Comparison of irreversible and reversible cluster formation

Covalent and reversible cluster molecules were synthesized by an A₃B₂ type gelation. Crosslinking of three-arm hydroxyl-terminated star polymers with 2,4-toluenediisocyanate gave branched polymers, while the reversible analogue was made by crosslinking of tertiary amine-terminated star polymers with bis(4-hydroxy-3,5-dinitrophenyl) adipate. Gelation process was followed by static and dynamic light scattering. The extent of reacted groups was measured with UV spectroscopy. Growth of the covalent clusters could be described in terms of percolation scaling laws. The experimental gel point (P_{OH, cr} = 0.70) was shifted significantly from the theoretical predicted gel point (P_{OH, cr} = 0.50), indicating extensive ring formation during the gelation. The reversible endlinking reaction gave no macroscopic gelation, though increase of the cluster dimensions was observed. Ring formation proved to be an important side reaction in both cases; however, the ring formation ability seems to change in a different manner during the course of a gelation.     

Distributions of cyclic oligomers formed by irreversible step-growth polymerisation. Results from kinetic modeling

The distribution of cyclic oligomer concentrations (examined up to the cyclic pentamer) in the process of step-growth polymerisation decreases, in concentrated solution, with i^−2.5 if the chains follow Gaussian statistics (i: degree of polymerization). This picture is analogous to that shown by cyclic oligomer distributions obtained under thermodynamic control and is in contrast with that shown by cyclic oligomer distributions obtained by irreversible chain-growth polymerisation which is characterized by a gradient of −1.5. This result has been initially obtained by numerical integration of the differential rate equations pertinent to a kinetic model relative to the reaction of a bifunctional reactant A—B under batchwise conditions, and then confirmed by analytical integration of the differential rate equations under the condition that propagation is much faster than cyclisation. The case in which the monomeric ring is strained has been also investigated. Also in this case the obtained distribution is analogous to that shown under equilibrium conditions. From an examination of the distributions of the ring yields, the best conditions for the preparation of either the cyclic monomer or the cyclic dimer are suggested.     

Formal similarity between irreversible and reversible bimolecular kinetics

A simple kinetic solution for reversible bimolecular associations is presented. Kinetic data of association alone often do not allow to distinguish between reversible and irreversible processes owing to the mathematical similarity of the two cases.

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Balance between reversible and irreversible processes during lithium intercalation in graphite

The effect of the measurements’ speed on reversible and irreversible processes occurring during intercalation and deintercalation of lithium in graphite out of a 1 M LiClO₄ solution in a propylene carbonate-dimethoxyethane mixture is studied by the chronopotentiometry and cyclic voltammetry methods. Dependence of reversible and irreversible capacity on the potential scan rate during potentiodynamic measurements is shown to be quite involved. Lithium diffusion coefficients in graphite are calculated by different methods.     

Temperature-modulated differential scanning calorimetry of reversible and irreversible first-order transitions

Temperature-modulated differential scanning calorimetry of first-order transitions has led to many new observations. Some of these involve non-linear processes or deal with transformations of practically instantaneous response. The latter may cause serious lags within the calorimeter due to limited thermal conductivity of the sample and the instrument. In both cases the “reversing heat capacity” or a “complex heat capacity” is not a precise representation of the transition since both are computed from abbreviated Fourier transforms, limited to the evaluation of the first harmonic component. One has in these cases to work in the time-domain with the raw output. But even from these analyses in the time-domain many interesting new insights about the transition and the calorimeter performance can be generated.     

A model for codependent reversible/irreversible growth processes

A model for codependent growth that combines reversible and irreversible bond formation is developed. The system is composed of two processes: A reversible process which is fast but does not lead to a stable growth by itself, while the irreversible process is stable but is too slow to occur by itself. Therefore, neither the reversible nor the irreversible growth processes will occur separately, but their combination is shown to yield a new type of stable, codependent growth. Using kinetic Monte Carlo techniques we simulate and analyze the general properties of this codependent growth. We discuss the general conditions for such growth and its applications to self-organization processes.     

