Synthesis of a Cylindrical Micelle from Hydrophilic Polymers Connected with a Single Supramolecular Structure-Directing Unit

Abstract

This article reports the facile synthesis of a cylindrical micelle from mostly hydrophilic polymers by a specific molecular interaction among a hydrophobic single supramolecular structure-directing unit (SSDU), appended at the chain terminal. The SSDU contains a naphthalene-diimide (NDI) chromophore, an amide group, and a hydrophobic wedge. H bonding, π stacking, and hydrophobic interaction among the SSDU lead to the formation of entropy-driven aggregates for appended hydrophilic polymers including one dendronized polymer. In water, the freshly prepared solution exhibits a spherical morphology, which gradually transforms into the cylindrical micelle for only one tested polymer having oligo-oxyethylene pendant chains. But no such transformation was noticed for the other two polymers having pendant hydroxyl groups. In the presence of 10% good solvent such as tetrahydrofuran (THF), the rate of such morphology transformation could be significantly enhanced for all of the tested polymers producing long (>5 μm) cylindrical micelles. The rate was inversely proportional to the sample concentration, indicating that the initially formed spherical species was not an intermediate but an off-pathway aggregate. In the presence of a good solvent, the mole fraction of the off-pathway aggregate reduced marginally and increased unimer population, which facilitated the nucleation for the thermodynamic product (cylindrical micelle). Isothermal titration calorimetry studies revealed the disassembly of the initially formed micelle upon dilution, but no such effect was noticed for the cylindrical micelle in the tested concentration window, suggesting enhanced stability. Noncovalent encapsulation stability, probed by fluorescence resonance energy transfer (FRET) studies, revealed fast chain exchange by the expulsion/insertion mechanism for the off-pathway spherical aggregate. In contrast, the average lifetime increased significantly for the cylindrical micelle with predominant splitting/merging of the micelle mechanism for chain exchange, similar to amphiphilic block copolymers, although in the present system, the hydrophobic SSDU was merely 5–8 wt %.