We now report that a dynamic combinatorial selection approach can quantitatively provide, from trivial building blocks, an architecturally complex organic material, in which carbon dioxide is reversibly but covalently incorporated as a guest with a mass content of 20%. Solid-state analyses combined with covalent disconnection and quantization of the liberated components allowed identification of a three-component monomeric unit repeated within a range of assembled oligomeric adducts whose repartition and binding capacity can be finely tuned through the starting stoichiometries. The self-assembly of these architectures occurs through the simultaneous creation of more than 25 covalent bonds per molecular entity. It appears that the thermodynamic selection is directed by the packing efficiency of these adducts, explaining the spectacular building block discrimination between homologues differing by one carbon unit. This selectivity, combined with the reversible nature of the system, provided pure molecular building blocks after a simple chemical disconnection, promoting CO2 as a green auxiliary to purify polyaldehyde or polyamine from mixtures of homologous structures. Moreover, the gas template could be expelled as a pure compound under thermodynamic control. This cooperative desorption process yielded back the initial libraries of high molecular diversity with a promising reduction of the energetic costs of capture and recycling.