Synthesis and Self-assembly of Hyperbranched Polymers

Hyperbranched polymers (HBPs), comprising of dendritic units, linear units and terminal units, are highly branched macromolecules with three-dimensional dendritic globular architecture. Compared with linear polymers, HBPs have shown the characteristics of a large population of terminal functional groups, low or no chain entanglement, small hydrodynamic diameter, low solution or melt viscosity and good solubility. Due to these characteristics, HBPs have attracted global interest from science to industry. In this paper, we review some progress in the synthesis, self-assembly and application of HBPs. The paper includes three parts. In the first part, a short history on the development of HBPs focusing on the introduction of "couple-monomer methodology (CMM)" to prepare HBPs was discussed. A typical example of CMM named as "AA' + B'B-2" method was described. In addition, the synthesis of a hyperbranched oxetanemethanol through ring-opening polymerization was also presented. In the second part, the self-assembly of amphiphilic HBPs was summarized according to the morphology of the self-assemblies from the spherical micelles, the vesicles, the sheets or films, the fibers or tubes, to complex supramolecular structures. The mechanism for the self-assembly of the micelles from HBPs was discussed in details. HBPs form unimolecular micelles (UMs) around 10 nm below the critical micellar concentration (CMC). Above the CMC, there are two possibilities for the formation of micelles: one is that HBPs undergo microphase separation and then form micelles with the diameter of the length of two HBPs (around 20 nm or larger), which is named as "microphase-separated small micelles" (ms-SMs) and similar to the star micelles of linear block copolymers in structure; the other is that HBPs form multimolecular micelles (> 50 nm) through a mechanism of "multimicelle aggregate" (MMA). The MMA mechanism can also be divided into two mechanisms of "unimolecular micelle aggregate" (UMA) and "small micelle aggregate" (SMA). The MAA mechanism is general and can also be used to explain the large multimolecular micelles self-assembled from other polymers or even small molecules. In the third part of this review, we introduced the applications of HBP self-assemblies in drug delivery, cytomimetic chemistry and monodisperse nanoparticle preparation. The HBP vesicles (named as branched-polymersomes) have displayed good properties like the simple preparation, good membrane fluidity and stability, and the facilely tuned and micrometer-scaled size, which make them the ideal model systems to mimic cell behaviors like fusion, fission, aggregation and so on. All these cytomimetic behaviors can be observed in real time under an optical microscope since the branched-polymersomes have a cell-like size.