Initial Stage Of Polymerization Research Articles

Structure of autocatalytically branched actin solutions.

The average branching number and cluster size in branched actin solutions with filament capping are evaluated using analytic theory and simulation methods. The average number of daughter branches per filament in steady state is much less than unity, regardless of the concentration of branching stimulant. Much more highly branched structures are obtained in the initial stages of polymerization.

Plausible guard effect on the active sites of heterogeneous Ziegler–Natta catalyst by coordinating monomers and growing polymer chains in the initial stage of propene polymerization

AbstractInvestigation of propene polymerization by a modified stopped‐flow technique using TiCl4/ethylbenzoate(EB)/MgCl2 Ziegler–Natta catalyst with or without pretreating the catalyst with triethylaluminium (TEA) within an ultra‐short period (ca 1 s) was conducted to gain new understanding of the nature of active sites related to TEA in the early stage of polymerization. When the catalyst was pretreated by a cocatalyst, deactivation behaviour was clearly observed, even within an extremely short pretreatment period. In contrast, without pretreatment, the deactivation of active sites can be neglected within the polymerization period indicating that the activated Ti species might be protected from deactivation by TEA when monomer is present in the system. A plausible guard effect on the active sites by coordinating monomer and growing polymer chains in the initial stage of polymerization is proposed to account for this phenomenon. Copyright © 2004 Society of Chemical Industry

End versus Side Branching by Arp2/3 Complex

We investigate the issue of end versus side branching of actin filaments by Arp2/3 complex, using a combination of analytic theory, polymerization assays, and quantitative modeling. The analytic theory shows that the effect of capping protein on the initial stages of actin polymerization in the presence of Arp2/3 complex depends strongly on whether new Arp2/3 complex-induced branches grow from the sides or ends of existing filaments. Motivated by these results, we measure and quantitatively model the kinetics of actin polymerization in the presence of activated Arp2/3 complex, for a range of concentrations of capping protein. Our model includes the most important types of events involving actin and actin-binding proteins, and can be adjusted to include end branching, side branching, or both. The side-branching model gives a better fit to the experimental data than the end-branching model. An end-plus-side model including both types of branching gives a moderate improvement in the quality of the fit. Another side-branching model, based on aging of subunits’ capacity for branch formation, gives a significantly better fit than the end-plus-side model. We discuss implications for actin polymerization in cells.

Cross-Linked Polyacrylonitrile Prepared by Radiation-Induced Polymerization Technique

Cross-linked polyacrylonitrile PAN granules were prepared at ambient temperature by γ-rays at a constant dose rate of 8.5 kGy/h. It was found that the rate of polymerization was linear with the radiation dose at the initial stage of polymerization, and then, at high conversion, it tends to level off above 35 kGy. Cross-linking of PAN is accompanied simultaneously with the polymerization process at high energy radiation. Characterizations of the cross-linked PAN prepared at different radiation doses were investigated by FT-IR, XRD, DSC, and TGA analysis. The appearance of an IR band at 1670 cm-1 confirmed the cross-linking of PAN via nitrile groups. The original crystal lattice and the entropy of melting ΔSm did not change significantly at high radiation doses.

The polyhomologation of 1-boraadamantane: mapping the migration pathways of a propagating macrotricyclic trialkylborane.

Trialkyl and triaryl organoboranes undergo multiple, repetitive homologations upon reaction with dimethylsulfoxonium methylide (1). This multiple homologation reaction, or polyhomologation, produces polymethylene in a living reaction. Applying the polyhomologation reaction to cyclic and polycyclic organoboranes permits the construction of unique oligomeric and polymeric architectures that are not readily accessible by standard olefin polymerization. The polyhomologation of 1-boraadamantane.THF (2) by ylide 1 generates novel macrotricyclic trialkylboranes (3). The oxidation of these macrocyclic organoboranes generates a three-armed star polymethylene polymer (4) incorporating a cis,cis-1,3,5-trisubstituted cyclohexane core. Interestingly, only one-third of the initiators lead to product formation, resulting in an observed degree of polymerization 3 times higher than expected. Close examination of the initial stages of polymerization show that 1-boraadamantane.THF reacts with 1 equiv of 1 to afford a monohomologated product. Subsequent homologations were found to contain branch points leading to isomeric tricyclic products after the third, fourth, and fifth methylene insertions. At these stages of homologation, all of the propagating species result in tricyclic trialkylborane cages with collapsed, inverted pyramidal boron centers that are substantially less reactive toward ylide. Approximately two-thirds of the species discontinue polymerization at these stages. However, one-third of these species continue to propagate and eventually result in the formation of giant macrotricyclic polymers of narrow polydispersity. Molecular modeling and kinetic simulation have aided in the analysis of the probable pathways through which the reaction proceeds.

