Three-Dimensional Synthons for Cage-Dense Inorganic Polymers and Materials

The relationship between the monomer structure and polymer properties is central to materials science. Here we discuss the notion of monomer dimensionality, as it refers to the presence of 1-dimensional chains, 2-dimensional rings, or 3-dimensional cages in inorganic polymers. We first highlight ring-containing systems, followed by classical 3-dimensional inorganic monomers (carboranes and polyhedral oligomeric silsesquioxanes, POSS). We then outline our development of modularly accessible phosphorus–nitrogen (PN) cages into polymerizable synthons to access linear or networked inorganic materials. Finally, we summarize the most important obstacles to advancement in the field and point toward areas where the most impactful discoveries are expected. Throughout the Perspective, an emphasis is placed on the properties that emerge when three-dimensional inorganic cages are present in the main chain, particularly as they contrast with materials containing monomers of lower dimensionality.