Electronic effects in early transition metal catalyzed olefin polymerization and the connection of regioselectivity and comonomer affinity

Steric effects dominate olefin polymerization catalysis; however, electronic effects have been shown to also be important.1 In fact, depending on the polymerization conditions and catalyst symmetry, they can even limit stereoselectivity.2 Here, we show that once steric effects are correctly accounted for, simple electronic descriptors can be identified that allow easy rationalization of how electronic effects influence regioselectivity and comonomer affinity of group 4 TM catalysts. In particular, while olefin insertion is a concerted reaction, it is essentially heavily asynchronous, initiated by the attack of an electron sink (polymeryl chain with partial anionic character) on an electron hole (olefin). In propene polymerization catalysis to isotactic polypropylene (iPP), the higher the carbanionic character of the polymeryl chain is, the less selective is the attack on the differently charged α- and β-atoms of propene. Comonomer affinity is similarly influenced. The higher the charge on the metal is, the higher is the preference for the comonomer. Higher ionicity however is detrimental as it lowers the preference between different electron holes. Unlike for stereoselectivity, where both competing reactions have the same chemoselectivity, the competing transition states for regioselectivity bring the propene methyl group in different areas of the active pocket; i.e., Analyzing the steric bulk requires deconvolution. Simple regression models can be built on these assumptions that transgress catalyst classes; electronic effects play minor roles but are essential to understand how changes in the central metal and ligand backbone influence chemoselectivity. This research forms part of the research programme of DPI, project #835.