Quantitative Comparative Kinetics of 1-Hexene Polymerization across Group IV Bis-Phenolate Catalysts

The kinetics of 1-hexene polymerization using a series of three Ti amine bis-phenolate catalysts, Ti[tBu-(ONO)-O-X]Bn-2 (X = THF (1), pyridine (2), NMe2 (3)), were investigated and compared to analogous Zr and Hf complexes. A model-based approach using a diverse set of data (including monomer consumption, molecular weight evolution, etc.) was employed to determine the reaction specific rate constants of the simplest mechanism. These catalysts exhibited similar mechanisms that include the elementary reaction steps of initiation, propagation via 1,2-insertion, misinsertion via 2,1-insertion, recovery from misinsertion by 1,2-insertion, and monomer independent chain transfer. Rate constants of the Ti catalysts are typically lower than those of the Hf and the Zr catalysts by 1 and 2 orders of magnitude, respectively. The percentage of regioerrors follows the trend of Ti > Hf > Zr for catalyst 1 while the trend of Ti > Zr > Hf occurs for catalysts 2 and 3. The ratio of the propagation rate to the termination rate at a constant monomer concentration exhibits the trend Zr > Ti > Hf for catalysts with the same X. This relationship was developed further by computing Mn values from the determined rate constants under fixed reaction conditions. A quantitative structure function relationship, similar to that found previously for Zr and Hf,is observed between the logarithm of the chain transfer rate constant and the Ti-X bond distance. These findings underscore the importance of comprehensive quantitative kinetic modeling in establishing structure-function relationships.