Session 6:

The Mechanism of Propylene Polymerization Using a Homogeneous Zirconium Catalyst


Question:

How do metal-based catalysts direct the polymerization of propylene monomers?


Text:

Organometallic zirconium complexes of the type used to produce polyethylene (see Session 3) are also effective catalysts for the synthesis of polypropylene (see Scheme 8).

Scheme 8

If we look closely at the structure of the polypropylene that is produced in the [cp2Zr(R)]+-catalyzed polymerization of propylene, we see that it results from head-to-tail polymerization. This, in turn, implies that each alkyl migration step in the mechanism must involve transfer of the alkyl group to the b-carbon (the methyl-bearing carbon) of the propylene, as shown in the scheme. However, although the propylene units are stitched together in a rigorous head-to-tail fashion, there is no regular pattern to the orientation of the methyl groups along the polymer chain. The methyl groups project toward us or away from us in a totally random fashion, i.e., the polymer is atactic. The randomness results from the fact that when propylene coordinates to the zirconium center, there is no preference for the methyl group to point up or down, because both orientations result in the same interactions between the methyl group and a cp ligand (see Figure 5). This is true whether the propylene coordinates to Zr in the front site or in the rear site.

Figure 5:

When propylene coordinates to [cp2Zr(R)]+, there is no preference for the methyl group to be oriented up or down.


Hands-On Activity:

Using Cochrane's Molecular Models, grow a polypropylene chain on a metal center. Are the propylene units joined in a head-to-tail fashion? How are the pendant methyl groups oriented?


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