Redox propagation

Besides ETC catalysis, atom-transfer chain (ATC) catalysis has been reviewed [2]. The initiation involves radicals and the redox propagation step exchanges an... 

This mechanism resembles the preceding one, but an atom, instead of an electron, is exchanged in the cross redox propagation. Taube s pioneering example concerns the incorporation of radioactive Cl into [Pt Cle], the initiation being most often carried out by irradiation (various means have been used) ... 

Here again, it is possible to forecast if the redox propagation step is thermodynamically favorable by comparing the relative strengths of the M-H bonds in the starting and final compounds. The M-H bond is indeed strengthened after CO substitution by a phosphine (because electron density is increased), which justifies the exergo-nicity of the cross redox step and the fact that this overall reaction works well. 

An exergonic cross redox propagation step should be planned. Therefore, if L is a better donor than L initiate by reduction if L is a less good donor than L initiate by oxidation. 

Generally the oxidant is compounded in one part of the adhesive, and the reductant in the other. Redox initiation and cure occur when the two sides of the adhesive are mixed. There also exist the one-part aerobic adhesives, which use atmospheric oxygen as the oxidant. The chemistry of the specific redox systems commonly used in adhesives will be discussed later. The rates of initiation and propagation are given by the following equations ([9] p. 221). 

Redox initiation is commonly employed in aqueous emulsion polymerization. Initiator efficiencies obtained with redox initiation systems in aqueous media are generally low. One of the reasons for this is the susceptibility of the initially formed radicals to undergo further redox chemistry. For example, potential propagating radicals may be oxidized to carbonium ions (Scheme 3.44). The problem is aggravated by the low solubility of the monomers (e.g. M VIA. S) in the aqueous phase. 

The field of modified electrodes spans a wide area of novel and promising research. The work dted in this article covers fundamental experimental aspects of electrochemistry such as the rate of electron transfer reactions and charge propagation within threedimensional arrays of redox centers and the distances over which electrons can be transferred in outer sphere redox reactions. Questions of polymer chemistry such as the study of permeability of membranes and the diffusion of ions and neutrals in solvent swollen polymers are accessible by new experimental techniques. There is hope of new solutions of macroscopic as well as microscopic electrochemical phenomena the selective and kinetically facile production of substances at square meters of modified electrodes and the detection of trace levels of substances in wastes or in biological material. Technical applications of electronic devices based on molecular chemistry, even those that mimic biological systems of impulse transmission appear feasible and the construction of organic polymer batteries and color displays is close to industrial use. 

In the theoretical treatment of ion exchange polymers the roles of charge propagation and of migration of ions were further studied by digital simulation. Another example of proven 3-dimensional redox catalysis of the oxidation of Ks[Fe(CN)5] at a ruthenium modified polyvinylpyridine coated electrode was reported... 

Anson FC, Ni CL, Saveant JM. 1985. Electrocatalysis at redox polymer electrodes with separation of the catalytic and charge propagation roles. Reduction of dioxygen to hydrogen peroxide as catalyzed by cobalt(II) tetrakis(4-A-methylpyridyl)porphyrin. J Am Chem Soc 107 3442. 

A discussion of the charge transfer reaction on the polymer-modified electrode should consider not only the interaction of the mediator with the electrode and a solution species (as with chemically modified electrodes), but also the transport processes across the film. Let us assume that a solution species S reacts with the mediator Red/Ox couple as depicted in Fig. 5.32. Besides the simple charge transfer reaction with the mediator at the interface film/solution, we have also to include diffusion of species S in the polymer film (the diffusion coefficient DSp, which is usually much lower than in solution), and also charge propagation via immobilized redox centres in the film. This can formally be described by a diffusion coefficient Dp which is dependent on the concentration of the redox sites and their mutual distance (cf. Eq. (2.6.33). 

Other methods of producing the initiating radicals include photochemical and redox reactions.) Initiation is followed by propagation of the radical by the successive additions of very large numbers (usually thousands) of monomer molecules... 

When the characteristic time for charge diffusion is lower than the experiment timescale, not all the redox sites in the film can be oxidized/reduced. From experiments performed under these conditions, an apparent diffusion coefficient for charge propagation, 13app> can be obtained. In early work choroamperometry and chronocoulometry were used to measure D pp for both electrostatically [131,225] and covalently bound ]132,133] redox couples. Laviron showed that similar information can be obtained from cyclic voltammetry experiments by recording the peak potential and current as a function of the potential scan rate [134, 135]. Electrochemical impedance spectroscopy (EIS) has also been employed to probe charge transport in polymer and polyelectrolyte-modified electrodes [71, 73,131,136-138]. The methods... 

From a theoretical point of view, charge propagation in films containing space-distributed redox centers can be achieved either by the physical displacement of the sites or by the transference (hopping) of electrons from neighboring reduced to oxidized sites or by the combination of both processes. In the case of free diffusing couples immobilized in oppositely charged polyelectrolytes, both processes occur and an apparent diffusion coefficient can be defined and measured [136, 142, 143] ... 

Metal transport and deposition capacities of mineral systems are closely linked to propagation of redox and related physicochemical gradients (pH, aH2, aHCI, aH2S, aS02, aC02, aCH4, aH20, etc) within mineral systems. For metals transported in solution, the rate of mineralization is a product of 3 factors (see Fig. 2) ... 

Other exceptions to the first-order dependence of the polymerization rate on the monomer concentration occur when termination is not by bimolecular reaction of propagating radicals. Second-order dependence of Rp on [M] occurs for primary termination (Eq. 3-33a) and certain redox-initiated polymerizations (Sec. 3-4H-2). Less than first-order dependence of Rp on [M] has been observed for polymerizations (Sec. 9-8a-2) taking place inside a solid under conditions where monomer diffusion into the solid is slower than the normal propagation rate [Odian et al., 1980] and also in some redox polymerizations (Sec. 3-4b-2) [Mapunda-Vlckova and Barton, 1978]. 

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