bluuebunny/crossref_metadata_2025_split huggingface dataset:
On exact and approximate methods of calculating an overall termination rate coefficient from chain length dependent termination rate coefficients
Authors: Russell Gregory T. 
Published: April 2003

AbstractIn this paper is tackled the problem of calculating the overall termination rate coefficient 〈kt〉 which follows from values of kti, j, by which is denoted the rate coefficient for termination between two free radicals of degrees of polymerization i and j, respectively. The significance of this problem is that polymerization experiments yield 〈kt〉 values, whereas microscopic models predict Kti,j values. An assumption‐free method is presented for computing the steady state 〈kt〉 corresponding to a set of Kti,j values. Parallel to this, approximate methods for calculating steady state 〈kt〉 values are developed: the so‐called short‐long approximation is used, and coarse graining of the radical chain length distribution is applied. Calculations are firstly carried out using a microscopic termination model which describes an intermediate conversion polymerization system, and then a set of calculations are performed with low conversion conditions in mind. Comparison of the completely exact solutions with the various approximate solutions reveals which of the approximate and therefore more tractable models is suitable for accurate, microscopic modeling of polymerization kinetics at the different physical conditions envisaged. In this respect the credentials of one of the coarse grain approaches are found to be persuasive. Also executed are calculations probing how values of 〈kt〉 depend on factors, such as the rate of propagation and of chain transfer to monomer, not traditionally regarded as having anything to do with termination rates; interesting results emerge. Because calculations are carried out with real systems very much in mind, the latter (and other) results are felt to be genuinely relevant to the mechanism of actual free radical polymerizations.
Quantum chemical characteristics of cationic active sites forming in the Bcl3‐α,ω‐dichloroalkyl systems
Authors: Yakimansky Alexander V. , Erussalimsky Boris L. , Zubkov Vladimir A. 
Published: April 2003

AbstractTaking as examples the cyclic halonium (I) and linear carbenium (II) model cations of equal composition, viz. [Cl(CH2)4]+, and products of propene addition to them (V and VI), the energetic characteristics of these structures were estimated by the MNDO(a) method. The energetic preference of I with respect to II was found to be much higher than that of V with respect to VI (ca. 97 and ca. 11 kJ/mole, respectively). This difference is suggested to reflect the decrease in the tendency of linear Cl‐containing carbenium ions to cyclization into halonium ions with an increase in the length of the linear hydrocarbon chain. The results obtained are applied to the discussion of the mechanism of polymerization in the 2‐methylpropene‐BCl3‐2,5‐dichloro‐2,5‐dimethylhexane system.
Quantum chemical PM‐3 study of the thermal stability of heterocyclic fragments of heteropolymers, 1. Five‐membered heterocycles: Azoles and imides
Authors: Zubkov Vladimir A. , Yakimansky Alexander V. , Bogdanova Svetlana E. , Kudryavtsev Vladislav V. 
Published: April 2003

AbstractSemiempirical PM‐3 calculations of the mechanisms of the decomposition of oxazole and maleimide have been carried out. It has been shown that thermal decomposition of the considered heterocycles proceeds via a consecutive breaking of two ring bonds, the breaking of the second bond being the rate‐determining step. Both an activation barrier of heterocycle decomposition and its dissociation energy (ED) can be used as a measure of heterocycle thermal stability. The correlation between ED values of azole and imide containing model compounds and the initial degradation temperatures of corresponding heteropolymers makes it possible to clarify some essential features of the thermal degradation of polyazoles, polybenzazoles and polyimides.
Quantum chemical PM‐3 study of the thermal stability of heterocyclic fragments of heteropolymers, 2. Six‐membered heterocycles
Authors: Zubkov Vladimir A. , Yakimansky Alexander V. , Bogdanova Svetlana E. , Kudryavtsev Vladislav V. 
Published: April 2003

