Ring-opening polymerization of α-amino acid N-carboxyanhydrides (NCAs) is a powerful synthetic methodology for generating well-defined functional polypeptides. However, conventional procedures require a compromise between obtaining controlled microstructures and employing the optimized polymerization conditions. Specifically, a versatile method to access sequenced cyclic polypeptides remains challenging due to the difficulty in site-specific cyclization. Here we describe a general and straightforward method for the synthesis of both linear and cyclic polypeptides using organocatalytic living polymerization of NCAs. The use of an air-stable organocatalyst, imidazolium hydrogen carbonate, allows for the rapid and controlled polymerization of a variety of NCAs, leading to high conversion within a few minutes under mild conditions. Linear and cyclic block copolypeptides are also accessible simply by controlling the type of initiators and the order of addition of NCA monomers.

p(His)-S-S-polyurethane-S-S-p(His) triblock copolymer nanodaisies exhibit efficient pH and glutathione responsive doxorubicin releasing behaviours under physiological conditions.

Smart nano-carriers have attained great significance in the biomedical field due to their versatile and interesting designs with different functionalities. The initial stages of the development of nanocarriers mainly focused on the guest loading efficiency, biocompatibility of the host and the circulation time. Later the requirements of less side effects with more efficacy arose by attributing targetability and stimuli-responsive characteristics to nano-carriers along with their bio- compatibility. Researchers are utilizing many stimuli-responsive polymers for the better release of the guest molecules at the targeted sites. Among these, pH-triggered release achieves increasing importance because of the pH variation in different organ and cancer cells of acidic pH. This specific feature is utilized to release the guest molecules more precisely in the targeted site by designing polymers having specific functionality with the pH dependent morphology change characteristics. In this review, we mainly concert on the pH-responsive polypeptides and some interesting nano-carrier designs for the effective theranostic applications. Also, emphasis is made on pharmaceutical application of the different nano-carriers with respect to the organ, tissue and cellular level pH environment.

Photo/pH dual‐responsive amphiphilic diblock copolymers with alkyne functionalized pendant o‐nitrobenzyl ester group are synthesized using poly(ethylene glycol) as a macroinitiator. The pendant alkynes are functionalized as aldehyde groups by the azide‐alkyne Huisgen cycloaddition. The anticancer drug doxorubicin (DOX) molecules are then covalently conjugated through acid‐sensitive Schiff‐base linkage. The resultant prodrug copolymers self‐assemble into nanomicelles in aqueous solution. The prodrug nanomicelles have a well‐defined morphology with an average size of 20–40 nm. The dual‐stimuli are applied individually or simultaneously to study the release behavior of DOX. Under UV light irradiation, nanomicelles are disassembled due to the ONB ester photocleavage. The light‐controlled DOX release behavior is demonstrated using fluorescence spectroscopy. Due to the pH‐sensitive imine linkage the DOX molecules are released rapidly from the nanomicelles at the acidic pH of 5.0, whereas only minimal amount of DOX molecules is released at the pH of 7.4. The DOX release rate is tunable by applying the dual‐stimuli simultaneously. In vitro studies against colon cancer cells demonstrate that the nanomicelles show the efficient cellular uptake and the intracellular DOX release, indicating that the newly designed copolymers with dual‐stimuli‐response have significant potential applications as a smart nanomedicine against cancer.