scholarly journals Ethylene Polymerization via Zirconocene Catalysts and Organoboron Activators: an Experimental and Kinetic Modeling Study

2020 ◽  
Author(s):  
Luis Valencia ◽  
Francisco Enríquez-Medrano ◽  
Ricardo López-González ◽  
Priscila Quiñonez-Angulo ◽  
Enrique Saldívar-Guerra ◽  
...  
Keyword(s):  
Active Sites ◽  
Optimization Procedure ◽  
Kinetic Mechanism ◽  
Least Square ◽  
Coordination Polymerization ◽  
Solvent Phase ◽  
Vapor Equilibrium ◽  
Kinetic Rate Constants ◽  
Rate Of Polymerization ◽  
Polymerization Behavior

After 40 years of the discovery of metallocene catalysts, there are still several aspects that remain unresolved, especially when the “conventional” alkylaluminum activators are not used. Herein, we systematically investigate the synthesis of PE via three different zirconocene catalysts, with different alkyl substituents, activated via different organoboron compounds. The polymerization behavior, as well as the properties of the materials, are evaluated. The results demonstrate that the highest catalytic activity is shown by Bis(cyclopentadienyl)dimethylzirconium activated by trityl tetra(pentafluorophenyl)borate. Also finding that toluene is the optimum solvent for these systems and at these reaction conditions. Moreover, to validate our experimental results, a comprehensive mathematical model is developed on the basis of thermodynamic and kinetic principles. The concentration of ethylene transferred to the solvent phase (toluene) in a liquid-vapor equilibrium (LVE) system is estimated based on the Duhem’s theorem. Arrhenius expressions for the kinetic rate constants of a proposed kinetic mechanism are estimated by a kinetic model, in which the rate of polymerization is fitted by a least-square optimization procedure and the molecular weight averages by the method of moments. The simulations of the coordination polymerization suggest the presence of two types of active sites, principally at low temperatures, and the reactivation of the deactivated sites via a boron-based activator. However, the effect of the temperature on the reactivation step is no clear; a deeper understanding via designed experiments is required.

scholarly journals Ethylene Polymerization via Zirconocene Catalysts and Organoboron Activators: An Experimental and Kinetic Modeling Study

Processes ◽  
2021 ◽  
Vol 9 (1) ◽  
pp. 162
Author(s):  
Luis Valencia ◽  
Francisco Enríquez-Medrano ◽  
Ricardo López-González ◽  
Priscila Quiñonez-Ángulo ◽  
Enrique Saldívar-Guerra ◽  
...  
Keyword(s):  
Active Sites ◽  
Optimization Procedure ◽  
Kinetic Mechanism ◽  
Least Square ◽  
Coordination Polymerization ◽  
Solvent Phase ◽  
Vapor Equilibrium ◽  
Kinetic Rate Constants ◽  
Rate Of Polymerization ◽  
Polymerization Behavior

Forty years after the discovery of metallocene catalysts, there are still several aspects that remain unresolved, especially when the “conventional” alkylaluminum activators are not used. Herein, we systematically investigated the synthesis of polyethylene (PE) via three different zirconocene catalysts, with different alkyl substituents, activated via different organoboron compounds. The polymerization behavior, as well as the properties of the materials, were evaluated. The results demonstrate that the highest catalytic activity is shown by bis(cyclopentadienyl)dimethylzirconium activated by trityl tetra(pentafluorophenyl)borate. Additionally, it was found that toluene is the optimum solvent for these systems and at these reaction conditions. Moreover, to validate our experimental results, a comprehensive mathematical model was developed on the basis of thermodynamic and kinetic principles. The concentration of ethylene transferred to the solvent phase (toluene) in a liquid–vapor equilibrium (LVE) system was estimated based on Duhem’s theorem. Arrhenius expressions for the kinetic rate constants of a proposed kinetic mechanism were estimated by a kinetic model, in which the rate of polymerization was fitted by a least-square optimization procedure and the molecular weight averages by the method of moments. The simulations of the coordination polymerization suggest the presence of two types of active sites, principally at low temperatures, and the reactivation of the deactivated sites via a boron-based activator. However, the effect of the temperature on the reactivation step was not clear; a deeper understanding via designed experiments is required.

