Chain-breaking Reactions Research Articles

The effect of wavelength on the polymerization of multi(meth)acrylates with disulfide/benzilketal combinations

As an alternative to asymmetric iniferters like XDT (p-xylylene bis(N,N-diethyldithiocarbamate), the use of DMPA (2,2-dimethoxy-2-phenylacetophenone) in combination with TED (tetraethylthiuram disulfide) has been proposed as a means of studying living radical polymerizations and creating crosslinked polymers without trapped radicals. This dual initiator system ideally could imitate XDT. The DMPA would photocleave and initiate the reaction via carbon radicals while the TED would photocleave and terminate via dithiocarbamyl (DTC) radicals. In this contribution, we report that for the case of TED and DMPA photoinitiated systems, the nature of the living polymerization and the polymerization kinetics are dependent on the wavelength of light used. Kinetic studies of TEGDMA (triethylene glycol dimethacrylate) photocured with monochromatic light at wavelengths 290, 310, 324, and 365nm were performed using differential scanning calorimetry. TED was added in incremental amounts to determine its effect on the polymerization as a function of wavelength and concentration. The results suggest that termination via DTC radicals becomes more prominent as the irradiating wavelength is increased. A model was developed that incorporates three modes of chain breaking, where carbon–carbon termination, carbon-DTC termination, and chain transfer to TED are all referred to as chain breaking reactions. The model fit the data well after the maximum rate of polymerization. Furthermore, the model predicts that termination involving TED and its derivatives dominates over carbon radical–carbon radical termination when the ratio of TED:DMPA is 1:2 or higher in TED. Therefore, TED:DMPA combinations can be used as an alternative to conventional iniferters as long as a sufficient amount of TED has been added so that carbon–sulfur chain breaking reactions dominate over traditional termination. Infrared analysis of several samples proved that TED:DMPA combinations greater than 1:2 can prevent trapped carbon radicals from persisting in polymer networks after cure.

Mechanism of Interaction between a Vortex Pair and a Premixed Flame

Abstract Colliding interaction of a vortex pair with a premixed flame is numerically studied with a two-step reaction model including chain-branching and chain-breaking reactions. The vortex pair has a maximum circumferential velocity ranging from 4.7 to 54.7 times the laminar burning velocity and has a core diameter ranging from 1.1 to 1.55 times the flame thickness. Besides well-known interacting behaviors such as the flame wrinkling by a weak vortex pair and the pocket formation by a moderate vortex pair, an elongation of the concave flame without entraining the burned gas appears in the interaction with a strong vortex pair. The different behaviors of interaction are attributed to the ratio of the moving velocity of the vortex pair to the burning velocity and the criterion is represented by the ratio of the maximum circumferential velocity to the burning velocity. For moderate and strong vortices, vorticity generation due to the baroclinic effect and reduction of distance between vortex cores result i...

Polypropylene functionalization with vinyltriethoxysilane

Polypropylene (PP) functionalization with vinyltriethoxysilane (VTES) was accomplished via a free radical process in a melt-mixer chamber, using dicumyl peroxide as the initiator. Fourier-Transform Infrared Spectroscopy (FTIR), Rutherford Backscattering Spectrometry (RBS), and gel permeation chromatography (GPC) were used to follow silane incorporation and product molecular weights. The influence of silane (0–10.0 wt %) and peroxide (0–1.0 wt %) concentrations on the functionalization degree and molecular weight of products was investigated. Chain-breaking reactions were present in all experiments, evidenced by smaller product molecular weights. This decrease was more pronounced for higher peroxide concentrations. Silane incorporation occurred even in peroxide absence, and it was seen to increase with increasing silane concentration. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1313–1319, 1999

Polypropylene functionalization with vinyltriethoxysilane

Polypropylene (PP) functionalization with vinyltriethoxysilane (VTES) was accomplished via a free radical process in a melt-mixer chamber, using dicumyl peroxide as the initiator. Fourier-Transform Infrared Spectroscopy (FTIR), Rutherford Backscattering Spectrometry (RBS), and gel permeation chromatography (GPC) were used to follow silane incorporation and product molecular weights. The influence of silane (0–10.0 wt %) and peroxide (0–1.0 wt %) concentrations on the functionalization degree and molecular weight of products was investigated. Chain-breaking reactions were present in all experiments, evidenced by smaller product molecular weights. This decrease was more pronounced for higher peroxide concentrations. Silane incorporation occurred even in peroxide absence, and it was seen to increase with increasing silane concentration. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1313–1319, 1999

