We report the application of the "internal trigger" approach to multistep microfluidic polymerization reactions conducted in droplets, namely, polyaddition and polycondensation. We hypothesized and experimentally established that heat generated in an exothermic free radical polymerization of an acrylate monomer (Reaction 1) triggers the polycondensation of the urethane oligomer (Reaction 2). Completion of two microfluidic polymerization reactions led to the continuous synthesis of polymer particles with an interpenetrating polymer network (IPN) structure. Use of this microfluidic synthesis allowed us (i) to conduct efficient screening of the compositions of the monomer mixtures; (ii) to achieve control of the stoichiometric ratios of reactants in Reaction 2 by varying the flow rates of liquids; (iii) to reach control over the morphology of the resulting particles; and (iv) to produce polymer particles with a narrow size distribution and a predetermined size.