scholarly journals Two view NURBS reconstruction based on GACO model

2021 ◽  
Author(s):  
Deepika Saini ◽  
Sanoj Kumar ◽  
Manoj K. Singh ◽  
Musrrat Ali
Keyword(s):  
Optimization Algorithms ◽  
Three Dimensional ◽  
Optimization Procedure ◽  
Least Square ◽  
Ant Colony ◽  
Free Form ◽  
Reconstruction Process ◽  
Nurbs Curve ◽  
Data Points ◽  
Ant Colony Optimization Algorithms

AbstractThe key job here in the presented work is to investigate the performance of Generalized Ant Colony Optimizer (GACO) model in order to evolve the shape of three dimensional free-form Non Uniform Rational B-Spline (NURBS) curve using stereo (two) views. GACO model is a blend of two well known meta-heuristic optimization algorithms known as Simple Ant Colony and Global Ant Colony Optimization algorithms. Basically, the work talks about the solution of NURBS-fitting based reconstruction process. Therefore, GACO model is used to optimize the NURBS parameters (control points and weights) by minimizing the weighted least-square errors between the data points and the fitted NURBS curve. The algorithm is applied by first assuming some pre-fixed values of NURBS parameters. The experiments clearly show that the optimization procedure is a better option in a case where good initial locations of parameters are selected. A detailed experimental analysis is given in support of our algorithm. The implemented error analysis shows that the proposed methodology perform better as compared to the conventional methods.

Kinetics of Isoprenepolymerization Initiated with TiCl4—Trialkyl Aluminum

1960 ◽  
Vol 33 (3) ◽  
pp. 696-698
Author(s):  
S. E. Bresler ◽  
M. I. Mosevitskiĭ
Keyword(s):  
Active Sites ◽  
Reaction Medium ◽  
Heat Evolution ◽  
Polymerization Reaction ◽  
Reaction Conditions ◽  
Low Viscosity ◽  
Rate Of Polymerization ◽  
Rate Of Reaction ◽  
Entire Experiment ◽  
Kinetics Of

Abstract For the study of the mechanism of polymerization by means of complexes of aluminum organic compounds with titanium chlorides, data on the kinetics of polymerization is of great interest. Up to the present time, the rate of polymerization of propylene has been studied but the interpretation of the kinetic data is difficult because the polymer, which is practically insoluble in the reaction medium, entraps the catalyst resulting in a rate of reaction which is dependent on the diffusion of monomer through the polymer to the active sites. In this work the polymerization of isoprene, which yields polymers soluble in the monomer, in saturated hydrocarbons and in benzene, was studied. The rate of the polymerization reaction was measured by the thermal effect in a calorimeter consisting of a 3.5 1. Dewar flask, with a lid, immersed in a thermostated air bath maintained at approximately the temperature of the reaction. Low viscosity spindle oil, heated to the temperature of the reaction (about 32°), served as the calorimeter fluid. The ampoule holder extended outside of the calorimeter and was connected to a shaking apparatus. The ampoule was divided by a thin partition into two sections each holding 45–50 cc. Into one section previously purified monomers and solvent were distilled. The other section was filled with catalyst components from a Shlenk container. The change in temperature of the calorimeter was determined with a Beckman thermometer with an accuracy of 0.01 °. When the temperature of the calorimeter containing the ampoule remained constant to within 0.01–0.02° for 30–40 minutes, the shaking apparatus was connected and the partition was broken with a striker. Intensive shaking was continued during the entire experiment resulting in mixing of the reaction mixture and of the calorimeter fluid. The rate of reaction was determined by the rate of heat evolution ; in other words, by the temperature rise in the calorimeter. For a rise of 0.1–0.5° the reaction conditions remained practically isothermal. This rise permits the kinetics of the reaction to be observed with sufficient accuracy. Adiabaticity of the calorimeter and the effect of mechanical heat were controlled in separate experiments.

scholarly journals Comparison of the active sites of the purified carnitine acyltransferases from peroxisomes and mitochondria by using a reaction-intermediate analogue

