Radical capture

Since polymer swelling is poor and the aqueous solubiUty of acrylonitrile is relatively high, the tendency for radical capture is limited. Consequentiy, the rate of particle nucleation is high throughout the course of the polymerization, and particle growth occurs predominantiy by a process of agglomeration of primary particles. Unlike emulsion particles of a readily swollen polymer, such as polystyrene, the acrylonitrile aqueous dispersion polymer particles are massive agglomerates of primary particles which are approximately 100 nm in diameter. 

DIrIgent protein Free radical capture and stereoselective coupling... 

SCHEME 12. Eree radical capture by a-methylstyrene dimer (225, MSD)... 

Oxidation of a snlfide to sulfoxide is known to be an electrophilic reaction, in contrast with nncleophUic oxidation of sulfoxide to sulfone. Since 2-nitrobenzenesulfinyl chloride/KOi oxidizes sulfides to sulfoxides selectively, intermediate 48 must be the actual active intermediate. Moreover, in the presence of l,4-diazabicyclo[2.2.2.]octane (DABCO), which is a radical capturing reagent, the oxidation of methyl phenyl sulfide to the sulfoxide was inhibited. In order to further detect the intermediate 48, pure 5,5-dimethyl-l-pyrroUne-l-oxide (DMPO) was used as a trapping reagent and spin adduct was obtained. The ESR spectrum of the DMPO spin adduct was obtained by the reaction of 02 with 2-nitrobenzenesulfinyl chloride (hyperfine coupling constants, aH = 10.0 G and aN = 12.8 G). 

We have carried out a limited study of the effect of metal dialkyl dithiophosphates on a hydroperoxide-autocatalyzed oxidation system. Table III summarizes induction periods for the oxidation of squalane at 140 °C. These results do not unambiguously reflect the peroxide-decomposing property of each dithiophosphate radical capture also occurs. 

This paper discusses three aspects of our extensive but as yet incomplete studies of the reaction oxygen uptake in rigorously purified materials, the effects of added iron complexes, and the influence of a well-known radical capture agent. An interesting feature of the last, anticipated from the work of Rosenwald (19) on inhibitor sweetening, was the large accelerating effect of N,N -di- ec-butyl-p-phenylenediamine on the co-oxidation reaction. 

Z. Xi, J. Rong, and J. Chattopadhyaya, Diastereospecific synthesis of 2 - or 3 -C-branched nucleosides through intramolecular free-radical capture by silicon-tethered acetylene, Tetrahedron 50 5255 (1994). 

The peroxide-free radicals capture a hydrogen ion from another fat molecule, forming a hydroperoxide and another fatty free radical ... 

It is accepted that the radical entry rate coefficient for miniemulsion droplets is substantially lower than for the monomer-swollen particles. This is attributed to a barrier to radical entry into monomer droplets which exists because of the formation of an interface complex of the emulsifier/coemulsifier at the surface of the monomer droplets [24]. The increased radical capture efficiency of particles over monomer droplets is attributed to weakening or elimination of the barrier to radical entry or to monomer diffusion by the presence of polymer. The polymer modifies the particle interface and influences the solubility of emulsifier and coemulsifier in the monomer/polymer phase and the close packing of emulsifier and co emulsifier at the particle surface. Under such conditions the residence time of entered radical increases as well as its propagation efficiency with monomer prior to exit. This increases the rate entry of radicals into particles. 

It was further developed the following year (22), and was based primarily on the scheme of Priest (12) with an idea from Gardon (9d). The latter suggested that the rate of capture of oligomeric radicals in solution by pre-existing particles, R, should be proportional to the collision cross-section, or tfie square of the radius of the particles, r. This has been called the "collision theory" of radical capture. In 1975 Fitch and Shih measured capture rates in MMA seeded polymerizations and came to the conclusion that R was proportional to the first power of the radius, as would e predicted by Fick s theory of diffusion (23). In his book, K. J. Barrett also pointed out that diffusion must govern the motions of these species in condensed media (10). 

In all of these considerations concerning the rate of radical capture and its effect on the rate of particle formation we have been hampered by the lack of a mathematical solution for the concentration of oligoradicals in the aqueous phase, Cs. 

Cg, is obtained. Further, Ay assuming that Cg is governed solely by termination in the aqueous phase, and by taking an average value for the rate constant for radical capture by particles, k, an analytical solution is obtained (30) for the particle number ... 

Comparison between Experimental Results and Model Predictions. As will be shown later, the important parameter e which represents the mechanism of radical entry into the micelles and particles in the water phase does not affect the steady-state values of monomer conversion and the number of polymer particles when the first reactor is operated at comparatively shorter or longer mean residence times, while the transient kinetic behavior at the start of polymerization or the steady-state values of monomer conversion and particle number at intermediate value of mean residence time depend on the form of e. However, the form of e influences significantly the polydispersity index M /M of the polymers produced at steady state. It is, therefore, preferable to determine the form of e from the examination of the experimental values of Mw/Mn The effect of radical capture mechanism on the value of M /M can be predicted theoretically as shown in Table II, provided that the polymers produced by chain transfer reaction to monomer molecules can be neglected compared to those formed by mutual termination. Degraff and Poehlein(2) reported that experimental values of M /M were between 2 and 3, rather close to 2, as shown in Figure 2. Comparing their experimental values with the theoretical values in Table II, it seems that the radicals in the water phase are not captured in proportion to the surface area of a micelle and a particle but are captured rather in proportion to the first power of the diameters of a micelle and a particle or less than the first power. This indicates that the form of e would be Case A or Case B. In this discussion, therefore, Case A will be used as the form of e for simplicity. 

If Case B is rather likely for , as the diffusion theory and the examination of experimental value of M /M predict, one must introduce the concept of radical capture e ficiency of a micelle relative to a polymer particle, a in the form of ak2 where k2 is the radical capture coefficient for a polymer particle. The approximate value of a is estimated to be 0.01 for emulsion polymerization of styrene because the value of is 1.28xl05, the value of... 

Longf ieict moaeiXwithout taking into account the radical capture efficiency of micelles(or the Nomura and Harada model with Case C for e) are given, for comparison, as follows ... 

Recently, additional radical capturing polymers have been proposed, in which, similar in substances to be discussed later, bifunctional derivatives of the active component are incorporated into polymeric chains directly by polycondensation. Substance 35, one of three polymers tested by Berlin (1 03), has been found to be the most active one inhibiting most effectively the development of leucosis. 

This allylic radical captures a molecule of oxygen at Cll to form a new oxyradical. The reaction occurs at one end of the delocalized radical so that the product is a conjugated diene and the new alkene is trans (E). 

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