Polyrotaxanes urethane-crown ether

Beckham and coworkers studied the dynamic mechanical properties of poly(urethane-crown ether rotaxane)s [138]. No difference was observed between the backbone and polyrotaxane, probably because of the low min value (0.02). However, 13C solid-state NMR detected die presence of the crown ether as a mobile structure at room temperature. The same observation was seen in polyrotaxanes with ether sulfone and ether ketone backbones (77-80) [114]. Although no detailed properties were reported, the detection of the liquid-like crown ether provided very important information in terms of mechanical properties, because these properties are the result of molecular response to external forces. For example, mobile crown ethers can play the role of plasticizers and thus improve impact strength. 

Similar to that in copoly(ester rotaxane)s 64, min for these poly(urethane rotax-ane)s increased with increasing BG, i.e. higher x values [116,117]. However, compared with the copoly(ester rotaxane), the dethreading occurred to lesser extent in these polyrotaxanes this is attributed to the fact that the NH groups retard dethreading by hydrogen bonding with the threaded crown ether as in structure 67. A linear relationship between min values and x was revealed. 

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