Patchy Supramolecular Bottle-Brushes Formed by Solution Self-Assembly of Bis(urea)s and Tris(urea)s Decorated by Two Incompatible Polymer Arms

In an attempt to design urea-based Janus nanocylinders through a supramolecular approach, nonsymmetrical bis­(urea)­s and tris­(urea)­s decorated by two incompatible polymer arms, namely, poly­(styrene) (PS) and poly­(isobutylene) (PIB), were synthesized using rather straightforward organic and polymer chemistry techniques. Light scattering experiments revealed that these molecules self-assembled in cyclohexane by cooperative hydrogen bonds. The extent of self-assembly was limited for the bis­(urea)­s. On the contrary, reasonably anisotropic 1D structures (small nanocylinders) could be obtained with the tris­(urea)­s (<i>N</i>_agg ∼ 50) which developed six cooperative hydrogen bonds per molecule. ¹H transverse relaxation measurements and NOESY NMR experiments in cyclohexane revealed that perfect Janus nanocylinders with one face consisting of only PS and the other of PIB were not obtained. Nevertheless, phase segregation between the PS and PIB chains occurred to a large extent, resulting in patchy cylinders containing well separated domains of PIB and PS chains. Reasons for this behavior were proposed, paving the way to improve the proposed strategy toward true urea-based supramolecular Janus nanocylinders

Self-Assembly and Critical Solubility Temperature of Supramolecular Polystyrene Bottle-Brushes in Cyclohexane

The formation of polystyrene (PS) supramolecular bottle-brushes by self-assembly in cyclohexane of hydrogen-bonding tris­(urea) units decorated by PS chains was investigated using light and neutron scattering. Atom transfer radical polymerization (ATRP) was used to control the length of the PS side-chains and allowed the straightforward synthesis of the targeted tris­(urea)­s. It was shown that their extent of self-assembly strongly depended on the degree of polymerization and chemical nature of the polymer side chains, in contrast with what was previously observed with cyclic oligopeptides, another type of self-assembling units. With sufficiently short PS side-chains, anisotropic supramolecular bottle-brushes could be obtained. Their critical solubility temperature, <i>T</i>_c, was measured in cyclohexane, proving experimentally for the first time that densely grafted PS bottle-brushes exhibit a much lower <i>T</i>_c than linear PS or even star-shaped PS of similar molecular weight

Competition Between Steric Hindrance and Hydrogen Bonding in the Formation of Supramolecular Bottle Brush Polymers

The formation of supramolecular bottle-brush polymers consisting of a noncovalent backbone assembled through directional hydrogen bonds and of poly­(isobutylene) (PIB) side-chains was investigated in cyclohexane by light scattering. Two limiting cases were observed depending on the balance between the favorable formation of hydrogen bonds and the unfavorable stretching of the PIB chains within the supramolecular bottle-brushes, in agreement with a theoretical model developed by Wang et al. On one hand, a bisurea self-assembling unit able to form four cooperative hydrogen bonds per molecule led to relatively short supramolecular bottle-brushes, the length of which could be varied by modifying steric hindrance or by using solvent mixtures. On the other hand, supramolecular bottle-brush polymers exhibiting persistent lengths of more than 300 nm could be obtained by using trisureas that are able to form six hydrogen bonds per molecule. Their easy synthesis and the fact that it is possible to control their self-assembly into long supramolecular bottle-brush polymers make polymer-decorated bisureas and trisureas an attractive alternative to cyclopeptides and shape-persistent rings for the creation of supramolecular nanostructures