One-Pot Synthesis of Amphiphilic Polymeric Janus Particles and Their Self-Assembly into Supermicelles with a Narrow Size DistributionThe National Science Foundation of China (NSFC) (Project Nos. 20574014 and 20528405) is acknowledged for supporting this research

There is increasing interest in the preparation of Janus particles owing to their potential for application in a number of fields, such as the development of microrheological probes, optical biosensors, functional surfactants, and electronic devices. Janus particles can also be used to fabricate complex superstructures that would not be accessible from uniformly functionalized particles. Microfluidic methods and flow-lithography techniques can be used to produce Janus particles of various shapes with good control over size distribution, but at present the particle size is limited to a diameter of approximately 5–100 mm. M&ller and co-workers reported the preparation of Janus micelles from ABC triblock copolymers and their self-assembly into supermicelles. However, the supermicelles coexist with the Janus micelles and a small number of large aggregates. Promising approaches in which an interface is used as a desymmetrization tool to prepare Janus particles have also been reported. As most interface-based techniques rely on the two-dimensional modification of a monolayer, the preparation efficiency is low. It is expected that the industrial future of these routes will depend mainly on the development of processes in which very large surfaces or interfaces are involved. Janus particles can be prepared in emulsion systems in potentially high yield. However, to our knowledge, Janus particles have been produced in large quantities by only two interface-based emulsion methods. The success of these methods lies in the restriction of the rotation of the precursor spheres at the interface of emulsion droplets. Nearly all interface-based approaches start with inorganic precursor spheres, which limits the structural variety of the Janus particles and their ability to form uniform superstructures. Clearly, efficient approaches to nanosized Janus particles capable of selfassembling into uniform superstructures need to be explored. Herein we report an efficient one-pot approach to the preparation of amphiphilic polymeric Janus nanoparticles that can self-assemble into supermicelles with a narrow size distribution. Our strategy is to use water-dispersible hybrid nanotubes (HNs) composed of an inorganic nanotube surrounded by a hydrophobic polymer layer (HPL) with watersoluble polymer chains grafted on the outer surface of the HPL as a desymmetrization tool. In an aqueous suspension of HNs, a hydrophobic divinyl cross-linker and a hydrophobic free-radical initiator were solubilized in the HPL, and a hydrophilic monomer remained in the aqueous phase. During the polymerization of the hydrophobic cross-linker, hydrophobic spheres composed of the resulting polymer were formed and grew in the HPL. One side of a sphere was exposed to the water phase when the diameter was larger than the thickness of the HPL. Polymeric free radicals at the hydrophobic-sphere/water interface then initiated the polymerization of the water-soluble monomer; the other side of the hydrophobic sphere was embedded in and protected by the HPL. This process led to the formation of amphiphilic Janus particles in which water-soluble polymer chains are grafted on one side of a hydrophobic sphere. We found that the Janus particles can self-assemble in water into supermicelles with a narrow size distribution. Under suitable conditions, the supermicelles can dissociate into individual nanosized Janus particles (Figure 1). The preparation efficiency of this method is relatively high as a result of the large area of the curved water/HPL interface. We believe that the rotation of the hydrophobic spheres at the interface is highly restricted, as the spheres are partially embedded in the semisolid HPL. Poly(ethylene oxide)-b-poly(4-vinylpyridine) (PEO113-bP4VP93) and yttrium hydroxide nanotubes (YNTs) with a diameter of approximately 200 nm and a length of 3–4 mm (see the Supporting Information) were used to form the HNs. PEO-b-P4VP and the YNTs were synthesized according to procedures reported previously. When the block copolymer and the YNTs were mixed in chloroform, the P4VP block chains were adsorbed onto the surface of the YNTs because of favorable hydrogen bonding between the pyridine units and the surface hydroxy groups of the YNTs, as determined by FTIR spectroscopy (see the Supporting Information). We thus obtained HNs in the form of YNTs coated with a layer of the diblock copolymer. When the chloroform solvent was exchanged for neutral water, we obtained HNs in which the P4VP HPL surrounds the YNTs and the PEO block chains are grafted on the outer surface of the HPL. With gentle stirring, HNs can be dispersed individually in water for weeks owing to the solubilization of the PEO block chains. However, without stirring, HNs will start to sediment after a few hours. This characteristic facilitates the convenient purification or removal of HNs. The polymer weight fraction in the purified [*] L. Nie, W. Shen, Prof. D. Chen, Prof. M. Jiang The Key Laboratory of Molecular Engineering of Polymers Department of Macromolecular Science, Fudan University Shanghai, 200433 (China) Fax: (+86)21-6564-0293 E-mail: [email protected]