New models to explain the stereoselectivity of propene polymerization by group 4 metal catalysts

The concept of “catalyst ligand design” for olefin polymerization has been deeply modified by the use of high throughput screening techniques in the last ten years. A special case is the new class of Hf(IV)-pyridylamido-based olefin polymerization catalysts discovered by a Dow/Symyx collaboration. These systems have attracted a lot of attention because of several unique features: a) they are industrially relevant producing highly isotactic polypropylenes with a solution process technology at temperature > 100 °C; b) in combination with a second catalyst they undergo reversible trans-alkylation, performing novel olefin block copolymers via ‘chain shuttling’. The stereoselectivity mechanism of propene polymerization promoted by such systems is still unknown . In this communication by means of computational methods we investigated the enantioselectivity of propene insertion for systems I-III reported in Chart 1. Surprisingly, the generally accepted model of "chiral growing chain orientation" was ineffective for this class of catalysts and a new model for the propene enantioselectivity has been sorted out.