Synthesis of linear copolymers and of polymer brushes substituted with spiropyran groups : a step towards light driven molecular walkers

摘要

This dissertation is concerned with the synthesis and properties of different well-defined complex polymer structures based on functional acrylates and methacrylates by means of modern controlled polymerization procedures such as atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer (RAFT) polymerization. The objective was to substitute different functional polymer architectures with photochromic spiropyran molecules to enable light induced energy transfer. These molecules undergo reversible, specific wavelength-induced changes in molecular properties such as colour shifting and conversion of light energy into mechanical switching. These changes are based on the light induced isomerization of the usually non-polar spiropyran species to zwitterionic merocyanine species. Different well-defined linear copolymers, like P(MMA-co-HEMA), P(BA-grad-HEMA-TMS), P(BA-co-HEA-TMS) and P(BMA-co-HEMA-TMS), were prepared via ATRP and RAFT polymerization. Even highly complex polymer brush architectures with different grafting densities, a highly dense grafted, a less dense grafted and a gradient type grafted polymer brush consisting of polymethacrylate backbones and P(BA-co-HEA-TMS) side chains, were synthesized using RAFT polymerization and ATRP. In all cases the microstructure of these polymers was determined via kinetic studies. Cleavage of the TMS protective groups gave access to different polymer architectures with hydroxyfunctional groups that have been successfully used to incorporate spiropyran derivatives. We observed that this polymer analogous reaction can be driven to quantitative conversion in case of linear functional copolymers but conversion decreases with increasing grafting density in case of polymer brushes with P(BA-co-HEA) side chains due to steric hindrance. In fact, in case of highly dense grafted polymer brush architectures the attempt to bind spiropyran molecules to the functional HEA repeating units of the side chains failed. The ability to use light energy to change the properties of the spiropyran substituted copolymers was demonstrated in different experiments using i.e. linear P(BMA-co-HEMA-spiropyran) chains and free spiropyran molecules in comparison in solution and adsorbed on specific substrates. The reversible photoswitching ability was also proven in polymer films coated on mica surfaces by measuring the fluorescence of the merocyanine species. A rather ambitious aim of this thesis was to explore a concept for a light driven molecular walker, which consists of a spiropyran substituted copolymer and uses the light energy for molecular motility. The key-step in the concept is to exploit the reversible switching from non-polar spiropyran to zwitterionic merocyanine species for control of the adsorption/desorption of the macromolecules on mica and thus to effect a macromolecular conformational change of the polymer, i.e. collapse and extension of the macromolecules due to the change in dipole-dipole/dipole-substrate interactions by means of light, following a segment and soldier principle. Scanning force microscopy (SFM) was used to image the different polymer brushes. So far, no light-induced conformational changes were observed upon irradiation. In the case of the brushes this might be due to the very small fraction of spiropyran groups. However, also in case of linear P(BMA-co-HEMA-spiropyran) molecules adsorbed on mica no conformational changes of polymers have been observed by SFM in situ during irradiation with UV- or Vis-light, which has been expected to causes photoswitching to P(BMA-co-HEMA-merocyanine). The attractive forces of the merocyanine dipoles are not high enough to cause the polymers to collapse or photo-switched zwitterionic merocyanine species are trapped by the polar mica surface. However, agglomeration of single P(BMA-co-HEMA-spiropyran) chains has been monitored when the spiropyran substituted macromolecules were irradiated in toluene solution. Here the dipole-dipole-interactions of the merocyanine species affected inter- and intramolecular forces after exposure to UV-light. Once adsorbed on mica, the molecules presented their state, i.e. agglomerated merocyanine polymers or extended spiropyran polymers, even when the merocyanine polymer were exposed to visible light to cause reisomerization to the spiropyran form. In situ SFM on mica demonstrated however that extended single P(BMA-co-HEMA-spiropyran) chains collapse reversibly to globules when they are exposed to ethanol/water vapours respectively. This is explained by changes of the spreading coefficient, which enabled motion of the macromolecules on the surface.