Phototriggered Ring-Opening Polymerization of a Photocaged l‑Lysine <i>N</i>‑Carboxyanhydride to Synthesize Hyperbranched and Linear Polypeptides

Abstract

Increasing efforts are being made on controlled photopolymerization methodologies; however, the previous polymerization systems need additional photoactive initiators or catalysts. The controlled synthesis of the hyperbranched polypeptide is still challenging, and developing a photopolymerization method to prepare a hyperbranched polypeptide is urgent for constructing biodegradable polymers and biomaterials. Without addition of any initiator/catalyst, we combine the inimer (initiator + monomer) ring-opening polymerization (ROP) and photocaged chemistry to prepare hyperbranched and linear polypeptides. The photocaged Nε-(<i>o</i>-nitrobenzyloxycarbonyl)-l-lysine-<i>N</i>-carboxyanhydride possesses intrinsic photosensitivity and will be transformed into an activated AB* inimer-type α-amino acid <i>N</i>-carboxyanhydride (NCA) containing a primary ε-amine, which further triggers ROP to produce linear and/or hyperbranched polypeptides in one pot and at room temperature. The microstructure and topology of the resulting polypeptide were clarified by means of mass spectroscopy and various NMR techniques including ¹H NMR, ¹H, ¹H–COSY, and quantitative ¹³C NMR. By tuning the UV irradiation time or intensity, this methodology can produce a linear polypeptide with a high <i>M</i>_w,GPC of 109 kDa and/or (hyper)­branched counterparts with tunable <i>M</i>_w,GPC’s of 1.4–73.5 kDa and degree of branching of 0.09–0.60