Polymerization of propylene by a new generation of iron catalysts: Mechanisms of chain initiation, propagation, and termination

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 degrees 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 l-propenyl end groups following beta-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%. C-13 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.