Well-defined star-shaped rod-coil diblock copolymers as a new class of unimolecular micelles: Encapsulation of guests and thermoresponsive phase transition

摘要

A series of star-shaped rod-coil diblock copolymers composed of poly(arylene ether sulfone) (PAES) as a core, and poly[2-(2-methoxyethoxy)ethyl methacrylate-co-oligo(ethylene glycol) methacrylate] [poly(MEO _2MA-co-OEGMA)] as a shell was synthesized by combination of chain-growth condensation polymerization (CGCP) and atom transfer radical polymerization (ATRP). In the presence of 4-arm initiator (1), CGCP of 4-fluoro-4′-hydroxydiphenyl sulfone potassium salt (2) via nucleophilic aromatic substitution (S_NAr) reaction yielded a well-defined star-shaped polymer with four fluorine end groups (C5-F and C8-F). Transformation of the fluorine groups into ATRP initiating sites produced a macroinitiator having four bromine groups (C5-Br and C8-Br). In the presence of the macroinitiator, random copolymerization of MEO_2MA and OEGMA via ATRP formed thermoresponsive shell blocks to produce a desired star-shaped PAES-b-poly(MEO_2MA-co-OEGMA) block copolymer with various block lengths. Dynamic light scattering (DLS) of the star-shaped diblock copolymers in aqueous solutions revealed that hydrodynamic diameters (D_h) of the polymers decreases significantly by addition of Nile Red due to the transition from polymeric aggregates to unimolecular micelles. Fluorescence spectroscopy confirmed that the polymers behave as unimolecular micelles in the encapsulation of Nile Red, but another transition into multimolecular polymer micelles was observed by UV/vis spectroscopy when the excess amount of Nile Red was used. Turbidity measurements of the polymer solutions indicated that unimolecular micellar state was necessary to exhibit lower critical solution temperature (LCST) if the shell length was relatively short compared to the core size. Macroscopic aggregation was observed above LCST, and removal of encapsulated guests from water was demonstrated by simple filtration.