pH- and Reductant-Responsive Polymeric Vesicles with Robust Membrane-Cross-Linked Structures: In Situ Cross-Linking in Polymerization-Induced Self-Assembly

Polymerization-induced self-assembly (PISA) has been established as a powerful strategy for fabrication of polymeric nano-objects in the past decade. However, in comparison with the traditional self-assembly method, PISA is unsatisfactory in preparation of vesicles with chemical versatility of membrane-forming block for tunable membrane properties, which limits the further application of PISA-based vesicles. Besides the stimuli-responsive property, structural integrity of the vesicles is another important concern for material applications. In situ cross-linking in PISA via copolymerization with multivinyl comonomers (cross-linkers) seems to be a straightforward and convenient method to afford stabilized nano-objects. However, it is hard to fabricate vesicles with cross-linked membrane via in situ cross-linking strategy because cross-linking greatly limits chain mobility of the produced copolymers and thus prevents morphology transition to form vesicles. In this article, in situ cross-linking in PISA for fabrication of pH- and reductant-responsive vesicles with robust cross-linked structure is realized via RAFT dispersion copolymerization of 2-(diisopropylamino)ethyl methacrylate (DIPEMA) and cystaminebismethacrylamide (CBMA). The cross-linking process is delayed to the late stage of polymerization after the formation of vesicles due to the lower reactivity of the cross-linker CBMA in comparison to the monomer DIPEMA, which is supported by nuclear magnetic resonance (NMR), dynamic light scattering (DLS), and transmission electron microscopy (TEM). The vesicles exhibit dual stimuli-responsive (pH and reductant) release of cargoes. The pH-regulated membrane permeability of the vesicles is due to the pH-responsive hydrophobic-to-hydrophilic transitions of the membrane-forming blocks. Reductant-responsive disaggregation of the vesicles is induced by cleavage of the disulfide linkages in the presence of dl-dithiothreitol (DTT) in acidic aqueous solution.