Degradable Thermoresponsive Polyesters by Atom Transfer Radical Polyaddition and Click Chemistry

Three types of linear polyesters containing monodisperse methoxy oligo(ethylene glycol)s (mOEG, x = 6, 7, or 8) side chains (PI-mOEG, P2-mOEG, and P3-mOEG) have been synthesized by atom transfer radical polyaddition and click chemistry. Degradable disulfide and ketal groups were incorporated into the polymer backbone of P2-mOEG and P3-mOEG respectively. All of these three series of mOEG-grafted polyesters are water-soluble and display reversible thermoresponsive properties with tunable cloud point (CP). Aqueous solution properties and thermo-induced phase transitions were studied by N-1 NMR, turbidimetry, DLS, and fluorescence probe method. The results indicate that these polyesters mainly adopt single chain conformations in aqueous solution below CP. OEG length has a significant effect on the CP, the CP increases by ca. 10 degrees C when the repeating units of OEG side chains increase from 6 to 7, and by ca. 6 degrees C when they increase from 7 to 8. The main chain structures also affect the CP values, which decrease from PI-mOEG to P3-mOEG due to the increased hydrophobicity of the backbone. Degradation of the polymers was conducted under basic, reductive and mild acidic conditions, respectively. The degradation products also show thermoresponsive behaviors, but the CP values vary from the precursor polymers depending on the alteration of hydrophilic/hydrophobic balance and shielding effect of the OEG side chain.