Cu- and Fe-mediated Atom-Transfer Radical Polymerization in Aqueous Solution
von Sebastian Smolne
Datum der mündl. Prüfung:2016-06-06
Betreuer:Prof. Dr. Michael Buback
Gutachter:Prof. Dr. Michael Buback
Gutachter:Prof. Dr. Philipp Vana
EnglischThe mechanism and the kinetics of metal-catalyzed radical polymerization were investigated by spectroscopic methods and by PREDICI® simulation. A particular focus was on Cu- and Fe-mediated atom-transfer radical polymerization (ATRP) in aqueous solution of poly(ethylene glycol)ether methacrylate (PEGMA) and on monomer-free model systems. The propagation kinetics of PEGMA in aqueous solution were determined between 20 and 77 °C via PLP–SEC. The significant dependence of kp on monomer content is due to the difference in the degree by which internal rotations of the transition state for propagation are hindered. Chain-length-dependent termination was analyzed in terms of the composite model for PEGMA in aqueous solution via SP–PLP–EPR. The termination rate coefficient for two radicals of chain-length unity, kt1,1, scales with the inverse viscosity of the solution prior to polymerization. The composite-model parameters for the short-chain and long-chain regime, αs and αl, respectively, are independent of water content, whereas the crossover chain length, ic, decreases toward higher dilution. Cu-mediated ATRP in aqueous solution of the monomer-free model systems and of PEGMA polymerizations with CuBr/2,2’-bipyridine acting as the catalyst and 2-hydroxyethyl 2-bromoisobutyrate (HEMA Br) as the initiator were studied via online Vis/NIR spectroscopy. In the monomer-free model system, PEGMA was replaced by poly(ethylene glycol) dimethylether (PEO) to mimic an ATRP situation. The SP–PLP–EPR technique was used for the first time to measure an ATRP deactivation rate coefficient, kdeact, in aqueous solution. Excess NaBr has been added to the polymerization system to avoid water-assisted dissociation of the Br-Cu species. The activation–deactivation equilibrium constant, KATRP, was measured at different water concentrations. In both the model system and the PEGMA polymerization, KATRP increases by about three orders of magnitude in passing from the bulk monomer toward a water environment. Since kdeact is independent of water content, the change in KATRP is essential due to the effect of the aqueous environment on the activation rate coefficient, kact. Kinetic analysis of the model system in conjunction with PREDICI® simulation under variation of NaBr concentration shows that NaBr does not affect kact and kdeact, and thus has no impact on KATRP. PREDICI® simulation of the ATRP systems however tells that the concentrations of water and NaBr influence dispersity and the degree of chain-end functionality. Addition of at least five equivalents salt with respect to the total catalyst concentration are essential for carrying out successful ATRP experiments in aqueous solution. Fe-mediated RDRP studies were performed with the bio-inspired protoporphyrin IX containing a ferric ion catalyst with an additional axial bromide ligand, Fe/Br mesohemin-(MPEG500)2. The catalyst was kindly provided by the Matyjaszewski group. The Fe-catalyst was studied by combined Mössbauer and online Vis/NIR spectroscopic analysis for the relevant Fe species. The interplay between ATRP and an organometallic reaction (OM), which includes the reaction of propagating radicals with FeII, may occur depending on the ratio of FeII/FeIII concentrations. The SP–PLP–EPR method was also applied to measure kdeact for the FeIII/Br-mesohemin-(MPEG500)2 catalyst in aqueous solution. Toward higher water content, kdeact increases by about one order of magnitude from 30 to 90 wt% H2O, which is beneficial for ATRP control in diluted aqueous solution. The activation–deactivation equilibrium and the addition of radicals to the FeII catalyst, kadd,Fe, were measured for the Fe/Br-mesohemin-(MPEG500)2 complex via UV/Vis spectroscopy in combination with stopped-flow injection. KATRP was found to be insensitive toward water content in the concentration range between 50 and 70 wt% H2O, whereas kadd,Fe exhibits an increase by a factor of five. It could be shown that kdeact exceeds kadd,Fe by almost one order of magnitude, and that the control operates exclusively by ATRP. The rate coefficients determined within this thesis allow for the prediction of dispersity, chain-end functionality and conversion vs time profiles for Cu- and Fe-mediated ATRP of PEGMA in bulk and aqueous solutions with the investigated catalysts and with catalysts of similar reactivity.
Keywords: ATRP; Cu-mediated ATRP; Iron-mediated ATRP; aqueous solution; Propagation and Termination; rate coefficients