Quantitative Structure-Activity Relationships for Unbridged Zirconocene Catalysts During Ethene Polymerization

Polymerization of ethene catalyzed by alkyl-substituted dicyclopentadienyl zirconium dichlorides [(R-Cp)₂ZrCl₂] was performed with methylaluminoxane as cocatalyst in toluene at T=50°C and P_Ethene=2 bar. A kinetic model which includes activation of the dichloride, propagation of the polymer chain, and (non-permanent) deactivation of the active site was used to extract rate constants from the time-dependent polymerization activity curves. A calibration set of nine catalysts (R=H; Me; l,2Me₂; l,3Me₂; lMe2Et;n-Pr; n-Bu; Me₄; Me₅) was used to establish quantitative structure-property relationships, both for the average polymerization activity over one hour and the propagation rate constant derived with the kinetic model. Structural parameters were obtained from low-energy conformations of γ-agostic (R-Cp)₂ZrC₄H₉+ cations, assumed to be reasonable representations of the active site. Geometries were optimized within the PM3(tm) semiempirical model, and electronic descriptors were computed via density-functional energy calculations on the PM3(tm) geometries. A principal-component analysis (PCA) followed by partial least-squares (PLS) regression indicates that the catalyst with R=1Me2Et is an “outlier”. Better linear models were found for the propagation rate constant than for the average polymerization activity. Several steric and electronic parameters have significant regression coefficients, indicating that the propagation rate is a complex function of molecular structure. Quantitative predictions were made for the propagation rate constant of catalysts that were not members of the calibration set: R=Et, n-Pen, t-Bu, and i-Pr, and Ind₂ZrCl₂.