This paper describes a process based on the ultrasound-assisted polymerization of a water-soluble monomer at a high concentration, which is known as a gel polymerization process. Polymerization is triggered by the ultrasound-assisted decomposition of the initiating species. This highly exothermic polymerization reaction can sustain a localized reaction front that propagates throughout an unstirred medium, as in frontal polymerization. Evidence for the controlled nature of thermal propagation alongside a gel process is presented by online 2D-infrared (IR) thermography monitoring. In addition, the effects of thermal propagation on the molar mass distributions are described. Specifically, the use of ultrasound not only suppresses the so-called "fingering" of frontal polymerization but also avoids any alteration of the molar mass distribution of the polymer.