Preparation And Supramolecular Structure Investigations Of Ionic Interaction Based Polyelectrolyte-Dendron Complexes

Dendronized polymers, with dendrons as the side chains, are a new kind of dendritic polymers and have been attracting more and more research interesting due to their unique molecular structures and inherent properties. Basing on the steric hindrance effect, one can control their chain conformation and flexibility by tuning the structures, types and generation of dendritic pendants, dendronization degree as well as the distance between the dendritic side chains and backbones. As the volume of dendritic pendants increases, the backbone conformation can be stretched, from random coils to semi-rigid or rigid column. Because of these factors, the studies on their chain conformation and flexibility are of fundamental importance in understanding polymer chemistry and physics. Moreover, the modification in the surface and interior of the molecular nano-columns is also a promising subject. So far some achievements have been made to these molecules, one can believe that in the near future, as a bottom-up mean of building nano-molecular materials, dendronized polymers will be applied in many fields, such as modification in the surface of nano-objects, nano-catalysts, photoelectric functional materials and biochemical materials and so on. However, the synthesis of this kind of polymers is still a challenge. The mostly used grafting strategy and macromonomer strategy have their own disadvantages, such as low yield, long reaction period, and strict stoichmetric ratio and so on. Even worse, sometimes the separation of target molecules from side-products is impossible. Thus one rather simple synthesis strategy for dendronized polymers is demanded to promote the studies on them and to expand their applications.Supramolecular chemistry refers to the area of chemistry beyond the molecules focuses on the chemical systems made up of a discrete number of assembled molecular subunits or components. While traditional chemistry focuses on the covalent bond, supramolecular chemistry examines the weaker and reversible noncovalent interactions between molecules. These forces include hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, pi-pi interactions and electrostatic effects. The study of non-covalent interactions is crucial to understanding many biological processes from cell structure to vision that rely on these forces for structure and function. The self-organization with the employment of Coulombic interaction is called ionic self-assembly (ISA) and has been used in the preparation of polyelectrolyte-surfactant complexes in the recent decades. This strategy has some advantages, such as facile preparation and separation, well-defined and stable result structures, and 1:1 stoichimetric ratio of anion and cation and so on. However, the surfactants adopted in this system are merely single- and double-tailed, and the limited structure of surfactants results in that the highly ordered supramolecular structures are usually of lamellar phase, which greatly confined the applications and studies on them. So far, very little works involved other complex structures in this system.Basing on the review on dendronized polymers and comb-shaped supramolecular polymers, we propose in this paper to prepare dendronized polymers via complexing ionic Frechet-type dendritic amphiphiles with oppositely charged polyelectrolyte through ionic interaction. The result first-, second- and third-generation dendronized polymers are labled as PG1, PG2 and PG3, respectively. Their chemical structures were confirmed by nuclear magnetic resonance (~1H-NMR), Fourier transform infrared spectroscopy (FT-IR), UV-visible spectroscopy (UV-vis) and elemental analysis (EA). According to EA results, the grafting degrees were also obtained as 83%, 74% and 64% for PG1, PG2 and PG3, respectively. This decrease is attributed to the steric hindrance caused by increasing volume of dendrons as the generation increases.The chain conformation of these complexes in dilute organic solutions is also investigated, revealing the effects of dendrons generation. Light scattering (LS) and atomic force microscopy (AFM) reveal that in dilute THF solution, all the three complexes take a similar flowerlike conformation with the formation of multiplets, which is much different with the conformation change of conventional covalently linked dendronized polymers. This chain conformation can be also used to explain that the low order of supramolecular structures formed by ionic interaction based dendronized polymers.Aggregation is another important property of these dendronized polymers complexes. Dynamic light scattering (DLS) showed addition of salts (LiCl) to complexes solution can lead to the conversion from individual chains to multi-layer reversed micelles then to thin membrane reversed vesicles. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and AFM combined with static light scattering (SLS) were employed to confirm the structure of these self-assemblies. The formed vesicles exhibited high stability upon dilution, and the polyelectrolyte properties of them were also observerd. In higher salinity, macroscopic precipation can occur, accompanying the appearance of huge vesicles with diameters of several micrometers.The thermal properties of resulting dendronized polymers in bulk were investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA), showing that these polymers have high thermal resistance and the crystallinity property of alkyl chains were changed due to the localization of dendrons. Small angel X-ray scattering (SAXS) as well as X-ray diffraction (XRD) both showed that the ordered supramolecular structure could be formed in bulk, lamellar phase by PG1 and columnar phase by PG3.In the last chapter, the relationship between molecular geometry and self-assemblies is explored based on two synthesized dendritic amphiphiles. We experimentally observed that the volume change of solvophilic segment can strongly affect the size of formed aggregates, and some theoretical explanations are also displayed. This work demonstrates the possibility of tuning the morphology and size of supramolecular structures by tailoring the molecular shape.