Selective synthesis of non-symmetrical bis-ureas and their self-assembly

Symmetrical bis-ureas composed of two urea functions linked together by a toluene ring were previously shown to form long supramolecular polymers and thus highly visco-elastic solutions, thanks to cooperative self-assembly involving four hydrogen bonds. In this paper, we report the direct and selective synthesis of non-symmetrical bis-ureas. Mono-isocyanate/mono-ureas were first obtained from a one-step selective reaction between one aromatic amine and one isocyanate function of 2,4-toluene diisocyanate. Then, non-symmetrical bis-ureas, tetra-ureas, and bis-urea functional polydimethylsiloxanes (PDMS) were obtained by reacting the mono-isocyanate/mono-ureas with well chosen amines. The chloroform solutions of these compounds were characterised by quantitative FTIR spectroscopy and viscosimetry. It was shown that non-symmetrical bis-ureas substituted on one side by an aromatic moiety and on the other by an aliphatic group combine the solubility of aliphatic bis-ureas and the strong association of aromatic ones. Moreover, the association of bis-ureas grafted onto polydimethylsiloxanes is efficient and leads to the physical cross-linking of these polymers, even in chloroform.