Facile synthesis and properties of thioamide-containing polymers

Abstract

Polythioamide, a variant of polyamide in which the carbonyl oxygen atom is replaced with sulfur, shows stronger metal affinity, weaker hydrogen-bonding interactions with water, and higher refractive indices in comparison with its counterpart polyamide. The development of efficient, versatile and economical strategies to prepare polythioamides along with gaining much deeper insights into their structure–property relationships, is crucial for creating new polythioamide-based functional materials. In this work, we describe a convenient and efficient platform to generate a variety of polythioamides. The commodity polymers poly(N-isopropylacrylamide) (PNIPAM), poly(N,N-dimethylacrylamide) (PDMA), poly(vinylpyrrolidone) (PVP) and polyurethane (PU), which have high-volume and low-cost production, can be readily thionated using commercially available Lawesson reagent to convert the amides to thioamides in a controlled manner with negligible chain degradation or crosslinking. For thionated PNIPAM, a higher level of thionation leads to a higher glass transition temperature and a lower decomposition temperature. The thionation of PNIPAM decreases its solubility in water. The LCST of thionated PNIPAM with 16% thioamide content decreases to 13 °C, and thionated PNIPAM with ≥44% thioamide content cannot be dissolved in water at temperatures as low as 0 °C. The addition of the metal ions Ag⁺, AuCl₄⁻ and Hg²⁺ increases the solubility of thionated PNIPAM owing to the strong coordinating capacity of thioamides toward Ag⁺, AuCl₄⁻ and Hg²⁺. Metal nanoparticles, for example, Ag and Au nanoparticles, can be embedded in thionated PU foams, which can be employed as reusable catalysts for the reduction of 4-nitrophenol to 4-aminophenol. This study provides an efficient and versatile platform and important guidelines for developing thioamide-containing polymers for diverse applications.