The potential for fast and specific process optimization by functional prototyping with 3D printed reactors was shown for a known continuous emulsion polymerization in a modular setup. Three tubular reactors, two with static mixing elements and a continuously stirred tank reactor (CSTR) were printed from polylactic acid. Thermal imaging of the reactors allows to map the basically adiabatic progress of the redox-started radical polymerization in a space-resolved way. The application of static mixing elements led to a higher conversion stability and a more uniform product, as expected for a better mass transfer. Polymerization in combinations of CSTR and a tubular reactor showed further improvements of process stability and product uniformity in terms of particle size and mol masses.