In an advance that may lead to new types of highly porous and conductive organic materials, researchers have developed a chemically flexible method to assemble organic building blocks into covalently bonded networks that form unusually large crystals (Nat. Chem. 2013, DOI: 10.1038/nchem.1730). Methods to string together molecular units into well-ordered extended networks have produced a large number of crystalline metal-organic frameworks (MOFs) and a smaller number of metal-free variants, such as covalent organic frameworks (COFs). The extreme porosities and gas-uptake capacities of some of these materials are leading manufacturers to commercialize them for gas separation and storage. Unlike MOFs, COFs tend to self-assemble via irreversible covalent bonding, an “unforgiving” type of bonding that leads to powdered products because crystallization is difficult to control. University of Montreal chemists Daniel Beaudoin, Thierry Maris, and James D. Wuest have now shown that covalently bonded networks can be prepared with improved control over crystallization by using a more forgiving type of bonding. Specifically, the team used reversible self-addition polymerizations to form covalent bonds between monomers with four tetrahedrally oriented nitroso groups and thereby prepared macroscopic single crystals of azodioxy COFs.