Using Diels–Alders (DA) chemistry as a dynamic-covalent linkage, we explored the kinetic growth mechanism of polymer structures including starlike materials and nanogels. The use of reversible addition–fragmentation chain transfer (RAFT) polymerization allows for precise control of DA linkages within the polymer backbone. Controlling the competition between intra- and intermolecular cross-linking reactions allows for the preparation of polymer structures that can expand their volume after cleavage of the DA linkages as determined by dynamic light scattering (DLS). Macroscopically, this would be analogous to untying a rope ball knot, commonly termed a “monkey’s fist”.

Poly­(ethylene oxide43-b-lysine62-b-leucine72) (wherein subscripts denote the degree of polymerization) was synthesized via ring-opening polymerization of N-carboxyanhydrides using an amine-terminated poly­(ethylene oxide) macroinitiator, with polypeptide blocks produced by sequential monomer addition. Infrared and circular dichroism spectroscopy indicated that the peptide blocks in this polymer formed α-helices in the solid and solution states, respectively. In the aqueous solution, this polymer self-assembled into spherical micelles with a hydrodynamic radius of approximately 90 nm at concentrations between 0.05 and 0.20% w/w and pH values between 2 and 6.5. Upon preparation of transmission electron microscopy (TEM) grids, the micelles at pH 2 underwent hierarchical assembly to produce fractal assemblies, whereas small clusters were observed for micellar solutions at pH 6.5. Cryogenic-TEM of solutions showed spherical micelles, and dynamic light scattering showed no large (∼1 μm) aggregates in the solution, which suggests that fractal formation was a result of the drying process, and that fractals were not present in the solution. This system provides a facile route to nanostructured surfaces, which can be used for applications such as modulating cell adhesion or promoting the growth of neurons.

The morphology of self-assembled block copolymer aggregates is highly dependent on the relative volume fraction of the hydrophobic block.

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