Frontal polymerization (FP) is a process in which a front propagates in a localized reaction zone, converting monomer into polymer through the coupling of thermal diffusion with the Arrhenius kinetics of an exothermic reaction. Fillers are added to control the rheological properties of the formulation and to enhance the mechanical properties of the product. However, the thermal and chemical effects of these fillers on the front propagation have not been thoroughly explored. Herein we report the thermal and chemical effects of fillers on free‐radical frontal polymerization. It was found that fillers with high thermal diffusivities, such as milled carbon fiber and boron nitride increased the front velocity. Despite their high thermal diffusivities, fillers such as aluminum and alumina decreased the front velocity. This is likely due to the radical‐scavenging ability of aluminum oxide, which was explored with clay minerals. It was found that the presence of water within clay fillers can also decrease the front velocity. To probe the chemical effects, acid‐activated clay minerals were utilized. The results demonstrate that some fillers can increase front velocity through their high thermal diffusivities while others decrease it by acting as radical scavengers.

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