Catechol-Containing Poly(2-isopropyl-2-oxazoline): Synthesis and Thermoresponsive Behavior in Aqueous Salt Solutions

Catechol-containing poly(2-isopropyl-2-oxazoline)s (PiPOx) were obtained either by functionalization of partially hydrolyzed PiPOx with 3,4-dimethoxybenzoyl chloride or by microwave-assisted cationic ring-opening copolymerization of 2-isopropyl-2-oxazoline with 2-(3,4-dimethoxyphenyl)-2-oxazoline, followed by quantitative deprotection of methylated catechols. All synthesized polymers were soluble in aqueous salt solutions at room temperature and showed thermoresponsive LCST behavior. The cloud point temperature (T_CP) remained virtually unchanged in the presence of different cations, while it was strongly affected by chaotropic and kosmotropic anions, which is in line with the Hofmeister series. Intriguingly, T_CP was lower for the catechol-containing PiPOx as compared to the protected precursor copolymer and PiPOx, which is rationalized by hydrogen bonding interactions between catechol and tertiary amide units of the polymer backbone, as indicated by Raman spectroscopy, decreasing the hydration level of the polymer.