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    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

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Record number 344240
Title Supramolecular coordination polymers in water: rings, chains and networks
Author(s) Vermonden, T.
Source Wageningen University. Promotor(en): Ernst Sudhölter, co-promotor(en): Ton Marcelis. - s.l. : S.n. - ISBN 9789085041481 - 128
Department(s) Organic Chemistry
Publication type Dissertation, internally prepared
Publication year 2005
Keyword(s) polymeren - moleculaire structuur - synthese - macromoleculen - supramoleculaire chemie - polymers - molecular conformation - synthesis - macromolecules - supramolecular chemistry
Categories Organic Chemistry
Abstract Supramolecular polymers are polymers in which the monomers are held together by non-covalent interactions. In solution these polymers can break and recombine reversibly yielding polymers with an average degree of polymerization. This thesis is devoted to water-soluble coordination polymers, in which the bonds between the monomers are based on metal ion coordination. The most successful ligands used in this research project to construct reversible coordination polymers are based on pyridine-2,6-dicarboxylate groups connected by oligoethylene oxide spacers of different lengths. These ligands were used for the research described in chapters 2, 3 and 4.

Chapter 2 deals with the formation of water-soluble reversible coordination polymers of Zn 2+ ions with bifunctional ligands that differ in spacer length. Besides linear chains also rings are formed. Viscosity measurements were used to follow the formation of chains and rings as a function of the ratio between metal ions and ligands, the total ligand concentration, and the temperature. To explain the experimental results a theoretical model was developed that accounts for the formation of both chains and rings. At low concentrations and at a 1:1 metal to ligand ratio, a large fraction of the ligand monomers is incorporated in small rings, with a small contribution to the viscosity. Rings are less important at higher concentrations, or if one of the two components is in excess. Also the length of the bifunctional ligands determines the amount of rings that are formed. The largest fraction of rings is found for bifunctional ligands that are just long enough to form a monomer ring around one metal ion. The fractions of monomers in chains and rings could be estimated from 1 H NMR measurements and they are in good agreement with the model predictions. With increasing temperature, the fraction of monomers in rings decreases. As a result, the reduced viscosity increases with increasing temperature.

In chapter 3, the formation of soluble supramolecular three-dimensional coordination polymers with Nd 3+ and La 3+ in aqueous solution is described for two bifunctional ligands that differ in spacer length.Neodymium(III) ions can bind three terdendate ligand groups.Viscosity measurements were used to monitor the network formation as a function of the ligand concentration and the ratio between metal ions and ligands. For corresponding conditions, solutionscontainingNd 3+ andligands with short spacersgave always much higher viscosities than solutionscontainingNd 3+ and ligands with longer spacers.The ligand with the longer spacer isflexible enough to bind with both chelating groups to only one metal ion (ring-formation). This causes the polymers to stop growing, resulting in smaller average sizes of the three-dimensional polymers. The ring-structures could be demonstrated by 1 H NMR spectroscopy using La 3+ at low concentrations.

At very high concentrations of the three-dimensional polymers, viscoelastic materials are obtained. The rheology of these reversible coordination polymer networks in aqueous solution is described in chapter 4. The polymers are formed byneodymium(III) ions and bifunctional ligands. The rheological properties of the viscoelastic materials can be described with the Maxwell model. The scaling of the elastic modulus, relaxation time and zero-shear viscosity with concentration are in good agreement with the predictions of Cates' model that describes the dynamics of linear equilibrium polymers. This indicates that the networks have only few cross-links and can be described as linear equilibrium polymers. The gels are also thermo-reversible. At high temperatures, fast relaxation was found, resulting in liquid-like behavior. Upon cooling, the viscoelastic properties returned immediately. From the temperature dependence of the relaxation time,an activationenergy of 49 kJ/mol was determined for the breaking and reptation of the polymers.

In chapter 5, the syntheses of four different ligand derivatives are described. These ligands are potential candidates for the construction and study of coordination polymers. The different 4-functionalised pyridine-based ligands were synthesized with aminomethyl, oxazolinyl, pyrazolyl and methylimidazolyl groups at the 2- and 6-position, respectively. The nitrogens of these groups together with the pyridine nitrogen can act as terdendate ligands for metal ions. Synthetic handles on the 4-position of the pyridine group were introduced via ether or ester bonds leading to monofunctional, bifunctional and amphiphilic ligands.

From the four synthesized bifunctional ligands described in chapter 5, only two were stable and soluble enough to study the coordination polymer properties with Zn 2+ . In water, a bifunctional ligand with two 2,6-bis(aminomethyl)pyridine groups as complexing groups was used. In an organic solvent (chloroform/acetonitrile), a bifunctional ligand with two 2,6-bis(methylimidazolyl)pyridine groups was used. The reversibility of the coordination bonds in these two coordination polymers was compared in the solvents mentioned. Viscosity measurements were used to follow the formation and breaking of the polymers as a function of the molar ratio for the ligands with 2,6-bis(methylimidazolyl)pyridine groups in an organic solvent. The breaking of the polymers made of the water-soluble ligands with 2,6-bis(aminomethyl)pyridine groups was shown by addition of monofunctional ligands. Viscosity measurements of the coordination polymers in water showed fast equilibration upon changes in concentration. In an organic solvent no changes in the size of the structures were found upon dilution. 1 H NMR measurements were used to monitor the ring-chain equilibrium of the coordination polymers containing the ligands with 2,6-bis(methylimidazolyl)pyridine groups in a chloroform/acetonitrile mixture. The coordination polymers in the organic solvent showed some exchange, but much slower than in the water-based system. Therefore, to prepare coordination polymers whose properties can be tuned rapidly by means of external changes, water is a more appropriate solvent than non-coordinating organic solvents. 

Since the best material properties for water-soluble coordination polymers in this thesis are obtained by using metal ions that can act as branch points, further research may be directed to the use of multifunctional ligand molecules as alternative. A first attempt in that direction is described in chapter 7. Two trifunctional ligand molecules with pyridine-2,6-dicarboxylate groups are synthesized and the viscosity of solutions containing mixtures of bifunctional and trifunctional ligands was studied as a function of molar ratio. To increase the viscosity in water by increasing the percentage of trifunctional ligand a very polar but also rather flexible trifunctional ligand is necessary.
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