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

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.