Thermoresponsive polymers that undergo Ca 2+ -induced phase transitions offer a versatile platform for designing tunable stimuli-responsive materials. In the present study, we investigated the influence of alkyl spacer length on the thermoresponsive behavior of phthalamic acid-functionalized poly( N -aminoalkyl acrylamide) (Pht-PAAAm) polymers bearing ethylene, butylene, or hexylene spacers. The polymers only exhibited lower critical solution temperature-type transitions in the presence of Ca 2+ ions. However, their cloud points did not follow the hydrophobicity trends expected from spacer length. Model compounds were analyzed using temperature-dependent 13 C NMR, which revealed that only the ethylene spacer forms intramolecular hydrogen bonds between adjacent amide groups. This unique intramolecular hydrogen bonding accounts for the nonmonotonous cloud-point behavior observed for the ethylene-spacer polymer. These findings highlight the critical role of side-chain architecture in modulating ion-responsive thermoresponsive properties.
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