Coagulation—control of particle number during nonaqueous emulsion polymerization of methyl methacrylate

Abstract

Methyl methacrylate (MMA) was polymerized in heptane in the presence of an amphipathic graft copolymer stabilizer to produce nonaqueous latices. The stabilizer was formed by copolymerizing a polyhydroxy stearate “macromer” and MMA. The initiator was cumene hydroperoxide/dodecylmercaptan redox system. The kinetics of particle formation were followed using a sampling dilatometer along with electron microscopy. Primary particles were presumed to be formed by homogeneous nucleation in analogy to the behavior of aqueous latex systems. Aggregation played an important role in the determination of the number concentration of particles and their rate of growth. Renucleation of subsequent generations of primary particles at later stages in the polymerization was observed. Calculations of the interaction potentials between particles, using the approximate theory of Hesselink, Vrij, and Overbeek, showed a secondary minimum. These were based on previous measurements of surface coverage by the stabilizer. The final number of particles was found to be determined by the amount of stabilizer and the fact that a fixed interfacial area per stabilizer soluble moiety is maintained. Particle formation kinetics are determined by three factors: dN dt = R i − R c − R f , in which R i is the rate of free radical generation, R c is the rate of oligomeric free radical capture by polymer particles, and R f is the rate of coagulation. These, in turn, were found to depend upon the stabilizer, initiator, and monomer concentrations.