Well-Defined Star-Shaped Rod−Coil Diblock Copolymers as a New Class of Unimolecular Micelles: Encapsulation of Guests and Thermoresponsive Phase Transition

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• 作者：Jeyoung Park ; Mihee Moon ; Myungeun Seo ; Hyungsam Choi ; Sang Youl Kim
• 刊名：Macromolecules
• 出版年：2010
• 出版时间：October 12, 2010
• 年：2010
• 卷：43
• 期：19
• 页码：8304-8313
• 全文大小：1066K
• 年卷期：v.43,no.19(October 12, 2010)
• ISSN：1520-5835

文摘

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(MEO₂MA-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 MEO₂MA and OEGMA via ATRP formed thermoresponsive shell blocks to produce a desired star-shaped PAES-b-poly(MEO₂MA-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.