Simplified treatment of cooligomerization kinetics

Equations for the degree of polymerization and the cooligomer composition in the styrene (A)–methyl methacrylate (B)–CCl₄(S) system were derived from the assumed reaction scheme by the use of some assumptions for simplification, and their appropriateness was examined. The chain transfer constants of the growing radicals of styrene (C_SA) and methyl methacrylate (C_SB) to CCl₄, which were estimated from the apparent chain transfer constants C_SAB in the cooligomerization system, agreed with the homooligomerization values. This means that the degree of the polymerization of the cooligomer can be expressed by the equation: where P_n is the degree of polymerization of the cooligomer and P_nO is that when no chain transfer agent (CCl₄) is added; r_A and r_B are the monomer reactivity ratios of monomers A and B in this system. The cooligomer composition deviated from the statistical steady-state composition on the low molecular weight side, and this deviation was explained by the equation:      

Computer simulation of the liquid crystal formation in a semi-flexible polymer system

Molecular dynamic simulations are reported for system of semi-flexible linear rod-like molecules. The molecules are composed of N_c tangent soft spheres, connected by elastic springs. Rigidity is introduced by additional springs between all pairs of spheres along the molecule. The formation of only a nematic LC phase is shown for all systems with N_c = 8 and different flexibility. The effect of flexibility on the order parameter and the volume fraction at the LC phase transition is compared with theoretical predictions by Khokhlov-Semenov and with available simulation data. The dependence of the anisotropy of diffusion on chain flexibility in LC phase was studied. The polymer brushes consisting of flexible and semi-flexible (composed of linear rod-like segments) chains were simulated at different grafting densities. Height of brush, order parameter, distribution of density and chain ends in brush were obtained in both cases and compared with theoretical predictions.     

Computer simulation of the chromatographic process

Summary A computer algorithm for the simulation of chromatographic processes is presented. It can be used to calculate elution curves and column profiles for linear, isotherms, non-linear isotherms, mixed retention mechanisms, different injection functions and different column efficiencies.     

Initiation and termination mechanism in the cooligomerization of the styrene–methyl methacrylate–carbon tetrachloride system

The analysis of the endgroups of the oligomers produced in the styrene (A)–CCl₄(S) system (system I), the methyl methacrylate(B)–CCl₄ system (system II), and the styrene–methyl methacrylate–CCl₄ system (system III) was carried out in order to clarify the mechanism of the initiation, transfer, and termination. In system I, the number of Cl atoms per oligomer molecule N_Cl increases with the molar ratio of [S]/[A] when the molar ratio of [S]/[A] is below unity and is about four when the molar ratio of [S]/[A] is above unity, and the number of initiator fragments per oligomer molecule N_I decreases with the increase in the molar ratio of [S]/[A]. In system II, N_Cl is about 0.45 over a considerably wide range of the molar ratio of [S]/[B]. In system III, N_Cl increases and N_I decreases with the increase in the molar ratios of [S]/([A] + [B]) and [A]/[B]. From the data of N_Cl and N_I, the fraction I_CC14 of the initiation by the tri-chloromethyl radical in the overall initiation reactions and the fraction T_CC14 of the chain transfer reaction of the growing radical of styrene in all the reactions which produce the cooligomer in the system III were calculated. I_CCl3 and T_CC14 both increase with the molar ratios [S]/([A] + [B]) and [A]/[B].     

Computer simulation of interactions in the NH3-CO2-H2O system

Computer simulation of interactions in the NH₃-CO₂-H₂O system was performed using a quantum-chemical method B3LYP/6-31G(d,p) for the simulation of the possible routes of the reactions and the estimation of the energy parameters: interaction energy between molecules in complexes, activation energy of forward and reverse reactions, and the heat of the reaction. A new version of termolecular reaction mechanism is proposed and investigated. The probability of realization of various paths of interaction in the NH₃-CO₂-H₂O system was shown to be determined by the temperature: at low temperatures the termolecular mechanism is more probable, while at the temperatures close to the standard conditions carbamate and bimolecular mechanisms are preferable.     

Linear cooligomerization of vinylsilanes with butadiene as a method for the synthesis of silicon-containing polyenes

Conclusions We have developed a method for the synthesis of silicon-containing polyenes from butadiene and trimethylvinylsilane or trimethoxyvinylsilane with a Ni-containing catalyst system. We suggest a reaction sequence for the cooligomerization process.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 599–604, March, 1979.     

