AbstractDuring the bulk agitated polymerization of styrene containing dissolved SBR, a phase inversion was found to occur at fairly low styrene conversion levels. Below this inversion point, the continuous phase was rubber‐in‐styrene, while above it the rubber‐in‐styrene became dispersed in a continuous phase of polystyrene‐in‐styrene. At the point of phase inversion, there was a marked reduction in bulk viscosity, large changes in polymer intrinsic viscosity, and in the light transmission of 5% polymer solutions in toluene. At partial styrene polymerizations, the dried prepolymer films were insoluble in 2‐butanone before phase inversion had occurred and soluble thereafter.After partial polymerization of the styrene in the bulk agitated stage, polymerization was completed in aqueous suspension. To get the ultimate dispersion of the rubber particles, which became crosslinked to form microgel, it was necessary to carry the bulk agitated polymerization well beyond the phase‐inversion point. The size of the final microgel particles was related to the initial viscosity of the SBR with the more viscous rubbers forming larger microgel particles. The desirable microgel particle size was indicated to be in the 1–10 μ range. When suspension polymerization was carried out before phase inversion was completed in the bulk agitated stage, the fully converted polymer contained very large networks of crosslinked grafted rubber gel and was in
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