Catalysts Stereorigidity and Site Epimerization

In the preceding sections, the term site epimerization was frequently used as the phenomenon responsible for the formation of the meso dyad (m) stereodefects and the lower stereoregularity and crystallinity of the s-PP polymers. To increase the stereoregularity and melting point of these polymers, obviously, the concentration of the rrmr pentads must be suppressed in the polymer chain s backbone by devising means that enable circumventing their formation during syndiospecific polymerization of propylene. 

Little was known initially about the driving forces underlying the phenomenon of site epimerization. Empirically, it has been observed that (in addition to low monomer concentration and higher polymerization temperature dependency) the catalysts formed with metallocene structures with inherently lower stereorigidity or higher flexibility (such as structures 5, 10, and 11) tend to undergo a more frequent site epimerization than those known to be less flexible, as can be seen from data presented in Table 13. Additionally, it was shown that the frontal substituents in more stereorigid catalysts systems impact the site epimerization rate (see 9/MAO rrmr in 

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