Rate-ratio asymptotic analysis of non-premixed methane flames

The asymptotic structure of laminar, non-premixed methane flames is analysed using a reduced four-step chemical-kinetic mechanism. Chemical reactions are presumed to take place in two layers: the inner layer and the oxidation layer. In the inner layer the fuel reacts with radicals and the main compounds formed are the intermediate species CO and H2. These intermediate species are oxidized in the oxidation layer. The structure of the oxidation layer is described by two second-order differential equations: one for CO and the other for H2. Two limiting cases are considered. At one limit the global step CO+H2⇌CO2+H2 is presumed to maintain partial equilibrium everywhere in the oxidation layer except in a thin layer adjacent to the inner layer. At the other limit the steady-state approximation is introduced for H2 everywhere in the oxidation layer except in a thin layer adjacent to the inner layer. This limit, called ‘slow CO oxidation’, has not been analysed previously. The structure of the inner layer is described by two second-order differential equations: one for the fuel and the other for the H radicals. This is a significant improvement over previous models in which either a steady-state approximation is introduced for the H radicals in the inner layer, or the reaction between the fuel and radicals is presumed to be very fast. The chain-breaking elementary reaction CH3+H+M→CH4+M is found to have a significant influence on the structure of the inner layer and on the scalar dissipation rates at extinction. The influence of this reaction was either neglected in previous models or was included as a perturbation to the principal elementary reactions taking place to the leading order in the inner layer. Using the results of the asymptotic analysis the scalar dissipation rates at extinction are calculated at a pressure of 1 bar. They are found to agree well with those calculated numerically using a chemical-kinetic mechanism made up of elementary reactions.

Asymptotic Analysis of the Structure of Moderately Rich Methane-Air Flames

The asymptotic structure of laminar, moderately rich, premixed methane flames is analyzed using a reduced chemical-kinetic mechanism comprising four global reactions. This reduced mechanism is different from those employed in previous asymptotic analyses of stoichiometric and lean flames, because a steady-state approximation is not introduced for CH 3. The aim of the present analysis is to develop an asymptotic model for rich flames, which can predict the rapid decrease of the burning velocity with increasing equivalence ratio φ. In the analysis, the flame structure is presumed to consist of three zones: a preheat zone with a normalized thickness of the order of unity, a thin reaction zone, and a postflame zone. The preheat zone is presumed to be chemically inert, and in the postflame zone the products are in chemical equilibrium and the temperature is equal to the adiabatic flame temperature T b . In the reaction zone the chemical reactions are presumed to take place in two layers: the inner layer and the oxidation layer. The rate constants of these reactions are evaluated at T 0, which is the characteristic temperature at the inner layer. In the inner layer the dominant reactions taking place are those between the fuel and radicals, and between CH 3 and the radicals. An important difference between the structure of the inner layer of rich flames and that of lean flames analyzed previously is the enhanced influence of the chain-breaking reaction CH 3 + H + ( M) → CH 4 + ( M) in rich flames. Here M represents any third body. This reaction decreases the concentration of H radicals, which in turn decreases the values of the burning velocity. In the oxidation layer of rich flames, the reactive–diffusive balance of O 2 is considered. This differs from the structure of the oxidation layer of lean flames where the reactive–diffusive balance of H 2 and CO was of primary interest. The burning velocities calculated using the results of the asymptotic analysis agree reasonably well with the burning velocities calculated numerically using chemical-kinetic mechanisms made up of elementary reactions. The values of the characteristic temperature at the inner layer T 0 are found to increase with increasing values of the equivalence ratio and to approach T b at φ = 1.36. When T 0 is very close to T b , the asymptotic analysis developed here is no longer valid and an alternative asymptotic analysis must be developed for even larger equivalence ratios.

Kinetic mechanism and chemical oscillations in the branching chain decomposition of nitrogen trichloride

On the basis of numerical simulation of the branching chain process of the decomposition of nitrogen trichloride it has been shown that suitable conditions for providing oscillating regimes are the following: (a) inclusion of adsorption–desorption of NCl3 on the reactor walls; (b) inclusion of nonlinear chain branching and breaking reactions.

