Controlled polymerization and self-assembly of poly(arylene ether sulfone)s

Abstract

This thesis deals with controlled polymerization and self-assembly of poly(arylene ether)s. Well-defined polymers based on poly(arylene ether sulfone)s with various compositions and architectures including rod-coil star-shaped block copolymers, core-functionalized multi-arm polymers, and sulfonated anionic random copolymers, were synthesized via chain-growth condensation polymerization ($S_NAr$) and/or atom transfer radical polymerization. Architectural variation of polymers was attempted to produce unimolecular micelles, supramolcular fibers, and anionic surfactants. Firstly, 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, CGCP of 4-fluoro-4’-hydroxydiphenyl sulfone potassium salt via nucleophilic aromatic substitution ($S_NAr$) reaction yielded a well-defined star-shaped polymer with four fluorine end groups (C5-F and C8-F). Trans-formation 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 mi-celles in the encapsulation of Nile Red, but another transition into multimolecular polymer micelles was ob-served 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 solu-tion 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. Secondly, a series of well-defined poly(arylene ether sulfone)s were synthesized (1P-4P) by chain-growth condensation polymerization with amide initiators having various numbers of initiating sites. Differential Scanning Calorimetry (DSC) study of the polymers revealed that branched polymers (3P and 4P) had higher glass transition temperatures ($T_g$) than linear polymers (1P and 2P) when they had the identical concentration of end groups. However, the viscosity of the polymers decreases as the number of branches increase due to the change of the hydrodynamic volume. Interestingly physical gelation of THF solution of these polymers was observed. Analyses of the self-assembled structure by FE-SEM, FE-TEM, temperature-dependent $ ^{1}H NMR$, FT-IR, and XRD indicated that the formation of fibrillar network was driven by the hydrogen bonding of aromatic amides. Thirdly, a new class of fluorescent organogelators, pyrene-containing poly(arylene ether sulfone)s, were synthesized via chain-growth condensation polymerization, and their gelation behavior and fluorescence properties were investigated. Interestingly, two fluorescence switching modes were observed in different gela-tion solvents. The gel formed from the THF solution exhibited excimer emission due to dimerization of the pyrene groups inside the fibrous structure. In contrast, emission due to the pyrene excimer was quenched after gelation in MC as a result of stacking among the pyrene groups, facilitated by strong intermolecular hydrogen bonds among the amide groups. Finally, a series of partially sulfonated poly(arylene ether sulfone)s synthesized via $S_NAr$ polycondensation was utilized as anionic surfactants for not only etching copper cation from Cu metal, but also capture ele-mental sulfur in an aqueous solution. $Cu_2S$ nanowires were grown on the surface of substrates, such as hydro-phobic silicon (Si) wafers and carbonious substrates, by the reaction between copper cation and sulfur in an aqueous solution. Interestingly, limited conditions such as no free Cu (II) cation or little sulfur contents can be overcome by an aqueous solution of polymers. Crystal structure of $Cu_2S$ nanowires was confirmed as hexagonal-phase chalcocite by TEM analysis.