Formation of 3 demonstrates that despite the stability of the core, quadruple functionalization on the periphery of 2 is possible to build molecular constructs extending from its central cage motif. We therefore envisioned the use of 2 as a platform for the synthesis of networked materials, as is known for scaffolds A and B. Thus, 2 was combined with two equivalents of p-n BuPhPCl2, which was selected as the electrophile due to the solubilizing nature of the linear butyl chain. Monitoring the THF solution by The porosity of the gel was confirmed by fully reversible absorption of ca. 100% of its mass in THF at room temperature (Figure ). The solvent swollen gel is supple, whereas the evacuated solventfree material is brittle and insoluble in all solvents tested. No evidence of degradation was noted when 5 was exposed to ambient atmosphere for ca. 6 months. While polyphosphazenes and cyclophosphazanes have previously been crosslinked by hydrocarbons, thereby unlocking a vast array of hybrid organic-inorganic functional network materials, 5 is, to the best of our knowledge, the only demonstrably porous material whose skeleton is constructed exclusively from inorganic phosphorusnitrogen bonds. In this context, conversion of 2 to 5 represents a topological generalization of hitherto linear (1D) and cyclic (2D) polyphosphazenes into the cage (3D) dimension.