Alternating current chronopotentiometry: Part II. Experimental studies of reversible and irreversible electrode processes

Experimental a.c. potential-time curves for the irreversible reduction of Ni(II) in 0.5 M KCNS and Co(II) in 0.5 M KCl have been measured by a.c. chronopotentiometry and values of the kinetic parameters have been obtained for these systems and compared with the reversible reduction of Tl(I) in 0.5 M KNO₃. The analysis of a.c. chronopotentiograms for the reduction of Cu(II) in 0.5 M KCl indicates the occurrence of a two-step process. Some potentialities and limitations of the technique are also pointed out.     

Kinetic modeling of controlled living microemulsion polymerizations that use reversible addition–fragmentation chain transfer

Reversible addition–fragmentation chain transfer (RAFT) polymerization is a useful technique for the formation of polymers with controlled architectures and molecular weights. However, when used in the polymerization of microemulsions, RAFT agents are only able to control the polymer molecular weight only at high RAFT concentrations. Here, a kinetic model describing RAFT microemulsion polymerizations is derived that predicts the reaction rates, molecular weight polydispersities, and particle size. The model predicts that at low RAFT concentrations, the RAFT agent will be consumed early in the reaction and that this will result in uncontrolled polymerization in particles nucleated late in the reaction. The higher molecular weight polydispersity that is observed in RAFT microemulsion polymerizations is the result of this uncontrolled polymerization. The model also predicts a shift in the conversion at which the maximum reaction rate occurs and a decrease in the particle size with increasing RAFT concentration. Both of these trends are also consistent with those observed experimentally. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6055–6070, 2006     

Advances in cycloaddition polymerizations

Cycloaddition reactions have been employed in polymer synthesis since the mid-nineteen sixties. This critical review will highlight recent notable advances in this field. For example, [2 + 2] cycloaddition reactions have been utilized in numerous polymerizations to enable the construction of strained polymer systems such as poly(2-azetidinone)s that can, in turn, afford polyfunctional beta-amino acid derived polymers. Polymers have also been synthesized successfully via (3 + 2) cycloaddition methods utilizing both thermal and high-pressure conditions. 'Click chemistry'--a process involving the reaction of azides with olefins, has also been adopted to generate linear and hyperbranched polymer architectures in a very efficient manner. [4 + 2] Cycloadditions have also been utilized under thermal and high-pressure conditions to produce rigid polymers such as polyimides and polyphenylenes. These cycloaddition polymerization methods afford polymers with potential for use in high performance polymers applications such as high temperature resistant coatings and polymeric organic light emitting diodes.     

A new method for the evaluation of the reversible and irreversible fouling propensity of MBR mixed liquor

In this paper, a new fouling measurement method is presented as a pragmatic approach to determine a mixed liquor's fouling propensity. The MBR-VFM (VITO Fouling Measurement) uses a specific measurement protocol consisting of alternating filtration and physical cleaning steps, which enables the calculation of both the reversible and the irreversible fouling resistances. The MBR-VFM principle, set-up and measurement protocol are described as well as the evaluation of the fouling measurement method. Finally, the MBR-VFM was validated by comparing the fouling propensity measured on-line by the MBR-VFM in a lab-scale MBR with the fouling of the MBR membranes themselves. Our experiments indicated that the MBR-VFM can accurately measure fouling and that it can even be detected earlier than can be seen from the on-line filtration data of the lab-scale system itself. Furthermore, the differences measured in reversible and irreversible fouling seemed to be related to the observed impact of physical and chemical cleaning respectively. Therefore, the application of the MBR-VFM as an on-line sensor in an advanced control system, enabling the deployment of the measured fouling data for the control of membrane cleaning, seems feasible and will be tested in the near future.     

A single layer artificial neural network type architecture with molecular engineered bacteria for reversible and irreversible computing

Here, we adapted the basic concept of artificial neural networks (ANNs) and experimentally demonstrate a broadly applicable single layer ANN type architecture with molecular engineered bacteria to perform complex irreversible computing like multiplexing, de-multiplexing, encoding, decoding, majority functions, and reversible computing like Feynman and Fredkin gates. The encoder and majority functions and reversible computing were experimentally implemented within living cells for the first time. We created cellular devices, which worked as artificial neuro-synapses in bacteria, where input chemical signals were linearly combined and processed through a non-linear activation function to produce fluorescent protein outputs. To create such cellular devices, we established a set of rules by correlating truth tables, mathematical equations of ANNs, and cellular device design, which unlike cellular computing, does not require a circuit diagram and the equation directly correlates the design of the cellular device. To our knowledge this is the first adaptation of ANN type architecture with engineered cells. This work may have significance in establishing a new platform for cellular computing, reversible computing and in transforming living cells as ANN-enabled hardware.

