An experimental and theoretical study on the thermal (spontaneous) polymerization of styrene in compartmentalized systems is presented. Experimental data on thermally polymerized miniemulsions varying temperature and droplet size is provided. This data is used to test a new model approach to describe the thermal polymerization process of a disperse phase. The mathematical model is based on a chemical master equation balancing radical species that are capable of desorption (monomeric radicals) and ones that cannot leave a particle. Reasonable agreement between theory and experiment for the data provided here and for literature data can be achieved. Part 1 of this series is a review on the thermal bulk polymerization of styrene.