Silsesquioxanes are compounds of formula (RSiO1.5)n, in which each silicon is bound to three bridging oxygens and to one organic group ^[1]. The most studied member is the cubic octamer, a cage of eight silicon atoms at the corners of a cube with oxygen along the edges and the organic substituent projecting outward ^[1]. The rigid inorganic core and the tunable organic shell make the molecule a hybrid building block, and the corner groups set solubility and compatibility with a matrix ^[2]. Because the cage carries one to eight reactive groups, it enters a material as a pendant unit, as a crosslinker or as the core of a star-shaped polymer ^[1]. Hydrolysis and condensation of trifunctional silanes give the cage, while incomplete condensation yields open or ladder structures ^[3].