Polymerization of Propylene by a New Generation of Iron Catalysts: Mechanisms of Chain Initiation, Propagation, and Termination

Abstract

A new series of iron catalysts bearing tridentate pyridine−bis(imine) ligands are reported for the isospecific polymerization of propylene (PP). The catalytic activities are moderate, with maximum activities exceeding 1600 kg PP/mol Fe h under 1 atm of propylene pressure at −20 °C . End-group analyses of the resultant materials are performed and indicate that the polymerization proceeds via a 2,1 mechanism of propagation leading to the exclusive formation of 1-propenyl end groups following β-H elimination/abstraction and chain transfer. The polymers are highly regioregular, but the regioregularity decreases with decreasing steric bulk of the tridentate ligands. Isotacticities are determined for a number of catalysts bearing ligands with variable ortho substituent patterns and symmetries. In all cases, isotactic material is obtained; the [m]4 methyl pentad content is always in the range of 55−67%. 13C NMR spectroscopy is used to demonstrate that the isotacticity arises from a chain-end control mechanism. These catalysts represent the first isospecific systems known to operate by a 2,1 mechanism of propagation.