A discussion on the kinetic behavior of Ziegler–Natta ethylene polymerization at early moments of the reaction via modeling

The gas-phase ethylene polymerizations with SiO₂-supported Ziegler–Natta (ZN) catalyst at early moments of reaction is modeled. The experimental data used in this work show that at initial stages of the polymerization there is a sharp reduction in the reaction rate and a sharp rise in molecular weight. In the modeling, multi-active sites assumption and diffusion limitations inside the particle are studied. Energy balance to calculate the temperature at early moment is applied. Kinetic model including initiation, propagation, chain transfers and active site deactivation steps are used to predict the reaction rate and molecular weights. Polymeric flow model (PFM) is applied for single particle model (SPM). The results show two distinct regions. First region with sharp gradient for reaction rate and molecular weights and then, shortly there is a smooth region in which the changes in rate and molecular weights are slow. To fit this sharp gradient followed by a nearly steady state behavior two types of active sites are necessary. A group of highly active sites which deactivated soon and the active sites with lower activity and relatively long-lasting.     

Gas-phase polymerization of ethylene over Ti-based Ziegler–Natta catalysts prepared from different magnesium sources

This study focuses on gas-phase polymerization of ethylene using the titanium-based Ziegler–Natta catalysts prepared from different magnesium sources including MgCl₂ (Cat A), magnesium powder (Cat B), and Mg(OEt)₂ (Cat C). During polymerization, different cocatalysts were also used. It was found that Cat C with triethylaluminum as a cocatalyst exhibited the highest activity. This was likely attributed to optimal distribution of active sites on the catalyst surface. It can be observed by increased temperature in the reactor due to highly exothermic reaction during polymerization. By the way, the morphologies of the polymer obtained from this catalyst were spherical, which is more preferable. Besides the catalytic activity, crystallinity and morphology were also affected by the different magnesium sources used to prepare the catalysts.     

Symmetry rules and reaction mechanisms of Ziegler–Natta catalysts

Symmetry rules obeyed by metallocenes having well-defined, rigid structures are in accord with reaction mechanisms proposed in the 1960s and 1970s for Ziegler–Natta catalysts with unknown structures.     

The Tafel–Heyrovsky route in the kinetic mechanism of the hydrogen evolution reaction

The examination of the dependence of surface coverage on overpotential for the kinetic mechanism of the hydrogen evolution reaction (HER) through the Volmer–Heyrovsky route shows that, under certain conditions, the Tafel re-adsorption can be produced. On this basis, the HER could be described with the contribution of Tafel and Heyrovsky elementary steps. The present communication derives and analyses the kinetic mechanism of the HER for the Tafel–Heyrovsky route, under Frumkin adsorption conditions.     

Polymerization control and fast characterization of the stereo-defect distribution of heterogeneous Ziegler–Natta isotactic polypropylene

Adjustment of the stereo-defects distribution of isotactic polypropylene (iPP) is important for obtaining better properties of the resins in Ziegler–Natta polymerization. In this study, in order to adjust the stereo-defect distribution of iPP prepared by the Spheripol process (Basell), the cocatalyst/external donor (triethylaluminium/dicyclopentyldimethoxysilane, Al/Si) mole ratio was gradually changed from 30 to 36, accompanied with hydrogen adjustment. And we obtained a series of iPP samples (A–E). The characterization of ¹³C NMR, xylene soluble fraction (XS) showed the average isotacticity of the obtained samples are nearly same, but the results of high resolution ¹³C NMR, temperature rising elution fractionation (TREF) and xylene solvent fractionation indicated that, as the Al/Si ratio increases, the amount of high isotacticity in iPP gradually decreases, the amount of medium and low isotacticity gradually increased, the distribution of stereo-defects becomes more uniform.A relative time-saving method, successive self-nucleation and annealing (SSA) fractionation was applied to evaluate the stereo-defect distribution of iPP. A calibration which can be used to convert SSA final melting curves to the average meso sequence length curves was constructed. A good agreement between ¹³C NMR, XS, TREF results and SSA fractionation results indicated that it is feasible by using SSA to provide quick characterize of the stereo-defect distribution of iPP.     

Advanced heterogeneous Pd catalysts for the Suzuki–Miyaura reaction with aryl bromides

The set of heterogeneous Pd catalysts containing different forms of Pd (PdCl₄^2– or Pd⁰) was prepared by chemical modification and laser electrodispersion using two types of ₊ supports, namely, SiO₂ modified by ionic liquid and γ-Al₂O₃. Testing of the synthesized catalysts in the Suzuki–Miyaura reaction with aryl bromides pointed out the possibility to achieve the prominent TOF and TON values. The dependencies of TOF on the catalyst loading indicate that only a fraction of loaded Pd was involved in the catalysis.     

The mechanism of ring-opening polymerization of L-lactide by ICl3 catalysts: Halogen bond catalysis or participating in reactions?

