One-pot, spontaneous, and in-situ incorporation of Ru(II) complexes into a microgel (solubilized nanometer-scale network) has been achieved in near quantitative efficiency by a polymer-linking reaction of linear living poly(methyl methacrylate) (PMMA) with a bifunctional methaerylate (ethylene glycol dimethacrylate or bisphenol A dimethacrylate; linking agent) and a phosphine-ligand monomer [diphenyl-4-styryl-phosphine (3); i.e., CH2=CH-C6H4-p-PPh2] in the RuCl2(PPh3)(3)-catalyzed living radical polymerization. The products were Ru-bearing. PMMA-armed star polymers with a microgel-core that consisted of a copolymer network of the linking agent and 3. Upon the network formation, the phosphine ligands efficiently encapsulated RuCl2(PPh3)(3), thus achieving a polymer catalyst directly from a polymerization catalyst. Colored dark brown-red, the star polymers exhibited UV-vis absorptions originating from the entrapped complex (3.1-7.4 x 10(-5) mol Ru/g of polymer), the incorporation efficiency being close to 100% with respect to the original polymerization-catalyst. Detailed spectroscopic characterization showed the following: an absolute molecular weight of 1.7 x 10(5) to 1.7 x 10(6), an arm number of 11-92 arms/polymer, and a radius of gyration of 8-19 nm (in DMF). Direct observation of the individual star molecules in solid state was achieved by transmission electron microscopy (unstained; 2-3 nm dark dots for the core) and atomic force microscopy (semi-circular images). (c) 2006 Wiley Periodicals, Inc.