Dynamic Monte Carlo Simulation of ATRP with Bifunctional Initiators

A dynamic Monte Carlo model was developed to simulate ATRP with bifunctional initiators in a batch reactor. Model probabilities were calculated from polymerization kinetic parameters and reactor conditions. The model was used to predict monomer conversion, average molecular weight, polydispersity and the complete CLD as a function of polymerization time. The Monte Carlo model was compared with simulation results from a mathematical model that uses population balances and the method of moments. We also compared polymerizations with monofunctional and bifunctional initiators to illustrate some of the advantages of using bifunctional initiators in ATRP. In addition, we used the model to investigate the effect of the control volume and several polymerization conditions on simulation time, monomer conversion, molecular weight averages and CLD. Our results indicate that computational times can be reduced without sacrificing the quality of the results if we run several simulations with small control volumes rather than one single simulation with a large control volume.

     

The characterization of the capability of a balance

A survey is given on important standardized definitions by which the capability of balances may be characterized. Some modifications are proposed with regard to the use of mass sensors for the continuous determination of mass variations. An important supplement is the relative resolution introduced by Jenemann. Optimum values are presented.I am indepted to H. R. Jenemann for his critical remarks.     

Mathematical Modeling of Bivariate Polymer Property Distributions Using 2D Probability Generating Functions, 1 – Numerical Inversion Methods

This is the first of two papers presenting a new mathematical method for modeling bivariate distributions of polymer properties. It is based on the transformation of the infinite mass balances describing the evolution of a two‐dimensional distribution using 2D probability generating functions (pgf). A key step of this method is the inversion of the transforms. In this work, two numerical inversion methods of 2D pgfs are developed and carefully validated. The accuracy obtained with both methods was very satisfactory. The inversion formulas of both methods are simple and easy to implement. A simple copolymerization example is used to show the complete procedure from the derivation of the pgf balances to the recovery of the bivariate molecular weight distribution.

     

Yes,I'm still eating

The profound knowledge of Analytical Chemistry is but one prerequisite for the food chemist. Many times the analytical problem itself is almost trivial whereas the problems involved with sampling, sample preparation and the conclusion drawn from the analytical results can be very complex. Two examples are described to illustrate the wide range of analytical methods necessary to tackle one problem: Experiments to investigate the occurrence of ethyl carbamate in alcoholic beverages and food illustrate applications of various analytical techniques: coarse methods for the quantitation of main components to sophisticated trace analyses of food contaminants; the analysis of aflatoxin in figs highlights the problems involved with sample inhomogeneity and the needs and uses of quick screening methods in food quality control.     

Modeling the Effects of Reactor Backmixing on RAFT Polymerization

The effects of reactor backmixing on the outcome of RAFT polymerizations are examined. The kinetic model for the RAFT reaction is based on the first principle of mass balances and the method of moments. Two reactor models were applied for the simulation of backmixing, i.e., a PFTR with loop and multiple CSTRs in series. The models predict the behavior of monomer conversion, molecular weights, their distributions as well as the polydispersities for various chain types at different levels of backmixing. Both models demonstrated that a significant impact of reactor backmixing on RAFT kinetics and polymer molecular weight development exists.

     

Modern vacuumbalances

Different types of vacuum balances have been developed corresponding to the various applications [ 1 ]:

Special beam balances are applied for metrology and thermogravimetry and for the investigation of physical and chemical reactions of solid samples with the gas phase. Today, with all these balances the beam deflection is observed by an inductive or photoelectric sensor and electromagnetically restored.

In corrosive atmospheres or under very clean conditions e.g. for investigations in ultrahigh vacuum the magnetic suspension balance should be favoured. The sample tube is separated from the balance and can be hermetically sealed from the environment. It contains only the pan with the sample connected to a permanent magnet, suspended at a stable position in the field of a controlled electromagnet. The carrying balance is electrodynamically compensated.

The spring balance is an inexpensive alternative, if a minor relative resolution can be accepted. The simple design utilizing only few materials enables measurements under very clean conditions. A quartz spring allows for investigations in corrosive atmosphere. The extension of the spring is observed optically or by means of an inductive sensor.

Quartz resonators are used to control vacuum metallizing and other evaporating and sputtering processes. Resonator systems of any type need no gravitational field and can be applied, therefore, in space technology. The mass determination is restricted to samples which are strongly connected to the surface of the sensor.

     

Monitoring the degradation and solubilisation of butyltin compounds during in vitro gastrointestinal digestion using “triple spike” isotope dilution GC-ICP-MS

