Self-Assembly Drives Rare-Earth Separations: M4L4 Cages and Polyamide Ionomer Networks

Abstract

Self-assembly is a spontaneous process that offers a general design principle for creating functional materials. It occurs not only in macromolecules, but also in small molecules and metal ions. Building on this principle, this thesis develops self-assembled platforms for rare-earth (RE) separations spanning two classes: discrete metal–organic cages and sulfonated polyamide ionomers.First, structure–property relationships in sulfonated polyamide ionomers, strictly alternating polar block and nonpolar blocks with exact space along polymer chains, were established. Increasing the aliphatic-amide content enables the amides hydrogen bonding to sulfonate motifs, suppresses ion segregation and crystallization, and thereby enhances solubility in water and methanol. The follow-up study on linear sulfonated ionomers with rare-earth bindings sites demonstrates room-temperature removal of rare-earth ions from water, underscoring the promise of ionomers for facile rare-earth removal. Second, a library of tritopic, tridentate ligands (L) that assemble RE3+ into M4L4 cages enabled studies of self-sorting and the creation of sorbent networks. Substituent studies on ligands reveal that rigidity or length of substituents modulate their cage formation abilities. In binary feeds, homometallic or heterometallic cages preferentially form with the smaller ions, with selectivity modulated by the ratios of metal ions and cage formation abilities of ligands. Thiol–ene coupling on pendant alkenes of polymerizable cages yields insoluble cage networks that can be demetallated to produce templated scaffolds, retaining cage-derived binding pockets and cooperative interactions from locked tritopic nodes. Comparative tests show that the free tritopic ligand outperforms a monotopic control ligand and L-embedded polyamide networks, consistent with solution-phase self-assembly and intrinsic self-sorting into homometallic cages. Together, these findings outline a modular, self-assembly-driven strategy for tunable RE separations.