1993 ◽  
Vol 294 (3) ◽  
pp. 645-651 ◽  
Author(s):  
N Nic a′ Bháird ◽  
G Kumaravel ◽  
R D Gandour ◽  
M J Krueger ◽  
R R Ramsay
Keyword(s):  
Active Sites ◽  
Random Order ◽  
Kinetic Mechanism ◽  
Product Inhibition ◽  
Binding Domains ◽  
Cell Compartments ◽  
Mitochondrial Inner Membrane ◽  
Kinetic Mechanisms ◽  
Carnitine Acyltransferases ◽  
Inhibition Studies

The carnitine acyltransferases contribute to the modulation of the acyl-CoA/CoA ratio in various cell compartments with consequent effects on many aspects of fatty acid metabolism. The properties of the enzymes are different in each location. The kinetic mechanisms and kinetic parameters for the carnitine acyltransferases purified from peroxisomes (COT) and from the mitochondrial inner membrane (CPT-II) were determined. Product-inhibition studies established that COT follows a rapid-equilibrium random-order mechanism, but CPT-II follows a strictly ordered mechanism in which acyl-CoA or CoA must bind before the carnitine substrate. Hemipalmitoylcarnitinium [(+)-HPC], a prototype tetrahedral intermediate analogue of the acyltransferase reaction, inhibits CPT-II 100-fold better than COT. (+)-HPC behaves as an analogue of palmitoyl-L-carnitine with COT. In contrast, with CPT-II(+)-HPC binds more tightly to the enzyme than do substrates or products, suggesting that it is a good model for the transition state and, unlike palmitoyl-L-carnitine, (+)-HPC can bind to the free enzyme. The data support the concept of three binding domains for the acyltransferases, a CoA site, an acyl site and a carnitine site. The CoA site is similar in COT and CPT-II, but there are distinct differences between the carnitine-binding site which may dictate the kinetic mechanism.

THE CONSTRAINED LEAST-SQUARE OPTIMIZATION OF SPHERICAL FOUR-BAR LINKAGES FOR RIGID-BODY GUIDANCE

1992 ◽  
Vol 16 (1) ◽  
pp. 47-60 ◽  
Author(s):  
Zheng Liu ◽  
J. Angeles
Keyword(s):  
Rigid Body ◽  
Numerical Procedure ◽  
Optimization Procedure ◽  
Least Square ◽  
Optimization Scheme ◽  
Loop Optimization ◽  
Linkage Error ◽  
General Basis ◽  
Set Up ◽  
Guidance Problem

The synthesis of spherical four-bar linkages for rigid-body guidance consists of the computation of the relevant dimensions of this type of linkage so that it can guide its coupler link to attain a set of prescribed orientations, Generalized coupler curves are used to describe the specified orientations and to evaluate the linkage error, defined as the sum of the errors between the corresponding computed and prescribed orientations. A two-loop optimization procedure is set up to minimize this error, results being obtained resorting to the orthogonal decomposition algorithm, an iterative numerical scheme introduced elsewhere. Continuation and damping techniques are used in the numerical procedure to enhance the convergence likelihood and its rate. The optimization scheme is developed on a general basis and can handle the spherical rigid-body guidance problem for any number of prescribed orientations. A numerical example is included in the paper.

scholarly journals Kinetic and structural evidence that Asp-678 plays multiple roles in catalysis by the quinoprotein glycine oxidase

2019 ◽  
Vol 294 (46) ◽  
pp. 17463-17470 ◽  
Author(s):  
Kyle J. Mamounis ◽  
Dante Avalos ◽  
Erik T. Yukl ◽  
Victor L. Davidson
Keyword(s):  
Active Sites ◽  
Catalytic Mechanism ◽  
Water Channel ◽  
Kinetic Mechanism ◽  
Multiple Roles ◽  
Nucleophilic Attack ◽  
Proton Relay ◽  
Initial Substrate ◽  
Stable Product ◽  
Glycine Oxidase

