Incompletely condensed polyhedral oligomeric silsesquioxane (POSS) with the highly reactive group of trisodium silanolate was used for the synthesis of two initiators for atom transfer radical polymerization, one with a 2-bromoisobutyl group and the other with a chlorosulfonyl group. These initiators were applied to solution polymerizations of styrene and methyl methacrylate in the presence of a copper complex. In both systems, polymerization proceeded in a living fashion, as indicated by the first-order kinetics of monomer consumption, the evolution of molecular weight in direct proportion to monomer conversion, the good agreement of molecular weight with the theoretical one, and the low polydispersity, thus providing tadpole-shaped polymers with an "inorganic head" of POSS and an "organic tail" of well-defined polymer. Thermogravimetric and differential scanning calorimetric studies showed that both thermal degradation and glass transition temperatures of the organic/inorganic hybrid polymers with molecular weights up to about 20 000 were enhanced as compared to those of model polymers without the POSS moiety.