Molecular Dynamics Study of ε-Caprolactone Intercalated in Wyoming Sodium Montmorillonite

The intercalation process of the e-caprolactone monomer into sodium montmorillonite clay is modeled with a combined molecular mechanics/molecular dynamics approach. This study is aimed at understanding the initial stages of caprolactone polymerization within the channels of the clay, to form highly dispersed nanocomposites. The theoretical method is first validated by modeling dry and hydrated Wyoming sodium montmorillonite; the caprolactone-intercalated clay is then investigated, with particular emphasis on the energetics of the intercalation process and the nature of the interactions building up between the organic molecules and the channel walls and sodium counterions.

Study on the phase behavior of EVA/PS blends during in situ polymerization

AbstractA triangular phase diagram of the polystyrene/styrene/ethylene vinyl‐acetate (PS/SM/EVA) system is established to account for the phase change during polymerization of SM. The phase behaviors for EVA and PS domains in the SM polymerization system are also studied by an optical microscope (OM) and a photometer. Both EVA and PS dissolve in SM quite easily, but PS and EVA are immiscible. Hence, the exclusion of PS from the EVA phase domain results during the polymerization. That is, the polymer solution will be phase‐separated at the initial stage of polymerization. Additionally, EVA phase inversion occurs as the conversion of SM increases to 15%, at which the volume ratio of EVA/PS is approximately unity. Monitoring the light transmittance level of EVA/PS/SM solution during polymerization, the EVA phase proves to have been completely inverted to the dispersed phase when the SM conversion reaches 20%.

Study on the initial stage of emulsion polymerization of vinyl acetate using poly(vinyl alcohol) as a protective colloid

A model experiment of emulsion polymerization of vinyl acetate (VAc) was carried out using polyvinyl alcohol (PVA) as a protective colloid, where a small amount of VAc (1 mL/100 mL water) was employed. This corresponds to the initial stage of the emulsion polymerization. In the presence of the large amounts of PVA, the number of new particles smaller than 80 nm continued to increase during the polymerization, while there was not much increase in the particle diameter. In contrast, in the absence of PVA, the particles formed at the very early stage continued to grow and the number of particles did not increase by so many. The fractionation of the polymers in the emulsion revealed that more than 90% of polymerized VAc and more than 70% of the PVA used were grafted. To realize these results, we calculated the reaction times of three elemental reactions, that is, the initiation reaction of the sulfate radical with VAc, the propagation of the PVAc radical, and the hydrogen abstraction from the PVA with the sulfate radical. The reaction times were τ1=10–7 s, τ2=10–3 s, and τ3=10–8 s, respectively, corresponding to the above reactions. The time of the entry of the sulfate radical into a particle was 10–4 s. Hydrogen abstraction from PVA with the sulfate radical is the fastest reaction, which results in the grafting onto PVA, while the initiation reaction, which results mainly in homo-polymer, is slower. The propagation of the PVAc radical in the aqueous phase is a much slower reaction. The grafted molecules coagulated with each other to become a particle.

ESR study of camphorquinone/amine photoinitiator systems using blue light-emitting diodes

New light-activation units equipped with high-illuminant blue light-emitting diodes (LEDs) have recently been proposed as a replacement for the halogen units that are widely used in dentistry to polymerize light-cured resins. The photoinitiators in light-cured dental resins, typified by the camphorquinone (CQ)/amine photoinitiator system, generate primary radicals with light irradiation that attack the double bonds of resin monomers. The physical properties of the cured resins are affected by the generation of primary radicals during the initial stage of polymerization. This study examined two types of photoinitiator systems, CQ/DMPT and CQ/DMAEMA, and three types of curing units, a new LED unit and two conventional halogen units. The primary radicals generated by irradiation were quantified using electron spin resonance (ESR) spectroscopy with a trapping method, using phenyl- tert-butyl nitrone as the trapping agent. The energy efficiencies of the LED and halogen units were compared by quantifying the generated radicals and emitted light energy (J/cm 2). The energy required to generate a given amount of radicals using the LED unit was smaller than that using the halogen units ( p<0.05). These results suggest that the new LED unit performs better than conventional halogen units with respect to light energy.