We created artificial neural network type architecture with engineered bacteria to perform reversible and irreversible computation. This may work as new computing system for performing complex cellular computation.     

Stable polydiacetylene/ZnO nanocomposites with two-steps reversible and irreversible thermochromism: the influence of strong surface anchoring

The influence of pH value on gold nanoparticle production in the presence of Pluronic stabilizers is systematically investigated. The reactions are studied as a function of pH and at fixed concentrations of the two reactants, HAuCl(4) and P123 block copolymer. Results indicate that the reaction pathway during the nanoparticle formation can be controlled by varying pH. The nanoparticles synthesized at pH=11.12 have an average diameter of 9.6 nm with a narrow size distribution, and the Pluronics are adsorbed on individual gold particle surfaces to form core-shell structures via hydrophobic interactions. The present work provides an economic way to improve the dispersion and stabilization of gold nanoparticles and throws further light on the understanding of gold nanoparticle production using block copolymers.     

Propagation kinetics in free-radical polymerizations

The combination of pulsed laser initiated polymerizations (PLP) with analysis of the generated polymer by size-exclusion chromatography (SEC) yields reliable individual rate coefficients for polymerizations of a large number of monomers in bulk and in solution. PLP-SEC experiments carried out in the presence of scCO₂ as a solvent show no unambiguous trend: while a significant reduction of k_p is seen for some monomers, e.g. acrylates, k_p for monomers such as vinyl acetate and styrene is not affected. It is suggested that the influence of CO₂ on acrylate k_p is not a true kinetic effect and that the experimental findings may be understood in terms of the occurrence of local monomer concentrations in the vicinity of the propagating radical. It is discussed that such local monomer concentrations may also contribute to a better understanding of why k_p increases with ester size within the acrylate or within the methacrylate family, and why k_p frequently is influenced by the initiating laser pulse repetition rate.     

Chain transfer in vinylpyridine polymerizations

Transfer constants to monomer at 55°C have been measured in 2 and 4-vinylpyridine radical polymerizations. In addition, the transfer constants to ethanol, octan-1-ol, toluene, benzyl alcohol, chloroform, and nitroethane have been measured. These values, which increase in the order above, show no significant variation between the two isomeric monomers.     

Fluorocarbon-hydrocarbon incompatibility in micellar polymerizations

A new approach to micellar polymerization is described. It is well known that hydrocarbons and fluorocarbons exhibit local phase segregation (demixing) owing to mutual antipathy; here this effect is employed in monomer swollen micelles with appropriate combinations of hydrocarbon and fluorocarbon monomers and surfactants. A matrix of these hydro- and fluorocarbon components has been investigated to delineate the effects of H–F antipathy on the outcomes of polymerization reactions to generate nanolatices of different size (and possibly morphology). Phase diagrams, ¹H NMR and small-angle neutron (SANS) data have been generated to characterize the systems, indicating new routes to influence nanolatex formation.     

Free radical entry in emulsion polymerizations

Measurements of the rate coefficients characterising the entry of free radicals into seed particles in styrene emulsion polymerizations has allowed the rate determining step for entry to be identified. This was found to be the rate of production of oligomeric species in the aqueous phase by monomer addition to the primary free radicals. Once formed the subsequent diffusion of these species to the latex particles (and their incorporation within these particles) is relatively fast, contrary to the assumptions of the previous diffusion controlled theories. The experimental results imply that the entering free radicals contain only two or three monomer units. Thermodynamic considerations show that such species should be both water soluble and surface active. Similar conclusions have been reached for other sparingly water soluble monomers, such as butyl acrylate and butyl methacrylate.     

Generalized ring-opening mechanism in cycloolefin polymerizations

Ring-opening polymerizations of cycloolefins under low strain induced by tungsten based Ziegler-Natta catalysts were previously shown to proceed via metathesis. Now, these investigations have been extended to highly strained four-membered rings and to molybdenum-, titanium-, and ruthenium-based ring-opening catalysts. Ozonolysis of 1[¹⁴C] cyclobutene-3-methylcyclobutene copolymers confirmed the general validity of the metathesis mechanism.