Living radical polymerization for the preparation of innovative macromolecular architectures

In this thesis I have dealt with the synthesis of different macromolecular structures in order to create innovative devices. The heart of the process of synthesis has been the RAFT polymerization, a recent polymerization technique which allows the compatibilization of various chemical systems. The aim of this work is the improvement of innovative devices already on the market with good performance, but that possess limitations both as what regards specific technical properties and commercial exploitation. The aspect which has to be improved isn’t related to the device’s functional materials, rather to the compatibilization between them. Often, materials with remarkable absolute performances are used in a device, but these state-of-the-art components suffer from a partial quenching of their properties when incorporated in the final device. For this reason, in recent years, many studies have focused on materials that compatibilize different chemicals structures. For example polymeric composite materials combine the various functional properties of inorganic materials (metals and metal oxides) with mechanical properties of structural polymers. The different chemical nature of these two classes of materials leads to incompatibility, un-mixing and then to the worsening of the final performance in the operating conditions of the device. So it is essential to find materials that allow the different structures to chemically recognize each other through their surfaces. The materials used in this context are the surfactants, namely compounds that possess both polar and non-polar moieties. The same mechanism is at the base of the natural world in which, for example, liposomes form cell membranes which are fundamental for life itself. With this in mind, I focused to the synthesis of amphiphilic materials that possess hydrophilic and hydrophobic parts, therefore affinity with inorganic materials, or water based, and organic materials. This type of structure can be found in macromolecular materials. Access to such complex polymer structures - and concomitantly access to carefully tunable polymer properties - has been greatly enhanced with the advent of living free radical polymerization (LFRP) protocols that allow for the synthesis of multifunctional “chain transfer” agents that can serve as molecular machinery for obtaining polymers with complex architecture. The most prominent among LFRP techniques are Reversible Addition–Fragmentation chain Transfer (RAFT), Atom Transfer Radical Polymerization (ATRP) , and Nitroxide-Mediated Polymerization (NMP). , In particular, the strength of RAFT chemistry lies in its high tolerance to functional monomers and the non-demanding reaction conditions (e.g. tolerance to oxygen and low temperatures) under which the polymerizations can be carried out. In addition, a wide range of monomers with varying reactivity can be used. RAFT polymerization offers substantial versatility when it comes to the synthesis of block copolymers, star polymers, polymer brushes, and other complex polymer systems. The critical key to their synthesis is the presence of chain transfer agent (RAFT agent or CTA) with a thiocarbonylthio end group. Undesired bimolecular termination reactions, high initiator concentrations, or chain transfer to monomer or solvent can reduce the amount of RAFT end capped polymer chains. If carefully designed, RAFT polymerization opens the door to a range of polymer architectures by variable approaches. Similar to other living radical polymerization techniques, block copolymers, star and comb polymers, as well as graft polymers are accessible by attaching the controlling moiety to a (multi)functional core linking moiety. In addition, block structures are obtained by chain extension of the RAFT moiety capped block. Unique to the RAFT process are the possible modes of attaching the RAFT group covalently to the (multi)functional moiety. The first aspect analyzed was the self-assembly of amphiphilic block copolymers into complex architectures. As known block copolymers in the solid state have a separation of phases in the order of nanometers . In addition, by varying the chemical composition of the blocks and their relationship, it is possible to generate a variety of morphologies (spherical, cylindrical, lamellar or gyroidal). This behavior is described by the diagram of Matsen and Schick (see Figure 1) and relates to polymers in their thermodynamic minimum. The introduction of a solvent in the system can be interpreted as a third dimension in the diagram. Thus, in addition to the variation of χN (Flory-Huggins interaction parameter times total number of monomer units) and fx (fraction of the monomeric units x), we can introduce the quantity of solvent. The interaction between the solvent and the chemistry of the block copolymer is a key parameter to determine in what way the self assembly occurs. In the case of complete solubility the polymer will be completely dissolved while selective (or partial) solubility occurs if a single block is dissolved. The latter leads to the formation of particular structures that depend on all previous parameters in addition to temperature and the environmental conditions. Therefore, amphiphilic block copolymers can self-assemble into structures such as micelles, spheres, worm-like assemblies, toroids and polymer gels, depending on the ratio of the selective solvent. With RAFT technique, I have synthesized diblock copolymers constituted by polystyrene and polydimethylacrylamide with different total block length and studied their self-assembly in different solvents and concentrations, with the aim of introducing functional molecules in incompatible matrices (Chapter 4). Also, I produced the triblock copolymer polystyrene-polyethylene oxide and used as a polymer gel for electrolyte in dye-sensitized solar cells (Chapter 5). The advantage of this technique is that the polymer is free from the contamination of metal catalyst. The second topic has been the functionalization of nanoparticles of metal oxides with different polymers. The surface chemistry has been modified by making it more similar to the host matrix, in this way a polymer nanocomposite is created with high performance limiting the de-mixing of the different components. Polymer grafting techniques provide a very versatile tool to tailor the surface of nanoparticles and thus the interfaces between nanoparticles and the matrix polymers. The RAFT technique provides control over the type of polymer to be grafted onto the particle surface, surface densities, and chain lengths at the nanometer scale. The technique of covalently grafting polymer chains onto particles can be categorized into “grafting from” and “grafting to”. The grafting to technique involves reacting the polymer, bearing an appropriate functional group, with the particles to chemically attach the polymer chains. Because of the steric hindrance imposed by the already grafted chains, it becomes increasingly difficult for the incoming polymer chains to diffuse to the surface against the concentration gradient of the existing grafted polymers, which intrinsically results in low graft densities. In contrast, the grafting from technique uses initiators that have been initially anchored to the particle surface, followed by the polymerization from the surface. Since the existing grafted polymers will not hinder the diffusion of the small-sized monomers, significantly higher graft densities can be achieved with this technique. In this study, I was involved in the growth of the polymer to the surface (grafting from) to ensure a high coating density. I have synthesized a polymer shell of polystyrene on nanoparticles of titania to create a nanocomposite TiO2/PS (Chapter 6), which has been tested as a material with high dielectric properties. I also polymerized isoprene on commercial SiO2 in order to introduce it in the production of compounds for tyres and thereby increase the dispersion and improve the dispersion of filler in the rubber matrix (Chapter 7).