Poly(urethane/crown ether rotaxane)s with Solvent Switchable Microstructures

Poly(urethane/crown ether rotaxane)s 7c−g were prepared from bis(p-tert-butylphenyl)bis[p-(2-(2‘-hydroxyethoxy)ethoxy)phenyl]methane (diol BG 4), tetrakis(ethylene glycol) (5) and 4,4‘-methylenebis(phenyl isocyanate) (MDI) (6) using 30-crown-10 (30C10) as solvent. It was found that threading efficiencies (m/n value, average number of cyclics per repeat unit) of 7c−g increased with the feed proportion of diol BG 4. Compared to poly(ester rotaxane)s in which the crown ethers can freely move along the backbone, the dethreading rate during polymerization is slow due to the H-bonds between in-chain NH groups and threaded crown ethers. In poly(urethane rotaxane)s, crown ethers are localized at the NH sites by H-bonding in chloroform, locked translationally and perhaps rotationally, to afford a well-defined microstructure. However, the cyclics tend to be in the vicinity of BG units in DMSO, which disrupts the H-bonding. Induction of shuttling between the two sites is caused by the use of mixtures of DMSO and CHC...