The mechanism of ring-opening polymerization of L-lactide by iodine trichloride (ICl₃) catalyst has been explored by using density functional theory (DFT) calculations and three catalytic pathways were proposed. The first and second pathways belong to the halogen bond catalysis, and the third pathway involves the ICl₃ catalysts participating in reactions. When the carbonyl group was maintained involved in the reaction and activated catalytically by the halogen bond, there are two possible pathways. The first pathway involves only one transition state, and the second pathway requires two transition states. There is another pathway in which ICl₃ directly participates in the reaction, it is named the third pathway. Two different transition states of the four-membered rings are generated successively, the transfer of I─O bonds determined the progress of the reaction. Theoretical calculations in this work provide the most basic understanding of ring-opening polymerization of L-lactide by ICl₃ catalysts. © 2019 Wiley Periodicals, Inc.     

Kinetic features of short-time polymerization of isoprene in the presence of supported titanium–magnesium catalyst

Short-time polymerization of isoprene under the action of supported titanium–magnesium catalyst is carried out. The pulsation mixing of the reagent flows and the unit design features allow one to reduce the average residence time of reagents in the reaction zone and to study the first 0.7 s of the isoprene polymerization. It is found that, very early in polymerization, the propagation of polyisoprene macromolecules proceeds on the surface of the primary aggregates of catalyst particles characteristic of a high trans-1,4 specificity via the “living” mechanism with a high rate. Furthermore, the fragmentation of the initial aggregates of the catalyst particles occurs, which results in formation of new polymerization centers, a decrease in the average molecular masses of polyisoprene, and a broadening of the polymer MMD. The results are explained by the existence of a range of the kinetic continuity of the rapid initiation stage and several subsequent stages of macromolecule propagation, followed by a significant decrease in the chain propagation rate constant compared to the initiation constant.     

Reaction of nitroanilines with aldehydes. Refinement of the Doebner–Miller reaction mechanism

Due to intramolecular hydrogen bonding between the amino and nitro groups, o-nitroaniline is incapable of forming Schiff bases in the reactions with acetaldehyde and crotonaldehyde but is converted to quinoline derivative under Doebner–Miller reaction conditions via addition to the C=C double bond of the α,β-unsaturated aldehyde. Under analogous conditions, p-nitroaniline possessing a free amino group gives rise to the product of Doebner–Miller quinoline synthesis through intermediate formation of Schiff base dimer. The reaction of p-nitroaniline with benzaldehyde also yields the corresponding Schiff base, whereas o-nitroaniline is converted to N-benzyl derivative.     

Structure–reactivity relationship in bimolecular elimination reactions based on the condensed graph of a reaction

By means of a structural representation of the chemical reactivity as a condensed graph a model predicting rate constants of the bimolecular elimination reaction is derived for the first time. The model developed enables the prediction of rate constants of reactions proceeding in different solvents or water-organic mixtures at different temperatures. It demonstrates a good predictive performance: a mean square deviation of predicted values from experimental ones is less than 0.7 logarithmic units. An outlier analysis shows that prediction errors are mainly due to the imperfection of the training data containing unique reactions. The model is available for users at arsole.u-strasbg.fr.     

Behaviors in ethylene polymerization of MgCl₂-SiO₂/TiCl₄/THF Ziegler-Natta catalysts with differently treated SiO₂

The present research focuses on investigation of the catalytic behaviors of MgCl?-SiO?/TiCl?/THF Ziegler-Natta (ZN) catalysts with fumed SiO? variously treated with silane compounds. The non-treated silica (NTS) and other silicas treated with dimethylsilicone fluid (TSDMSF), dimethyldichlorosilane (TSDMDCS), and hexamethyl-disilazane (TSHMDS) were employed. It was found that the Cat-TSDMDCS and Cat-TSHMDS exhibited remarkably high activity, even with a similar bulk Ti content as the others. Thus, the more powerful technique of XPS analysis was used to determine the Ti content at the catalyst surface. It was evident that the surface concentrations of Ti could play important role on the catalyst activity. As the result, the increased activity is proportional to the surface concentration of Ti. It was mentioned that the change in surface concentration of Ti with different treated silica can be attributed to the effect of silane spacer group and steric hindrance. The distribution of Ti on the external surface can be also proven by means of EDX mapping, which matched the results obtained by XPS analysis. The treated silica also resulted in narrower molecular weight distribution (MWD) due to the more uniform active sites produced. There was no significant change in polymer morphology upon treatment of the silica.     