An in vitro gastrointestinal digestion approach in combination with species-specific isotope dilution analysis has been employed for the first time to study the transformation reactions as well as the solubilisation of butyltin species throughout a simulated human digestion. Different sample preparation procedures were assayed in order to avoid problems derived from lack of isotope equilibration between the endogenous and the isotopically-enriched added species. A triple spike approach, which can be used to calculate the corrected concentrations of mono-, di-, and tributyltin (MBT, DBT and TBT, respectively), as well as six interconversions, was employed throughout this work. In order to calculate and compare the species degradation factors, a triple spike solution containing each butyltin species enriched in a different isotope was added to the simulated gastric and intestinal fluids before the digestion procedures in the presence and in the absence of a solid biological matrix (commercial mussel tissue). Additionally, by analysing the soluble and insoluble fractions resulting from the simulated digestion of a commercial mussel tissue (gastric and gastric plus intestinal digestion), total mass balances for each butyltin compound could be derived. For this purpose, the isotopically-enriched species were added after the enzymatic digestions in order to avoid problems derived from lack of isotope equilibration. The mass balances provided information not only about the solubilisation but also about the degradation of the butyltin species during the digestion procedures. Good agreement between the degradation factors calculated under all experiments performed in this work and between those reported in previous works were obtained. The most serious degradation observed was that of DBT to produce MBT, whereas slight degradations of TBT and MBT were detected. Moreover, a worrying 61% of the original total butyltin content present in a commercial mussel tissue was found to be solubilised after complete simulated gastrointestinal digestion, with minimal degradation of TBT.     

Hydrogen Exchange Mass Spectrometry of Related Proteins with Divergent Sequences: A Comparative Study of HIV-1 Nef Allelic Variants

Hydrogen exchange mass spectrometry can be used to compare the conformation and dynamics of proteins that are similar in tertiary structure. If relative deuterium levels are measured, differences in sequence, deuterium forward- and back-exchange, peptide retention time, and protease digestion patterns all complicate the data analysis. We illustrate what can be learned from such data sets by analyzing five variants (Consensus G2E, SF2, NL4-3, ELI, and LTNP4) of the HIV-1 Nef protein, both alone and when bound to the human Hck SH3 domain. Regions with similar sequence could be compared between variants. Although much of the hydrogen exchange features were preserved across the five proteins, the kinetics of Nef binding to Hck SH3 were not the same. These observations may be related to biological function, particularly for ELI Nef where we also observed an impaired ability to downregulate CD4 surface presentation. The data illustrate some of the caveats that must be considered for comparison experiments and provide a framework for investigations of other protein relatives, families, and superfamilies with HX MS.

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Graphical Abstract ?

     

Kinetics of the solvolysis of thetrans-dichlorobis (Diaminoethane)-cobalt(III) ion in aqueous ethylene carbonate and aqueous propylene carbonate

The kinetics of the solvolysis of trans-[Coen₂Cl₂]⁺ have been followed in mixtures of water with either ethylene carbonate or propylene carbonate over a range of temperatures. Both the enthalpy and entropy of activation for the first order loss of a chloride ion to give [Coen₂Cl]²⁺ in water + ethylene carbonate show a maximum at low mole fractions of ethylene carbonate. As similar extrema in H and S for the same process for this complex and others in water +2-propanol and in water + t-butanol correlate well with extrema in the physical properties of the mixtures which are influenced by changes in solvent structure, it is suggested that these new extrema can be attributed to solvent structure effects. The application of a free energy cycle to the loss of the chloride ion in water and in the mixtures suggests that, although changes in solvent structure influence the cation in the transition state more than the cation in the initial state in water + ethylene carbonate, in water + propylene carbonate the influence of changes in solvent structure approximately balances. This is compared with the application of the free energy cycle to the same process in mixtures of water with a range of cosolvents using kinetic data available in the literature.     

Development of a Kinetic Model for Thermal Crosslinking of Rubbery Polymers

Summary: A kinetic model for the thermal crosslinking of rubbery polymers is presented. The reaction mechanism used to develop the model includes thermal radical generation producing a polymeric radical and a primary radical, crosslinking from attack of a polymer radical to any inactive polymer molecule, bimolecular radical termination among chains of any degree of branching, and radical termination between a polymer radical and a primary radical from the thermal radical generation. The overall polymer population is divided into “generations”, which are defined in terms of the number of primary (linear) chains of original rubbery polymer attached to the polymer molecule. Mass balances for each generation are written down, and expressions to calculate fraction of branched molecules, gel fraction, and number and weight average chain lengths are derived. Model simulations, parameter sensitivity analyses and preliminary parameter estimation studies are presented, taking the thermal crosslinking of linear polybutadiene as a case study.

Development of a kinetic model for thermal crosslinking of rubbery polymers: reaction mechanism, model equations and parameter sensitivity analyses.     

Studies of trace element transport in plants using high resolution γ-ray spectroscopy and perturbed directional correlation measurements

Two examples are given to illustrate how modern experimental techniques may extend the scope of possibilities of radiotracer applications. The first example refers to the use of a Ge-detector -ray spectrometer to measure the transport in plants of 15 elements simultaneously. The second example presented is an in-vivo study of the binding of Cd-ions in plants using meansurements of perturbed --directional correlations.     

A rapid spreadsheet method for the estimation of dead time

Summary A method has been developed for the rapid estimation of chromatographic column dead time. This utilises a commercial spreadsheet running on a Personal Computer, allowing automated dead time calculation in less than 30 seconds. Retention data from the literature have been used to illustrate the accuracy of this carbon number method. The method was also validated by using GC retention data experimentally recorded during this work.     

