Diacetylene monomers are known to undergo solid-state 1,4-addition polymerization when their crystal packing satisfies strict geometric criteria; however, the influence of bulky terminal protecting groups on the lattice adjustments required for bond formation remains insufficiently understood. Here, we synthesized amide derivatives of 2,4-hexadiyne-1,6-diamine, crystallized them via antisolvent vapor diffusion, and evaluated their thermal and photochemical reactivity. Single-crystal analysis shows that Boc-protected monomers (Boc-DA) form hydrogen-bond-directed parallel stacks that align diyne units in geometries nominally consistent with topochemical polymerization, yet they exhibit negligible photoreactivity under ambient UV irradiation. Structural inspection indicates that steric congestion from the tert-butoxycarbonyl termini restricts the subtle axial contraction and molecular shifts required for bond formation. Reducing steric bulk or applying combined thermal and photochemical activation enables polymerization of these diacetylenes. These findings demonstrate that globally favorable packing arrangements can coexist with local steric barriers that impose kinetic constraints on reactivity. Modulating terminal-group size and applying multimodal activation therefore provide a simple and tunable strategy to control diacetylene polymerization, offering design principles for switchable polydiacetylene materials in crystal engineering.
Ding et al. (Mon,) studied this question.