Free radical polymerization has been revolutionized by the advent of methodologies that can significantly extend the lifetimes of the propagating radicals and minimize termination events, thus providing access to well-defined polymer structures with pre-determined molecular weights and low polydispersities. Most importantly, these protocols can be employed to construct complex macromolecular architectures of variable shape and form ranging from linear block copolymers to star and palm-tree like structures. We mainly employ reversible addition fragmentation chain transfer (RAFT) chemistry, arguably the most versatile of the living/controlled protocols. At the same time, we also use alternative methodologies such as ring opening polymerization (ROP) and atom transfer radical polymerization (ATRP) to complement RAFT chemistry.

While we rely on existing control methodologies for polymerization processes, we constantly strive to expand the existing technologies (e.g. by the design of novel agents for reversible addition fragmentation chain transfer (RAFT) chemistry) or develop entirely new approaches to effectively control the position and polydispersity of the molecular weight distribution of variable (functional) polymers. Typical examples include polymerization control via spin trapping reagents (e.g. thioketones or nitrones).

To develop an effective synthetic approach to polymers with well-defined properties, an in-depth understanding of the underpinning reaction mechanism and kinetics is important. When the reaction mechanism of a polymerization (including the rate coefficients that govern the individual rate coefficients) is known, one can compute the entire polymerization process including the molecular weight and sequence distribution in advance without ever entering a laboratory. In our group, we use the powerful PREDICI software package to model polymerization processes and deduce kinetic rate coefficients of key reaction steps in a close interplay between experiment and simulation. Key objects of interests are the formation processes of complex architecture polymers (such as stars) and the understanding of acrylate reaction kinetics.

While many complex architectures are accessible via living/controlled free radical polymerization methods, some polymer structures can only be constructed by linking two specifically functionalized polymer strands. In addition, modern conjugation protocols allow for the connection of biological molecules to synthetic polymer strains. Typical conjugation methods (so called Click and Clack chemistry) are 1,3-dipolar Huisgen cycloadditions or (hetero) Diels-Alder reactions. In our group, we develop specific conjugation methods that are particularly adapted to RAFT chemistry and operate in an atom efficient fashion. Typically, the carbon-sulfur double bond in the thiocarbonly thio agents acts as a dienophile in a hetero Diels-Alder (HDA) reaction. Such RAFT-HDA reaction sequences can be employed to rapidly construct complex architectures under mild reaction conditions.

Related to the computer based modeling of polymerization processes, we have a keen interest in experimentally deducing the rate coefficients that govern the individual reaction steps in polymerization processes. These include the difficult to obtain termination rate coefficient, where we have developed a novel approach based on living radical polymerization to arrive in a relatively facile fashion at chain length dependent (CLD) termination rates (the so-called RAFT-CLD-T method). Similarly, we are deducing propagation rate coefficients for unusual monomer systems by employing a high frequency (500 Hz) pulsed laser system in conjunction with absolute molecular weight determination via triple detection size exclusion chromatography.

Analysis of the full molecular weight distribution of polymers generated by kinetic experiments allows access to a number of important kinetic rate coefficients in free radical polymerization. Classical detectors used for the determination of molecular weight distributions by size exclusion chromatography (SEC) yield accurate information about the polymer concentration. The molecular weight axis though is uncertain in SEC and existing calibration procedures may introduce errors of up to 30% in the obtained molecular weights. Chromatographic band broadening in SEC imposes further bias on the measured molecular weight distributions. Mass spectrometry has the potential to yield exact molecular weights of individual molecules. However, today highly accurate molecular weight distributions (MWD) with errors of less then 1% of synthetic polymers can only be obtained in very limited cases. This is because synthetic polymers do not exhibit one uniform chain length but rather a distribution of molecular weights. Although in mass spectrometry the molecular weight axis is certain, due to instrumental bias and a potential dependence of ionization efficiency on molecular weight and charge-state, abundances of oligomeric ions are not an accurate description of oligomer concentration in the analyzed sample. This research theme aims at deriving accurate molecular weight information on polymers by coupling of size exclusion chromatography with ESI-MS and refractive index (RI) detection. Use is made of the high accuracy of individual molecular weights obtained by mass spectrometry and the possibility of using ESI-MS to depict the concentration profiles of oligomers eluting from the chromatographic column. Absolute concentration information is gained solely from a concentration sensitive (RI) detector. A sophisticated deconvolution approach is applied for data processing. During the course of investigations, additionally to accurate molecular weight distributions, valuable information about the electrospray ionization process of polymers are be gained.

We enjoy generating interesting and complicated polymer structures via sophisticated chemistries. However, it is also important to develop methods which can arrive at complex architectures in straight-forward fashions, using few reagents and those preferably in one pot reactions. Typical examples of chemistries that we have developed in this field include a one pot reversible addition fragmentation (RAFT)/ring opening polymerization (ROP) process that gives facile access to comb block copolymers featuring degradable and non-degradable polymer strands. Other approaches include the initiator free fabrication of polymerizable macromonomers and their subsequent usage as macromolecular scaffolds in conjugation reactions to arrive at palm tree and pom-pom like polymer structures.

A convenient route to generate free radicals – both industrially and academically – is via ultraviolet (UV) initiated processes. Typically, radical fragments are generated by either the direct formation of radicals from solvent and/or monomer units or by the decomposition of an added photoinitiator molecule. UV initiated polymerization – whilst having industrial applications in the curing of coatings – has especially been employed in conjunction with pulsed UV-laser initiation to obtain propagation and termination rate coefficients via the so-called pulsed laser polymerization – size exclusion chromatography (PLP–SEC) and single pulse – pulsed laser polymerization (SP-PLP) techniques. In the context of these techniques, initiation processes have been studied and the reactivity of the generated initiator radical fragments has been estimated. We employ soft ionization MS techniques to accurately map the groups generated under well-defined UV laser initiated polymerizations; the results support the idea that some initiators generate fragments of very variable reactivity, e.g. an initiator fragment that is mainly responsible for initiation processes and one which is almost exclusively undergoing termination. It is the aim of this research strand to employ SEC-ESI-MS to map UV initiated polymerizations with respect to the generated polymer end groups for a range of initiator systems as well as monomers in solvent and bulk systems. The overall aim is to arrive at a quantitative map of how photolytically generated radical fragments react with specific vinyl double bonds. Molecular weight control in UV-initiated systems (so that molecular weight of the polymers is suitably low for ESI-MS analysis) can be achieved via the RAFT processes (provided the RAFT agent is chosen judiciously to not decompose under UV radiation) as well as via the fast pulsing action of a UV laser.