Mechanism and Modeling of Brookhart-Type Polymerization Catalysts

Dean M. Philipp, Richard P. Muller, and William A. Goddard, III.

Materials and Process Simulation Center, California Institute of Technology

Mark McAdon, Michael Mullins, and Joey Storer

The Dow Chemical Company

We used first principles density functional theory (B3LYP/6-31G**) to investigate the energetics of the chain propagation steps in olefin polymerization by Brookhart-type homogeneous olefin polymerization catalysts. In particular we considered:

but with the mesityl ligands simplified. For the Fe and Co pyridine-bis(imine) cases, we also did full quantum mechanics (QM) calculations for catalysts including the bulky mesityl sidegroups. Full QM calculations on such large molecules are very costly and time-consuming, and thus the impending implementation of mixed quantum mechanics/molecular mechanics (QM/MM) methods to do such calculations will be briefly discussed.

We find that the mechanism generally proceeds as follows for pyridine-bis(imine) systems:

  1. the growing polymer is bound covalently to the metal at an angle perpendicular to the metal ligand plane, say below (structure A)
  2. the olefin then complexes to the metal at a site trans to the ligand (structure B)
  3. the metal-polymer bond and the olefin-metal p bond rotate by 90° from the original orientation so that the olefin lies perpendicular to the metal ligand but above this plane (structure C); this is the rate-determining step
  4. the polymerization step is a 2s + 2s addition of the olefin p bond to the metal-polymer s bond (structure D), resulting in the new metal-polymer bond lying 90° above the metal ligand plane (structure E)

 

We have also investigated trends in the catalytic behavior of these systems as the metals, coordinating ligands, and monomer units (ethylene or propylene) are varied. Finally, we considered the regioselectivity and stereo-selectivity of the propylene catalytic pathways.

This research was funded by the Dow Chemical Company, NSF (95-22129), and by the MSC.

Selected Results