AbstractDissociation energies (ED) of various heterocyclic fragments of repeating units of thermally stable heteropolymers have been calculated by the semiempirical PM‐3 method. Fragments with heterocycles containing carbonyl and/or amine groups and/or oxygen (or sulfur) atoms (benzazoles, phthalimides, benzoxazinones, etc.) have fairly close ED values. The correlation between ED values of fragments and the initial degradation temperature (TD) of heteropolymers comprising these fragments reveals that the TD values depend both on ED of corresponding heterocycles and on the manner of heterocycle decomposition: whether it leads to an immediate weight loss (polyimides, polybenzoxazinone, etc.) or just to a chain scission (polybenzazoles). Quinoline and quinoxaline are characterized by higher ED values, but it seems that decomposition of these compounds during the initial degradation of polyquinoline and polyquinoxaline proceeds via mechanisms different from the straightforward homolytic decomposition characteristic of other considered heterocycle decompositions.
Theoretical study of free‐radical ring‐opening polymerization of 2‐(o‐chlorophenyl)‐4‐methylene‐1,3‐dioxolane. A molecular and quantum mechanics approach
Authors: Morariu Simona , Surpateanu Gheorghe , Simionescu Bogdan C. 
Published: April 2003

AbstractThe methylene derivatives of 1,3‐dioxolane were proved to yield homo‐ and binary or ternary copolymers via free‐radical polymerization, depending on initiation type and reaction conditions. While different authors reported experimental data on these compounds, no explanation of such polymerization behaviour was provided. The present paper is aimed to propose, through molecular mechanics and quantum mechanics approaches, a valid explanation of the already published experimental results.
Persistence lengths of semiflexible chains — methods and approximations
Authors: Otto Matthias , Eckert Jochen , Vilgis Thomas A. 
Published: April 2003

AbstractSeveral models and methods for stiff polymer chains are discussed. The basic idea is to develop approximate solutions to the problem of the presistence length of stiff polymers. It turns out that the persistence length can be regarded as a measure for the quality of approximations. Mean‐field methods for field theoretical calculations of the persistence length show similarities of 1/d expansions in statistical physics (d being the space dimension) and saddle point approximations become reliable in various limits. Gaussian approximations become — as well known for the Ising model — simple extensions of random walks as trivial renormalisations of the Wiener‐Edwards model for bosonic strings.
On a quantity describing the degree of chain entanglement in linear polymer systems
Authors: Kröger Martin , Voigt Harald 
Published: April 2003

AbstractIn the present paper we define a quantity which describes how strong a polymer system is entangled. In MD computer simulations we apply this definition to samples of polymer melts which were generated with a specific large‐scale structure, as characterized by their radius of gyration, end‐to‐end distance and the “degree” of mutual entanglement of the chains. The quantities mentioned above are monitored over the relaxation of the samples towards equilibrium.
A simple model for finitely extensible polymers
Authors: Carl Wilfried 
Published: April 2003

AbstractA simple model for the calculation of configurational and rheological properties of finitely extensible polymers in flow is introduced. The finite extensibility of the chain is incorporated into the common Rouse model by varying the spring constant such that a constant contour length is maintained for every flow strength. For elongational flow, a comparison with Monte Carlo simulations of a bead‐rod chain with 100 links yields qualitative agreement. For shear flow, this model predicts non‐Newtonian flow behaviour.
Optimization of isothermal batchwise bulk free radical polymerization with and without chain transfer agent for minimum reaction time
Authors: Sajjadi Shahriar , Jahanzad Fatemeh 
Published: April 2003

AbstractIn this work the minimum reaction time was studied for a specific final monomer conversion and number‐average chain length by adjusting the amount of initial initiator concentration in the presence of a fixed amount of chain transfer agent at the best isothermal temperature. A new method for the determination of the best initial initiator concentration ([I]0) and isothermal temperature (T) in the presence of chain transfer agent was developed by the application of a simple optimization algorithm, based on the Lagrangian multiplier, to the basic free radical kinetics. Numerical examples for the polymerization of styrene are presented. It is shown that in the presence of a fixed amount of chain transfer agent the best isothermal policy for minimum time would deviate from dead‐end polymerization. As the concentration of chain transfer agent increases, a stronger deviation from dead‐end polymerization is observed. It was found that for a given pair of desired conversion and number‐average chain length the usage of a chain transfer agent would result in a longer reaction time. The results give insights into operation of batchwise bulk free radical polymerization for minimum reaction time under isothermal conditions.
Molecular weight distribution in random crosslinking of polymers: Modality of the molecular weight distribution
Authors: Tobita Hidetaka , Yamamoto Yoshiyasu , Ito Kenji 
Published: April 2003

