Transient Salt-Bridge-Based Supramolecular Polymers: Experiments and Theory

The smooth decarboxylation under basic conditions of activated carboxylic acids (ACAs) is exploited to achieve a transient supramolecular polymer based on hydrogen bonds reinforced by electrostatic interactions. In particular, it is proved that when the aliphatic α,ω-diamine 3, namely, 1,8-diamino-3,6-dioxaoctane, reacts with an equimolar amount of the activated dicarboxylic acid 1H₂, i.e., a difunctional derivative of 2-cyano-2-phenylpropanoic acid, a supramolecular polymer of the kind -AB─BA─AB- is immediately formed in chloroform solution. The A─A and B─B monomers are held together by salt bridges (hydrogen bonds reinforced by electrostatic interactions) between ammonium and carboxylate functions. The larger the concentration of the added materials, the higher the polymerization degree (DP) of the polymer. Under the given experimental protocol, such a polymer disaggregates over time due to decarboxylation, and at the end of the process, only diamine 3 and waste product 4, which cannot interact with one another anymore, remain in the solutions. DOSY spectra recorded at different reaction times definitely demonstrate the phenomenology described above. The trend of the degree of polymerization as a function of monomer concentration has been clarified in the light of the ring-chain equilibrium theory. The application of the theory enables the accurate evaluation of the distribution of linear and cyclic oligomers as well as the critical concentration, c_crit, above which polymerization rapidly becomes more extensive due to the saturation of macrocyclic species. Notably, the ACA is not used just as a stimulus for a dissipative system, but as one of its structural components.