Threading/Dethreading Exchange Rates as Structural Probes in Polypseudorotaxanes

Abstract

First, the complexation of rigid bis(paraquat)-based cyclophane host 1 with linear MDI-based polyurethane guests 2−5 prepared from tetra(ethylene glycol) was studied. Through detailed analysis of 1H NMR data, we have shown a relationship between the length of the linear polyurethanes and the fraction of all threaded cyclophanes rapidly exchanging with solution upon the 1H NMR time scale. In other words, the molecular weight of the polymer can be estimated from the proportion of cyclic threaded but exchanging rapidly, i.e., those near the ends, versus those that are threaded but exchanging slowly because they do not have ready access to the ends of the macromolecule; this is essentially an end group analysis. And then in a similar study of branched polyurethanes 7−11 derived from tetra(ethylene glycol) and glycerol, it was shown that the cyclophane can be used to determine the (accessible) fraction of the polymer between a branch point and a terminus and the average length between a branch point and a terminus. Third, inclusion of bisphenol A diethoxylate units in linear polyurethanes 12−16 provides effective barriers to room-temperature threading and dethreading, allowing a “slippage” method for polyrotaxane synthesis at elevated temperature (60 or 90 °C), yielding polyrotaxanes stable for extended periods in solution at room temperature.