AbstractThe molecular weight distribution (MWD), formed during random crosslinking of polymers whose initial molecular weight distribution is given by the Schulz‐Zimm distribution, is proposed. The obtained equation can be considered as a good approximation even for nonrandom crosslinking reactions such as free‐radical and living crosslinking copolymerizations. When the initial molecular weight distribution is very narrow, namely, the polydispersity index (Pw/Pn) is smaller than about 1,1, a multimodal distribution can be obtained. Quite often the MWD is represented solely by the average molecular weights such as number‐ and weight‐average molecular weights in conventional approaches; however, such types of measure can not give enough description for multimodal distributions. Multimodal distributions may be obtained for the primary polymer molecules formed via living polymerizations. On the other hand, for the primary polymer molecules formed by free‐radical polymerization whose polydispersity index must be larger than 1,5, it is impossible to form a bimodal distribution as long as nonideal kinetics such as the size and structure dependence of crosslinking reactions are neglected.
Perturbation expansion of polymers in shear flow
Authors: Carl Wilfried 
Published: April 2003

AbstractThe effect of shear flow on the excluded volume properties of bead spring chains is calculated via the perturbation expansion of the mean‐square end‐to‐end distance with respect to the excluded volume parameter z. The coefficient of the series 〈R2〉/〈R2〉0 = 1 + C1 · z − … vanishes for large flow rates but shows a strong dependence on the shear rate in the intermediate regime.
Tension and mobility of a DNA fragment in the lakes‐straits model
Authors: Loomans Dirk , Sokolov Igor M. , Blumen Alexander 
Published: April 2003

AbstractWe consider numerically, in the framework of the lakes‐straits model, the mobility of a DNA chain under field‐inversion gel‐electrophoresis (FIGE). Here we investigate the role of the gel's structure on the mobility. We consider two situations: (i) a DNA chain fragment is bounded by two straits (narrow gateways) at a fixed distance, but is otherwise free, and (ii) the fragment is, in addition, confined with in a closed volume (pore). We calculate the tension on the DNA fragment in the two cases. For Gaussian chains we evaluate the corresponding statistical weights exactly, using a cellular automation algorithm. We find that the resulting tension differs drastically in the two models considered. Nonetheless this difference influences only weakly the overall FIGE mobility.
Conformational analysis of highly extended poly(ethylene terephthalate) chains by Monte Carlo calculations
Authors: Auriemma Finizia , Corradini Paolo , Guerra Gaetano , Vacatello Michele 
Published: April 2003

AbstractThe conformational disorder compatible with the highly extended chains found in mesomorphic poly(ethylene terephthalate) has been studied by Monte Carlo calculations on model oligomers confined inside cylindrical tubes. The distribution of torsional angles for such extended chains is characterized by OCCO bonds being always in the trans domain, while the COCC bonds show an approximately similar probability of being found in trans and gauche states, the probability maxima being centered at 90° and −90° in the latter cases. At variance with the torsional angles of the OCCO and the ester bonds, always very close to 180°, the distributions for all other torsional angles show flat and broad probability maxima, indicating the possibility of substantial deviations from the average value inside each domain. This is also true for the fictitious OC˙˙˙CO bonds across the phenylene rings, for which a nearly trans geometry is preferred in extended conformations.
Simulation of free radical addition reactions modified by the presence of cobalt macrocycles
Authors: Davis Thomas P. , Kukulj Dax , Maxwell Ian A. 
Published: April 2003