Computer expert system for spectral line simulation and selection in inductively coupled plasma atomic emission spectrometry

This paper is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta, Part B (SAB). This hardcopy text, comprising the main body and an appendix, is accompanied by a disk with programs, data files and a brief manual. The main body discusses purpose, design principle and usage of the computer software for the inductively coupled plasma atomic emission spectrometry (ICP-AES) expert system. The appendix provides a brief instruction on the manipulation of the demonstration program and relevant information on accessing the diskette.The computer software of the expert system has been developed in C++ language to simulate spectra and to select analytical lines in ICP-AES. This expert system is based on a comprehensive model of non-LTE ICP-AES, which includes expertise in plasma discharges, analyte ionization and excitation, and spectral-line shapes. The system also provides several databases in which essential elemental and spectral data are stored. A logic reasoning engine is utilized for selection of the best analytical line with a main criterion of minimizing the true detection limit. The system is user-friendly with pop-up menus, an editor for database operation, and a graphic interface for the display of simulated spectra. The system can simulate spectra and predict spectral interferences with good accuracy.     

Computer simulation of liquid crystals

Summary We review recent progress in the computer simulation of liquid crystals, with special emphasis on hard particle models. Surprisingly, the simplest molecular models, taking account only of molecular size and shape, are sufficient to generate a wide variety of liquid crystalline phases, closely analogous to those observed in real life. Thermodynamic stability of different phases is very sensitive to shape, and presumably will also be sensitive to further details of intermolecular interactions as they are incorporated into the model. Realistic atom-atom potential models of liquid crystals are available, but the associated simulations are quite expensive. Thus, while idealized models may be used to study quite general, fundamental properties of mesophases, the modelling of specific liquid crystal systems in a realistic way remains a great challenge. Progress continues to be made on both these fronts.     

Computer simulation of asymmetric transformations

In order to obtain the best strategy for organic synthesis, the simulation of the reaction course can be helpful. In this letter we present a facile mathematical system for the simulation of the reaction course based on the reaction kinetics. Reaction courses are described and displayed easily by dividing the whole process into several small intervals. Exemplarily this is demonstrated for kinetic resolutions and desymmetrizations. The Microsoft^® Excel tables used for these simulations are available herein.     

Computer simulation of macromolecular materials

Computer simulation of model systems with Monte Carlo methods enables the detailed study of structure and thermodynamic properties of these systems and thus constitutes a link between analytic theory and experiment. Typical applications that are discussed include polymer blends, dynamics of local motions in polymer melts, and the adsorption of polymers on walls.Invited talk at the Tagung der Deutschen Physikalischen Gesellschaft (DPG), Fachausschuß Polymerphysik, Hamburg, March 14–16, 1988.     

Computer simulation of cluster assembling

Density functional theory is used to investigate the assembling of metallic clusters to yield stable or metastable cluster solids. Motivated by the observed high stability of the Al₁₃H cluster, which has a substantial highest occupied and lowest unoccupied molecular orbitals (HOMO–LUMO) gap, we have modeled the assembling of those clusters. For a favorable relative orientation of each cluster with respect to all its neighbors, a cluster solid is predicted and its structure appears to be stable at least up to 150 K, which is the highest temperature in our simulations. We have also studied the chemical bonding in the stoichiometric solid alloys PbA, where A is one of the alkali elements Na, K, Rb, or Cs. Those crystals exist in an ordered phase formed by tetrahedral Pb₄ clusters surrounded by the alkali atoms. The study of this family of natural cluster compounds reveals the coating role played by the cations, providing further insight into the favorable conditions required for the formation of cluster solids. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Protein-like copolymers: Computer simulation

The model ofAB copolymers with a “protein-like” primary sequence was developed. This type of copolymers was obtained in a computer experiment. First, the conformation of a collapsed dense homopolymer globule was generated and then, based on this conformation, the primaryAB sequence was determied by denoting the monomeric units located near the surface of the globule as unitsA and those constituting the core of the globule as unitsB. After that, the primary structure of the chain was fixed, and different interaction potentials for theA andB units were introduced. Drawing an analogy of this model to aqueous solutions of globular proteins,A units were interpreted as hydrophilic, andB units were regarded as hydrophobic. By means of Monte Carlo simulation using the bond fluctuation model, the coli—globule transition in “protein-like”AB copolymer, induced by an increase in the attraction between the hydrophobicB units, was studied. The coil—globule transition in a copolymer with the “protein-like” primary sequence occurs at a higher temperature and has higher rate and is sharper than that in a random copolymer with the sameA/B composition and in a random block copolymer with the sameA/B composition and the same “degree of blockiness”. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 884–889, May, 1998.     

