Hierarchical polymer architectures governed by diffusion–reaction competition Decoupling morphology, packing and density in covalent systems

<title>Abstract</title> <p>Understanding how molecular systems encode higher-order structure remains a central challenge in chemistry. Here, we show that hierarchical architectures in covalent polymers are governed by the competition between reaction kinetics and mass transport, captured by an effective Damköhler number. Using phase-transition polymerization of C₃-symmetric monomers as a model system, we observe a continuous transition from lamellar flakes to solid spherical architectures with increasing reaction temperature. Cross-sectional electron microscopy reveals that these apparently hollow-like particles are in fact fully solid, yet exhibit internal heterogeneity with a radially heterogeneous outer region and inner core. Despite pronounced morphological evolution, X-ray diffraction and density measurements show that molecular packing and bulk density remain essentially invariant, demonstrating a clear decoupling between morphology, packing and density. Extending this analysis across chemically diverse monomer systems, we find that structural dimensionality can be consistently rationalized within an effective Damköhler-number framework rather than by chemical identity alone. These results establish a general diffusion–reaction framework for polymer assembly, in which chemical interactions are translated into structure through kinetic–transport competition. This work provides a conceptual basis for rationalizing and guiding the design of hierarchical polymer architectures by controlling the interplay between reaction and diffusion.</p>