RAFT-Synthesized Citrulline-Derived Polymethacrylates with Tunable UCST for Dye Encapsulation and a Fluorescent FITC-Tagged LCST Conjugate

The library of stimuli-responsive materials is rich in amino acid-based polymers due to their inherent biocompatibility and ease of functionalization, both of which are essential for developing biomaterials with significant potential in drug delivery and therapeutics. Polymers derived from noncanonical amino acids (ncAAs) expand the functional pool of these amino acid polymers, as ncAAs provide unique side chains with chemical or structural properties not found in canonical amino acids (cAAs). Thus, this work aims to synthesize a citrulline-based thermoresponsive polymer, a type of ncAA. To achieve this, we design and synthesize the Boc-l-citrulline-ethyl methacrylate (Boc-cit-EMA) monomer, followed by its RAFT polymerization to poly(Boc-cit-EMA). Detailed studies of the RAFT kinetics reveal a first-order rate up to very high monomer conversions, providing precise control over the molecular weight (Mn) and dispersity. P(Boc-cit-EMA) exhibits reversible upper critical solution temperature (UCST)-type thermoresponsive phase behavior with a tunable cloud point (TcU) in various aliphatic alcohols. TcU increases with an increase in the alkyl chain length of the alcohols. P(Boc-cit-EMA) is soluble in alcohols and various other organic solvents, but insoluble in water. Upon adding water, the transparent alcoholic solution of P(Boc-cit-EMA) turns into a turbid suspension, which becomes transparent again upon heating, confirming UCST-type thermoresponsive behavior. The cloud point of P(Boc-cit-EMA) in both neat alcohols and alcohol–water mixtures can be tuned by adjusting the concentration, Mn, type of alcohol, and water content in the mixed solvent. P(Boc-cit-EMA) molecules can encapsulate Nile red (NR) dye during their association into spherical aggregates upon cooling below the cloud point, forming insoluble dyed particles, as evidenced by time-correlated single-photon counting measurements. Furthermore, conjugation of fluorescein isothiocyanate (FITC) to the Boc-deprotected P(cit-EMA) yields a fluorescent, thermoresponsive conjugate that exhibits a distinct lower critical solution temperature (LCST)-type behavior.