Analysis of Stereochemistry Control in Homogeneous Olefin Polymerization Catalysis

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.