Cooperative, bottom-up generation of rigid-rod nanostructures through dynamic polymer chemistry
Résumé
A set of carbazole- and benzene-derived di(aldehyde) and di(acylhydrazine) monomers containing hexaglyme groups to impart water solubility has been synthesized. Mixing a given di(aldehyde) and di(acylhydrazine) pair in acidic aqueous solution causes polymerizationthroughreversibleacylhydrazonecondensation.Thestructuresof theresultantamphiphilicpolyacylhydrazones have been studied using 1H NMR spectroscopy, matrix-assisted laser desorption ionization mass spectrometry, small-angle neutron scattering, transmission electron microscopy, size exclusion chromatography/multi-angle laser light scattering (SECMALLS) andUV-visibleandfluorescence spectrophotometries. All theavailabledatasupport theexistenceof structurallyrelated rod-like nanostructures of variable lengths and constant diameters of approximately 5 nm in all cases,which are interpreted as corresponding to individually folded polymer chains. On the basis of these studies, molecular models are proposed in which the hydrophobic, aromatic polymer backbones assume helical conformations allowing for hydrophobically driven π-stacking, while exposing the hydrophilic hexaglyme groups to the solvent. The molecular models are in agreement with the observed physical dimensions of the nanostructures, and are further supported by the observation of strong hypochromic effects on changing the solvent from dimethylformamide to water. Additionally, the reversible polymerization process is found to be cooperative. 1H NMR and SEC-MALLS studies reveal severe deviations from statistically predicted product distributions under imbalanced stoichiometry, which are characteristic of nucleation–elongation behaviour.
