Bottom-up Strategies to Molecular Containers: Multicompartment Micelles from ABC Triblock Copolymer Miktoarm Stars.

University of Minnesota graduate student Zhibo Li, working in the laboratories of MRSEC investigators Tim Lodge and Marc Hillmyer and in collaboration with Ellina Kesselman and Yeshayahu Talmon from the Technion-Israel Institute of Technology, have design block copolymers that self-assemble into customized containers with adjustable shapes and sizes. The customized containers have promise for controlled drug delivery, light harvesting, and templating of functional nanostructures. Furthermore, these new materials, which can be dispersed in water, compartmentalize space in a way that mimics the structure of living cells.

The essence of the Minnesota discovery is a set of remarkable micellar structures that form in dilute solutions of multicomponent block copolymers. Covalent connection of a hydrophobic polymer to a hydrophilic polymer gives an (AB) amphiphilic macromolecule that will spontaneously self-assemble in aqueous solution. The micelles formed are aggregates of these molecules with the hydrophobic portion on the inside of the micelle (protected from the water) and with the hydrophilic component as the water-compatible corona. A variety of self-assembled structures (e.g., spheres, cylinders, bilayers) can be formed from amphiphilic AB diblock copolymers. However, with only two components the structures simply consist of an inside and an outside. Further division of space within the hydrophobic core would be a first step toward mimicking the structural and functional complexity of eukaryotic cells with multiple functional units (e.g., nucleus, organelles, etc.).

To prepare a "multicompartment micelle" using block copolymers, Zhibo incorporated a third block (C) that is both hydrophobic and incompatible with the other hydrophobic block. In this way, the two hydrophobic materials are forced to share the same space inside the micelle but do mix. To further "frustrate" the system, each of the blocks was connected at one junction point thus giving an ABC mikto (mixed) arm star polymer (Figure 1). By tying all three chains to a single juncture point, a remarkable array of multicompartment micellar structures was formed. A particular interesting structure is the segmented worm-like micelles that contain alternating disks of the two hydrophobic components. Direct evidence for these structures was obtained by cryo transmission electron microscopy as shown in Figure 1. The nature of the structures could be tuned by controlling the molecular parameters in the triblock copolymers. This "bottom up" engineering of nanostructures is a particularly appealing approach for the preparation of controlled supramolecular assemblies.

This approach to muticompartment micelles represents the remarkable versatility of self-assembled amphiphilic block copolymers. The combination of exquisite control over the molecular components and chain architecture with detailed and careful characterization of the resultant structures has led to the discovery of a truly new class of self-assembled structures that have enormous potential in various technologies ranging from controlled release to energy harvesting.

[Li, Z.; Kesselmann, E.; Talmon, Y.; Hillmyer, M. A.; Lodge, T. P. "Multicompartment Micelles from ABC Triblock Copolymer Miktoarm Stars" Science 2004, 306, 98.]

Figure 1. Cryo-TEM images of 1 wt % aqueous solutions of four different ABC star structures with varying compositions (scale bars indicate 50 nm).