Synthesis and self-assembly of thiophene-based all-conjugated amphiphilic diblock copolymers with a narrow molecular weight distribution

Abstract

A series of amphiphilic poly(3-hexylthiophene-b-3-(2-(2-{2-[2-(2-methoxy ethoxy)-ethoxy]-ethoxy}-ethyl))thiophene) (P(3HT-b-3EGT)) polymers was synthesized via a nickel-catalyzed quasi-living polymerization. Size exclusion chromatograms (SEC) revealed that the molecular weight distributions of the P3HT blocks in the block copolymers were comparable with those of the polystyrene standard (monodisperse). NMR spectra revealed that the P3HT and PEGT units in the block copolymers were well-defined and did not form compositionally mixed regions at the boundary between the blocks and the highly regioregular P3HT units. The correlations among the block ratio, the amphiphilicity, and the self-assembled nanostructures of the block copolymers in thin films and in solution were examined. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies revealed that the crystallinity of the BP93 composed of 93 mol % P3HT blocks was higher than the crystallinity of the P3HT alone due to the packing effects caused by repulsion among the hydrophobic hexyl and hydrophilic ethylene glycol oligomer side chains. A long relaxation time was required to observe the ordering among P3HT blocks in the BP26 composed of 26 mol % P3HT blocks, suggesting that self-assembly could occur if induced on the molecular level. We demonstrated that the molecular-level self-assembly of BP26 (at dilute concentrations) via a slow dialysis method produced highly ordered polymer vesicles 200-250 nm in size under thermodynamic control. The size could be tuned via competitive hydrophobic interactions using polystyrene. In contrast, kinetic control via a rapid precipitation method yielded 5-20 nm micelles.