Enantiofacial misinsertion

This mechanism will produce perfectly isotactic polypropylene as long as there are no enantiofacial misinsertions. However, the difference in energy between insertion of the re and si enantiofaces is fairly small ( 2 kcal/mol) for the chain-end control mechanism. The stereocenter resulting from each misinsertion will cause the opposite enantioface of the alkene to be preferred for the next insertion the result is a stereoblock structure with one isolated r defect, mmmmmrmmmm (Figure 1.6). The signature for a chain-end control mechanism is therefore the presence of the mmrm and mmmr pentads in a 1 1 ratio in the NMR of the polypropylene synthesized. ... 

SCHEME 2.2 Site epimerization and enantiofacial misinsertion are two principal sources of stereoerrors commonly observed in syndioselective polymerizations. The bridge substituents have been omitted for clarity P represents the growing polymer chain and M = cationic Zr+ or Hf ", generally. 

FIGURE 2.6 Site epimerization and enantiofacial misinsertion produce defective pentads in characteristic ratios. (Reprinted with permission from Miller, S. A. Bercaw, J. E. Organometallics 2004, 23, 1777-1789. Copyright 2004 American Chemical Society.)... 

Naturally, this catalyst is not perfect and stereoerrors are invariably present in the formed polymer. The principal stereoerrors arise from enantiofacial misinsertion at the more stereoselective site and site epimerization. Such mistakes give rise to mr and rm triads and m and r dyads in an otherwise consistent procession of mm and rr triads (Figure 2.17). Thus, perfect hemiisotactic polypropylene... 

FIGURE 2.17 Enantiofacial misinsertion and site epimerization stereoerrors give rise to pentads containing isolated m and r dyads (in bold) such pentads are forbidden in perfectly hemiisotactic polypropylene. 

SCHEME 4.2 Mechanisms for stereoerror formation in propylene polymerization enantiofacial misinsertion, site epimerization, and chain epimerization (with or without site epimerization). (P represents the polymer... 

SCHEME 4.4 Enantiofacial misinsertion in isotactic propylene polymerization generates double m errors (i.e., mm triads). 

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