Versatile Azo-Coupling Route to Amphiphilic Poly(2-Oxazoline) Graft Copolymers for Multi-Responsive Micelles and Enzymatic Demicellization

Abstract

The development of stimuli-responsive azocopolymer amphiphiles, especially those responsive to pH, light, thermal, and enzyme stimuli, as well as their micellization/demicellization, remains a challenging endeavor for their application in drug delivery. In this work, first, a tailor-made p-aminobenzoic acid end-capped poly(2-ethyl-2-oxazoline) (PABA–PEtOx) is synthesized by a one-pot technique involving cationic ring-opening polymerization (CROP) followed by chain-termination. It is then allowed for diazotization to PEtOx–PABA–N₂⁺Cl^– for coupling with electron-rich phenolic polymers, such as poly(l-tyrosine) or poly(4-vinylphenol) to produce azo-bridged water-soluble PTyr–N₂–PEtOx or PVPh–N₂–PEtOx graft copolymers. The grafted PEtOx segments introduce tunable lower critical solution temperature (LCST)-type phase behavior into copolymers in response to the pH and polymer concentration. The absorbance of trans- and cis-azobenzene peaks in copolymers varies with pH and the irradiation time of light. Amphiphilicity drives the self-assembly of copolymers into spherical micelles, with hydrophobic azo-bridged PTyr/PVPh segments in the core exhibiting unusual emission in water as well as in DCM. Intriguingly, the temperature above which a significant jump in emission intensity is observed, due to copolymer’s micellar agglomeration, can be correlated with the cloud point. The micellar size increases with increasing pH and shape deforms upon light irradiation. The acidic treatment of PVPh-N₂–PEtOx produces poly(4-vinylphenol)-N₂–poly(ethylene imine) (91% hydrolyzed) copolymer with no LCST in water. The copolymer micelles with low cytotoxicity disintegrate with azoreductase treatment, which triggers the release of Nile red (NR) dye from the NR-loaded micelles. The synthetic approach is simple, yet versatile and can be extended for preparing thermoresponsive azo-bridged bioconjugate by coupling PEtOx–PABA-N₂⁺Cl^– with BSA protein.