Dual Location, Dual Acidic pH/Reduction-Responsive Degradable Block Copolymer: Synthesis and Investigation of Ketal Linkage Instability under ATRP Conditions

Stimuli-responsive degradation (SRD) undergoing chemical transition through the cleavage of labile linkages has been proved to dramatically increase the versatility of stimuli-responsive block copolymers. In particular, dual or multiple stimuli-responsive degradable block copolymers that can be triggered by two endogenous stimuli of acidic pH and reduction are in high demand. Here, a new strategy utilizing atom transfer radical polymerization (ATRP) is reported to synthesize a dual acidic pH/reduction-responsive degradable block copolymer (DLDSRD) labeled with an acidic pH-labile ketal linkage at the block junction and pendant reductively cleavable disulfide groups in hydrophobic block at dual locations. A robust route with multiple steps utilizing carbamate chemistry to endow stability during protection/deprotection steps enables the synthesis of a novel poly­(ethylene glycol)-based ATRP macroinitiator labeled with a ketal linkage (PEG-ketal-Br macroinitiator). Conducting ATRP allows for the synthesis of a series of DLDSRD diblock copolymers consisting of a hydrophilic poly­(ethylene glycol) block covalently conjugated through a ketal linkage with a hydrophobic polymethacrylate block having multiple disulfide pendants. Analysis shows an unexpectedly high degree of polymerization of the hydrophobic polymethacrylate block that could be attributed to the instability of ketal linkages under ATRP conditions. The preliminary results from aqueous micellization and dual acidic pH/reduction-responsive cleavage of ketal and disulfide linkages suggest the feasibility of DLDSRD-based nanoassemblies toward effective drug delivery exhibiting precisely controlled release in response to dual stimuli at dual locations (core and interfaces)