Advanced families of NPMs, such as metal-organic frameworks (MOFs) , covalent organic frameworks (COFs) , porous polymer networks (PPNs) , porous organic cages (POCs) , and metal-organic polyhedra (MOPs) , are constructed modularly from molecular building blocks. The copiousness of compatible building blocks within many topologies, together with post-synthetic modifiability , make the number of possible NPM structures extremely large .