XPS and UPS characterization of the reactions of Al(111) with tetrahydrofuran and propylene carbonate

Tetrahydrofuran (THF) and propylene carbonate (PC) condensate layers were formed by dosing the Al(111) surface at 135–140 K. The reaction of each molecule with Al was followed by both core level (XPS) and valence band (UPS) spectroscopies as a function of temperature on heating to successively higher temperatures. Considerable desorption of solvent molecules occurs after heating to their respective bulk melting points but in both cases a multilayer of solvent molecules remains on the surface. Al catalyzes the polymerization of THF at its melting point, resulting in a relatively thick, e.g. 30–60 Å, film remaining on the surface to temperatures more than 100 K above the melting point. Above about 220 K the poly-THF film reacts with Al in two stages: the first is a chain-breaking reaction eliminating ethylene to the gas phase and producing propylene alkoxide (AlOCHCH 2), and the second is decomposition of this adsorbate by oxygen extraction to form an “oxide”-like AlO layer. The latter reaction begins at temperatures above about 320 K. At 470 K the carbon from the THF is present in an elemental and/or CH x form which on heating to 620 K further reacts to form a “carbide”-like state mixed with the AlO oxide layer. At or near its melting point some fraction of the PC reacts with Al to form fragments of PC from which the carboxyl oxygen has been removed. The deoxygenated fragments interact strongly with unreacted PC preventing their desorption on heating above the melting point. Extraction of carbonyl oxygen by Al occurs to a greater extent as the temperature is increased until at ambient temperature no unreacted PC remains on the surface. The overlayer becomes completely deoxygenated on heating to 470 K, ultimately producing at 620 K a thicker film of similar composition to that produced by THF. Al 2O 3 and Al 4C 3 are the thermodynamically expected products of THF and PC in equilibrium with metallic Al and these appear to be the ultimate products of the reaction. The results are consistent with the chemical intuition that THF is a much less reactive solvent than PC.

Unimolecular and bimolecular exchange reactions in controlled radical polymerization

Abstract : Well defined polymers can be produced by radical polymerization using three different approaches: degenerative, transfer, reversible homolytic cleavage of covalent species, and reversible homolytic cleavage of persistent radicals. The first method requires high degenerative transfer coefficients but allows for slow initiation; the other two methods require that the equilibrium be strongly shifted towards dormant species, and that initiation is fast. All three approaches may provide controlled polymerizations, but not living polymerizations, because chain breaking reactions can not be eliminated although their contribution can be strongly reduced.

Spontaneous ignition limits of silane and phosphine

Spontaneous ignition limits of silane and phosphine have been investigated at relatively low concentrations. For silane, the spontaneous ignition occurs if the mixture concentrations is such that the silane/oxygen ratio is higher than a certain threshold limit value. In other words, the mixture is not stable if the ratio is higher than a certain value. On the other hand, in the case of phosphine the threshold limit line has been found to be a little curved, though the reason for the fact is not clear. At any rate, it is concluded that the spontaneous ignition of both silane and phosphine occurs as a result of a competition of chain branching and chain breaking reactions, in a way that is qualitatively similar to that in hydrogen oxidation.

Development of a method for fabrication of modified viscose fibre containing quaternary ammonium salts

