Polymer ChemistryPolymer chemistry at JCNS-1 is primarily concerned with the synthesis of model systems, such as:
- classical and deuterated polmers
- functional materials
- complex polymer architectures
Polymer-colloid composites can be synthesized as core-shell particles consisting of an inner solid, 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. The resulting chlorosilane groups are used in the next step to anchor the anionically produced polymers with the still active head groups on the nanoparticle surfaces. Both steps in the reaction take place without irreversible particle aggregation. The polymer chain density obtained by the new technique is around 1 chain per nm2 of particle sruface and is similar to standard controlled radical "grafting to" techniques. Furthermore, the new technique offers access to composites that cannot be produced using controlled radical methods, such as composites based on polydienes or those with very high molecular polymers.
The physical properties of a polymer are influenced not only by the type 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. Whereas all other structures possess free end groups, these do not exist in cyclic polymers. For this reason, the dynamics of the polymer chains are fundamentally different from processes which occur in linear or branched chains.
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 on a multi-gram scale. The synthetic strategy uses anionic polymerization for the preparation of α,ω-heterodifunctionalized polymers and subsequent ring closure under highly diluted conditions. Through the chemical modification of the chain ends of the non-cyclized linear chains, it should then be easier to separate them from cyclic polymers.
Supramolecular Polymers/Self-healing Materials
Supramolecular polymers are polymers whose building blocks are held together by relatively weak intermolecular interactions such as hydrogen bonds, ionic interactions or metal ligand interactions.
Their 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.
The challenge lies in the synthesis of functionaliized polymers. 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).
Through the combination of reversible and permanent networking, dual networks can be built. The supramolecular part of the network can open up when exposed to mechanical load and thus, by reversible stress relief, the tearing of the material at an early stage can be avoided.
As semiconducting polymers, polyalklthiophenes (PATs) are potential candidates for organic light-emitting diodes (OLEDs), field-effect transistors, photodiodes and polymer-based solar cells. The special properties of PATs, such as high conductivity (doped up to 1000 S/cm) or solvatochromism resulting from a conjugated π system of individual monomer units. Furthermore, PATs demonstrate a considerably higher solubility in organic solvents compared to polythiophenes and are therefore easier to work with (thin films, for example). With their relatively accessibility and modifiability, they offer an ideal area of study for organic electronics.
There are numerous possible synthetic processes for PATs, for example, using coupling reactions, by electrochemical means, or by Fe-catalysed polymerisation. The method developed in the 1990s by McCullough et al., the Kumada Type Coupling Polymerisation (KCTP, often referred to as the Grignard metathesis GRIM) is the most promising method, due to a defined molecular weight, a narrow molecular mass distribution, (PDI ca. 1.10) and regional tactivity (2.5' head-to-tail coupling, HT). In virtually living Ni-catalysed polymerisation, the monomer is transformed into a Grignard bond. In addition, this method enables the functionalisation of start and end groups, made posible by an access point for block co-polymers, which then forms defined nanostructures through supramolecular organization.
The production of deuterated polymers normally takes place through the polmerisation of deuterated monomers. The subsequent replacement of hydrogen with deuterium (H/D replacement) in the polymer is often avoided, as due to the frequently harsh conditions, alterations in the polymer structure may occur.
The necessary deuterium-enriched monomers are either synthesized from commercially deuterated chemicals or produced using an H/D exchange of organic molecules catalysed by a transition metal in a deuterated solvent. For subsequent use in anionic polymerisation, the monomers must meet high levels of purity and deuteration.