Multi-responsive γ-methylene-γ-butyrolactone/N-vinyl caprolactam copolymers involving pH-dependent reversible lactonization

Abstract

Poly(N-vinyl caprolactam) is one of the most popular thermo-responsive polymers used in the biomedical field. For application purposes, its transition temperature can be adjusted by copolymerizing N-vinyl caprolactam with comonomers of various hydrophilicity levels. Insertion of carboxylic acid containing vinyl monomers in PNVCL also imparts some pH-responsiveness to the latter, although, at low pH, H-bonding between the protonated carboxylic groups and the amide functions of NVCL induces aggregation phenomena. The present work describes a unique multi-responsive PNVCL system based on γ-methylene-γ-butyrolactone as a comonomer involving a peculiar pH-dependent reversible lactonization reaction. A series of narrowly distributed statistical NVCL/γMγBL-based copolymers with different molar masses and compositions were prepared via organometallic-mediated radical polymerization. Hydrolysis of the pendant lactones led to the corresponding hydroxy acid-containing copolymers allowing the tuning of the cloud point temperature of the polymer solution over a wide range of temperatures. In contrast to the previously reported acid-bearing PNVCL derivatives, acidification of the hydroxy carboxylate containing NVCL-based copolymers induced the lactonization reaction leading to the regeneration of the starting NVCL/γMγBL-based copolymers, which ultimately prevents aggregation via hydrogen bonding. The synthesis and responsiveness of NVCL/γMγBL-based block copolymers were also considered and discussed. Overall, this unusual multi-responsive methylene lactone-based system constitutes a promising platform for tuning the solution properties of PNVCL, and possibly of other smart polymer sequences, opening new application prospects.