Chain-Length-Dependent Hydrogen-Bonded Self-Assembly of Terminally Functionalized Discrete Polyketones

Here, we report the synthesis and the chain-length-dependent self-assembling behaviors of discrete di-, tetra-, and hexaketones terminally functionalized with hydrogen-bonding carboxyl (C1, C2, and C3) and 3-acylaminopyridine groups (P1, P2, and P3). These polyketones were prepared by the coupling reactions of silylated analogues of 3,3-dimethylpentane-2,4-dione and t-butyl 2,2-dimethyl-3-oxobutanoate and the subsequent hydrolysis or amidation with 3-aminopyridine. Single-crystal X-ray diffraction analysis revealed that C1 and C2 form helical assemblies in which the components are connected by the dimerization of terminal carboxyl groups, whereas the longer C3 showed infinite hydrogen-bonded chains mediated by 1,4-dioxane used as a crystallization solvent. Pyridine-terminated P1 exhibited a three-dimensional hydrogen-bonded network owing to multiple NH<middle dot><middle dot><middle dot>N(pyridine) hydrogen bonds in the solid state. P2 generated a double-helix-like fiber structure in the crystalline state. Among the pyridine-terminated polyketones P1-P3, only P2 showed gelation behavior in chloroform (100 mM concentration) at 25 degrees C. The scanning electron microscopy measurement of xerogel P2 revealed the formation of rod-like structures with a thickness of approximately 0.5-3.5 mu m. These results demonstrate that the precise control of the polyketone chain length can significantly alter hydrogen-bonded self-assembly in the solid state and in solution even with the same terminal structures.