Visualization and force measurement of branching by Arp2/3 complex and N-WASP in actin filament

To determine whether the Arp2/3 complex activated by N-WASP (VCA) branches actin filaments at the side (side branching), or at the barbed (B-)end (end branching) of the mother filaments, we have directly observed the branching process of actin filaments and examined single-molecule unbinding under optical microscope. We found that side branching was predominant, though not exclusive. At the initial stage of polymerization, the branching at the B-end occurred and subsequently the side branching started to occur. In either type of branching, the mother and daughter filaments elongated at nearly the same rate (growing type). Independently of the stage of polymerization, branching due to the direct coupling of filaments with an acute angle to the mother filaments (a coupling type) occurred. Phalloidin suppressed the growing type of branching but not the coupling type, implying that actin monomers are required for the former but not the latter. We found, by single molecule measurements using optical tweezers, that the Arp2/3 complex attaches to the side of actin filaments and the N-WASP appears to detach from the actin–Arp2/3 complex at 6–7 pN.

Anionic polymerization of d, l-lactide initiated by lithium diisopropylamide

The anionic polymerization of d, l-lactide initiated by lithium diisopropylamide.monoTHF complex was performed in different solvents and at various temperatures. No polymerization took place in THF solution. In dioxane at 25°C the polymerization is fast and goes up to completion in few minutes. In toluene, due to solubility requirements, polymerizations were run at 70°C. At initial stage of polymerization M w/ M ns were quite narrow, but at higher conversion a significant broadening occurred due to transesterification reactions. The latter were identified by 13C NMR analysis, while MALDI-TOF spectra revealed the simultaneous presence of cyclic and linear oligomers substantiating the occurrence of both intra and inter ester exchange processes. At low conversion, polymers exhibited a highly syndiotactic microstructure.

Radical Polymerization of Methyl Methacrylate in Presence of Ferrocene. Monte Carlo Simulation

Monte-Carlo technique is used for the numerical simulation of radical polymerization kinetics of methyl methacrylate in the presence or absence of ferrocene and exposition of molecular-weight distribution during the process. The peculiarities of polymeric radical concentration changes at the initial stage of polymerization are shown. Satisfactory correspondence between calculated and experimental data is obtained. The data of numerical experiment have allowed to evaluate the influence of ferrocene on kinetic parameters of polymerization.

Radical polymerization of methyl methacrylate and styrene in the presence of ferrocene

The influence of ferrocene on the kinetic parameters of methyl methacrylate (MMA) or styrene polymerization is investigated. It is shown, that in the presence of ferrocene the growth of the initial rate of polymerization is accompanied with significant decrease of the polymerization degree of polymer obtained when peroxides are used as the polymerization initiators. The polymerization kinetics parameters are evaluated. Its demonstrated that the process of polymerization in the presence of a ferrocene has a low activation energy, that allows to carry out the polymerization at lower room temperatures. The influence of the ferrocene on the molecular-mass characteristics of poly(methyl methacrylate) (PMMA) was studied and it was found that the influence of ferrocene on PMMA molecular mass is determined by the nature of the initiator used. The decomposition temperature of PMMA increases with the increase of ferrocene concentration. Monte-Carlo technique was used for numerical simulation of MMA radical polymerization kinetics in the presence and in the absence of ferrocene and to follow the change of molecular-weight distribution during the polymerization. The peculiarities of polymer radical concentration changes at the initial stage of polymerization are shown. Satisfactory correspondence between calculated and experimental data was obtained.

Gradient Composites of Alkaline Poly(6-hexanelactam) with Graphite: One-Step Synthesis, Structure, and Mechanical Properties

In a one-step synthesis of gradient composites, molten monomer of 6-hexanelactam was mixed with graphite (5 wt.-%) and alkaline polymerization was performed under quasi-isothermal conditions at about 170°C. The following initiator/activator system insensitive to traces of water and other low-molecular-weight compounds adsorbed on filler surface was used: the sodium salt was prepared through the reaction of sodium dicaprolactamobis(2-methoxyethoxo)aluminate with 6-hexanelactam; N-acyllactam was formed in situ in molten monomer by solving flexible or rigid polyurethane foam consisting of either toluene diisocyanate or diphenylmethane-4,4'-diisocyanate, and a poly(propylene oxide) based polyol. To obtain gradient composites with a compositional variation between plane-parallel surfaces, the incorporated filler underwent sedimentation due to gravity during initial stages of polymerization. The graphite-free surface is suitable for treatment with adhesives, while the graphite-rich surface layer (containing about 11 wt.-% of graphite) possesses improved friction characteristics. Graphite slightly (i) reduces the polymer yield and the mean spherulite diameter; (ii) increases the crytallinity due to its nucleation activity; (iii) decreases the compliance, but does not affect its time dependence given by the matrix and (iv) reduces the yield strength, tensile strength and elongation at break. The friction coefficient of the graphite-rich surface is reduced to almost 50% of that found for the graphite-free surface; composites with cross-linked matrix also show better wear properties.

