Star-shaped polymers that consist of well-defined poly(methyl methacrylate) [poly(MMA)] arms and a microgel core of a divinyl compound were prepared in high yield with the RuCl2(PPh3)(3)-catalyzed living radical polymerization followed by direct in-situ polymer linking reactions with divinyl compounds. In typical examples, these star-shaped polymers had the arm number (f) of 10-30 and (M) over bar (w)- (star) > 10(5). The following factors were effective in the star-shaped polymer formation: (a) a monofunctionalized chloride initiator; (b) a linking agent with a soft aliphatic or a rigid long aromatic spacer between the two methacryloyl groups.