Grigori A. Medvedev

Mechanistic Detail Revealed via Comprehensive Kinetic Modeling of [rac-C2H4(1-indenyl)2ZrMe2]-Catalyzed 1-Hexene Polymerization

Thorough kinetic characterization of single-site olefin polymerization catalysis requires comprehensive, quantitative kinetic modeling of a rich multiresponse data set that includes monomer consumption, molecular weight distributions (MWDs), end group analysis, etc. at various conditions. Herein we report the results obtained via a comprehensive, quantitative kinetic modeling of all chemical species in the batch polymerization of 1-hexene by rac-C(2)H(4)(1-Ind)(2)ZrMe(2) activated with B(C(6)F(5))(3). While extensive studies have been published on this catalyst system, the previously acknowledged kinetic mechanism is unable to predict the MWD. We now show it is possible to predict the entire multiresponse data set (including the MWDs) using a kinetic model featuring a catalytic event that renders 43% of the catalyst inactive for the duration of the polymerization. This finding has significant implications regarding the behavior of the catalyst and the polymer produced and is potentially relevant to other single-site polymerization catalysts, where it would have been undetected as a result of incomplete kinetic modeling. In addition, comprehensive kinetic modeling of multiresponse data yields robust values of rate constants (uncertainties of less than 16% for this catalyst) for future use in developing predictive structure-activity relationships.

Effects of Pendant Ligand Binding Affinity on Chain Transfer for 1-Hexene Polymerization Catalyzed by Single-Site Zirconium Amine Bis-Phenolate Complexes

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.

Development of a stochastic constitutive model for prediction of postyield softening in glassy polymers

A stochastic constitutive model has been developed that explicitly acknowledges the nanometer size dynamic heterogeneity of glassy materials, where the distribution of the viscoelastic relaxation times emerges naturally as a result of the dynamic heterogeneity. A set of stochastic differential equations for local stresses and entropy describing behavior of a mesoscopic domain are developed, and the observed macroscopic response of the material is obtained as an average of an ensemble of domains. The stochastic constitutive model naturally predicts and provides a mechanism for the postyield stress softening and its dependence on physical aging that is observed during constant strain rate uniaxial deformations.

TIME-DEPENDENT MECHANICAL BEHAVIOR OF CARBON BLACK FILLED ELASTOMERS

Abstract An extensive set of time-dependent mechanical data was obtained for several filled SBR elastomers, including Mullins experiments, cyclic loading experiments, and stress relaxation. These c...

Mechanistic study of a manganese porphyrin catalyst for on-demand production of chlorine dioxide in water

A water-soluble manganese porphyrin complex was examined for the catalytic formation of chlorine dioxide from chlorite under ambient temperature at pH 5.00 and 6.90. Quantitative kinetic modeling allowed for the deduction of a mechanism that accounts for all experimental observations. Catalysis is initiated via an OAT (Oxygen Atom Transfer) reaction resulting in formation of a putative manganese(V) oxo species, which undergoes ET (Electron Transfer) with chlorite to form chlorine dioxide. As chlorine dioxide accumulates in solution, chlorite consumption slows down and ClO2 reaches a maximum as the system reaches equilibrium. In phosphate buffer at pH 6.90, manganese(IV) oxo accumulates and its reaction with ClO2 gives ClO3-. However, at pH 5.00 acetate buffer proton coupled electron transfer (PCET) from chlorite to manganese(IV) oxo is fast and irreversible leading to chlorate formation only via the putative manganese(V) oxo species. These differences underscore how PCET rates affect reaction pathways and mechanism. The ClO2 product can be collected from the aqueous reaction mixture via purging with an inert gas, allowing for the preparation of chlorine dioxide on-demand.