Chemists can now exert a bit more control over the synthesis of polymer nanostructures. Ian Manners of the U.K.’s University of Bristol, Mitchell A. Winnik of the University of Toronto, and coworkers have developed a way to grow cylindrical polymer micelles in just one direction (Science, DOI: 10.1126/science.1221206).
Up to now, chemists could grow wirelike polymer micelles of a specific length, but they could not always control the makeup and architecture because the nanostructures would grow at both ends simultaneously, which limits their utility. Manners, Winnik, and coworkers say the new method will help chemists give aggregated cylindrical nanostructures any length and composition required for their potential use as drug-delivery vehicles and molecular electronics components.
“One of the grand challenges associated with the assembly of synthetic nanostructures is to prepare asymmetric nanoscale objects,” says Craig J. Hawker, whose research group synthesizes polymer nanostructures at the University of California, Santa Barbara. The new work by Winnik and Manners, he says, “changes our understanding of what can be accomplished.”
The new approach is based on crystallization-driven self-assembly, a process the researchers developed in 2007 for creating cylindrical micelles from block copolymers (Science, DOI: 10.1126/science.1141382). At that time, they realized that, using the right solvent, they could get one of the blocks of a copolymer to crystallize and form an ordered, cylindrical core. Like the bristles on a pipe cleaner, the other polymer block branches out from the solid core into solution.
To add a polymer of a different composition to the initial nanostructure is easy, Manners says, as long as the crystal structures of the two core-forming blocks match well enough to lock together. The problem, he explains, is that normally the second polymer (B) adds onto both ends of the original cylinder (A). “So you get a B-A-B structure with centrosymmetry,” an architecture, Manners points out, that isn’t useful for all applications.
With the new approach, noncentrosymmetric micelle structures are possible. To obtain them, the researchers begin with a small B-A-B micelle. A is a diblock copolymer composed of polyferrocenyldimethylsilane (PFS) and polydimethylsiloxane. B is another diblock copolymer made of PFS and polyisoprene. The PFS units on A and B crystallize to create the cylindrical core of the whole micelle.
The team then cross-links the vinyl groups in the polyisoprene chains with a silane reagent to make the ends of the B-A-B micelle unreactive to addition of further polymer. Finally, the researchers selectively dissolve the A tract of the B-A-B micelle with well-chosen solvents. The treatment leaves behind the two small B ends, each with a single reactive face to which a series of new polymers of various compositions can be added in succession.
“The ability to control nanostructure in this way is most interesting,” says Marc A. Hillmyer, a polymer chemist at the University of Minnesota, Twin Cities. “The combination of advanced synthetic polymer chemistry, solution self-assembly, and post-assembly modification” in this work is “a tour de force, in my view.”
The research team sees the method as a new synthetic tool and has some possible applications in mind.
“If you’ve got that B-A-B centrosymmetric structure,” Manners explains, “you can’t make a nanostructure that’s polar”—where one end is hydrophobic and the other end is hydrophilic. But the team is now working on generating polar micelles with its new approach, he says . “We’re hoping to do some really neat things with them,” he adds, suggesting that the polar structures might be used to make new membrane materials.
The researchers are also trying to use the new method to form semiconducting nanostructures made of multiple polymers, each with a distinct electronic property, Manners says. These materials might someday be used to make electronic circuit components such as diodes.