Reversible Thermoresponsive Host–Guest Binding of Pillar[n]arenes with Guest End-Functionalized Poly(2-n-Propyl-2-Oxazoline)s

Abstract

Host–guest based thermoresponsive supramolecular polymer assemblies have found significant interest in recent years as they provide unique opportunities for the reversible release of the guest upon thermally controlled collapse of the polymer. However, the reported systems are mostly based on polyacrylamides and polyacrylates in combination with the positively charged cyclobis(paraquat-p-phenylene) host. To investigate whether such thermally induced guest release can be more generally developed, here we investigated the thermoreversible host–guest association of guest-functionalized poly(2-n-propyl-2-oxazoline) (PⁿPrOx) with various pillar[n]arene hosts. Two types of thermoresponsive poly(2-n-propyl-2-oxazoline) (PⁿPrOx) consisting of a hydrophilic 4,4′-bipyridinium unit (PⁿPrOx-MV) or a 5-aminopentanenitrile unit (PⁿPrOx-CN; introduced as novel guest for percarboxylato pillar[5/6]arenes) as the end-group were prepared. The complexation of these guest-functionalized PⁿPrOx with water-soluble pillar[5/6]arenes containing anionic carboxylate groups (WP5 and WP6) or nonionic methoxytriethylene glycol (TP5 and TP6) solubilizing groups was investigated in water. The 1:1 host–guest complexes, i.e., WP6–PⁿPrOx-MV and WP5–PⁿPrOx-CN, were formed by electrostatic interactions at 283.15 K with the association constants of (3.84 ± 0.25) × 10⁵ M^–1 and (0.96 ± 0.04) × 10⁴ M^–1, respectively. In contrast, the nonionic TP5 and TP6 only showed weak binding with the polymers. When heated above its cloud-point temperature, PⁿPrOx collapses and precipitates due to the effect of lower critical solution temperature (LCST), which is demonstrated to lead to host–guest dissociation, providing thermal control over the complexation and decomplexation of the WP6–PⁿPrOx-MV and WP5–PⁿPrOx-CN host–guest complexes. The reversible, temperature-controlled threading and dethreading process suggest potential applications in sustainable separation and recovery processes, as well as in the development of smart materials with temperature-responsive functionalities.