A continuous one-stage method for fabrication of modified viscose fibre containing quaternary ammonium groups was developed. Ways of increasing the CMGP were defined. It was shown that CMGP increases sharply on simultaneous addition of Na2SO 4 and MBAA (N,N'-methylenebisacrylamide) (MBMAA: N,N'-methylenebismethacrylamide) was developed. A viscose fibre containing up to 80 wt. % graft PDMVPMS with CMGP of up to 98 % was obtained. The efficiency of ion-exchange cellulose fibre materials (CFM) for different purposes is well known [1-3]. The expediency and technical and economic efficiency of using materials containing ionizable groups are determined by different factors, but simplicity of fabrication is one of the most important. Of the different types of CFM containing basic groups, CM-A2 fibre, fabricated by a two-stage periodic method by graft polymerization of 2-methyl-5-vinylpyridine with subsequent quaternization of the nitrogen atom of tertiary amino groups with epichlorohydrin, has been most widely used [4]. The study described here was conducted to develop a method for fabrication of an anion-exchange fibre containing quaternary ammonium groups which could be done according to a one-stage continuous scheme. 1,2-Dimethyl-5vinylpyridinium methyl sulfate (DMVPMS), which is a water-soluble, nonvolatile monomer with low toxicity manufactured in Russia, was used for modification of CFM. We previously [5] investigated the characteristics of modification of viscose fibre by graft polymerization of DMVPMS according to a method based on the reaction of chain transfer from initiator radicals, where diffusion of the initiator into the solution of monomer is hindered. To fabricate the anion-exchange fibre, viscose fibre (in the form of a twist) was impregnated with an aqueous solution of DMVPMS containing ammonium peroxydisulfate (APS) for several seconds, squeezed, heated in an oven at 100-140°C for 10 min, washed with water, and dried. It was shown in [5] that adding NaCI to a solution of the monomer containing APS increases conversion of the monomer in the graft and homopolymerization (CMGP and CMHP) reaction by approximately two times, while addition of Na2SO 4 sharply increases the amount of homopolymer due to formation of associates of Na2SO 4 with molecules of DMVPMS and poly-l,2-dimethyl-5-vinylpyridinium methyl sulfate (PDMVPMS), an increase in the viscosity of the system, and a decrease in the rate of the chain-breaking reaction. The possibility of increasing CMGP by adding a bifunctional comonomer capable of forming a sterically cross-linked copolymer with the cellulose macromolecule and graft DMVPMS to a solution of DMVPMS was examined in the present article. Since inorganic nonpolymerizable electrolytes (NaC1 and Na2SO4) have an important effect on graft polymerization, the possibility of increasing the CMGP of DMVPMS by simultaneously adding a nonpolymerizable electrolyte and a bifunctional comonomer to the solution of monomer was also investigated. Water-soluble N,N'-methylenebismethacrylamide (MBMAA) was used as the bifunctional comonomer. The amount of DMVPMS applied on CFM was determined* after extraction of the monomer and homopolymer with water to characterize graft polymerization and the composition of the modified CFM. The concentration of unreacted monomer and water-soluble homopolymer (CHP) after the graft polymerization reaction was determined by analyzing their concentration in the aqueous extract by spectrophotometry at k = 250 and 275 nm and was calculated with specially derived equations. CMGP and CMHP were calculated with the results of these analyses. The amount of MBAA (MBMAA) applied to CFM and not reacting was determined in the aqueous extract by spectrophotometry at k = 210 nm. The modified CFM prepared with this method

Premixed hydrogenoxygen flames. Part I: Flame structure near the flammability limits

A numerical investigation of flammability limits and structure of adiabatic planar flames in premixed hydrogenoxygen gases is carried out with a detailed chemistry and using a pseudo-stationary code. The well-known kinetic mechanism responsible for the second limit of explosion results from a competition between chain branching and chain breaking reactions, concerning mainly the H radical. In the flame propagation problem, this mechanism defines a cross over flame temperature. The corresponding critical values of the equivalence ratio indicates a change in the chemical kinetic regime sustaining the flame propagation. The one-dimensional laminar flame structure and the burning velocity are studied by direct numerical simulations close to these limits. When they are approached, the flame velocity decreases to very small values (∼ mm/s) and the global activation energy increases. These pure kinetic mechanisms, which are studied here, are essential for explaining the phenomena which are observed at the flammability limits. Reduced schemes describing these effects have been also identified. The hydrogen rich flame is found to be well represented by a reduced scheme of two steps involving one radical H, while the lean flame requires a reduced scheme of three steps involving two radicals H and OH.

Ranking living systems

The living ystems are ranked quantitatively based on the ratio of the rate constants of transfer and termination to that of propagation and also on the absolute values of the rate constants of the chain breaking reactions

Living Polymers in Ionic and Radical Polymerizations: Recent Developments

Abstract The discovery of living polymers, that is, assemblies of polymer molecules formed by anionic polymerization which may grow without chain-breaking reaction and may react subsequently with other monomers and various reagents through their end-groups, has led to great progress in the knowledge of the mechanism of anionic polymerization and to the synthesis of a large variety of well-defined block copolymers, graft co-polymers, and polymers with functionalized end-groups. Since only a limited number of the current monomers are polymerizable by an anionic mechanism, many attempts have been made to obtain similar results by polymerizing other monomers by cationic, radical, and Ziegler polymerization. Systems making it possible to work at temperatures higher than those used for many anionic and most cationic polymerizations would be particularly interesting.

