Relative Rates of Branching in Emulsion and Miniemulsion Polymerization

This communication describes the potential advantages of using a miniemulsion rather than an emulsion process for the polymerization of synthetic rubbers in which the polymerization is ended (short‐stopped) considerably before full conversion in order to limit excessive branching brought on by a high polymer‐to‐monomer ratio in the polymer particles. Because the polymer‐to‐monomer ratio in the particle at low monomer conversion is much lower in a miniemulsion, a miniemulsion can be polymerized to a significantly higher conversion than a conventional emulsion while maintaining an equivalent degree of branching. Short‐stopping at a higher monomer conversion will result in substantially reduced processing costs associated with recovery and recycle of unpolymerized monomer. In emulsion polymerization, the thermodynamic equilibrium between polymer particles and monomer droplets results in polymer particles that contain high concentrations of polymer, even at low conversion. This causes enhanced chain branching which is often detrimental in synthetic rubber processes. A mathematical model is used to show that a miniemulsion process for rubber polymerization may suppress branching.