Block copolymers : Nanoreactors for the synthesis of coordination polymers

Abstract

This thesis aims for the synthesis and characterisation of coordination polymer (CP)/block copolymer (BCP) nanocomposites. The used coordination polymers are either based on Schiff base-like ligands with an iron(II) centre connected by bis(monodentate) bridging ligands. Alternatively, zinc(II) complexes are also reacted with bis(monodentate) bridging ligands. The diblock copolymer consists of one block polystyrene and another block poly(4-vinylpyridine) resulting in polystyrene block poly(4 vinylpyridine) (PS b P4VP). The PS-b-P4VP polymer self-assembles in suitable solvents like tetrahydrofuran or toluene to micelles where PS is building up the shell of the micelles and P4VP is forming the core. These block copolymer micelles are used as a size template for the formation of nanoparticles of the respective coordination polymer. The size of the micelle cores can be enlarged by raising the percentage of P4VP of the block copolymer. Above a certain ratio also the shape of the micelles can be altered to rods and worm-like structures. The synthesis method for the formation of nanocomposites is as follows: The block copolymer is dissolved together with the respective complex in the suitable solvent and the reaction mixture is heated to reflux. After the addition of the bridging ligand and the subsequent heating the solvent can either be removed by cold distillation or the complex and the bridging ligand can be added simultaneously up to four times. Thus, several different nanocomposites were obtained. The sizes in the solid state and in solution, the crystallinity, the composition and, additionally for iron(II)-based CPs, the spin crossover (SCO) properties were analysed. Nanocomposites of three different one-dimensional iron(II) CPs [FeL(bpea)]n@BCP, [FeL(bpee)]n@BCP, and [FeL(bpey)]n@BCP with varying bridging ligands were synthesised. Their size, magnetic, and SCO properties were investigated. Transmission electron microscopy (TEM) images and dynamic light scattering (DLS) revealed that the sizes of the nanoparticles were equal in size independent from the formed CP (TEM: ~50 nm, DLS: ~150 nm). Microcrystals were observed for some samples in TEM images. The appearance of microcrystals was explained by the stability of the CPs regarding their ligand field splitting, their electronic configuration, and the rigidity of the bridging ligands. The magnetic measurements showed that samples with microcrystals exhibit a bulk-like behaviour, whereas the nanocomposites without microcrystals undergo a gradual spin transition. In the case of the nanocomposite [FeL(bpey)]n@BCP a gradual, two-step spin transition was found whereas the bulk [FeL(bpey)]n features an abrupt, half complete spin transition with a hysteresis width of 10 K. Powder X-ray diffraction explained the variation in the spin transitions of the nanocomposite which showed a different polymorph than the bulk material. By utilising another BCP as template, the particle core size of the BCP and the nanocomposite with the CP [FeL(bipy)]n could be reduced to 15 nm and 16…