Sulfate ion-radicals

An expanding development is the use of peroxodisulfates as oxidants in organic chemistry (80,81). These reactions are initiated by heat, light, gamma rays, or transition-metal ions. The primary oxidising species is usually the sulfate ion radical, P hskip -3pt peroxodisulfate anion... 

Water is extensively used to produce emulsion polymers with a sodium stearate emulsifrer. The emulsion concentration should allow micelles of large surface areas to form. The micelles absorb the monomer molecules activated by an initiator (such as a sulfate ion radical 80 4 ). X-ray and light scattering techniques show that the micelles start to increase in size by absorbing the macromolecules. For example, in the free radical polymerization of styrene, the micelles increased to 250 times their original size. 

The persulfate ion undergoes homolytic cleavage producing two sulfate ion radicals. 

The sulfate ion radical then initiates polymerization, here with a styrene monomer eventually forming a PS oligomer radical and eventually a PS radical. 

Sugar-derived ketones, catalyzed dioxirane epoxidation, 1147 Sulfanilamide, TEARS assay, 667 Sulfate ion radical, peroxydisulfate organic salts, 1014 Sulfides... 

Since sulfur from the persulfate does not appear in the polymer, two possible reactions were suggested. The sulfate ion-radicals may initiate the polymerization to produce fluoroalkyl sulfuric acid esters which would be very rapidly hydrolyzed. 

Alternatively, the sulfate ion-radicals may hydrolyze to give hydroxyl radicals which could initiate the polymerization. 

One possibility is hydroxyl endgroups, which may be formed by a side reaction of sulfate ion-radicals to form hydroxyl radicals (9) or hydrolysis of the surface sulfate groups. To determine if hydroxyl groups were present, the ion-exchanged latexes were oxidized by heating with persulfate and 10 silver ion at 90°, then ion exchanged and titrated conductometrically to determine the carboxyl groups. Table II (9) shows that some sul-... 

The extent of the side reaction of sulfate ion-radicals with water to produce hydroxyl radicals was postulated to increase with decreasing pH. Therefore, polymerizations were carried out using persulfate initiator but with the pH of the polymerization adjusted to values in the range of pH range 2-8 (9). Table III (9) shows that, at the lowest pH, the endgroups were about 90% hydroxyls and 10% sulfates at pH 7-8, they were all sulfates. 

We know that the particle formation mechanisms can be much more complex, especially for monomers such as MMA and VA. The sulfate ion radical formed from persulfate initiator is not likely to be strongly attracted to either micelles or polymer particles due to its charge and hydrophilic nature. Instead, this radical will begin to polymerize monomer which is dissolved in the water phase. As monomer units are added, the aqueous phase ion radical will become less hydrophilic and more surface active. During the period of surface activity, it may well adsorb on a micelle, a polymer particle, or a monomer drop. 

Here, M stands for monomer, as usual. The oligomers produced by reaction (8-7) could meet several fates. If such a radical encounters another radical in the aqueous phase it may undergo termination by combination or disproportionation (Section 6.3.3) to yield a molecule with one or two sulfate ion ends, respectively. The end groups may also reflect whether the second reactant in the termination reaction is a hydroxyl radical (reaction 8-2) or sulfate ion radical (reaction 8-10), or results from hydrolysis, as in ... 

When polymerization is carried out at relatively high temperatures (t > 40° G.), thermally unstable compounds are frequently used as a source of free radicals. The persulfate ion, for example, decomposes with formation of two sulfate ion radicals 804 , a reaction which is strictly first-order at least in the absence of soap. In addition, another type of decomposition occurs at low pH where the H+ ion reacts with persulfate (37). Also soap is known to influence the rate of decomposition of persulfate. These last reactions, however, do not produce radicals (2, 38,63). 

Table 2.6. Relative Rates of Reactions of the Sulfate Ion-Radical With Some Monomers ...

The Smith-Ewart mechanism does not take into account any polymerization in the aqueous phase. This may be true for monomers that are quite insoluble in water, like styrene, but appears unlikely for more hydrophilic ones like methyl methacrylate or vinyl acetate. In addition, it was calculated by Flory that there is insufficient time for a typical cation radical (like a sulfate ion radical) to add to a dissolved molecule of monomer like styrene before it becomes captured by a micelle. This was argued against, however, on the ground that Flory s calculations fail to consider the potential energy barrier at the micelle surfaces from the electrical double layer. This barrier would reduce the rate of diffusion of the radical ions into the micelles. ... 

Do the same as question 1, but with a redox mechanism, showing a sulfate ion radical adding to vinyl acetate. 

Table 3.6 Relative rates of reactions of the sulfate ion-radical with some monomers [51, 52]...

Polymerizations initiated with persulfate-ferrous ion and others initiated with bisulfite-ferric ion have shown that both sulfate ion radicals and bisvilfite radiccils are effective in prcxooting polymerization. We have not yet been able to assign a differ t... 

The predicted concentrations of sulfate ion radicals for two surfactant levels during the course of polymerization are shown in Figure 2. These curves are for equimolar concentrations of persulfate and bisulfite/ so that the total radical oonoentratioi is just about double that of the one radical species shewn. The radical concentration rises r idly at first and readies a sonaximum in about 20 minutes. About 85% of the persulfate and bisulfite are still present after 100 minutes so that a plentiful svpply of free radicals is available throw iout the course of the reaction. 

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