Bimetallic mechanism of polypropylene polymerization.
According to this theory, the active site has the structure shown in the entry POLYPROPYLENE POLYMERIZATION MECHANISMS IN FIGURE 3.6. in Figure 3.6. The olefin addition sequence in the polymer chain is given by the mechanism indicated in the lower part of the figure.
This mechanism explains the orientation of the propylene molecule through the interaction of the olefin double bond with the Titanium-Carbon bond, while the stereospecificity (the stereoscopic orientation of the methyl group) is commanded by the steric interaction with the ordered groups next to the Aluminum atom.
However, the biggest criticism of this mechanism is that the insertion of the monomer 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, is further complicated in the case of supported catalysts.
Figure 3.6: Bimetallic Mechanism for Polymerization of Propylene