. For our
purposes, the cp ligand can be viewed simply as a bulky group
that occupies coordination sites around the zirconium center. It
can be conveniently represented as a ball.
The proposed mechanism for ethylene
polymerization, using ion 1 as catalyst, is outlined in
Scheme 3. The first step involves coordination of the ethylene to
the zirconium center. Like the cp ligand discussed above,
ethylene uses its p-electrons to coordinate to the metal center. At this stage,
the metal atom is formally bonded to both carbons of the olefin.
The second step involves "migration" of the alkyl group
(R) from the Zr atom to a carbon atom of the ethylene. This step
results in destruction of the ethylene p-bond, and the Zr atom is left bonding
only to one carbon. Note that intermediate 3 bears a very
close resemblance to catalyst 1; in addition to the two
cyclopentadienyl ligands, intermediate 3 possesses an
alkyl ligand and an open coordination site. The alkyl group is
now two carbons longer and is projecting out towards us, while
the open site (o ) is situated in back, behind the plane of the
paper. At this stage, rotation about the Ca -Cb single
bond of the extended alkyl group occurs (step 3). This moves R
out of the way so that another ethylene molecule can coordinate
to zirconium (step 4). Another alkyl migration (step 5) then
leads to intermediate 6, which is identical to 1,
except that the alkyl chain is four carbons (two ethylene units)
Continued olefin coordination/alkyl
migration/bond rotation steps will cause the polymer to grow
longer and longer. What will ultimately terminate polymer chain
growth? One natural termination step is b-hydride elimination, which involves
migration of a hydrogen atom on the b-carbon of the alkyl (growing polymer)
chain back to the metal center, followed by dissociation
of the polymer from the metal center (see Scheme 4).
The organometallic species that results
from these steps (cmpd. 9) is now set up to coordinate
another ethylene molecule and begin growing a new polymer.
Polymer chain growth can also be terminated by addition of
hydrogen (H2) gas to the reaction system (Scheme 5).
The H2 cleaves the bond
between the metal and the growing polymer chain. This produces a
polymer with a saturated end group and the same organometallic
species discussed above (cmpd. 9). Again, addition of
ethylene begins the chain growth process anew.
It is interesting that the properties of
the polymers that are generated via these two different
termination steps are not significantly different, even though in
the first case the polymer has a terminal double bond while in
the second case the end of the chain is saturated (C-C single bond). The properties of
the polymer are dominated by the chain length and by the
arrangement of the atoms (e.g., linear vs. branched).
Using Cochrane's Molecular Models,
grow a polyethylene chain on a metal center. Be sure to do each
individual step (coordination, migration, rotation).
Designer Plastics Module Table of Contents