For the first time, ATRP was successfully employed for homopolymerization of a commercial methacrylate-functionalized polyhedral oligomeric silsesquioxane (POSS) monomer, iBuPOSSMA, to high molecular weights. It was found that iBuPOSSMA has a low ceiling temperature (T-c); therefore, low temperatures and/or high initial monomer concentrations need to be used in order to avoid low degrees of polymerization that had been observed previously. The values of T-c, as well as of the polymerization enthalpy Delta Hp and entropy Delta Sp were determined to be 130 degrees C (at [M](0) = 1 M), -41 kJ mol(1), and -101 J mol(1) K-1, respectively. Under optimized conditions, poly(iBuPOSSMA) homopolymers having low dispersity and high M-n, ranging from 23 000 to 460 000, were obtained in a well-controlled ATRP process. Moreover, various block copolymers having high-M-n poly(iBuPOSSMA) blocks were prepared by copolymerization of iBuPOSSMA with methyl methacrylate and styrene.