Properties of Pd–Ag/C catalysts in the reaction of selective hydrogenation of acetylene

Samples of Pd/C and Pd–Ag/C, where C represents carbon nanofibers (CNFs), are synthesized by methane decomposition on a Ni–Cu–Fe/Al₂O₃ catalyst. The properties of Pd/CNF are studied in the reaction of selective hydrogenation of acetylene into ethylene. It is found that the activity of the catalyst in hydrogenation reaction increases, while selectivity decreases considerably when the palladium content rises. The obtained dependences are caused by the features of palladium’s interaction with the carbon support. At a low Pd content (up to 0.04 wt %) in the catalyst, the metal is inserted into the interlayer space of graphite and the catalytic activity is zero. It is established by EXAFS that the main share of palladium in catalysts of 0.05–0.1 wt % Pd/CNF constitutes the metal in the atomically dispersed state. The coordination environment of palladium atoms consists of carbon atoms. An increase in the palladium content in a Pd/CNF catalyst up to 0.3 wt % leads to the formation of highly dispersed (0.8–1 nm) Pd particles. The Pd/CNF samples where palladium is mainly in the atomically dispersed state exhibit the highest selectivity in the acetylene hydrogenation reaction. The addition of silver to a 0.1 wt % Pd/CNF catalyst initially probably leads to the formation of Pd–Ag clusters and then to alloyed Pd–Ag particles. An increase in the silver content in the catalyst above 0.3% causes the enlargement of the alloyed particles and the palladium atoms are blocked by a silver layer, which considerably decreases the catalytic activity in the selective hydrogenation of acetylene.     

Formation of acridones by ethylene extrusion in the reaction of arynes with β-lactams and dihydroquinolinones

N-Unsubstituted β-lactams react with a molecule of aryne by insertion into the amide bond to form a 2,3-dihydroquinolin-4-one, which subsequently reacts with another molecule of aryne to form an acridone by extrusion of a molecule of ethylene. 2,3-Dihydroquinolin-4-ones react under the same reaction conditions to afford identical results. This is the first example of ethylene extrusion in aryne chemistry.     

Chiral mesoporous templated silicas as heterogeneous inorganic–organic catalysts in the enantioselective alkylation of benzaldehyde

New mesoporous templated silicas of the MCM-41 family covalently linked to chiral ephedrine are used as heterogeneous chiral auxiliaries in the enantioselective alkylation of benzaldehyde by diethylzinc. Promising results are obtained with these inorganic–organic catalysts. Various factors which may affect the activity, selectivity and enantioselectivity are studied. Comparison with homogeneous catalysis is carried out.     

Analysis of homogeneous–heterogeneous reactions in a micropolar nanofluid past a nonlinear stretching surface: semi-analytical approach

In the present scenario, the application of reactive agents in industries plays a vital role in the production processes of various materials such as paper production, drawing of wires, etc. Keeping in mind it is necessary to discuss the application of heterogeneous and homogeneous chemical reactions in a micropolar nanofluid flow over a stretching surface where the sheet is considered to be nonlinear. In addition to that, the inclusion of heat generation/absorption in the energy equation of an electrically conducting fluid also affects the fluid flow phenomena. In the present analysis, the Maxwell model thermal conductive is considered with Fe₃O₄, CuO nanoparticles, and water is treated as base fluid. It is very much transparent that the nonlinear dimensional form of the PDEs gets transformed into ODEs and a semi-analytical approach is employed, i.e., Adomian decomposition method (ADM) for those transformed ODEs. The computation for several characterizing parameters is obtained using the mathematical package MAPLE and these are displayed via graphs and tables. An excellent concurrence with the earlier established result is found which validates the result with the current methodology.

     

Selfcondensation of 2-methylpropanal with homochiral BINOL catalysts as a model asymmetric aldol–Tishchenko reaction

Binaphtholate catalysts were used to investigate the stereochemistry of 2-methylpropanal selfcondensation. Several steps of this condensation were found to be affected weakly or moderately by homochiral catalysts, but with opposite enantioselectivities.     

Immobilization of palladium nanoparticles as a recyclable heterogeneous catalyst for the Suzuki–Miyaura coupling reaction

This work describes the synthesis of Pd nanoparticles that are stabilized on CaAl-layered double hydroxide functionalized with Tris (tris(hydroxymethyl)aminomethane). The synthesized catalyst is characterized by several different analyses and has been successfully applied to the Suzuki–Miyaura reaction.     

On the mechanism of C ⇄ N migration of alkoxycarbonyl groups in reactions of pyridinium ylides with isocyanates

New examples of reversible C N migrations of alkoxycarbonyl groups, which occur in the reactions of pyridinium and 3-(diethylcarbamoyl)pyridinium ylides, viz., derivatives of dimethyl and diethyl malonates, with aryl isocyanates were studied. The mechanism of migration of the methoxycarbonyl group from the carbon atom to the nitrogen atom was considered on the basis of quantum-chemical calculations. The product of the primary attack of the isocyanate group by pyridinium ylide was established to be rearranged with low potential barriers to form carbamate without formation of cyclic intermediate compounds.