|185. Liu, Y.; Abetz, V.; Müller, A.H.E.: Janus Cylinders, Macromolecules 36, 7894 (2003)|
Introduction. Currently, amphiphilic copolymers with nonlinear and noncentrosymmetric architectures start to attract much interest due to the synthetic challenges and their unprecedented properties.1-3 The particular asymmetric character of amphiphilic particles having two faces was termed as "Janus".4-6 Depending on their shape, Janus particles can be divided into Janus spheres, cylinders, and sheets (Figure 1). These amphiphilic nanoparticles should have novel properties in solution, at interfaces, or in the bulk. Because of their amphiphilic nature, the formation of structures of higher hierarchical order is expected. Proteins in the shape of "amphiphilic helices" are observed in nature, and they form oligomeric superstructures.7 Various strategies can be designed to construct the structures in Figure 1. One strategy is based on the self-assembly of block terpolymers (i.e., ABC triblock copolymers) in the bulk.8-10 Depending on their composition, they can form, for example, lamellae-sphere, lamellae-cylinder, or lamellae-lamellar morphologies in the bulk. Selective cross-linking of the central block and redissolution leads to nanoscopic particles where the corona is forced to be compartmentalized. This approach was used by Erhardt et al. in order to synthesize spherical Janus micelles with polystyrene (PS) and poly(methyl methacrylate) (PMMA) hemicoronas.3 These micelles form larger aggregates in nonselective organic solvents, on a silicon surface, and at the air/water interface.3,11 Hydrolysis of the PMMA ester groups led to amphiphilic Janus micelles with PS and poly(methacrylic acid) hemicoronas which again lead to superstructures and giant particles.12 Another strategy to form Janus micelles is based on the self-assembly of block copolymers in solution.13 In one approach, block terpolymers can form micellar solutions in a solvent that dissolves the outer blocks but is a nonsolvent for the middle block. Cross-linking of the core again should form Janus micelles.14 However, phase separation of the outer blocks may not be as pronounced as in the bulk approach. Another self-assembly approach is based on the electrostatic interactions of AB and CD diblock copolymers where B and C form insoluble complexes, e.g., inter-polyelectrolyte complexes, in a solvent for blocks A and D. The first reported approach has led to nonsymmetric vesicles rather than to Janus spheres.15 Alternatively, great efforts have been taken into the synthesis of Janus-like particles through pure organic synthetic approaches in recent years. Hadjichristidis et al. and other groups investigated the synthesis of miktoarm star copolymers through living anionic polymerization,1,2 controlled radical polymerization,16 or ring-opening metathesis polymerization.17 Schmidt and co-workers obtained wormlike cylinders through radical copolymerization of macromonomers poly(4-vinylpyridine) and poly(methyl methacrylate).18 Ishizu and co-workers also prepared alternate heteroarm copolymer brushes by free radical copolymerization of macromonomers, -vinylbenzylpolystyrene and a -methacryloylpoly(ethylene oxide), and afterward studied their dilute solution properties and aggregation behavior.19,20 Schlüter and co-workers synthesized poly(p-phenylene)s with hydrophilic and hydrophobic dendrons through polycondensation.21 Laschewsky and co-workers reported the partial modification of cyclodextrins and obtained amphiphilic cylinders.22 In all these works, the synthetic routes are complicated or not easy to be operated. Moreover, because of the steric effects and the reactivity of functional groups, in many cases, both the density and the length of side chains are rather low; it is difficult to reach high overall molecular weights. Also, spontaneous phase separation of the chemically different chains may not always occur easily in these systems.23 In conclusion, the synthesis of Janus structures through chemical design is still a challenge to chemists. In the present paper, we report first results on the synthesis and characterization of Janus cylinders where the separation plane is parallel to the cylinder axis. These particles were obtained in a similar way as the Janus spheres by using a polystyrene-block-polybutadiene-block-poly(methyl methacrylate) (SBM) block terpolymer forming the lamellae-cylinder morphology (Scheme 1).