In emulsion polymerization to reduce coagulation, you have to make sure that particles are well stabilized.
1. Coagulation of particles can be decreased by increasing surfactant amount in the recipe. Better is that you feed extra surfactant during the process (semi-batch). Adding all of it during nucleation phase would increase number of particles which will not have enough surfactant for stabilization, meaning more coagulation.
2. If you are using ionic surfactant, reducing total number of ions (consider thermal initiator also one of the ion increasing ingredient) would reduce coagulation.
3. You may use so called "miniemulsion polymerization" process, which usually results in more stable final dispersion compared to conventional emulsion polymerization.