Leveraging Molecular Weight Dispersity for Scalable, Precise Synthesis of Nanocylinders Via Quasi‐Polymerization‐Induced Self‐Assembly

The scalable and precise synthesis of polymeric nanostructures represents a significant challenge due to the inherent trade‐off between production efficiency and morphological control. Here, we introduce a quasi ‐polymerization‐induced self‐assembly ( quasi ‐PISA) strategy that integrates in situ nucleation‐growth kinetics to fabricate uniform cylindrical micelles from liquid crystalline block copolymers at high concentrations (up to 10 wt.%). Counterintuitively, we demonstrate that the broad molecular weight distribution of PISA, often considered a drawback, can be harnessed as a key variable for precise assembly. By modulating polymerization conditions, fibril lengths were well controlled from 100 nm to over 3 µm (with length dispersity < 1.10) without requiring pre‐formed seeds. This enables the one‐pot synthesis of triblock comicelles and gram‐scale production. Rheological analysis revealed a scaling relationship between solution viscosity and fibril length ( η ∝ L 0.667 ). This work advances the scalable fabrication of length‐controlled cylindrical nanostructures, offering design principles for their practical applications in diverse fields.