Effect of oligo(ethylene glycol) length on properties of poly(oligoethylene glycol terephthalate)s and their cyclic oligomers

We present here the effect of oligo(ethylene glycol) (OEG) segmental length on properties of cyclic oligo(oli-goethylene glycol terephthalate)s (COEGTs) and their corresponding polymer counterparts, poly(oligoethylene glycol terephthalate)s (POEGTs). COEGT monomers were synthesized from terephthaloyl chloride with corre-sponding diethylene glycol, triethylene glycol, and tetraethylene glycol under pseudo-high-dilution conditions. As those large ring cyclic monomers are hard to be polymerized via traditional ring-opening polymerization, a cascade polycondensation-coupling ring-opening polymerization method is utilized to obtain high molecular weight POEGTs. The cascade polymerization mechanism is confirmed by the kinetics study. The structures of COEGTs and POEGTs were well characterized and their properties were studied to reveal the structure-property relationship. With the increment of OEG length in repeating units, the chain flexibility of POEGT and COEGTs is enhanced. As a result, the melting temperatures of COEGTs and glass transition temperatures of POEGTs decrease. On the other hand, the thermal degradation temperature of POEGTs and COEGTs increases due to the enhanced ether fraction and reduced ring strain. Solid-state polymer electrolytes based on POEGTs was prepared by blending with lithium bis(trifluoromethanesulfonyl)imide. The measured ionic conductivity increased with the OEG length in POEGT repeating units. Our results revealed the effect of OEG length on the structure-property relationship in cyclic oligo(ether ester)s and copoly(ether ester)s, providing a facile way to tune their properties.