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).
 
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Scheme
8
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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.
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Figure
5:
When propylene coordinates to [cp2Zr(R)]+,
there is no preference for the methyl group to be
oriented up or down.
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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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