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Stereochemical Effect of Trans/Cis Isomers on the Aqueous Solution Properties of Acid-Labile Thermoresponsive Polymers

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posted on 22.07.2008, 00:00 by Xiaonan Huang, Fusheng Du, Dehai Liang, Shrong-Shi Lin, Zichen Li
Two acid-labile, thermoresponsive poly(methacrylamide)s with the pendant cyclic orthoester moieties of trans and cis configurations, PtNEM and PcNEM, were synthesized via free radical polymerization of the corresponding trans and cis isomers of N-(2-ethoxy-1,3-dioxan-5-yl)methacrylamide (NEM). The thermally induced phase transition/separation behaviors of both polymers as well as the aqueous solution properties below and above their phase transition temperatures were investigated by means of turbidimetry, DSC, 1H NMR, microscopy, fluorescence probe, and dynamic light scattering. Both PtNEM and PcNEM showed aggregation behaviors below their respective LCSTs, and the former formed more hydrophobic microdomains which had greater capability to solvate pyrene molecules compared with PcNEM. These two polymers exhibited thermally induced sensitive and reversible phase transitions in aqueous solution. PtNEM showed a little lower cloud point but much greater phase transition enthalpy compared to PcNEM. The results of DSC, 1H NMR, and microscopy measurements revealed that PcNEM exhibited a liquid−liquid phase separation while PtNEM likely underwent a liquid−solid transition. Furthermore, the pH-dependent hydrolyses of both polymers were studied by the 1H NMR and turbidimetric approaches. The results indicated that both PtNEM and PcNEM showed acid-triggered hydrolysis behaviors, and the hydrolysis products were affected by the configurations of the pendant cyclic groups. On the basis of these results, we can conclude that the stereochemical structures of the pendant cyclic orthoester groups in these poly(methacrylamide)s greatly affect their aqueous solution properties as well as their hydrolysis behaviors.