Postpolymerization Modification of Poly(2-isopropenyl-2-oxazoline) with Thiols: Scope and Solvent Effects

Poly(2-isopropenyl-2-oxazoline) (PIPOx) has emerged as a unique postpolymerization modification platform; however, only the relatively slow PIPOx reactions with carboxylic acids are currently routinely exploited. Here, we provide the first comprehensive investigation into PIPOx reactivity with different aliphatic and aromatic thiols. We revealed that these reactions are dramatically accelerated in water, affording high degrees of modification (DM) faster and/or under milder reaction conditions than in organic solvents. This was particularly important for the generally less-reactive aliphatic thiols, where we demonstrated high DM values for the first time. Aromatic thiols were found to be comparatively more reactive, with reactivity differences ascribed largely to the thiol acidity governed by substitution effects. Besides enhancing the reaction rate, the aqueous medium also greatly simplified product isolation in many cases. Furthermore, a precise stoichiometric control over DM, enabling precision tailoring of (co)polymer composition, was manifested for model thiols. Finally, we exemplified how thiol reactions can be used for the rapid and clean incorporation of a range of useful functionalities into the parent polymer, imparting important functional properties or creating reactive sites for further polymer modifications, including cross-coupling and para-fluoro-thiol click reactions. Our findings extend significantly the PIPOx-based postpolymerization modification platform.