The kinetics of 1-hexene polymerization using a family of five zirconium amine bis-phenolate catalysts, Zr[tBu-ON(X)O]Bn2 (where X = THF (1), pyridine (2), NMe2 (3), furan (4), and SMe (5)), has been investigated to uncover the mechanistic effect of varying the pendant ligand X. A model-based approach using a diverse set of data including monomer consumption, evolution of molecular weight, and end-group analysis was employed to determine each of the reaction specific rate constants involved in a given polymerization process. The mechanism of polymerization for 1-5 was similar and the necessary elementary reaction steps included initiation, normal propagation, misinsertion, recovery from misinsertion, and chain transfer. The latter reaction, chain transfer, featured monomer independent β-H elimination in 1-3 and monomer dependent β-H transfer in 4 and 5. Of all the rate constants, those for chain transfer showed the most variation, spanning 2 orders of magnitude (ca. (0.1-10) × 10(-3) s(-1) for vinylidene and (0.5-87) × 10(-4) s(-1) for vinylene). A quantitative structure-activity relationship was uncovered between the logarithm of the chain transfer rate constants and the Zr-X bond distance for catalysts 1-3. However, this trend is broken once the Zr-X bond distance elongates further, as is the case for catalysts 4 and 5, which operate primarily through a different mechanistic pathway. These findings underscore the importance of comprehensive kinetic modeling using a diverse set of multiresponse data, enabling the determination of robust kinetic constants and reaction mechanisms of catalytic olefin polymerization as part of the development of structure-activity relationships.

Kinetic modeling using a population balance approach has been performed in order to identify a mechanism and a set of rate constants that describe the batch polymerization of 1-hexene by the homogeneous single-site catalyst Zr(tBu-ON NMe 2 O)Bn 2 activated by B(C 6 F 5 ) 3 in toluene. The mechanism and rate constants were determined by making use of a multiresponse data set, including (i) monomer concentration versus time for various initial concentrations of monomer and catalyst, (ii) the time evolution of the molecular weight distribution, (iii) active site concentrations versus time, and (iv) vinyl end group concentrations versus time. The overall mechanism requires slow chain initiation compared to propagation, 2,1-misinsertion and recovery, and two chain transfer pathwaysone forming vinylidene end groups and the other forming vinylene end groups. The quantitative analysis of kinetic data clearly shows that a significant fraction of the catalyst does not participate in the chain growth process. The quantitative analysis is carefully detailed to provide a general procedure for kinetic model discrimination and the assignment of rate constants that can be used for other single-site catalysts.

The kinetics of 1-hexene polymerization using a family of three zirconium and hafnium amine bis-phenolate catalysts, M[t-Bu-ONXO]Bn2 (where M = Zr (a) or Hf (b), and X = THF (1), pyridine (2), or NMe2 (3)), have been investigated to uncover the mechanistic effect of varying the metal center M. A model-based approach using a diverse set of data including monomer consumption, evolution of molecular weight, and end-group analysis was employed to determine each of the reaction-specific rate constants involved in a given polymerization process. This study builds upon the mechanism of polymerization for 1a–3a, which has been previously reported by applying the same methodology to the hafnium containing analogues, 1b–3b. It has been observed that each elementary step-specific rate constant that involves the insertion of a monomer is reduced by an order of magnitude. As previously reported for catalysts 1a–3a, a quantitative structure–activity relationship was uncovered between the logarithm of the monomer-independent chain transfer rate constants and the Hf–X bond distance for catalysts 1b–3b. However, this dependence on the pendant ligand is 2.7 times weaker for the Hf-containing analogues versus those containing Zr. These findings underscore the importance of comprehensive kinetic modeling using a diverse set of multiresponse data, enabling the determination of robust kinetic constants and reaction mechanisms of catalytic olefin polymerization as part of the development of structure–activity relationships.

The kinetics of ethylene trimerization by a chromium N-phosphinoamidine (Cr-(P,N)) precatalyst activated by modified methylaluminoxane (MMAO) has been investigated by high-pressure NMR techniques. An in-depth kinetic analysis of this metallacyclic mechanism has been conducted. It was found that an intermediate in the trimerization catalytic cycle, proposed in this study as the chromium alkenyl hydride species, degrades into a polymer active site where this degradation step is independent of ethylene concentration and is first order in catalyst. Additionally, we report that at least one of the first two ethylene coordination steps must be reversible in order to predict the features of the monomer consumption profiles. The reaction order in ethylene is dependent on the reversibility of the ethylene coordination steps. The observation of these details of the mechanism explains many of the challenges inherent in the examination of this and similar catalyst systems and emphasizes the usefulness of operando high-pressure NMR studies and a quantitative kinetic modeling approach in the study of such systems.

The kinetics of 1-hexene polymerization using a series of three Ti amine bis-phenolate catalysts, Ti­[tBu-ONXO]­Bn2 (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 M n 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.