Bimetallic mechanism of polypropylene polymerization.
According to this theory, the active site has the structure shown in the entry POLYMERIZATION MECHANISMS OF POLYPROPYLENE in Figure 3.6. The sequence of olefin addition to the polymer chain is governed by the mechanism shown at the bottom of that figure.
This mechanism explains the orientation of the propylene molecule through the interaction of the olefin’s double bond with the titanium–carbon bond, while the stereospecificity (the stereoscopic orientation of the methyl group) is governed by the steric interaction with the groups arranged adjacent to the aluminum atom.
However, the main criticism of this mechanism stems from the fact that monomer insertion would be hindered by steric hindrance from the growing polymer chain itself, since the only active site is virtually blocked by the living polymer.
The overall picture, in terms of the polymerization mechanism, becomes even more complicated in the case of supported catalysts.
Figure 3.6: Bimetallic Mechanism for Propylene Polymerization