Site-controlled stereospecific polymerizations heterogeneous catalysts

Hence, there can be four stereospecific polymerization mechanisms in primary polyinsertion, all of which have been documented with metallocene catalysts (Scheme 13) the two originated by the chiralities of the catalyst active sites, referred to as enantiomorphic site control (isospecific and syndio-specific site control), can be relatively strong, with differences in activation energy (AA. ) for the insertion of the two enantiofaces up to 5 kcal/mol. A value of 4.8 kcal/mol has been found by Zambelli and Bovey for a Ti-based heterogeneous catalyst. 

At the same time, the fact that the homogeneous catalyst precursors are structurally well-defined has provided an extraordinary opportunity to investigate the origin of stereospecificity in olefin polymerization at a level of detail that was difficult if not impossible with the conventional heterogeneous catalysts. For example, NMR analysis of the isotactic polymer produced with HI revealed the stereochemical errors mmmr, mmrr, and mrrm in the ratios of 2 2 1 (Fig.5). This observation is consistent with an enantiomorphic site control mechanism, where the geometry of the catalyst framework controls the stereochemistry of olefin insertion.6 30,31 These results established unambiguously a clear experimental correlation between the chirality of the active site, which could be established by x-ray crystallography of the metallocene catalyst precursor, and the isotacticity of the polymer produced. 

The classical heterogeneously catalyzed propene polymerization as discovered hy Natta is a stereospecific reaction forming a polymer with isotactic microstructure. During the development of single-site polymerization catalysts it was found that C2-symmetric chiral metallocene complexes own the same stereospecificity. An analysis of the polymer microstructure hy means of NMR spectroscopy revealed that misinsertions are mostly corrected in the next insertion step, which suggests stereocontrol (Figure 6) hy the coordination site, as opposed to an inversion of stereospecificity hy control from the previous insertion steps (chain-end control). In addition, it was found that Cs-symmetric metallocene catalysts lead to syndio-tactic polymer since the Cosee-Arlmann chain flip mechanism induces an inversion of the stereospecificity at every insertion step. This type of polymer was inaccessible by classical heterogeneous systems. 

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