Rate-Controlling Events for Radical Exit in Electrosterically Stabilized Emulsion Polymerization Systems

The mechanism controlling radical loss by exit (desorption) in electrosterically stabilized emulsion polymerization particles was obtained from kinetic studies. Using RAFT-controlled radical polymerization techniques, polystyrene particles stabilized with differing lengths of poly(acrylic acid) chains bound to the surface were synthesized, with the hydrophilic block of low polydispersity, and of various degrees of polymerization. After removal of the RAFT agent, these latexes were used in seeded emulsion polymerization experiments with styrene, with the radical loss kinetics studied through the use of γ-radiolysis dilatometry. The size of the particles is such that they follow “zero−one” kinetics, so the sole rate-determining step for radical loss is by exit. The rate coefficient for exit (k) of these latexes was obtained directly from the non-steady-state relaxation period. A significant decrease in k occurs (by a factor of 10) relative to ionically stabilized latexes of corresponding size, even for particles with very small hydrophilic layers. The value of k was smaller for latexes with greater length of the hydrophilic block, consistent with the hypothesis that exit in electrosterically stabilized systems is bound by a restricted diffusion through the hydrophilic polymeric layer on the surface. Modification of the Smoluchowski treatment for diffusion-controlled rate coefficients to allow for diffusion through two different regions provides an expression for the rate coefficient of radical desorption out of a particle in these systems; semiquantitative agreement with experiment was obtained.