Modified cubic spherosilicate cages of the type [Si8O20](8-) were used as rigid, inorganic cores for the synthesis of macroinitiators for thermal and photoinduced free radical and controlled radical polymerizations. Two different routes to these macroinitiators were investigated: the direct modification of the octaanion with chlorosilane-functionalized initiators and the hydrosilation of SiH-substituted cages. The latter synthesis of the macroinitiators resulted in more defined reaction products. With these compounds, the polymerizations of styrene and methyl methacrylate were carried out. The free radical polymerizations showed broad polydispersities based on coupling reactions, whereas the copper-mediated atom transfer radical polymerizations (ATRP) revealed that good polymerization control could be achieved with the prepared initiators.