Fluorescent Fluorenylmethoxycarbonyl-Protected Serine-Derived Homo- and Copolyacrylates with UCST/LCST Behavior: A Platform for Smart Responsive Materials and Their Applications

Amino acid-based polymers are promising stimuli-responsive materials owing to their biocompatibility, structural diversity, and functional tunability, with applications in drug delivery and therapeutics. The incorporation of fluorescent tags further extends their potential for bioimaging and optical sensing. Herein, we describe the rational design of a fluorescent 9-fluorenylmethoxycarbonyl-l-serine-based acrylate monomer and its subsequent RAFT polymerization to afford poly(Fmoc-serinyl acrylate) [poly(Fmoc-SerA)]. Unlike the monomer, poly(Fmoc-SerA) exhibits upper critical solution temperature (UCST)-type behavior in organic solvent–water mixtures, where the TcU can be readily tuned by solvent polarity, water content, molecular weight, and polymer concentration. Below the TcU, intermolecular hydrogen bonding and π–π stacking interactions drive the association of the polymer chains into spherical nanoaggregates that efficiently encapsulate Nile Red in solution. Additionally, under basic aqueous conditions in the presence of bulky organic cationic salts, P(Fmoc-SerA) exhibits an LCST-type phase behavior. Furthermore, it efficiently debundles and disperses MWCNTs in organic solvents, organic solvent–water mixtures, and water. Upon forming a charge-transfer poly(complex) with 1,2,4,5-tetracyanobenzene (TCNB), the TcU increases relative to that of noncomplexed P(Fmoc-SerA) and induces the formation of dendritic nanostructures. Furthermore, a random copolymer, P(Fmoc-SerA)-ran-P(SerA), synthesized via conventional radical polymerization (CRP), displays UCST-type behavior in water and efficiently debundles MWCNTs. The copolymer’s noncytotoxicity and inherent fluorescence make it a promising candidate for future applications as a sensory material and bioimaging probe.