Molecular Weight Control in Frontal Ring‐Opening Metathesis Polymerization

Abstract In this study, we push the limits of bulk frontal ring‐opening metathesis polymerization (FROMP) to control polymer molecular weight and dispersity without the need for solvents, deoxygenation, or post‐polymerization purification. By tuning inhibitor loadings and introducing inhibitory comonomers, we enable enhanced control over molecular weight (39–700 kg mol −1 ) and achieve dispersity as low as Ð = 1.07, surpassing previous limitations while preserving the thermal properties of the materials. This increased inhibition also leads to the first demonstration of closed‐mold, nonlinear propagation modes (i.e., spin modes) in FROMP of norbornene‐type monomers, where the reaction front spirals down the tube, revealing the intricate interplay between thermal transport, reaction kinetics, and gravitational effects. Furthermore, we constructed materials with gradient compositions and spatially defined variations in molecular weight, dispersity, composition, and patterning. These advances position controlled FROMP as a powerful tool for rapid, scalable, and self‐regulating material design, merging the fields of controlled polymerization and self‐propagating material manufacturing.