Hydrogen bonding induced order in supramolecular polymers

The self-assembly of small and relatively simple molecules has proven to be a powerful tool for the development of complex supramolecular nanostructures of defined size and shape. More recently, the understanding of supramolecular polymeric aggregates has become a research aim, with the aim to increase the knowledge of the non-covalent interactions in factors that govern self-assembly in these systems. This thesis addresses the development of supramolecular polymers, which are based benzene-1,3,5-tricarboxamides (BTAs) and traditional polymers. BTAs comprising alkyl side chains form helical supramolecular polymers in dilute solution and in the solid state as a result of the threefold helical arrangement of the intermolecular hydrogen bonds. The self-assembly properties of BTAs combined with traditional polymers has led to the development of novel supramolecular materials. Because the chirality of monomeric BTAs is expressed at the supramolecular level, these systems allow the use of a wide range of spectroscopic techniques to study the self-assembly. As a result, valuable insights into the formation of the supramolecular structure of the resulting polymers was obtained, which are crucial to further develop the field of supramolecular polymers. In Chapter 1, an overview is given of supramolecular systems that self-assemble by the formation of hydrogen bonds. Special attention is paid to hydrogen bond strength and to approaches to increase the binding strength. In addition, three classes of supramolecular assemblies, in which the formation is hydrogen bonding driven, are discussed based on literature examples. This short literature overview includes systems such as supramolecular polymeric networks, disc-shaped motifs with the ability to form long elongated supramolecular polymers and the final class describes supramolecular assemblies, which are limited in size and shape as a result of their design. In Chapter 2, the self-assembly of BTAs into helical aggregates when end-capped to or copolymerized with low molecular weight polyethylene butylene is investigated in solution and in the solid state. Self-assembly is evaluated with a variety of spectroscopic techniques such as CD, UV and IR. The introduction of BTAs results in the formation of phase segregated nanorods. In some cases, this leads to drastic improvements of the material properties, as revealed by tensile testing and oscillating shear rheology measurements. A systematic study on the role of polarity on the self-assembly of BTAs in solution and on their ability to phase segregate in the solid state is performed in Chapter 3. In dilute solution, the polarity is varied by mixing polar and apolar solvents. In the solid state, a wide range of backbone polarities is covered by end-capping of telechelics of varying polarity with the BTA motif. In both cases, an increase of polarity leads to a significant decrease of the stability of BTA aggregates, which in the solid state eventually results in the loss of nanorod formation This study gives a detailed understanding of the scope and limitations of BTA based polymers for various applications. In Chapter 4, the use of BTA functionalized polymers as hydrogelators is explored. BTAs equipped with aliphatic side chains are end-capped to polyethylene glycol via a short apolar spacer. In water, long entangled nanorods were formed as a result of intermolecular hydrogen bonding stabilized by hydrophobic interactions. At higher concentrations, strong and transparent gels were obtained. The supramolecular structure and the material properties of the gel are determined with CD spectroscopy and with oscillating shear measurements. A supramolecular polymer system that consists of a mixture of BTAs and UPys, end-capped to monofunctional polymers is introduced in Chapter 5. Both supramolecular motifs self-assemble in an orthogonal fashion in two separate types of phase segregated nanorods. After the addition of an a, ¿-telechelic polymer containing both the BTA and UPy motif (compatibilizer) to this system, a cross-linked network is formed. This is observed by the transformation of a viscous sticky liquid into a solid material with elastomeric properties as evidenced by oscillating shear rheology measurements. The preliminary results presented in this study show the potential of orthogonal self-assembly based on hydrogen bonding motifs. In the final Chapter, the ability of BTAs grafted to the side chain of polymethacrylate to form well-defined nanosized objects comprising an internal helical architecture is investigated. These polymeric structures form ordered chiral single-chain polymeric nanoparticles in a controlled fashion. Controlled folding is achieved by making use of photolabile deprotection chemistry and with the aid of heating and cooling steps. This process is quantitatively followed with CD spectroscopy. The high stability and chiral conformation of the folded particles make them excellent candidates for compartmentalized catalytic systems.