In-situ Investigation of Transition Metal Catalyzed Polymerization Processes
The catalytic polymerization of olefins is an important branch of commercial polyolefin production. The material properties of the resulting polymers can be controlled by the design of the active type of these catalysts, accessible through the activation of transition metal complexes. Methylaluminoxane (MAO) represents in this context one of the most important activators. Although MAO has been used in polyolefin production for many years, there is little knowledge available about the structure of MAO and the relationship between MAO’s structure and the activation of the transition metal complex. Within the MAO Robots project, small angle scattering is used as tool for the structural analysis of MAO. Using in-situ measurements by means of small angle neutron scattering and nuclear magnetic resonance spectroscopy, the influence of MAO in the catalytic synthesis of polyolefinis can be investigated.
Polymer-colloid composites can be synthesized as core-shell particles consisting of an inner solid and inorganic core surrounded by a soft polymer shell. A new "grafting to" technique, developed in our laboratory, allows the functionalization of silica particles with anionically-produced polymers.
Based on a two-step procedure, the silica nanoparticles are first modified with multifunctional chlorosilanes. This procedure allows the original Si-OH surface groups to be replaced by Si-Cl groups. Second, the anionically-synthesized polymers are linked to the Si-Cl functionalized nanoparticle surface. Both the chlorosilane functionalization of the nanoparticles and the subsequent reaction with living polymers can be carried out without irreversible particle aggregation. The grafting densities obtained are similar to the ones reported using "grafting from" techniques which are mainly based on controlled radical polymerization. Our new approach offers the possibility to obtain hybrid materials containing polymers which cannot be produced via controlled radical techniques, such as polydienes. In addition, the new technique allows polymers with molecular weights of up to 500,000 g/mol to be grafted and still maintain narrow molecular weight distributions.
The physical properties of a polymer are influenced not only by the choice of monomer or the degree of polymerization but also to a large extent by the polymer architecture. Among the many different possible architectures, that of cyclic polymers occupies a special position. While most polymer architectures involve branched polymers and thereby increase the number of end groups, cyclization eliminates the end groups of a linear polymer, which gives rise to the unique properties of cyclic polymers. This leads to completely different polymer dynamics compared to the processes existing in linear or branched polymers.
Although macrocycles are a common by-product in many polymerization and polycondensation reactions, the specific synthesis of cyclic polymers is much less common. Our goal is to obtain thermally stable, cyclic polymers of high purity, controlled molecular weight and with a narrow molecular weight distribution. The synthetic strategy uses anionic polymerization for the preparation of α,ω-difunctionalized polymers and subsequent ring closure under highly diluted conditions. The chemical modification of the chain ends of the remaining linear polymers allows the efficient separation from their cyclic counterparts.
The building blocks of supramolecular polymers are held together by relatively weak intermolecular interactions such as hydrogen bonds, ionic interactions or metal ligand interactions. The reversible bonds permit the self-healing of structural damages. The success of this process depends on different conditions. One of these is self-assembly. Only when suitable bonding motives fit together, can a repair take place successfully.
Text field: Formation of a supramolecular (a) polymer and (b) networkOne challenge comprises the synthesis of monomers with functionalized end-groups The polymer scaffold, based on polyethylene oxide (PEO) or polybutylene oxide (PBO), is extended by groups capable of forming hydrogen bonds so that these end-groups are able to form reversible bonds. In this way, both supramolecular polymers and networks are synthesized (see figure).