Polymerization as a structuring strategy for hierarchical order and function

Abstract Polymerization has long been used as a tool for structural fixation: liquid or soft precursors are converted into solids. Recent advances in polymerization-based processing technologies have greatly expanded this role, now allowing complex external architectures to be inscribed from submicrometer to macroscopic length scales, thereby contributing to the maturation of polymer mechanical metamaterials. Importantly, the field is moving beyond ideal periodic architectures toward disordered design, in which irregularity and heterogeneity are increasingly treated as programmable design variables rather than fabrication errors. This trend resonates with the history of polymer materials because polymers intrinsically contain hidden internal architectures, including entanglements, network topology, phase-separated domains, and molecular alignment, that govern mechanical functions such as deformation, fracture, fatigue, and energy dissipation. Polymer science is therefore entering a stage in which external geometry and internal architecture can be considered together across hierarchical length scales. In this Focus Review, we discuss polymerization as a hierarchical structuring strategy that not only fixes predesigned external geometry but also actively generates internal architectures with controlled heterogeneity through chemical reaction, molecular diffusion, flow, alignment, and arrest. We highlight the importance of spatiotemporal polymerization fields for hierarchical structuring and focus on both frontal polymerization and scanning wave photopolymerization (SWaP) as representative approaches based on localized and moving reaction fields. This perspective suggests that the next step in polymer material design is to treat polymerization not only as a chemical platform for solidification but also as a programmable field for inscribing hierarchical order and emergent function.