Analysis of Stereochemistry Control in Homogeneous Olefin Polymerization Catalysis
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We propose a new method, using statistical analysis, to quantify factors contributing to stereo- and regiocontrol in olefin polymerization catalysis and competing β-hydrogen transfer to monomer. The method has been applied to three rather different Zr-based catalysts: (1) rac-Me2Si(3-Me-C5H3)2ZrR+, a representative of “standard” ansa metallocenes; (2) [2,2′-bis(2-indenyl)biphenyl]ZrR+, a sterically congested and rather atypical metallocene; and (3) [1,2-(2-O-3-tBu-C6H3CH2NMe)2-C2H4]ZrR+, an example of an octahedral “ONNO”-type catalyst. The analysis produces separate numeric values for repulsive ligand–chain, chain–monomer, and ligand–monomer interactions. For insertion in the Zr–iBu bond of 1 and 3, the ordering is syn (∼2.3 kcal/mol) > ligand–chain (∼1.6) > ligand–monomer (∼1.3), while for more crowded system 2 the three interactions are all around 2.0–2.5 kcal/mol. Despite the non-negligible magnitude of the ligand–chain interaction, the standard Corradini model of stereocontrol was found to apply for all cases. Our results also indicate that the stereocontrol penalties are sensitive to the nature of the chain and the olefin and that extrapolation from H or Me “chains” to more realistic chains is not warranted.
