Mapping Nanoscale Order–Disorder Transitions to Optimal Topochemical Polymerization Across Alkyl Diacetylene Monolayers

On‐surface reactions within self‐assembled molecular networks offer a powerful strategy for nanoscale interface design, but must balance molecular layer stability with the local dynamics required for bond formation. Here, we develop a framework for identifying optimal reaction conditions in the topochemical polymerization of diacetylene monolayers on 2D materials. By systematically varying H‐bonding headgroup chemistry (COOH, OH, and NH 2 ), we examine how bulk and surface‐confined structural transitions can be used to establish a window of optimal on‐surface polymerization efficiency. Combining temperature‐dependent polymerization measurements with atomic force microscopy, differential scanning calorimetry, and molecular dynamics simulations, we relate bulk melting measurements ( T m (TCD‐COOH) = 56°C, T m (TCD‐OH) = 51°C, T m (TCD‐NH 2 ) = 26°C), to observed surface‐confined structural transitions. Across all monomers, polymerization efficiency increases above room temperature with a maximum near the onset of surface disorder or near solid–solid transitions that alter distances between bond‐forming carbons (e.g., TCD‐OH herringbone phases): T max (TCD‐COOH) = 65°C, T max (TCD‐OH) = 45°C, and T ma x (TCD‐NH 2 ) = 45°C. These results demonstrate the relationship between optimal on‐surface reactivity and pre‐melting increases in alkyl chain mobility, establishing a broader strategy for maximizing on‐surface topochemical polymerization efficiency.