Synthesis and properties of block polypseudorotaxanes by threading cucurbit[6]uril

Abstract

A novel water-soluble block polypseudorotaxane (4) was synthesized in water from cucurbit[n]uril (n = 6), designated as CB[6], and a block copolymer of methoxy poly(ethylene glycol)-b-poly-[N 1-(4-vinylbenzyl)-pentane-1,5-diamine dihydrochloride]-b-methoxy poly(ethylene glycol) (P3). Driven by the hydrophobic/hydrophobic interaction and the association between the diamine and glycoluril units, CB[6] beads are localized on the pentamethylene units in the side chains of P3 as found by NMR studies. The degree of threading, i.e., the average number of CB[6] beads per repeat recognition unit of P3, can be controlled from 0.25 to 1.0 by varying the amount of CB[6] added. This molecular feature leads to interesting aggregation behavior of the polypseudorotaxanes in aqueous solutions, as revealed by dynamic light scattering measurements, transmission electron microscopy observations, ultraviolet/visible spectroscopy (UV–vis) and fluorescence spectrometry. The average hydrodynamic radius (R h), the intensity of UV–vis absorption band and the fluorescence intensity (I f) of the block polypseudorotaxanes in solution increase with the increasing of threaded CB[6]. With the solution temperature increasing, the size of the aggregates in water increased and the fluorescence intensity (I f) of the solution decreased, which shows the polypseudorotaxanes can sensitively respond to temperature changes. This opens the door for the application of the block polypseudorotaxanes in various fields such as drug delivery and controlled release.