Advances in supramolecular polymer chemistry : well-defined terpyridine-functionalized materials

Abstract

Controlled/living polymerization techniques have attracted enormous attention in the field of polymer science since they have opened an avenue to the preparation of well-defined materials with precisely designed molecular architectures like random, block, graft and comb copolymers. These techniques facilitate the creation of new materials for specialty applications as well as for elucidating the corresponding structure-property relationships. The research described in this thesis was focused on the preparation of well-defined polymers by employing controlled radical and living anionic polymerization processes. In addition, chelating ligands were incorporated at the end of the polymer backbone capable of connection different polymer chains together via non-covalent metal-ligand interactions. The introduction of directional supramolecular motifs in synthetic polymers represents a promising synthetic approach for the development of smart materials which combine the (reversible) binding behavior of supramolecular interactions and the processing advantages of polymers. Their remarkable properties are based on reversible selfassociation and, hence, they posses the capability of self-healing which makes them interesting for advanced applications in fields of nanotechnology, plastic electronics and biomedical purposes. This new methodology provides access to highly complex molecular structures that are extremely difficult or even impossible to synthesize with current covalent techniques. The terpyridine moiety forms octahedral bis-terpyridine complexes with a variety of transition metal ions, such as iron, cobalt, nickel and ruthenium. Special focus in this thesis was on the synthesis of bis-terpyridine ruthenium(II) complexes since this transition metal ion allows the formation of homoleptic as well as heteroleptic complexes in a straightforward fashion. First, different synthetic strategies were applied to prepare bis-terpyridine ruthenium(II) model complexes which were subsequently characterized by means of 1H NMR spectroscopy, UV-vis spectroscopy, MALDI-TOF mass spectrometry, elemental analysis, gel permeation chromatography and, in some cases, single crystal x-ray analysis. These investigations were the foundation for the construction of various polymer architectures, such as polymer monocomplexes, homo dimers, di-, tri- and tetrablock copolymers. For this purpose, the terpyridine ligand was inserted at the chain end(s) of a polymer. Various synthetic strategies were employed including (1) end-group modification of hydroxy end capped polymers by etherification, (2) in situ functionalization of polymers derived by anionic polymerization as well as (3) the utilization of a terpyridine-functionalized initiator suitable for controlled radical polymerization methods, more precisely nitroxide mediated polymerization. The latter approach was intensely explored for the preparation of terpyridine-functionalized homopolymers and block copolymers with well-defined molecular characteristics (predetermined molar mass, narrow molar mass distribution, end group control and architecture). Monomers belonging to different monomer families, e.g. styrenes, acrylates and acrylamides, were polymerized in a controlled fashion and subsequently connected to terpyridine-functionalized polyethylene glycol via ruthenium(II) metal ions. The resulting amphiphilic metallo-supramolecular block copolymers were primarily characterized by 1H NMR spectroscopy, UV-vis spectroscopy and GPC. GPC measurements were performed using an optimized GPC system (5 mM NH4PF6 in DMF as eluent) that suppresses the interaction of the charged metallo-supramolecular complex. In order to prove the formation of the desired block copolymer, a photo-diode array detector was connected to the GPC. The respective 3- dimensional GPC chromatograms revealed the characteristic metal-to-ligand charge transfer band at 490 nm. A versatile post-modification reaction was applied to polymers consisting of pentafluorostyrene building blocks. Using this approach, a variety of multifunctional graft copolymers can be designed simply by reacting substituted amino-compounds carrying the desired functionality. The scope of this approach is seemingly unlimited since it can be applied in any area that includes polymers. Well-defined terpyridine-functionalized polymers were also obtained by employing living anionic polymerization. This strategy allows the access to differently composed polymers including homopolymers, alternating copolymers as well as block copolymers by sequential monomer addition. It was found that the use of 1,1-diphenylethylene as end capper is necessary to promote the functionalization with 4’-chloro-terpyridine. The alternating copolymers were fully characterized by 1H NMR spectroscopy, GPC, elemental analysis, UV-vis spectroscopy and MALDI-TOF mass spectrometry revealing the successful incorporation of the terpyridine moiety at the end of the polymer chain. Moreover, analytical ultracentrifugation (in combination with density and viscosity measurements) was applied revealing an excellent agreement with respect to the molar masses obtained from analytical techniques mentioned before. The morphologies of the amphiphilic metallo-supramolecular block copolymers were investigated in solution. The micellar aggregates formed from these copolymers in water or polar organic solvents have been studied by several analytical techniques, such as atomic force microscopy (AFM), transmission electron microscopy (TEM) and dynamic light scattering (DLS). In particular, one A-B-[Ru]-C triblock copolymer consisting of a hydrophobic (A), a fluorophilic (B) and a hydrophilic block (C), was investigated towards the influence of the solvent on the micelle formation. All basic micellar morphologies (spherical micelles, wormlike micelles and vesicles) were obtained by changing the polarity of the solvent. Moreover, the morphology of the micellar aggregates could be reversibly tuned as a function of temperature due to the upper critical solution temperature (UCST) behavior of the fluorinated middle block. A smaller part of the thesis was dedicated to the preparation of iridium containing polymers. For this purpose, terpyridine-functionalized polymers were reacted with iridium(III) precursor complexes which form, upon bridge-splitting, polymeric mixed ligand iridium complexes. As a result of the metal-to-ligand based radiation, these materials exhibit different emission colors depending the functionality of the introduced N,N-chelating ligand or N,C-cyclometallating ligand, respectively. These luminescent polymeric materials are of special interest due to their potential application in light-emitting devices and solar cells. The optical properties of the iridium containing polymers were investigated by absorption and emission spectroscopy. In general, it can be concluded that the combination of controlled/living polymerization methods and supramolecular chemistry represents a powerful strategy to design complex macromolecular architectures. Metallo-supramolecular polymers provide a basis for potential applications as smart materials due to the fact that the non-covalent bond can be reversibly broken under certain conditions. This characteristic binding behavior can be exploited for the preparation of nanoporous materials and hollow nanocages which could be of interest for applications in catalysis as well as waste water treatment. Moreover, the described triblock copolymers consisting of a hydrophilic, a hydrophobic and a fluorophilic block are promising materials with respect to the incapsulation of various guest molecules into the different core domains. This class of nanomaterials may be used in the fields of drug delivery, catalysis as well as nanotechnology.