Chelation-Induced Hierarchical Self-Assembly Of Flexible Polymers

The manipulation of nanoscale structural hierarchy/complexity is considerably attractive in the fields of supramolecular chemistry and nanoscience. Hierarchical and complex superstructures have been achieved via self-assembly driven by noncovalent interactions, including metal coordination, solvation effect and electrostatic interactions. The superstructures of small molecules can be achieved by coordination-induced self-assembly. However, coordination-induced hierarchical self-assembly of hydrophilic flexible polymers has not been reported to date. NH2-motifs in synthetic polymer can coordinate with first-row transition metal ions. Thus, the chelation can be introduced to explore protein-alike superstructures. In this thesis, we present the nanoscale structural hierarchy/complexity of flexible polymers via copper chelation-induced self-assembly（CCISA） in water. The factors and corresponding assembling pathway were intensively studied in this thesis.In view of amazing aqueous solubility, simplicity and versatile utility of aliphatic NH2-motifs, hydrophilic flexible block copolymers, including poly（2-hydroxypropylmethacrylamide）-block-poly（2-aminoethylmethacrylamide hydrochloride）(PHPMA₆₃-bPAEMA₆₄, ethylene spacer in AEMA unit), poly（2-hydroxypropylmethacrylamide）-blockpoly（6-aminohexylmethacrylamide hydrochloride）(PHPMA₆₃-b-PAHMA₆₄, hexylene spacer in AHMA unit) were synthesized by reversible addition-fragmentation chain transfer（RAFT） polymerization in aqueous solution under visible light irradiation at 25 oC. PAEMA/PAHMA acted as chelating block, and PHPMA block was utilized to stabilize hydrophobic domains assembled by CCISA. In both cases the polydispersity indices are rather low（PDI<1.12）.These copolymers were molecularly dissolved in water over the wide range of pH 4.7- 11.5. Upon increase of solution pH in the presence of stoichiometric copper ions, hydrophilic NH3+-block underwent simultaneously NH3+-to-NH2 transition and Cu（II）-NH2 coordination. Copper-chelation promoted the dehydration and thereby self-assembly of hydrophilic block copolymer under stabilization of hydrophilic PHPMA shell. The aqueous solubility and dehydration were examined by ^H NMR and dynamic light scattering（DLS）. Degree of chelation was investigated by UV-vis spectroscopy. The phase transition was tracked by DLS. Surface charge of as-assembled nanoparticles was examined by aqueous electrophoresis, using their zeta potential parameters as the indicatives. The morphologies were visualized using transmission electron microscopy（TEM） and atomic force microscopy（AFM）. The results demonstrated that the copper coordination and phase transition of PAHMA block took place at high solution pH, as compared to those of PAEMA block due to relatively long aliphatic spacer. Chelation in PAEMA led to formation of spherical micelles, colloidal networks and spherical micelles. In contrast, chelation in PAHMA resulted in formation of spherical micelles, colloidal networks and compartmentalized disks. All these results demonstrate that the nanoscale structural hierarchy/complexity of the hydrophilic flexible polymers can be achieved by deliberate control of the aliphatic spacer length and solution pH.Furthermore, pathways forward to hierarchically-ordered colloidal networks and multicompartmentalized disks were examined in this thesis. DLS results unveiled that the phase transition occurred instantaneously and proceeded in unambiguous stepwise manner. TEM and AFM results revealed a stepwise-growth mechanism, in which the polymer chains assembled into spherical micelles immediately, and interfacial chelation led to the supracolloidal self-assembly into three-dimensional supracolloidal networks. Nevertheless, the multicompartmentalized disks were achieved through instantaneous formation of cylindrical micelles, followed by lateral alignment of cylindrical micelles with selfcorrection and self-sorting behaviours, intermittent interfacial association of cylindrical subdomains, and ultimately into compartmentalized disk-like architectures. All these results demonstrate that CCISA underwent an instantaneous supramolecular self-assembly to supracolloidal assembly in a programmed manner.Therefore, by deliberate control of the aliphatic spacer length and solution pH, the nanoscale structural hierarchy/complexity of hydrophilic polymers has been achieved by CCISA, under supramolecular-to-supracolloidal stepwise-growth mechanism, into colloidal network and multicompartmentalized disk-like superstructures. This unique CCISA strategy underlines an amazing prospect forward to protein-alike hierarchy and complexity of hydrophilic flexible polymers in aqueous solution.