Multi-responsive γ-Methylene-γ-Butyrolactone/<i>N</i>-Vinyl Caprolactam Copolymers Involving pH-depend Reversible Lactonization

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. 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 ?-methylene-?-butyrolactone as comonomer involving a peculiar pH-depend reversible lactonization reaction. A series of narrowly distributed statistical NVCL/?M?BL-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 to tune the cloud point temperature of the polymer solution over a wide range of temperature. 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/?M?BL-based copolymers, which ultimately prevents aggregation via hydrogen bonding. The synthesis and responsiveness of NVCL/?M?BL-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 applicative prospects.