Polymerization-induced self-assembly (PISA) has been established as a powerful strategy for fabrication of polymeric nanoobjects 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 PISAbased 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. Reductantresponsive disaggregation of the vesicles is induced by cleavage of the disulfide linkages in the presence of DL-dithiothreitol (DTT) in acidic aqueous solution.

Stimuli-responsive polymeric vesicles have attracted great attention in drug delivery due to their intrinsic hollow structures and "on demand" release of drugs upon environmental stimuli. The drug-release kinetics from polymeric vesicles, which is usually dependent on the stimuliresponsive behaviors of the polymeric vesicle, has great impacts on the therapeutic efficacy. Over the past decade, polymerization-induced selfassembly (PISA) has been demonstrated to be a powerful strategy to prepare the polymeric vesicles. However, fabrication of stimuli-responsive vesicles with adjustable drug release kinetics via PISA has been rarely reported, which may be due to the poor selectivity of functional membrane-forming polymers in the PISA system. Herein, a series of vesicles with different pH-responsive behaviors were fabricated via RAFT dispersion copolymerization of (diisopropylamino)ethyl methacrylate (DIPEMA) and benzyl methacrylate (BzMA). Both the content of DIPEMA units in hydrophobic P(DIPEMA-co-BzMA) blocks and the DP of P(DIPEMA-co-BzMA) have great influence on the pH-responsive behaviors of the vesicles, which can be used to adjust the responsive release rate of RhB from the vesicles upon the same pH switch.

Polymerization-induced self-assembly (PISA) has been established as an efficient method to fabricate polymeric vesicles. In most PISA cases, the formation of vesicles is solely driven by the solvophobic interactions. Besides solvophobic effects, many other noncovalent interactions can also drive/influence self-assembly of block copolymers. In this work, PISA driven by the synergistic effects of solvophobic and aromatic interactions is investigated. 7-(2-Methacryloyloxyethoxy)-4-methylcoumarin (CMA) is selected as a model monomer for RAFT dispersion polymerization to fabricate vesicles with both solvophobic and aromatic interactions existing in the membrane-forming blocks. Controlling vesicular size in the range of sub-100 nm to micrometer in PISA is realized by copolymerization of CMA with three different comonomers (aromatic or not) in various proportions to adjust the intermolecular interactions. Moreover, polymeric tubes with adjustable aspect ratios (from about 2 to 20) are produced by applying the cooperativity of aromatic and solvophobic interactions.

This review discusses the strategies of core-cross-linking in most of the PISA literatures (including post-polymerization cross-linking, photo-cross-linking and in situ cross-linking) and the applications of the cross-linked nano-objects.