Thermal Switching of Polymer Topology Enables Programmable Mechanical Properties in Soft Materials

Abstract Soft materials with on‐demand mechanical tunability remain challenging to realize, particularly those capable of large, reversible, and programmable changes within a single material system. In this work, a synthetic elastomer is designed that undergoes thermally reversible topological network reconfiguration, switching between brush‐ and linear‐like architectures, thereby enabling a reversible transition from soft to stiff mechanical states. This reconfiguration is achieved by grafting crystallizable side chains onto a polymer backbone via Diels‐Alder (DA) adducts at low annealing temperatures to form brush‐like networks, while retro‐DA reactions at higher temperatures release the side chains, yielding a linear topology. The brush architecture suppresses crystallization, whereas the linear form facilitates crystallinity to form an additional crystalline framework, leading to a reversible rubbery‐to‐glassy transition. As a result, the elastomers undergoing annealing cycles between 60 and 130 °C exhibit reversible enhancements in stiffness and strength by up to 286‐fold and 25‐fold, respectively. Coarse‐grained molecular dynamics (CGMD) simulations reveal that the significantly improved stiffness and strength originate from the formation of a crystalline framework that effectively bears mechanical load and impedes crack propagation. This thermally programmable strategy enables dynamic control of mechanical behavior, offering a novel paradigm for designing intelligent materials with tailored and on‐demand performance.