Kinetics and Mechanism of Cu-Catalyzed Atom Transfer Radical Polymerization
von Nicolai Sörensen
Datum der mündl. Prüfung:2015-05-26
Betreuer:Prof. Dr. Michael Buback
Gutachter:Prof. Dr. Michael Buback
Gutachter:Prof. Dr. Philipp Vana
EnglischThe kinetics and mechanism of Cu-mediated atom transfer radical polymerization (ATRP) were investigated with particular emphasis on the ATRP deactivation rate coefficient, k_deact . The method of pulsed laser polymerization (PLP) induced by a laser single pulse (SP–PLP) was applied in conjunction with electron paramagnetic resonance (EPR) for the analysis of radical concentration. As the ATRP equilibrium is superimposed on a conventional radical polymerization scheme, the decay of radical concentration after instantaneous laser-induced production of an intense burst of radicals occurs as a consequence of both deactivation and radical-radical termination. The measurement of k_deact thus consists of two steps: First, termination is measured via SP–PLP–EPR for the Cu-free monomer system. Within a second step, the decay of radical concentration is monitored with the Cu(II) complex being present. The novel strategy has been applied to homopolymerizations of butyl acrylate (BA) and dodecyl methacrylate (DMA). HMTETA, PMDETA and TPMA were selected as the ligands to Cu. The first two are a linear tri- and a tetradentate ligand, respectively, whereas TPMA is a branched tetradentate ligand. In the Cu-free DMA system, the crossover chain length i_c showed a strong decrease toward higher temperature. It is the first time that a temperature effect on i_c was observed. In case of DMA polymerization with TPMA, k_deact was found in close agreement with the number for the monomer-free model systems, whereas with HMTETA and PMDETA, k_deact for the polymerizing system was lower by about one order of magnitude which effect is assigned to steric strain on the polymer chain due to the α-methyl group. The values of k_deact for BA polymerization are close to the ones determined for the model system. With BA, an additional reaction channel occurs in which the Cu(I) species reacts with acrylate radicals. Thus, SP–PLP–EPR experiments on BA in the presence of an ATRP Cu catalyst were performed and the produced Cu(II) species was detected. In addition, timeresolved spectra were recorded. The results suggest that the formation of organometallic species mainly depends on the ligand to Cu and is enhanced for highly active catalysts.
Keywords: atom-transfer radical polymerization (ATRP); reversible-deactivated radical polymerization (RDRP); copper; deactivation; termination; chain-length dependence; electron paramagnetic resonance (EPR); kinetics; spectroscopy; pulsed-laser polymerization (PLP)