We present herein an original tool to monitor polymerization kinetics. It consists in a droplet-based millifluidic approach where the use of aqueous droplets of monomer can be seen as polymerization microreactors. Acrylic acid mixed with sodium persulfate at low pH was used as a fast and exothermic polymerization model. By using a nonintrusive spectroscopic system with our millifluidic system, we were able to safely investigate harsh polymerization conditions. As expected, polymerizations exhibited higher order with respect to monomer concentration than the usual first-order kinetics for ideal free-radical polymerization, and a 1/2 dependence with respect to the initiator concentration. The overall activation energy for polymerization rate was also estimated to 47.0 kJ/mol. Also, it has been shown that Er becomes temperature dependent due to the solution viscosity, i.e., low temperatures and high concentrations. These results set the stage for further studies of polymerization reactions where detailed basic kinetic data must be acquired in conditions which cannot be investigated by using conventional batch glassware, i.e., high temperatures or concentrations. This versatile approach can also be used as an efficient high throughput screening tool.