Free-radical polymerization exhibits nonlinear phenomena, such as the Trommsdorff effect and thermal autocatalysis. A propagating front, an interface between reacted and unreacted zones, thus can be observed in the free-radical polymerization system. The frontal polymerization is usually started by attaching a mixture of monomer and initiator to an external heat source. In this chapter, we introduce that the propagating front can be generated at the center of the reaction system by the local accumulation of the heat of reaction. Polymerization of methyl methacrylate was carried out in a test tube in a thermostatic oil bath. Under optimum conditions, the front spontaneously generated at the center was propagated to complete the polymerization without forming bubbles. The phenomena might be an alternative to an interfacial gel polymerization for preparing functionally gradient materials.