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 NH2), 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 (Tm(TCD-COOH) = 56°C, Tm(TCD-OH) = 51°C, Tm(TCD-NH2) = 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): Tmax(TCD-COOH) = 65°C, Tmax(TCD-OH) = 45°C, and Tma x(TCD-NH2) = 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.