Synthesis, dynamic mechanical, and solid-state NMR spectroscopy of crown-ether-based linear aliphatic polyurethane rotaxanes

Abstract

Linear aliphatic polyurethanes were synthesized from hexamethylene diisocyanate (HDI) and either ethylene glycol (EG) or diethylene glycol (DEG). Polymerization of HDI and EG in a 30-crown-10 (30c10) solvent afforded poly[(ethylene hexamethylenediurethane) n -rotaxa-(30-crown-10) x ] with x/n = 0.02 (4% by mass). Polymerization of HDI and DEG in a 36-crown-12 (36c12) solvent provided poly[(oxydiethylene hexamethylenediurethane) n -rotaxa-(36-crown-12) x ] with x/n = 0.15 (23% by mass). For both polyurethane-rotaxanes, the melting temperatures are lower than those of the unthreaded backbones, and the glass temperature decreases when the threading efficiency is sufficiently high. The dynamic mechanical (DM) spectrum of the EG-30c10-containing polyrotaxane is identical to that of its unthreaded backbone. For both EG-based polymers, vacuum drying led to the disappearance of a sub-T g mechanical loss around -85 °C (1 Hz). The DM spectrum of the DEG-36cl2-containing polyrotaxane contains additional loss peaks in its tan δ versus temperature curves, as compared to its unthreaded backbone. Solid-state 2D WISE NMR revealed the new sub-T g loss process around -53 °C to be due to the onset of 36-crown-12 mobility in the polyrotaxane.