title: Insight into Group 4 Metallocenium-Mediated Olefin Polymerization Reaction Coordinates Using a Metadynamics Approach.
abstract: We report here the first application of the computationally efficient metadynamics approach for analyzing single-site olefin polymerization mechanisms. The mechanism of group 4 metallocenium catalysis for ethylene homopolymerization is investigated by modeling the ethylene insertion step at the cationic (η(5)-C5H5)Zr(CH3)2(+) center using molecular dynamics simulations within the Density Functional Theory (DFT) framework. In particular, the metadynamics formalism is adopted to enable theoretical characterization of covalent bond forming/breaking processes using molecular dynamics ab initio tools. Analysis of the ethylene insertion step free energy surface indicates a slightly exoergic process (-3.2 kcal/mol) with a barrier of 8.6 kcal/mol, in good agreement with conventional ab initio static calculations. Analysis of the structural and dynamic aspects of the simulated reaction coordinate reveals a preferred olefin configuration which aligns parallel to the Zr-CH3 vector in concert with insertion and a slightly bent conformation of the product n-propyl chain to avoid nonbonded repulsion between methylene groups. It is found that the unsaturated/electrophilic CpZr(CH3)2(+) center drives the insertion step, thus promoting the formation of the Zr-alkyl bond. The metadynamics analysis uniquely encompasses all energetically possible reaction coordinates, thus providing a more detailed mechanistic picture. These results demonstrate the potential of metadynamics in the conformational and geometrical analysis of transition metal-centered homogeneous catalytic processes.
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Catania , and INSTM, UdR Catania, Viale A. Doria 6, 95125 Catania, Italy.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Motta
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Catania , and INSTM, UdR Catania, Viale A. Doria 6, 95125 Catania, Italy.
authors.forename: Ignazio L
authors.initials: IL
authors.lastname: Fragalà
authors.affiliation: Department of Chemistry, Northwestern University , Evanston, Illinois 60208-3113, United States.
authors.forename: Tobin J
authors.initials: TJ
authors.lastname: Marks