A Novel Approach to Population Balance Modeling of Reactive Polymer Modification Leading to Branching

The mathematical treatment of polymer modification systems, described by population balances containing convolution is discussed. The two‐dimensional case (molecular weight vs. number of branch points) was considered by utilizing approximations of distributions, expanding them in terms of Gaussian basis functions. Three branching reactions were addressed: chain backbone to chain end point coupling; three‐functional coupling of chain ends; and crosslinking. The results were compared to those of Monte Carlo (MC) simulations. Good agreement was observed, although the quality of a distribution as generated by the numerical approach is much better in view of the strong scatter in the MC data.

     

Study of the thermal decomposition of 3-cyclopentenone by using the AM1 semiempirical method

Summary RHF/AM1 and UHF/AM1 low-dimensionality surfaces were calculated by applying the reaction-coordinate technique to the thermal decomposition of 3-cyclopentenone. Several stationary points were accurately located within the entire coordinate space. Although the two formalisms used predicted asynchronous reaction pathways for the process, our results illustrate the limitations of single reference treatments of two-bond reactions while the nature of the reaction path remains uncertain.     

An investigation of convective diffusion to a stationary electrode in salt melts

Methods are proposed for determining the thickness of the diffusion layer and the ion diffusion coefficient D in melts, using potential-time and current-time curves. In developing the equations used for calculation, current ideas on the mass transfer of the depolarizer to the electrode are used. To illustrate the possibilities of the method, the diffusion of lead and cadmium ions is investigated in a eutectic mixture of potassium and lithium chlorides at 450C. The use of liquid electrodes of the same metals eliminates experimental errors due to chemical overpotential and capacity currents. The results obtained reveal that the equations proposed are applicable over the whole range of the experimental curves, and give reliable values for and D.     

Intrinsic chirality of triple‐layered naphthalenophane and related graphs

We prove that the graph of triplelayered naphthalenophane and an infinite class of related graphs are all intrinsically chiral. We also give examples to illustrate that not all graphs which are contractible to a Möbius ladder with three rungs are necessarily intrinsically chiral.     

Branching and Crosslinking in Coordination Terpolymerizations

A general kinetic method, based upon population balances of generating functions, is applied to the prediction of the microstructure and molecular size of non‐linear terpolymers obtained through the coordination polymerization of two monovinyl monomers and a non‐conjugated diene. A rather complex kinetic scheme involving crosslinking and long‐chain branching is considered. It is shown that even in these conditions it is possible to carry out the prediction of molecular size and mass distributions, sequence size distributions, and z‐average mean‐square radius of gyration of the polymers. The influence of some kinetic parameters on the properties of the products is studied, considering a homogeneous operation in a semi‐batch reactor. The used simulation method is able to predict these properties before and after gelation whenever it occurs.

     

The garner heavy duty quartz fiber microbalance

Summary A Garner balance is described which by means of a vertical suspension fiber is said to be capable of weighing a 5-g load with a sensitivity better than that attainable with the ordinary knife-edge balance. Its weighing chambers were especially designed to accommodate light absorption tubes for the determination of carbon and hydrogen. A practical system for eliminating vibration and furnishing a suitable mounting for sensitive quartz fiber balances in the industrial laboratory is also reported. Under a 5-g load, the Garner balance was found to give strict proportionality within±0.001 mg between torsion and weight. It reproduces within ±0.26g when weighing a 0.4-g platinum boat with the torsion fiber under continuous strain.

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Zusammenfassung Eine Garner Mikrowaage mit Quarzbalken wird beschrieben, deren Empfindlichkeit die der üblichen, auf Schneiden ruhenden Feinwaagen übertrifft und die doch eine Last von 5 g tragen kann, da der Balken von einem vertikalen Quarzfaden getragen wird. Die Gehänge und Gehängekammern wurden zur Aufnahme von leichten Absorptionsapparaten speziell entworfen. Eine für das Industrielaboratorium geeignete, praktische Aufstellung der Waage, die störende Vibrationen ausschließt, wird beschrieben. Unter Maximalbelastung von 5 g wurde Proportionalität innerhalb±0,001 mg zwischen Gewicht und kompensierender Torsionskraft festgestellt. Das Gewicht eines Platinschiffchens (0,4 g) konnte mit einem mittleren Fehler von ±0,26g reproduziert werden, wenn der horizontale Torsionsfaden aus Quarz ständig unter Torsionsspannung gehalten wurde.

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Résumé On décrit une balance de Garner qui, au moyen d'un fil de suspension vertical, est capable de peser une charge de 5 g avec une sensibilité meilleure que celle que l'on obtient avec une balance à couteau ordinaire. Ses compartiments ont été spécialement agencés pour loger les tubes d'absorption légers pour le dosage du carbone et de l'hydrogène. On donne aussi un système pratique pour éliminer les vibrations et pour fournir un montage convenable pour les balances à fil de quartz sensibles dans les laboratoires industriels. Sous une charge de 5 g, on a trouvé que la balance de Garner donne une proportionnalité rigoureuse à moins de±0,001 mg entre la torsion et le poids. Elle donne une valeur reproductible à moins de±0,26g pour une nacelle de platine de 0,4 g pour le fil de torsion soumis à un effort constant.