Asymptotic Analysis of the Ignition of Hydrogen by a Hot Plate in a Boundary Layer Flow

Abstract In this paper we have analyzed the steady-state process leading to ignition of a combustible mixture of hydrogen, oxygen and nitrogen by a hot flat plate in a boundary layer flow. For plate temperatures larger than the crossover temperature dictated by the competition between the chain branching reaction H + 02 → OH + O and the chain breaking reaction H + 02 + M → H02 + M, the ignition event corresponds to a typical chain branching explosion with negligible heat release, in a first approximation. The boundary layer equations are solved using the fact that the activation energy of the chain branching reaction H + 02 → OH + O is relatively large, employing the reduced kinetic mechanism appropriate for this regime. The equations reduce to a single integro-differential equation for the concentration of atomic hydrogen. The ignition condition can be assumed to be reached when one of the shuffle reactions reaches partial equilibrium. On the other hand, for low plate temperatures, the ignition event is characterized by a thermal runaway. The governing equations reduce to a single one-parameter integro-differential equation, to be solved numerically.

Photolysis of sulphur-containing polymers

Photolysis of polymers with -S-, -S-S- and -SO- groups in their backbone was investigated. It was shown that the polymers studied undergo photodestruction under UV irradiation ( λ = 220–270 nm). The quantum yield of the chain breaking reaction depends on the structure of the polymer and on the irradiation conditions, and amounts to 0.05 and 0.09 for polypropylenesulphide (solution) and 0.2 (film); 0.16 and 0.21 for a polymer containing 80% disulphide bonds, and 0.19 for polypropylenesulphoxide.

Mechanism of repeated chain breaking and the intermediate products of transformation of aromatic amines in oxidized isopropanol and ethylbenzene

1. Aminyl and nitroxyl radicals and quinone imines and the products of their transformation participate in repeated chain breaking in inhibition of oxidation of isopropanol by aromatic amines. One set of intermediate products takes place in cyclic chain-breaking reactions as a function of the structure of the amine. 2. The composition of the products of transformation of aromatic amines in oxidized ethylbenzene and isopropanol are different. The quinone imine and nitroxyl radical are formed from diphenylamine in ethylbenzene, while there is no quinone imine in isopropanol; quinone imine and the nitroxyl radical are formed from dimethylbis(4-diphenylaminophenoxy)silane in ethylbenzene, while quinone imine alone is formed in isopropanol. 3. Quinone monoanilide is reduced in the reaction with the isopropanol peroxide radical, and the radical formed in this reaction reacts with the peroxide radical.

Preparation and reactivity of metal-containing monomers. Communication 6. Polymerization of metal-containing monomers based on alkoxy derivatives of Ti(IV) and some properties of the products obtained

1. Polymerization of Ti(IV) monomers in benzene is characterized by a reaction order of 0.5 with respect to the monomer, which is due to coordination of the radicals formed with the monomer and their predominant participation in chain breaking reactions. 2. The metal-containing polymers based on Ti(IV) have elevated thermal stability due to the formation of rare cross links between the macromolecules.

Fluid dynamic-chemical interactions at the lean flammability limit

Laminar flame propagation at the lean flammability limit is analyzed in terms of a four step kinetic model that accounts for the competition of the chain branching reaction H + O 2 → OH + O with the chain breaking reaction H + O 2 + M → HO 2 + M. Assuming large activation energies for the chain-branching reaction, the structure of the reactive-diffusive zone is derived. It consists of a thin fuel consumption layer where radicals are formed and an even thinner radical consumption layer. Within the fuel consumption layer the radical profiles are described by the steady state approximation. The radical consumption layer lies ahead of the fuel consumption layer toward the burned mixture at the position where the rate of the chain breaking reaction equals that of the chain-branching reaction. When the temperature decreases by a lowering of the fuelair ratio, the radical consumption layer moves toward the burned gas and takes all radicals out of the reaction zone. This leads to a vanishing flame velocity. Thus the kinetic model describes chemical extinction. The same effect is produced by a negative temperature perturbation for flames close to the flammability limit. It is shown how those flames will eventually be extinguished by flame stretch or heat loss.

Chain propagation and breaking reactions in photooxidation of cellulose

1. Photooxidation of hydrated cellulose (HC) is a chain process with short chains. The kinetic parameters of this process were determined. 2. The radicals responsible for the reaction of chain propagation in oxidation of HC were identified. 3. Photostable acceptors of alkyl radicals could be the most effective photostabilizers of HC.