"Grafting-from" polymerization inside a polyelectrolyte hollow-capsule microreactor.

Over the last decade versatile, electrostatic layer-by-layer (LbL) assembly methods involving polyelectrolytes have been widely used in the preparation of hollow capsules. These capsules can be fabricated by the stepwise adsorption of polyelectrolytes with charges that are opposite from their aqueous media onto templating cores, especially organic or inorganic particles and biopolymers, followed by dissolution of the templating cores to give polyelectrolyte hollow capsules that are replicas of the templates. A variety of microcapsules with customized physicochemical properties can be produced by incorporating one or more functional components, such as nanoparticles, biomacromolecules, lipids, photosensitive dyes, inorganic crystals, and multivalent ions, onto the capsule wall or into the interior of the capsule. Hollow capsules consisting of alternating layers of strong and weak polyelectrolytes can also be used for the targeted encapsulation and controlled release of macromolecules by tuning the porosity upon changing the pH value. 17] Most of these studies have focused on the development of synthetic methodologies and the characterization of the hollow capsules. Relatively little attention has been directed to reactions in the confined interior of these hollow capsules. In these studies, hollow capsules were used as microreactors for spatially restricted inorganic synthesis. However, only a limited number of reports have appeared that demonstrate that polymerization inside hollow capsules could provide new opportunities for tuning the properties of the products. For example, polyelectrolyteloaded capsules have been used to tune the physicochemical properties of the products formed in the interior of a nanoreactor capsule, 22] and enzyme-loaded capsules have been employed for the synthesis of polymers within microcapsules. Recently, the so-called “grafting-from” polymerization technique has attached a great deal of attention because polymer brushes can be generated with high grafting densities of surface-attached neutral macromolecules, as well as polyelectrolytes. Grafting-from polymerization at solid surfaces can be performed directly at the surface by using an immobilized initiator and various polymerization techniques. The initiator system is immobilized on the surface of a solid substrate and the polymer layer is then generated by polymerization in situ. To date, these grafting-from polymers or polymer brushes have been largely prepared on flat substrates. Herein we report the grafting-from polymerization to generate end-attached polyelectrolyte brushes inside a hollow-capsule microreactor. To the best of our knowledge, polymer brushes grown from the inner wall of hollow capsules have not yet been reported. This novel fabrication method for hollow capsules was performed by preparing polyelectrolyte hollow capsules whose inner layer was coated with a watersoluble initiator, followed by polymerization of a styrene sulfonate (SS) monomer on the inner wall. The shape of the capsules after polymerization could be controlled by varying the SS content. Furthermore, we also demonstrate that these hollow capsules can be used as a novel submicroreactor to control polymerization behavior. Figure 1 shows the fabrication of polyelectrolyte hollow capsules with initiator bound on the inner wall, and the grafting-from polymerization. Poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS) were used as the positive and negative polyelectrolytes, respectively, and potassium peroxodisulfate (KPS) was used as the watersoluble initiator. The capsule reactors were fabricated by the layer-by-layer (LbL) assembly technique on sacrificial [*] W. S. Choi, J.-H. Park, H. Y. Koo, J.-Y. Kim, Prof. B. K. Cho, Prof. D.-Y. Kim Center for Frontier Materials Department of Materials Science and Engineering Gwangju Institute of Science and Technology 1 Oryong-dong, Buk-gu, Gwangju 500-712 (Korea) Fax: (+82)62-970-2304 E-mail: [email protected]