Session 7:

Propylene Polymerization Using Modified Homogeneous Zirconium Catalysts


Question:

How can metal-based catalysts be designed to produce polypropylene with a highly regular arrangement of pendant methyl groups.


Text:

One of the attractive features of homogeneous, molecular catalysts is that chemists can tinker with them and modify their catalytic activity in rational, predictable ways. A case in point is the organometallic zirconium catalysts that were discussed in Sessions 3 and 6. When simple cyclopentadienyl ligands are used in the catalyst, atactic polypropylene is produced because the methyl substituent on propylene has no preference for being oriented up or down. However, chemists can modify the cyclopentadienyl ligands in ways that force the propylene methyl group to orient in a particular way. This, in turn, can lead to the production of isotactic or syndiotactic polypropylene.

Shown in Figure 6 is an example of a zirconium complex with modified cyclopentadienyl (cp) ligands.

Figure 6:

Bridged bis-tetrahydroindenyl-Zrcatalyst. Hydrogen and carbon labels are omitted for clarity.

In this molecule, a second ring (a six-membered ring) has been fused to each cp group to form "tetrahydroindenyl" groups and the tetrahydroindenyl groups have been linked together with a CH2-CH2bridge. This new ligand is called a bridged bis-tetrahydroindenyl ligand, but for our purposes it can be viewed simply as a rigid, bulky ligand that occupies sites around the zirconium center. Significantly, this ligand possesses C2 symmetry, which means that a 180o rotation about an axis passing through the zirconium atom and the middle of the –CH2–CH2 bridge will result in no change in overall ligand geometry (i.e., the ligand will look the same before and after the 180o rotation). We can conveniently represent a tetrahydroindenyl group as two fused balls, with the smaller ball representing the 5-membered cp ring and the larger ball representing the 6-membered fused ring.

With this new ligand attached to zirconium, the propylene methyl group now has a strong orientational preference (see Figure 7).

Figure 7:

When propylene coordinates to the bridged bis-tetrahydroindenyl-Zr catalyst, there are strong orientational preferences for the methyl group.

When propylene coordinates in the front site, the methyl group prefers to be oriented up in order to avoid unfavorable contacts with the six-membered ring (large ball) of the lower tetrahydroindenyl group. On the other hand, when propylene coordinates in the rear site, the methyl group prefers to be oriented down so as to avoid bad contacts with the six-membered ring (large ball) of the upper tetrahydroindenyl group. Given these orientation preferences, we can now work through the stepwise mechanism of polymer chain growth (see Scheme 9). We find that as the polymer grows, all of the methyl group line up on the same side of the polymer backbone, i.e., isotactic polypropylene is produced.

Scheme 9

The bridged bis-tetrahydroindenyl ligand is chiral, that is, it is not superimposible on its mirror image. In the same sense, your left hand is chiral; it cannot be superimposed on its mirror image, your right hand. In principle, the introduction of any chiral ligand into the zirconium complex could lead to the production of isotactic polypropylene. In practice, however, only chiral ligands with bulky groups in the right places will result in highly regular isotactic polypropylene.

Syndiotactic polypropylene can also be produced using rationally designed zirconium-based catalysts. For example, the molecule shown in Figure 8 has been designed and synthesized for this purpose.

Figure 8:

Bridged cyclopentadienyl-fluorenyl-Zr catalyst. Hydrogen and carbon labels are omitted for clarity.

In this complex, the two cyclopentadienyl-type ligands have again been linked together by a carbon bridge and the lower cp has been replaced by a "fluorenyl" group, in which six-membered rings have been fused onto both sides of the C5 cyclopentadienyl core. As before, we can conveniently represent the cp ring as a ball and the fluorenyl group as three fused balls. Again, this ligand will force the propylene methyl group to orient in such a way that unfavorable contacts are avoided (see Figure 9). And in this case, the preferred orientation is always with the methyl group pointing up toward the smaller cp ligand and away from the bulkier fluorenyl group.

Figure 9:

When propylene coordinates to the cyclopentadienyl-fluorenyl-Zr catalyst, the methyl group is always oriented up toward the smaller cp ligand.

If we work through the stepwise mechanism of polymerization (Scheme 10), taking into consideration these orientational preferences, we see that syndiotactic polypropylene results, i.e., the methyl groups alternate on opposite sides of the polymer chain. The bridged cyclopentadienyl-fluorenyl ligand is not chiral; it is superimposible on its mirror image. The key to its ability to orchestrate the production of syndiotactic polypropylene is its asymmetry – the fact that one end is much bulkier than the other.

Scheme 10


Hands-On Activity:

Using Cochrane's Molecular Models grow isotactic and syndiotactic polypropylene on a metal center. Pay particular attention to how the propylene is oriented in each coordination step.


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