AbstractFree radical addition reactions in the presence of cobaloximes and related compounds have been modelled. Several mechanisms are presented and similarities with the “persistent radical effect” noted by Daikh and Finke are discussed. Cobaloximes and salophenSystem. name: N‐acetyl‐p‐aminophenyl salicylate.
derivatives are widely used in organic synthesis to build carbon‐carbon bonds, whereas in polymer synthesis they are used as catalytic chain transfer agents in the production of oligomers. This work shows that these reactions are closely related and also demonstrates the influence that an external radical source has on the overall reaction kinetics.
Quantum chemical PM‐3 study of the thermal stability of heterocyclic fragments of heteropolymers, 3. Thermal stabilities of different components of heteropolymer repeating units
Authors: Zubkov Vladimir A. , Bogdanova Svetlana E. , Yakimansky Alexander V. , Kudryavtsev Vladislav V. 
Published: April 2003

AbstractDissociation energies (ED) of different components of repeating units of heteropolymers (HPs) have been calculated by the semiempirical PM‐3 method. Comparison of ED values of heterocycles (HCs) and of single bonds contained in phenyl‐substituted benzoheterocycles have shown that for polybenzazoles and HPs with carbonyl‐containing HCs the initial degradation in the absence of weak links (uncyclized units, structural defects, etc.) preferably starts with HC homolytic decomposition. For these two groups of HCs the relationship between the ED of HCs and the initial degradation temperature of corresponding HPs is, on the whole, similar to the one found upon calculation of benzoheterocycles. In HPs based on quinoline and quinoxaline the single bond between HC and phenyl (Ph) group is less stable than the HC, and the breaking of that single bond can accompany the initial HP degradation. An analysis of relative stabilities of X‐Ph bonds of Ph‐X‐Ph fragments of HPs with X = O, S, CH2, CO and of the relationship between their stabilities and those of other fragments of the HP repeating unit has been performed by using both calculated ED values and those obtained from thermochemical data. ED values for Ph‐X‐Ph decompositions turned out to be close to those for some HCs.
Persistence lengths of aromatic polyamides: a computer simulation approach
Authors: He Chaobin , Windle Alan H. 
Published: April 2003

AbstractA range of polyamides has been modelled using Monte Carlo routines based on carefully determined bond‐rotation potentials, bond length and bond angle. For each polymer type, a large number of chains has been built for different simulated temperatures, and the persistence lengths determined as an average over all models for a given set of conditions. The simulations predict mesogenicity of aromatic polyamides to be smaller than that of equivalent aromatic polyesters, while the experimental values of persistence length obtained from polyamides in solution are large on account of the strong bonding between polymer and solvent.
Unusual finite size effects in the Monte Carlo simulation of microphase formation of block copolymer melts
Authors: Micka Uwe , Binder Kurt 
Published: April 2003

AbstractExtensive Monte Carlo simulations are presented for the Fried‐Binder model of block copolymer melts, where polymer chains are represented as self and mutually avoiding walks on a simple cubic lattice, and monomer units of different kind (A, B) repel each other if they are nearest neighbors (εAB> 0). Choosing a chain lengthN= 20, vacancy concentration Φv= 0,2, composition ƒ = 3/4, and aL×L×Lgeometry with periodic boundary conditions and 8 ≤L≤ 32, finite size effects on the collective structure factorS(q) and the gyration radii are investigated. It is shown that already above the microphase separation transition, namely when the correlation length ξ(T) of concentration fluctuations becomes comparable withL, a nonmonotonic variation of bothS(q) and the radii withLsets in. This variation is due to the fact that the wavelength λ*(T) of the ordering (defined from the wavenumberq*whereS(q) is maximal at λ*= 2 π/q*) in general is incommensurable with the box. The competition of two nontrivial lengths ξ(T), λ*(T) withLmakes the straigthforward application of finite size scaling techniques impossible, unlike the case of polymer blends. Since also the specific heat is found to have a broad rounded peak near the transition only, locating the transition accurately from Monte Carlo simulations remains an unsolved problem.
Calculation of phase diagrams not requiring the derivatives of the Gibbs energy demonstrated for a mixture of two homopolymers with the corresponding copolymer
Authors: Horst Roland 
Published: April 2003

