Sulfate endgroups

Monodisperse Polystyrene Latexes Surface Charge and Number of Sulfate Endgroups/Polymer Molecule (3,5,9)... 

The stability of latexes during and after polymerization may be assessed at least qualitatively by the theoretical relationships describing the stability of lyophobic colloids. The Verwey-Overbeek theory (2) combines the electrostatic forces of repulsion between colloidal particles with the London-van der Waals forces of attraction. The electrostatic forces of repulsion arise from the surface charge, e.g., from adsorbed emulsifier ions, surface sulfate endgroups introduced by persulfate initiator, or ionic groups introduced by using functional monomers. These electro-... 

It must be noted that the process of seeded emulsion polymerization does not lead to an equilibrium structure. Hence, the sharp interface between PS and PMMA observed in the above core-shell particles cannot be explained by thermodynamic arguments. A possible mechanism maybe sought in the adsorption of oligo(methylmethacrylate) radicals from the water phase onto the PS-seed particles [45]. The temperature of the seeded emulsion polymerization (80 °C [45]) is well below the glass transition temperature of polystyrene and the adsorbed chains bear a sulfate endgroup. The adsorbed oligomers will therefore remain at the surface of the core particles and in consequence there is no extended interface between PS and PMMA in these. particles. 

Therefore, we decided to remove the adsorbed emulsifier as completely as possible and to rely upon the sulfate endgroups to give the particle the required stability. This would give an ideal model colloid, i.e., monodisperse spheres of constant and known surface charge arising from chemically-bound strong-acid surface groups. 

After ion exchange, the latex particles are stabilized only by the sulfate endgroups of the polymer chains, all of which are in the H form. Thus, their number can be determined by titration with base. Generally, these titrations were followed conductometrically, by recording the voltage drop across s. 10 resistor in series with a dip-type conductance cell and in parallel with a 6-volt transformer, while 0.01 N NaOH was added continuously from a constant... 

Using this method of latex characterization, we determined the number of sulfate endgroups on the particle surface and inside the... 

Figure 4. Comparison of Different Methods for Determination of Sulfate Endgroups of Latex A-2 0 dye partition,...

Table II shows that 45-100% of the sulfur incorporated into the polymer is present on the particle surface. In terms of the surface charge, these amounts cannot be neglected as a factor in latex stability, both during and after polymerization. (This accounts for the fact that polymerizations with hydrogen peroxide initiator and low emulsifier concentrations frequently coagulate at low conversions). For latexes B-1, B-2, and D-4, the charge density due to sulfate endgroups is, respectively, at least 4, 8, and 1.6 times the charge due to adsorbed emulsifier. For the other latexes, it amounts to 20-100% of the emulsifier adsorbed. Thus, this permanent charge is an important factor that has been overlooked in earlier studies of latexes at low emulsifier coverage (16, 17).

The foregoing results indicate that the 176oX-diameter particles with a number of residual sulfate endgroups corresponding to a surface charge density of 1.7TJC/cm are relatively stable without... 

For initiation in the aqueous phase to produce a monodisperse latex, the primary particles generated early in the reaction must act as nuclei to capture all primary radicals formed thereafter and these nuclei must grow without flocculation until the end of the polymerization. Therefore, the emulsifier must adsorb rapidly enough to stabilize these initial nuclei but not so rapidly as to stabilize the primary particles formed later in the reaction. This condition may be met if the emulsifier concentration is relatively low or if the emulsifier is omitted the sulfate endgroups introduced by the persulfate initiator are often sufficient to stabilize latex particles at relatively low monomer-water ratios (30). Table III gives the increase in particle size and surface charge resulting from the flocculation of the primary particles described in... 

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. 

Monodisperse Polystyrene Latex A-2 Oxidation of Sulfate and Hydroxyl Endgroups 10 5n silver nitrate 6 hours at 90°)... 

To determine the effect of different polymerization conditions on the polymer endgroups produced, polymerizations were carried out using the standard bicarbonate buffer as well as other variations. Table V (13,16) shows that the use of the persulfate-bicarbonate combination with and without emulsifier gave latexes of final pH 7-8 with only sulfate groups. The addition of 10 5 silver ion gave a latex of pH 8.5, but with weak-acid groups, presumably because of oxidation of the sulfate groups. 

Sulfonate anion is not suitable when control of endgroups by termination is desired. The THF tertiary oxonium ion reacts irreversibly with the sulfate anion to form sulfates and disulfates, which can be hydrolyzed but are otherwise stable. 

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