Bimolecular coupling reaction, radical center

With the idea of avoiding the potential bimolecular coupling reaction of the radical centers in the solution-phase chemical oxidation reactions, a photochemical approach was adopted. Diazo compounds 10 and 12 were treated with the Rh catalyst under basic conditions to give the poly(acetylene)s 43 [R and m- /7-(/i-C,9H39)C6H4]C(N2)] of 200000 [8]. While photolysis of the diazo groups proceeded smoothly on neat films at 2 K and broad EPR si-... 

At some point, the propagating polymer chain stops growing and terminates. Termination with the annihilation of the radical centers occurs by bimolecular reaction between radicals. Two radicals react with each other by combination (coupling) or, more rarely, by... 

Bimolecular reaction between a pair of chain radicals accounts for annihilation of active centers. Two obvious processes by which this may occur are chain coupling (or combination)... 

Hexenyl bromide 22 can be activated similarly to 5-hexenyl radical 22A, which cyclizes to cyclopentylmethyl radical 22B with a rate constant of 2.3 x 105 s 1 [91]. For this slower reaction, a competition between direct trapping of 22A to 24 and 5-exo cyclization to 22B followed by coupling to 23 is often found. These unimolcular reaction steps are in comparison to bimolecular transition metal-centered transformations faster or at least as fast. 

Termination. Just as peroxy radicals are key to the propagation sequence, so the bimolecular recombination of these radicals is the major termination process in the unstabilized polymer. The existence of an intermediate tetroxide has been established in solution (25). Several factors influence the competitive pathways of subsequent decomposition to form alcohols, ketone and singlet oxygen or to form alkoxy radicals which can couple before separation from the reaction center to form a peroxide. This latter process is a route to crosslinking in the case of polymeric peroxy radicals. The effect of steric control, viscosity and temperature have been studied in solution. However, in the solid phase the rates of bimolecular processes which require the mutual diffusion of the reactant groups will be limited by the diffusion process. As a standard, we have assumed a value close to that determined from oxygen absorption (26) and by ESR spectra (27) for oxidized polypropylene films. 

In all of the photochemistry of UO2 , the nature of the bimolecular product formed by association of the reactant with the excited state U02 is an important facet of the chemistry. In aqueous solution the uranyl ion exists as the hydrate, U02(H20)5, where the five water molecules are complexed in the equatorial plane. Replacement of one or more of these complexed water molecules by an incoming substrate allows for bimolecular photoreaction to occur in the vicinity of the metal center. The reaction types can be summarized by the series of reactions shown in Scheme 8.5, where the radical pair can undergo dissociation, or either forward or back electron transfer reactions. Since the potentials for the (U02 /U02) and the (UOj/U ) couples are 0.06 V and 0.55 V, respectively, the UO2 ion can either be readily oxidized back to UOi, or it can act as a one-electron oxidant to a substrate that reacts as a reducing agent. By observing the reactions... 

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