Selective Degenerative Benzyl Group Transfer in Olefin Polymerization

The kinetics of 1-hexene polymerization was investigated using a previously studied zirconium amine bis-phenolate catalyst, Zr­[tBu-ON^THFO]­Bn₂, where the effect of substoichiometric amounts of activator on the polymerization was studied to more clearly elucidate the mechanism of degenerative benzyl-group transfer. Comprehensive kinetic analysis was performed for a diverse set of data including monomer consumption, evolution of molecular weight, and end-group counts over a range of activator to precatalyst ratios, where the analysis determined the rates of association and dissociation of a binuclear complex (BNC) intermediate through which degenerative transfer proceeds. Kinetic modeling indicates that the benzyl-group transfer inside the BNC is rapid, as supported by ¹H NMR. Rapid association and dissociation of the BNC enable complete activation of all precatalysts even under the condition of substoichiometric amounts of activator through a degenerative benzyl-group transfer. Through the use of a novel experimental technique wherein a labeled catalyst is introduced during a normal polymerization reaction, this process has been observed to instantaneously activate all incoming precatalyst and effectively shut down the misinsertion pathway. The kinetic analysis shows that BNC has a faster initiation rate than a typical catalyst–ion pair, which may be due to the anion being previously displaced by the incoming unactivated precatalyst

Zwitterionic Ring-Opening Polymerization: Models for Kinetics of Cyclic Poly(caprolactone) Synthesis

Investigations of the kinetics of zwitterionic ring-opening polymerization of ε-caprolactone by N-heterocyclic carbenes (NHC) were carried out to illuminate the key reaction steps responsible for the formation of high molecular weight cyclic poly­(caprolactones). Modeling of both the decay in monomer concentration as well as the evolution of molecular weights and polydispersities were necessary to identify the key reaction steps responsible for initiation, propagation, cyclization and chain-transfer. Nucleophilic attack of the NHC on ε-caprolactone to generate reactive zwitterions is slow and reversible. The modeling indicates that less than 60% of the carbenes are transformed to active zwitterions, but that these zwitterions rapidly add monomer and cyclize by intramolecular backbiting of the terminal alkoxides on internal esters of the zwitterions. This cyclization event maintains the concentration of active zwitterions. The reactivation of cyclized chains by active zwitterions is a key step that leads to high molecular weight poly­(caprolactones)

Non-Heme Manganese Catalysts for On-Demand Production of Chlorine Dioxide in Water and Under Mild Conditions

Two non-heme manganese complexes are used in the catalytic formation of chlorine dioxide from chlorite under ambient temperature at pH 5.00. The catalysts afford up to 1000 turnovers per hour and remain highly active in subsequent additions of chlorite. Kinetic and spectroscopic studies revealed a Mn^III(OH) species as the dominant form under catalytic conditions. A Mn^III(μ-O)­Mn^IV dinuclear species was observed by EPR spectroscopy, supporting the involvement of a putative Mn^IV(O) species. First-order kinetic dependence on the manganese catalyst precludes the dinuclear species as the active form of the catalyst. Quantitative kinetic modeling enabled the deduction of a mechanism that accounts for all experimental observations. The chlorine dioxide producing cycle involves formation of a putative Mn^IV(O), which undergoes PCET (proton coupled electron-transfer) reaction with chlorite to afford chlorine dioxide. The ClO₂ product can be efficiently removed from the aqueous reaction mixture via purging with an inert gas, allowing for the preparation of pure chlorine dioxide for on-site use and further production of chlorine dioxide

Comparison of Selected Zirconium and Hafnium Amine Bis(phenolate) Catalysts for 1‑Hexene Polymerization

The kinetics of 1-hexene polymerization using a family of three zirconium and hafnium amine bis-phenolate catalysts, M­[<i>t</i>-Bu-ON^XO]­Bn₂ (where M = Zr (<b>a</b>) or Hf (<b>b</b>), and X = THF (<b>1</b>), pyridine (<b>2</b>), or NMe₂ (<b>3</b>)), 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 <b>1a</b>–<b>3a</b>, which has been previously reported by applying the same methodology to the hafnium containing analogues, <b>1b</b>–<b>3b</b>. 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 <b>1a</b>–<b>3a</b>, 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 <b>1b</b>–<b>3b</b>. 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

Interaction between Two Active Sites of the Same Catalyst for Macromonomer Enchained Olefin Polymerization

A zirconium amine bis­(phenolate) catalyst that is capable of simultaneously producing both oligomers and polymers of 1-hexene was investigated. It was found that the polymer produced has more branching than the commonly encountered poly­(1-hexene), suggesting an oligomer macromonomer enchainment. The polymer weight fraction and molecular weight are weakly dependent on conversion and are not affected by increasing the concentration of free oligomer macromonomers in solution. After considering a number of possible mechanisms, the most plausible supported by spectroscopic and kinetic data is a second-order reaction between an oligomer forming site and a polymer forming site, resulting in the transfer of oligomer macromonomers into the growing polymer chain. The ratio between oligomer and polymer products can be precisely tuned by varying the precatalyst activation conditions