Radical macroscopic kinetics

For revealing of influence of conformational state of macro-radicals on kinetics of radical polymerization of acrylate- and methacrylate-guanidines in water mediums with the help of viscosimetry method the values of macroscopic viscosities in solutions modeling reaction mixtures at low conversion degrees were measured and obtained data were compared with kinetic ones. 

Cage effect dynamics or kinetics of geminate recombination was observed for the first time under photodissociation of aC C dimer of aromatic radicals in a viscous media. A suggestion has been made that at least in a number of smdied cases the mumal diffusion coefficient of radicals in the pair is approximately 10 times lower than the sum of macroscopic diffusion coefficients of the individual species. In other words, a geminate recombination proceeds considerably longer than expected. 

At least two points should be especially emphasized, (i) From the solvent part, the parent radical cations exist in a non polar surrounding. Hence, the cations have practically no solvation shell which makes the electron jump easier in respect to more polar solvents. In a rough approximation the kinetic conditions of FET stand between those of gas phase and liquid state reactions, exhibiting critical properties such as collision kinetics, no solvation shell, relaxed species, etc. (ii) The primary species derived from the donor molecules are two types of radical cations with very different spin and charge distribution. One of the donor radical cations is dissociative, i.e. it dissociates within some femtoseconds, before relaxing to a stable species. The other one is metastable and overcomes to the nanosecond time range. This is the typical behavior needed for (macroscopic) identification of FET ... 

During ultrasonic irradiation of aqueous solutions, OH radicals are produced from dissociation of water vapor upon collapse of cavitation bubbles. A fraction of these radicals that are initially formed in the gas phase diffuse into solution. Cavitation is a dynamic phenomenon, and the number and location of bursting bubbles at any time cannot be predicted a priori. Nevertheless, the time scale for bubble collapse and rebound is orders of magnitude smaller than the time scale for the macroscopic effects of sonication on chemicals (2) (i.e., nanoseconds to microseconds versus minutes to hours). Therefore, a simplified approach for modeling the liquid-phase chemistry resulting from sonication of a well-mixed solution is to view the OH input into the aqueous phase as continuous and uniform. The implicit assumption in this approach is that the kinetics of the aqueous-phase chemistry are not controlled by diffusion limitations of the substrates reacting with OH. 

Since plasma contains electrons, ions, photons, radicals, and excited molecules, it becomes important to identify the reactive species controlling the propagating process of the polymerization. A number of workers have reported on kinetic models of plasma polymerization. Our current xmderstanding of the chemical and physical mechanism of the process remains limited because the extreme complexity of the plasma environment resists efforts toward a generalization and characterization. The bulk of the research has been concentrated on establishing the dependence of the macroscopic and spectroscopic properties of the product on the major process variables, e.g., rf power, monomer type, and gas flow rate. 

Ozol-Kalnin and co-workers [25] derived the kinetic equation that describes the polymerisation on assumption that the reaction proceeds via stepwise addition of the monomer of one kind. To describe the formation of a macroscopic network, the criterion for gelation, snggested previonsly for free radical polymerisation of nnsaturated componnds ... 

Few people realize that when they stretch a rubber band - or inflate tires on their cars - they do chemistry. In this example of vulcanized rubber, the chemistry is simple - homolysis of C-C, C, or S-S bonds, but the underlying principle is anything but simple. The probability of ethyl disulflde spontaneously dissociating into radicals at room temperature is negligible. Yet this probability can increase by many orders of magnitude for the same molecular moiety when it is as part of amorphous material under mechanical load. In other words, translation of macroscopic objects (e.g., our hands) that compress, stretch, or twist polymeric material can directly control reaction rates of material building blocks. Such control of course is not seen in the vast majority of reactions studied by chemists and as a result is not accommodated in any of the existing models of chemical kinetics. 

Unfortunately, the increasing complexity of radical polymerization processes (which may contain hundreds or thousands of kinetically distinct reactions) can signihcantly hinder experimental efforts to extract this information for all but the simplest systems. The fundamental problem is that experimental techniques can only measure the observables of a process—typically the time-dependent concentrations of some of the major species or (more often) some of the major functional groups. Linking this macroscopic information to the microscopic properties of the process (i.e., the rate coefficients of the individual reactions) requires model-based assumptions, which can be subject to signihcant errors [6]. 

In this paper we examined the effect of CLDP on kinetics in low-conversion free-radical polymerization. We have shown that although the chain length dependence of the individual fej, does not extend beyond for common systems, a significant macroscopic effect may be observed in systems with DPa up to 100. This observation leads us to draw some preliminary conclusions regarding CLDP (a) it should probably not be ignored in living radical polymerizations with low DPa ( 0>... 

A special feature of the kinetics of reactions initiated by electron pulses is that in the early stages the reaction proceeds mainly in relatively small confined regions of the solution, known as spurs , within which the concentrations of electrons, radicals and excited molecules are larger by orders of magnitude than those that obtain in the bulk solution. That this is the state of affairs may be expected from the physics of absorption of electrons by liquids sketched above. Much experimental evidence on product yields shows good agreement with a model based on the assumption of equilibrated macroscopic values of rate constants within the spurs, coupled with diffusion-controlled transfer of product molecules to the bulk solution. A brief account of the theory follows. 

The discussion of the rate of bimolecular termination has, up to now, been mainly of a qualitative nature. The scaling of average or macroscopic kt values with viscosity, solvent effects and coil dimensions were discussed without much attention for the chain-length dependence of this process. This dependence originates from the simple fact that free-radical termination is a diffusion-controlled process. Consequently, the overall mobility of polymer chains and/or polymer chain ends determine the overall rate of radical loss in a polymerizing system. As small chains are known to be much more mobile than large ones, the chain length of radicals can be expected to have a profound effect on the termination kinetics. 

Photo-oxidation of some aaylic-urethane thermoset networks was induced by chromophoric impurities that absorb UV light and produce radicals, initiating a radical oxidation of the polymer [145]. The authors introduced a quantitative kinetic model based on the identified mechanisms and a multi-scale approach from the molecular to the macroscopic level. 

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