Stopped-Flow Techniques in Ziegler Catalysis

The stopped-flow method, by which a quasi-living polymerization process can be realized within an extremely short period (ca. 0.2 s), has been proven to be one of the most powerful techniques for studies on the nature of the active sites and elucidation of the olefin polymerization mechanism based on the information from the polymers obtained in the initial stage of polymerization. It has been demonstrated that a better and deeper understanding of many controversial problems in Ziegler catalysis has been achieved, such as arguments concerning the non-uniformity of the active sites, the intrinsic kinetic parameters, the effects of catalyst preparation conditions, the role of cocatalysts, hydrogen and electron donor, etc. The successive modifications to the basic stopped-flow system have led to extensive applications in investigating the catalyst pretreatment effects induced by various reagents, as well as developing a series of novel block copolymers.

Effect of water-soluble cross-linker on the growth and properties of ethyl acrylate-methacrylic acid emulsion opolymer particles

Model ethyl acrylate–methacrylic acid copolymer latices and latices of particles cross-linked by copolymerizing small amounts of water-soluble N,N′-methylenebisacrylamide were prepared by nonseeded semicontinuous emulsion copolymerization. Dynamic and static light scattering measurements indicated a slightly higher degree of polydispersity in the case of cross-linked particles, especially in the initial stages of polymerization. The hydrodynamic volume of the alkalinized particles controlling the viscosity properties of the dispersions decreased with the time of polymerization and in the case of cross-linked copolymer almost reached a constant value at about 1 h. The different character of the particle structure was confirmed by differences in particle disintegration after alkali addition or in the presence of methanol.

Self-Assembly of Narrowly Distributed Carboxy-Terminated Linear Polystyrene Chains in Water via Microphase Inversion

We found by surprise that ω-metal carboxy-terminated linear polystyrene chains could self-assemble into stable surfactant-free colloidal nanoparticles if the polymer solution in THF or DMF is added dropwise into an excess of water. Our results showed that the self-assembly was controlled by competition between the intrachain contraction and interchain association. There is a delicate balance between thermodynamic and kinetics. Inside the self-assembled micelle-like nanostructure, the polymer chains were in a metastable state and kinetically frozen. The self-assembly resembles the initial stage of surfactant-free emulsion polymerization. The size of the nanostructures was essentially governed by the surface area occupied per ionic end group. A variation of the initial conditions, such as the polymer concentration in THF and DMF, solvent, and counterions, can make either the intrachain contraction or interchain association dominant so that the self-assembly can be tailored.

Using the Toms effect for rheokinetic study of the initial stage of polymerization

Traditionally, the Toms effect was widely used in its direct application for drag reduction in channel flow or for the movement of solids through a liquid medium. In this paper, however, we have considered a possibility for using this effect as a method allowing one to monitor the very beginning of the polymerization process. Our approach was based on the high sensitivity of the Toms effect to minor changes in polymer concentration in a low-molecular-weight solvent. We have centered our discussion on chemical reactions that occurred in very dilute solutions. As a typical example, we have examined ionic polymerization of hexene. Application of the Toms effect gave us a unique chance to investigate chemical processes happening when the polymer concentration is on the order of tens of ppm. This concentration range was much lower that the ranges studied by traditional chemical or physical methods.

Effect of additives on the initial stage of emulsion polymerization of methyl methacrylate using polyvinyl alcohol as a protective colloid

Effect of additives on the initial stage of emulsion polymerization of methyl methacrylate using polyvinyl alcohol as a protective colloid

Study of Kinetics of Dispersion Polymerization of Styrene in Organic Media

The kinetics of dispersion polymerization of styrene in alcohol/methyl or butyl cellosolve was investigated with dried-weigh methods. The reaction parameters, such as concentration of initiator, polymerization temperature, and solvent, play an important role in determine polymerization rate. It was found that polymerization rate increases with the reaction temperature. The apparent activation energy is of 42.2kJ/mole and 52.6kJ/mole for the initial polymerization stage and the stationary polymerization interval. The polymerization rate increases with the concentration of the initiator with approximately 0.67 order dependence at conversion about 5%. It was described that the relation of conversion with the Hansen Parameters of media in detail by analysis of solvent dispersion, polarity and hydrogen bonding contributions. More significant was the result that polymerization rate versus conversion curve consisted of 3 intervals (2 non-stationary and 1 stationary one). The plateau of polymerization rate was observed in the curve of polymerization rate vs. monomer conversion.