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Mechanisms of association of supramolecular polymers

The mechanisms of association of supramolecular polymers are strongly dependent on the non-covalent interactions that participate in the self-assembly. In supramolecular polymerization process, there are three main growth mechanisms: isodesmic model, ring-chain model, and intermolecular cooperative model (Figure 8) [23] [24].
The isodesmic model (Figure 8-a) of polymerization is similar to step growth polymerization in conventional polymers, the strength of growing polymer chains is unaffected by the length of the chain. The energy of assembly is equivalent at each step, thus each step of decrease of the corresponding free enthalpy is the same. This is reflected in a single association constant for the whole polymerization process, as shown below. M1 represents the monomer, whose molar equilibrium constant is K. Assuming no ring formation in the process, the degree of polymerization (DP) only depends on the association constant Ka and the total concentration of monomer [M], which will be approximately proportional to (Ka [M])1/2 [25]. Furthermore, no critical concentration or temperature is required for polymerization process [26] [27] [28].

Properties of supramolecular polymers

By comparison with conventional bonded polymers, a variety of non-covalent interactions play their part in supramolecular polymers, which can lead to multiple structures and formation models (Figure 9).
While supramolecular polymers can’t compete with covalent polymers in terms of mechanical strength or the elasticity of plastics, their dynamic and reversible nature endow them new properties in the biological field, electronic field, and mechanical field [21] [37] [38] [39].

Optoelectronic properties

Natural photosynthesis process proved as source of inspiration for researchers to develop optoelectronic materials to achieve the light-to-charge conversion for a wide range of applications such as electronic devices and smart windows. Simple organic molecules display elaborately designed amphiphilic interactions, quadrupole interactions, and hydrogen bonding. These supramolecular polymers show excellent properties in switching speeds, optical contrast and power conversion efficiency which are important factors for optoelectronic devices. For example, Nakayama et al. prepared a barbiturate oligo-butylthiophene based supramolecular polymer, which bore hydrogen bonds to precisely govern self-organization in the nano-level. The tapelike supramolecular array was directly observed by STM at a liquid–solid interface and further formed into helical nano-fibers in solution and bulk states by TEM, AFM and XRD (Figure 10). They gave powerful proofs that showed a high conversion efficiency of 4.5% [40].

Biological properties

Supramolecular polymers self-assemble by means of the combination of non-covalent interactions, which are highly vulnerable to depolymerization under external stimulus, so they are good candidates as the bioactive materials to accomplish high control over both stability and dynamics. Biological applications based on supramolecular polymers mainly involve the fields of drug delivery, bioimaging, protein/gene delivery, gene transfection, and tissue engineering.
Dankers et al reported the design of ureidopyrimidinone-based multicomponent supramolecular polymers in aqueous solution. They synthesized different classes of monomers through linking the ureidopyrimidinone (UPy) unit to oligo ethylene glycol (OEG) chains by three ways: amine group as end-functionalized group to obtain the cationic monomer; acetamide as end-functionalized group to obtain the neutral monomer; Cy-5 as end-functionalized group to obtain the fluorescently labeled monomer (Figure 12). They demonstrated the ability of their system for intracellular siRNA delivery and provided a potential platform for bioimaging and biosensing [42].

Driving forces for supramolecular polymers

Supramolecular polymers are constructed from small molecules as building blocks, which interact through the same reversible non-covalent interactions defined previously for supramolecular chemistry in general (Figure 13). This section will discuss different bonding motifs that are employed for supramolecular polymerization: hydrogen bonds [43] [44] [45], coordination bonds with a metal ligand and host-guest interactions [46] [20].

Multiple hydrogen bonds

Multiple hydrogen bonds, the first type of non-covalent bonds, were adopted to assemble supramolecular polymers. Supramolecular polymer based on hydrogen bonds was reported by Jean-Marie Lehn for the first time in 1990. The polymer synthesized via a triple hydrogen bonds show very good performance in liquid crystal [47] [48]. In 1997, Meijer et al. prepared a linear supramolecular polymer formed via quadruple-hydrogen-bonding arrays [49]. The polymer presented a higher association constant and degree of polymerization in CHCl3 solution. Since the discovery of construction of supramolecular polymers employing hydrogen bonding units, the researchers have developed numerous other examples. Now multiple hydrogen bonds, a valuable type of “intermolecular glue,” are one of the most widely applied non-covalent interactions.

