Aqueous Solution Properties of the Acid-Labile Thermoresponsive Poly(meth)acrylamides with Pendent Cyclic Orthoester Groups

Abstract

A series of poly(meth)acrylamide derivatives with pendent six-member cyclic orthoester groups, i.e., poly(N-(2-alkyloxy-1,3-dioxan-5-yl)methacrylamide)s and poly(N-(2-alkyloxy-1,3-dioxan-5-yl)acrylamide)s, have been synthesized and characterized. The difference between these polymers lies in the type of alkyl substitutes (R3), the stereochemical structures of the pendent cyclic orthoester groups (trans vs cis), and the main chain structures (polymethacrylamide vs polyacrylamide). Aqueous solution properties and pH-dependent hydrolysis behaviors of these polymers were studied by various methods including turbidimetry, fluorescence probe, DSC, 1H NMR, microscopy, and light scattering. The results show that these polymers except PtNPM can be dissolved in water at low temperature, and all of the water-soluble polymers are thermosensitive with different lower critical solution temperatures (LCSTs) and susceptible to hydrolysis in mildly acidic conditions. Both thermosensitive properties and acid-triggered hydrolysis behaviors of the polymers are closely related to the polymer structures. In general, polymethacrylamides display higher cloud points (CPs) than polyacrylamides. In addition, the polymers with larger R3 and trans configuration have a lower CP and greater magnitude of dehydration and exhibit a liquid−solid phase transition, while those with smaller R3 and cis configuration have a smaller magnitude of dehydration and undergo a liquid−liquid phase separation. In addition, a two-stage transition process is observed for the polymers with R3 being methyl. 1H NMR results reveal that the acid-triggered hydrolysis rate of the pendent orthoesters increases as R3 changed from methyl to isopropyl, and the configuration changed from cis to trans. The synergetic effect of R3 and stereochemical structure of the pendent groups on the hydrolysis products of the polymers were also observed