A Schlenk vessel equipped with a stir bar was sealed, evacuated, weighed, taken into the drybox and loaded with (Me3Sn)8Si8O20 (~ 2 g, CUBE). Then, the vessel was evacuated and weighed again to determine the exact masses of the vessel and the reagent. The vessel was reconnected to the manifold, the solvent (toluene or THF, 20 cm 3 ) was added by a syringe and the resulting solution was cooled to -80 °C. In the case of compounds L-AlMe 3 (L = py, Et 3 N, TEPO) and [Me 4 N] [AlCl 4 ], a weighed 20 cm 3 screw-top vial with a PTFE septum was loaded with the corresponding Al complex (1.5 or 3.0 equivalents of the reactive groups per CUBE) in the drybox, taken out, weighed and the solvent (5 cm 3 ) was introduced by a syringe to produce a solution. In the case of L-AlCl3, (L = THF, py, and TEPO), the ligand and AlCl3 were loaded into separate weighed vials and the solvent was first used to dissolve the ligand before it was transferred to the AlCl 3 vial to generate the complex in situ. Care was taken to ensure the ratio of L/Al ≥ 1 to avoid uncoordinated AlCl3. The AlCl3 vial was initially cooled by liquid N2 to dampen the highly exothermic contact of the ligand with the strong Lewis acid. The room-temperature solution of an Al complex was then added dropwise to the cooled solution of the CUBE with vigorous stirring over 10 min. Another portion of the solvent (5 cm 3 ) was used in several doses to ensure a quantitative transfer of L-AlCl 3 . The Schlenk vessel was sealed and allowed to slowly warm up in the cooling bath to room temperature over 12-18 h. Fast warm-ups generally lead to the formation of inhomogeneous precipitates. After 24 h, the solution was heated at 60/100 °C for another 48 h, at which point a clear to hazy solution of oligomers was obtained. All volatiles were removed under dynamic vacuum at 60/100 °C to obtain a glassy residue, which was broken to powder by a magnetic stirrer and outgassed further for 48 h at 60/100 °C. The evacuated Schlenk vessel was then weighed and taken into the drybox for product handling. The solids were analyzed by 1 H, C, Al, Si, and 31 P MAS NMR and FT-IR spectroscopies, as well as ICP-OES and N 2 adsorption porosimetry. The volatiles were collected in a Schlenk-type cold trap and analyzed by solution NMR (Supporting Information, Section S2).