Metal coordination bonds

The organization of metal coordination bonds into metallosupra of molecular polymers through non-covalent interaction has been extensively studied. Because of the mixed of properties from organic polymers and those of metal properties (ions, magnetic, optical, electronic or catalytic), metal coordination polymers have been a flourishing interdisciplinary research topic [20] [50] [51].

NMR-Rotating-frame nuclear Overhauser Effect SpectroscopY (NMR-ROESY)

In NMR spectroscopy, the Nuclear Overhauser Effect (NOE effect) provides accurate information on the relative orientation of a molecule in a supramolecular polymer, such as a protein or other large biological molecule with a three-dimensional structure. The NOE effect is based on spin relaxation from two atoms interaction. And it is only related to the proximity of two atoms in the space rather than through bond J couplings. In other words, this effect is almost determined by the inter-nuclear distance (distance limited to 6Å) [72] [73].
The two experiments, called NMR-NOESY and NMR-ROESY, have the same principle and are performed in two dimensions, which can detect this effect by visually measuring the integration of the coupled atoms. The frequency of the spectrometer (ω) and molecules rotational correlation time (τc) are two important parameters for the NOE intensity. And the rotational correlation time (τc) directly depended on solvent viscosity and the molecular weight. Higher viscosity and larger molecular weight correspond to longer relaxation time. For small sized molecules (MW<600), the NOE is positive; for medium sized molecules (MW between 700-1200), NOE is zero; for large sized molecules (MW>1200), NOE is negative (Figure 19). Hence for molecules with a molecular weight between 700-1200 g·mol-1, the NMR-ROESY is used to measure the NOE effect rather than the NMR-NOESY [72] [74].

NMR-Diffusion Ordered SpectroscopY (NMR-DOSY)

Two-dimensional NMR-DOSY becomes a powerful tool for characterization of supramolecular polymers [76] [77], which is a technique for qualitatively estimating the diffusion coefficient (D) of species in sample solution. A large diffusion coefficient corresponds to small supramolecular polymer, when lowering of the diffusion is associated to the growing size of the supramolecular assembly. Moreover, the size of supramolecular polymers can be calculated according to the Stokes-Einstein equation.
D Coefficient of diffusion (m·s-1).
kB Boltzmann constant (J·K-1).
T Kelvin temperature (K).
η Viscosity of the solvent (Pa·s) r.
The radius of the spherical particle (m).
The result at the basis of the relationship mentioned above holds true for many systems. However, the disadvantage of this method is that a rough estimate of the average molar weight of the supramolecular polymer causes the different supramolecular polymer system to badly meet the requirements of the Stokes-Einstein equation model [20].
The DOSY mechanism is based on the Pulsed Field Gradient Spin Echo (PFG-SE) experiments [78]. First a 90° magnetic pulse ensures all spins to be placed in the same direction. Then applying a pulsed field gradient (PFG) of duration (δ) gives rise to a phase shift of the global magnetization. That is to say, the different vertical positions of the molecules in the NMR tube result in the application of different magnetic pulses. After diffusion delay, a period of Δ/2, the same gradient of the pulsed field in the opposite direction then applied to reverse the magnetization. In the absence of diffusion phenomenon, the spin echo obtained is the same as the beginning of the experiment (Figure 21-a); on the other hand, in the presence of diffusion phenomenon, the faster the molecular diffusion in the tube, the smaller the spin echo obtained (Figure 21-b) [79] [80].

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Isothermal Calorimetric Titration (ITC)

ITC [81] is a physical technique that allows to determine the interaction between two molecules thermodynamic parameters of interactions in solution, so it is the quantitative method of multiple molecules of supramolecular polymer systems and diverse bio-molecular interactions. This method works by directly measuring the absorbed or released heat during the two compounds binding process. Particularly, it is the only method that can give all binding parameters at the same time in a single experiment. The parameters involve binding constant, reaction stoichiometry, enthalpy (ΔH) and entropy (ΔS).
The device of the thermal core includes two isolated cells with an adiabatic shield (Figure 22-a). The two cells are filled with a certain concentration of the sample solution and the solvent as a reference, respectively. Taking a host-guest system as an example, firstly, two cells are set to the desired experimental temperature and kept at the same temperature during the whole experiment. Then, a given concentration of guest solution is loaded into the micro-syringe which locates above the sample cell. A series of heat changes in the cell during the formation of the inclusion complex caused by the injection are detected and measured (Figure 22-b). ITC experiment not only characterizes the affinity constant between two molecules but also explains the mechanisms of molecular interactions.

