Session 3:

The Mechanism of Ethylene Polymerization Using a Homogeneous Zirconium Catalyst


Question:

How do metal-based catalysts orchestrate the formation of polyethylene?


Text:

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.

Scheme 2

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) longer.

Scheme 3

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).

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).

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).


Hands-On Activity:

Using Cochrane's Molecular Models, grow a polyethylene chain on a metal center. Be sure to do each individual step (coordination, migration, rotation).


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