Post-synthetic modification of polyvinyl alcohol with a series of N-alkyl-substituted carbamates towards thermo and CO2-responsive polymers

Abstract

Intensive efforts have been devoted to the synthesis of thermoresponsive polymers with terminal N-alkyl-substituted groups. Furthermore, an investigation into polymers with a lower critical solution temperature (LCST) controlled by CO₂ tuning is important to many applications; yet, this technology cannot advance without understanding the critical impact that N-alkyl-substituted groups has on the polymer. These polymers have been synthesized by polymerization of (co)monomers (e.g., acrylamide or methacrylate) with N-alkyl-substituted groups in a block or random configuration. Complicated synthesis procedures and unconfirmed biocompatibility may limit their desired properties. In this work, a general post-synthetic modification strategy is developed to functionalize poly(vinyl alcohol) (PVA) with a series of pendant alkyl or N-alkyl-substituted carbamates under facile conditions. The resulting PVA derivatives show excellent biocompatibility. The mechanism study shows that the LCST behavior of the PVA derivatives with pendant N-alkyl-substituted carbamates is attributed to the hydrogen bonding effect and hydrophobic interactions with terminal tertiary amines. However, a PVA derivative without terminal tertiary amines will likely exhibit globular conformation in solution, even at low temperatures (e.g. 15 °C). Heating induces slow agglomeration, but no changes in the hydrogen bonding occur. In addition to the conformation changes, heating can induce fluctuations in the pK_a of tertiary amines contained in PVA derivatives, which facilitates the CO₂ absorption/desorption cycles. Below the phase transition temperature, most amines in PVA derivative solutions are capable of being protonated in the presence of CO₂, resulting in a significant increase in the LCST. Above the phase transition temperature (70 °C), the hydrophilic-to-hydrophobic transition of PVA derivatives lowers the pK_a, thereby resulting in an almost complete desorption of CO₂ upon N₂ bubbling. This work demonstrates a general post-synthetic modification in the synthesis of responsive polymers, the significance of the alkyl amine motif in the solution behavior and the potential of thermoresponsive polymer solutions to capture CO₂.