Dynamic Light Scattering (DLS)

DLS is a well-established tool that provides information (hydrodynamic radius) about the particle size distribution of supramolecular polymers. This technique is suitable for particle with hydrodynamic radius in the range of 2-500 nm [82].
The principle of the DLS experiment is based on the difference in light diffusion as a function of the size of the molecules. The DLS experiment works by measuring the scattered laser light intensity during the illuminating the liquid sample over time. The molecules diffusion subjects to Brownian motion. As a result, it means the larger the particle, the slower the diffusion, and vice versa (Figure 23). This method only results in an approximation of the species size range rather than an exact value. In addition, the size distribution information of small species solutions cannot be truly reflected due to the possibility of interference from a small amount of large particles in the solution. This might lead to misleading results for the size distribution of analytes by DLS [83].

Table of contents :

1. Supramolecular Chemistry
1.1. Introduction
1.2. The origins of supramolecular chemistry
1.3. From molecular to supramolecular chemistry
1.4. Nature of supramolecular interactions
1.4.1. Ionic-dipolar interactions
1.4.2. Van der Waals interactions
1.4.3. π-Interactions
1.4.4. Hydrogen bonding
1.4.5. Hydrophobic effects
2. Supramolecular Polymer
2.1. Definition
2.2. Mechanisms of association of supramolecular polymers
2.3. Properties of supramolecular polymers
2.3.1. Optoelectronic properties
2.3.2. Mechanical properties
2.3.3. Biological properties
2.4. Driving forces for supramolecular polymers
2.4.1. Multiple hydrogen bonds
2.4.2. Metal coordination bonds
2.4.3. Host-guest interaction
2.5. Characterizations of supramolecular polymers
2.5.1. NMR spectroscopy 1H-NMR nuclear Overhauser Effect SpectroscopY NMR-Diffusion Ordered SpectroscopY (NMR-DOSY)
2.5.2. Isothermal Calorimetric Titration (ITC)
2.5.3. Dynamic Light Scattering (DLS)
2.5.4. Viscometry
2.5.5. Small-Angle Neutron Scattering (SANS)
3. Cyclodextrins Based Supramolecular Polymers
3.1. Cyclodextrin (CD)
3.1.1. Structure and properties of cyclodextrin
3.1.2. Properties of cyclodextrin cavity Inclusion complex of cyclodextrins Guest of inclusion complex of cyclodextrins
3.1.3. Reactivity of cyclodextrins
3.2. Supramolecular polymers based on cyclodextrin in solution
3.2.1. Supramolecular polymers of AnBm type
3.2.2. Supramolecular polymers of AB type
4. Conclusion
1. Selective Functionalization of Cyclodextrins Debenzylation
1.1. Synthesis of cyclodextrin diol with diisobutylaluminum hydride
1.2. Mechanism of DIBAL-H mediated debenzylation
1.3. Selectivity rationalization for α- and β-cyclodextrins debenzylation
2. Our Previous Work: Supramolecular Polymerization Based on β-Cyclodextrin- Adamantane in Aqueous Solution
2.1. Initial considerations and preliminary experiences
2.2. Further experiences with new strategies
3. System Design
4. Synthesis of Functionalized CD/Adamantane Monomers
4.1. General retrosynthesis
4.2. Synthesis of the common precursor: bi-azide cyclodextrin
4.3. Synthesis of bridged neutral cyclodextrin/adamantane monomer
4.4. Synthesis of bridged cationic cyclodextrin/adamantane monomer
4.5. Synthesis of difunctionalized bridged β-cyclodextrin-adamantane monomer: functionalized 1-deoxynojirimycin derivative
4.6. Conclusion of synthesized cyclodextrin/adamantane monomers
5. Conclusion
1. Characterization of the Supramolecular Assembly
1.1. Characterization of supramolecular assembly by 1H-NMR
1.2. Characterization of supramolecular assembly by NMR-ROESY
1.3. Characterization of supramolecular assembly by NMR-DOSY
1.4. Characterization of supramolecular assembly by ITC
1.5. Characterization of supramolecular assembly by viscosity
1.6. Characterization of supramolecular assembly by DLS
2. Study Influence Factors on Polymerization of Supramolecular Polymer
3. Conclusion
1. General Conclusion
2. Perspectives
1. General Procedures
2. Nomenclature for Protons:
3. Synthesis


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