Computer simulation of polyolefin interfaces

Interfaces, thin films and amorphous cells of several hydrocarbon polymers are studied with atomistic computer simulation. Solubility parameters, surface tensions and interfacial tensions are evaluated from the computed structures. These structures are constructed using Molecular Dynamics and Molecular Mechanics techniques. The various components of the energetic interactions (torsional, vdW, etc.) are examined in detail in order to gain an insight into the nature of the regular and anomalous interactions between these films.The polymers studied in this article are all are atactic linear hydrocarbons and have approximately 200 carbons in the backbone chain. These include polypropylene (PP), head to head polypropylene (hhPP), a polymer which is termed P78 (a random copolymer composed of 22% of a linear (–CH₂CH₂CH₂CH₂–) monomer and 78% of a branched (–CH₂CH(C₂H₅)–) monomer), polyethylenepropylene (PEP), and PE2P2 (an alternating copolymer consisting of two ethylene (E) and two propylene (P) monomers in the repeating pattern (–(EEPP)_n–)). The primary difference between the different polymers studied is the type of side group (ethyl or methyl) and the location, frequency and spacing of that group.Interfaces formed with the same polymer type (PP–PP, PEP–PEP, P78–P78) yield a surface tension close to zero as expected. Attractive interactions between some of the pairs of films is found in the simulation. The distribution among the energetic terms seems to indicate that this attractive interaction is brought about by favorable local intramolecular energetic terms rather than simply by van der Waals interactions alone.     

Computer simulation of colloidal dispersions

This paper discusses recent applications of statistical mechanics to dispersions with particular emphasis on the computer simulation of the dynamic properties.Fundamental to any computation on a colloidal dispersion is the knowledge of the potential of mean force for at least a pair of suspended particles. At low-to-moderate particle concentrations for stable dispersions, statistical mechanical calculations based on the normal DLVO pair potential produce reasonable agreement with experiment for a number of equilibrium properties of simple latex dispersions. This phenomenon indicates that under these conditions the DLVO pair potential is a reasonable effective pair potential. However, recent Monte Carlo simulations and experimental measurements with liquids of spherical molecules suggest that the force between a pair of dispersed particles at very small separation may differ significantly from that predicted by DLVO theory.The computation of dynamic properties of dispersions involves problems not encountered in the above equilibrium calculations. In particular, one must include the effects of indirect hydrodynamic as well as direct interactions among the particles. This computation may be easily accomplished at moderately low particle concentrations and the results of such calculations are able to give a very detailed analysis of the results of Photon Correlation Spectroscopy measurements on ion exchanged polystyrene latex suspensions at low concentration. These computations also, once again, emphasize the usefulness of DLVO pair potentials as effective pair potentials for systems of strongly interacting particles.     

Computer simulation of stress relaxation

Molecular dynamics simulations of stress relaxation have been performed for models of metals and polymers. The method which employs coupling between the simulation cell and an external thermal bath as well as an applied stress has been used. Two-dimensional models of the materials are defined with interactions described by the Lennard-Jones (Mie 6–12) and harmonic potentials. A special method is employed to generate chains in dense polymeric systems. Simulated stress relaxation curves are similar for metals and polymers, while there exist essential differences in the stress-strain behavior of the models. During the relaxation, trajectories of the particles in different materials display a common feature: there exist domains in which movement of the particles is highly correlated. This observation supports the cooperative theory of stress relaxation developed by one of us. The results of the simulations do not significantly depend on the number of particles in the system nor on other simulation details.     

Computer simulation of surfactant solutions

Major advances have been made at several levels of computer simulation of surfactant solutions. Atomistic level studies of preassembled surfactant structures have become fairly routine. The development of structure in surfactant solutions has now been studied using atomistic, coarse grain and mesoscopic models. Coarse grain and mesoscopic simulations have been used to determine phase diagrams. The challenges involved in treating complex surfactant solutions will continue to drive this field forward.     

Computer simulation of the chemical properties of copolymers

A new method for modelling polymerization kinetics is described in which numerical integration is used to provide the time scale for the polymerization experiment and Monte Carlo methods are employed to calculate the instantaneous molecular weight distribution, detailed chain composition, and chemical heterogeneity of the polymer as a function of time/conversion. The method has been applied in simulations of styrene-methyl methacrylate copolymerization and styrene-hydroxyethyl acrylate-butyl acrylate terpolymerization.