We present the synthesis and UCST-type thermoresponsive properties of helical polypeptide based single-chain nanoparticles which displayed increased solution phase transition temperature and improved biocompatibility.

Homopolypeptide bearing azobenzene and triethylene glycol spacers, and 1‐butylimidazolium pendants, namely P(Azo‐OEG3‐ImX) (X = Cl, I, BF4) are prepared by 1,3‐dipolar cycloaddition and subsequent ion‐exchange reaction. Both 1H NMR and FTIR confirm the polymer structures and reveal a high grafting efficiency (97%). P(Azo‐OEG3‐ImI) shows reversible upper critical solution temperature (UCST)‐type thermoresponsive property in ethanol while P(Azo‐OEG3‐ImI/BF4) shows a UCST in ethanol/water solvent mixtures. P(Azo‐OEG3‐ImI) shows UCST‐type phase transitions in ethanol or ethanol/water solvent mixtures. After UV irradiation, the UCST‐type cloud point temperature (Tcp) remains constant in ethanol. However, it decreases after UV irradiation and increases after visible light irradiation in ethanol/water solvent mixtures due to the trans–cis isomerization of azobenzene moieties. P(Azo‐OEG3‐ImCl/I) shows NaI‐induced UCST in water. The Tcp of the P(Azo‐OEG3‐ImCl/I) aqueous solution is highly related to the polymer/salt concentration and nature of counteranions. P(Azo‐OEG3‐ImCl) also shows reversible light‐responsive properties in NaI aqueous solutions. Higher NaI concentration is needed to induce a UCST for P(Azo‐OEG3‐ImCl) after UV irradiation. Moreover, ciprofloxacin‐loaded hydrogel containing P(Azo‐OEG3‐ImCl) shows more efficient drug release while the UV irradiation leads to slightly low drug release.

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