PlGoxA from Pseudoalteromonas luteoviolacea is a glycine oxidase that utilizes a protein-derived cysteine tryptophylquinone (CTQ) cofactor. A notable feature of its catalytic mechanism is that it forms a stable product-reduced CTQ adduct that is not hydrolyzed in the absence of O2. Asp-678 resides near the quinone moiety of PlGoxA, and an Asp is structurally conserved in this position in all tryptophylquinone enzymes. In those other enzymes, mutation of that Asp results in no or negligible CTQ formation. In this study, mutation of Asp-678 in PlGoxA did not abolish CTQ formation. This allowed, for the first time, studying the role of this residue in catalysis. D678A and D678N substitutions yielded enzyme variants with CTQ, which did not react with glycine, although glycine was present in the crystal structures in the active site. D678E PlGoxA was active but exhibited a much slower kcat. This mutation altered the kinetic mechanism of the reductive half-reaction such that one could observe a previously undetected reactive intermediate, an initial substrate-oxidized CTQ adduct, which converted to the product-reduced CTQ adduct. These results indicate that Asp-678 is involved in the initial deprotonation of the amino group of glycine, enabling nucleophilic attack of CTQ, as well as the deprotonation of the substrate-oxidized CTQ adduct, which is coupled to CTQ reduction. The structures also suggest that Asp-678 is acting as a proton relay that directs these protons to a water channel that connects the active sites on the subunits of this homotetrameric enzyme.

scholarly journals Fully Bio-Based Elastomer Nanocomposites Comprising Polyfarnesene Reinforced with Plasma-Modified Cellulose Nanocrystals

Polymers ◽  
2021 ◽  
Vol 13 (16) ◽  
pp. 2810
Author(s):  
Ilse Magaña ◽  
Dimitrios Georgouvelas ◽  
Rishab Handa ◽  
María Guadalupe Neira Neira Velázquez ◽  
Héctor Ricardo López López González ◽  
...  
Keyword(s):  
Cellulose Nanocrystals ◽  
Plasma Reactor ◽  
Plasma Generator ◽  
Model Systems ◽  
Characteristic Relaxation Time ◽  
Coordination Polymerization ◽  
Polymerization Behavior ◽  
Characteristic Features ◽  
Tunable Mechanical Properties ◽  
Surface Modified

This article proposes a process to prepare fully bio-based elastomer nanocomposites based on polyfarnesene and cellulose nanocrystals (CNC). To improve the compatibility of cellulose with the hydrophobic matrix of polyfarnesene, the surface of CNC was modified via plasma-induced polymerization, at different powers of the plasma generator, using a trans-β-farnesene monomer in the plasma reactor. The characteristic features of plasma surface-modified CNC have been corroborated by spectroscopic (XPS) and microscopic (AFM) analyses. Moreover, the cellulose nanocrystals modified at 150 W have been selected to reinforce polyfarnesene-based nanocomposites, synthesized via an in-situ coordination polymerization using a neodymium-based catalytic system. The effect of the different loading content of nanocrystals on the polymerization behavior, as well as on the rheological aspects, was evaluated. The increase in the storage modulus with the incorporation of superficially modified nanocrystals was demonstrated by rheological measurements and these materials exhibited better properties than those containing pristine cellulose nanocrystals. Moreover, we elucidate that the viscoelastic moduli of the elastomer nanocomposites are aligned with power–law model systems with characteristic relaxation time scales similar to commercial nanocomposites, also implying tunable mechanical properties. In this foreground, our findings have important implications in the development of fully bio-based nanocomposites in close competition with the commercial stock, thereby producing alternatives in favor of sustainable materials.

Parameter Identification for Multibody Dynamic Systems

1997 ◽  
Author(s):  
Radu Serban ◽  
Jeffrey S. Freeman ◽  
Dan Negrut
Keyword(s):  
Sensitivity Analysis ◽  
Parameter Identification ◽  
Dynamic Systems ◽  
Time History ◽  
Optimization Procedure ◽  
Optimal Choice ◽  
Least Square ◽  
Unknown Parameters ◽  
Multibody Dynamic ◽  
Dynamic Sensitivity Analysis

Abstract This paper presents a parameter identification technique for multibody dynamic systems, based on a nonlinear least-square optimization procedure. The procedure identifies unknown parameters in the differential-algebraic multibody system model by matching the acceleration time history of a point of interest with given data. Derivative information for the optimization process is obtained through dynamic sensitivity analysis. Direct differentiation methods are used to perform the sensitivity analysis. Examples of the procedure are presented, applying the technique both to perfect data; i.e. data produced by the assumed model with the optimal choice of parameters, and to experimental data; i.e. data measured on the real system and thus subject to noise and modelling imperfections.

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