Steric and Solvation Effects on Polymerization Kinetics, Dormancy, and Tacticity of Zr-Salan Catalysts

1-Hexene polymerization using four zirconium based amine bis(phenolate) catalysts, Zr[2-R-4-tBu-ONNO]-Bn-2 (where R = methyl (1), ethyl (2), isopropyl (3), and tertbutyl (4)), was examined to establish the effect of ligand sterics on kinetics and stereocontrol of olefin polymerization. For each catalyst, a rich data set was obtained including monomer consumption, molecular weight evolution, end-group analysis, and active site counts. A set of reaction specific rate constants was determined for catalysts 2-4. In addition to the standard elementary reaction steps of initiation, propagation, mis-insertion, and chain transfer, these catalysts undergo isomerization leading to dormancy. This isomerization is particularly prominent in bromobenzene and for less sterically hindered complexes. Across the series 2-4, ligand steric properties have a systematic effect on both insertion and chain transfer rate constants. This relationship was rationalized using the Sterimol steric parameters. The rates of propagation and mis-insertion were found to be similarly sensitive to steric effects with sensitivities of 0.9 and 0.76 angstrom(-1), respectively, while initiation was found to be slightly more sensitive (0.99 angstrom(-1)) and chain transfer to be considerably less sensitive (0.58 angstrom(-1)) owing to the reduced steric demand for monomer independent chain termination compared to bimolecular insertion. The rates of propagation and misinsertion in bromobenzene were found to be more sensitive to sterics (1.24 and 0.98 angstrom(-1), respectively). In addition, ligand sterics play a significant role in the tacticity of the resulting polymer, the difference in polymers resulting from catalysts 1-4 was also systematic and could be compared to the Sterimol steric parameters. All kinetic data were critical in the determination of the full mechanism for this series; thus, this study illustrates the importance of obtaining a complete kinetic data in order to confidently establish quantitative structure activity relationships (QSARs).