Self-assembly and disassembly are the basic principles of nanostructure construction in nature. At the same time, this reversible process plays an important role in the maintenance of biological organism functions, such as metabolism and self-replication of cellular tissue. For example, the globular actin (G-actin) binds to adenosine triphosphate (ATP) in non-covalent interactions to form microfilamentous structures in the cells. These microfilaments can depolymerize to generate adenosine diphosphate (ADP) via ATP hydrolysis and produce G-actin again. This reversible recombination process is essential for cell division and cytoplasmic recycling. As one of the important scientific frontiers in supramolecular chemistry, supramolecular polymerization is an important method for the precise preparation of hierarchical complex nanostructures. However, its opposite process, supramolecular depolymerization, i.e., the decomposition of supramolecular structures into assembled units, has been less studied. In particular, the mechanism and kinetics of supramolecular depolymerization are not clear.

Figure 1. Micelle units are formed by the supramolecular depolymerization of nanowires

   Recently, the group of Prof. Jiaping Lin, East China University of Science and Technology, reported the thermally induced supramolecular depolymerization of one-dimensional nanowires synthesized by supramolecular polymerization. Nanowires are formed by pre-assembled micelles through supramolecular stepwise polymerization. As the temperature increases, the micelle interactions at the nanowire nodes, i.e. hydrophobic interactions, are disrupted, leading to random depolymerization of nanowires into nanowire fragments and micelle units (Fig. 1). The transfer of polymer molecular chains between micelles is accompanied by depolymerization, which is one of the characteristics of this type of supramolecular depolymerization. The researchers also proposed a theoretical model to describe the process of supramolecular depolymerization. Kinetic theory shows that the rate constant of supramolecules' thermal depolymerization increases with increasing temperature. In addition, the interaction between micelles gradually increases and the depolymerization of nanowires becomes difficult with the increase of water content in the nanowire solution. This temperature-induced supramolecular depolymerization has potential applications in the biomedical field. Notably, this work also provides important information for understanding the fundamentals of supramolecular degradation and can aid in the design and preparation of various types of complex functional nanostructures. This work was done by Gao Hongbing, a Ph.D. student, and Gao Liang, a postdoctoral fellow at East China University of Science and Technology, under the supervision of Prof. Chunhua Cai and Prof. Jiaping Lin. It was published in Macromolecules (DOI: 10.1021 /acs.macromol.0c00146) under the title of Supramolecular Depolymerization of Nanowires Self-Assembled from Micelles.

      In addition, Prof. Jiaping Lin's group has made several advances in supramolecular polymerization in recent years. The team extended the concept of polymerization to supramolecular polymerization of assemblies and discovered and reported the phenomena of supramolecular polymerization of micellar units, supramolecular cyclization, and supramolecular living polymerization. For example, pre-assembled micelles of polypeptide graft copolymers that undergo supramolecular polymerization to form hierarchical nanowire structures under low-temperature induction, and a kinetic model for supramolecular polymerization is proposed (Macromolecules 2019, 52, 7731); cylindrical micelles formed by rigid-flexible block copolymers can undergo seed growth behavior after secondary feeding. The kinetic theory of liquid-crystallization-driven self-assembly (LCDSA) to achieve supramolecular living polymerization and the origin of homogeneity of supramolecular aggregates were proposed by theoretical simulations (Nano Letters 2019, 19, 2032). In addition, Prof. Jiaping Lin's group recently published a review article entitled "Self-Assembly of Copolymer Micelles: Higher-Level Assembly for Constructing Hierarchical Structure" in Chemical Reviews (Figure 2), which describes the research progress and development prospect of copolymer micelle self-assembly of our research group and peers in recent years. In particular, the behavior of micellar supramolecular aggregation occurring under different driving mechanisms is discussed, as well as the important role played by theoretical simulations in revealing the mechanism of such assembly.

Figure 2. Multistage self-assembly behavior of copolymers

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