AbstractA method is presented which allows the calculation of phase diagrams (spinodal, binodal and tie lines) on the basis of the Gibbs energy of mixing ΔG. No derivatives of ΔG with respect to the composition variables are required. This method is particularly useful in cases where the composition dependence of ΔG is very complex and no analytical representation of the derivatives can be given. The method is applied to a ternary mixture of two homopolymers with a copolymer consisting of the same monomers. The sequence distribution of the copolymer is kept constant between random and purely alternating, and phase diagrams are calculated for different chemical compositions of the copolymer. The complex phase separation behavior resulting for a 1 : 1 copolymer becomes much simpler as one monomeric unit starts to predominate in the copolymer.
The kinetics of free radical polymerizing systems at low conversion, 1. On the rate determining step of the bimolecular termination reaction
Authors: Russell Gregory T. 
Published: April 2003

AbstractUsing a styrene bulk system as a model, this paper examines rates of termination at very low conversions in bulk and solution polymerizations. No definitive answer to the question of what determines such rates of termination is arrived at. Indeed, it is argued that on the basis of existing kinetic information, no such definitive answer is possible. However several things may be said with conviction. To begin with, it is rigorously shown that low conversion rates of termination cannot be explained by assuming that all radical chain end encounters result in termination, and then using center‐of‐mass diffusion coefficients of polymer in free solution to calculate rates of chain end encounter. However this does not mean that rates of center‐of‐mass diffusion do not determine rates of low conversion termination, as is shown; the idea that it may be the case that not all chain end encounters result in termination, this a manifestation of a spin multiplicity effect, is especially worthy of mention. It is also possible to explain low conversion rates of termination, as has traditionally been done, in terms of chain end motions being hindered by the presence of another polymer chain. However in concentrating on interactions between overlapping long chain macroradical coils, this traditional picture is certainly inaccurate, for it is shown that most termination interactions must involve at least one radical of shorter than expected degree of polymerization. This has the important consequence that an understanding of overall rates of dilute solution termination must be founded on an understanding of the diffusional behavior of the ends of short and intermediate length polymer chains.
The kinetics of free radical polymerizing systems at low conversion, 2. On the influence of the monomer and initiator concentrations
Authors: Russell Gregory T. 
Published: April 2003

AbstractIn this paper are calculated variations of steady state rates of polymerization Rpol with initiator concentration [I] and monomer concentration [M]. For these calculations a chain length dependent termination model which provides a microscopic description of zero conversion bulk and solution polymerization of styrene at 20°C is used. Despite the calculations being necessarily specific in this respect, the principles that are uncovered are general, and so are germane for polymerization of all monomers at all conversions. The central such principle is that any chain length dependence of termination rate coefficients may alone be expected, in the first instance, to induce non‐classical polymerization behavior; specifically, Rpol will scale with a weaker than square root dependence on [I] and a stronger than linear dependence on [M]. Further, while the extent of deviation from classical behavior does depend on the severity of the chain length dependence of termination rate coefficients, it does not depend on whether a Smoluchowski or geometric mean model of termination is operative, a finding which counsels reappraisal of some commonly held perceptions regarding low conversion termination. It is also shown that regardless of the chain length dependence of the termination rate coefficient, the occurrence of chain transfer to monomer will act to promote classical polymerization behavior, and it is argued that this has been a factor in many low conversion experimental investigations appearing to have found an at most rather weak chain length dependence of termination; it is further argued that existing experimental data for the variation of Rpol with [I] and [M] is not inconsistent with termination being center‐of‐mass diffusion controlled at low conversions. Close attention is paid to experimental data throughout this paper, which concludes by discussing aspects of experimental design, this in the hope that simple measurements of steady state Rpol might be more profitably used to elucidate the complexities of free radical polymerization kinetics.