| dc.description.abstracteng | This thesis provides an in-depth investigation into the chain-length dependent termination kinetics of radical homo- and copolymerization by using the most powerful method: single pulse–pulsed laser polymerization (SP–PLP) in conjunction with electron paramagnetic resonance (EPR) spectroscopy. In combination with the kinetic simulation package PREDICI®, more detailed insights into the complex copolymerization kinetics were obtained.
For the first time, the composite parameters for the radical homopolymerization of n-pentyl methacrylate (PnMA) in bulk were determined over a wide temperature range. The composite parameters αs and αl, which describe the strength of the chain-length dependence of the termination, perfectly agrees with both literature values for other methacrylates and the theoretically predicted values. Furthermore, the activation energy EA(kt1,1) for the termination rate coefficient of two monomeric radicals was obtained by an Arrhenius plot. Here, kt1,1 showed a clear relationship with the viscosity η and it was observed that the product kt1,1·η is insensitive toward temperature and that the value of kt1,1·η depends on the hydrodynamic radius. All the results of the PnMA polymerization fitted perfectly into the trends within the methacrylate family.
Furthermore, the homopolymerization of 2-ethylhexyl methacrylate (2-EHMA) and dodecyl methacrylate (DMA) was extensively studied in this thesis with the focus on the temperature dependency of the crossover chain length ic. A sigmoidal behavior of the crossover chain length could be observed for both monomers where ic decreases with increasing temperature. Such a temperature dependency of ic could not be observed for PnMA. Hence, it could be demonstrated that the size of ester side chain significantly influences the crossover chain length. This was also confirmed by the determined inflection points of the sigmoidal fits of the
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experimental data for 2-EHMA and DMA. The inflection point of 2-EHMA was at a significantly lower temperature than for DMA. On this point, more experimental data on the chain-length dependent termination kinetics for several methacrylates (PnMA, 2-EHMA and DMA) were obtained within this work. Furthermore, parts of the experimental setup had to be replaced. Thus, these changes could be validated by the investigated methacrylates in this work because they fit perfectly into the tendencies of the other methacrylates.
In the second part of this thesis, the kinetics of the radical copolymerization of styrene and MMA was thoroughly investigated. First, EPR spectra were measured for different compositions. It is worth-mentioning that fully deuterated styrene-d8 was used to simplify the EPR spectra. By comparing the corresponding homopolymerization spectra to the spectrum obtained for the copolymerization, a clear assignment of the different signals in the copolymerization spectra to the macroradicals with the different terminal units was achieved. Moreover, the radical fraction of styrene was directly determined by fitting the EPR spectra via Matlab®. The so-obtained radical fraction of styrene-d8 was significantly higher than the feed fraction of styrene-d8. For understanding this behavior, the penultimate model was applied in conjunction the literature known copolymerization parameters. However, this approach failed to describe the radical fraction of styrene. If the copolymerization parameters were adjusted to the radical fraction of styrene, the model were not able to describe the literature known propagation rate coefficient of the copolymerization and copolymer composition. To address this issue, it was manually analyzed whether a set of copolymerization parameters can describe all experimental data simultaneously. This approach was successful for both penultimate models. In this way, more reliable copolymerization parameters were received. Furthermore, single pulse experiments were reevaluated with a refined PREDICI® model. To do so, four simulation approaches were applied considering the following four aspects: (A) viscosity of the copolymerization mixture, (B) termination reactions, (C) copolymerization parameters and (D) chain lengths of macroradicals. Depending on the simulation variant, kt,cross1,1 and kt,copo1,1was determined via PREDICI® modelling based on the implicit penultimate model with a
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parameter estimation. If the termination reactions were separated into cross-termination and homo-terminations under consideration of the viscosity of the copolymerization (simulation variant A), no realistic values were obtained for cross-termination rate coefficient kt,cross1,1. Therefore, the termination reactions were treated equally in another simulation (simulation variant B). Thus, a more realistic kt,copo1,1 was received. Since the copolymerization parameters from literature could not describe the radical fraction of styrene, a further simulation (simulation variant C) was performed with the manually fitted copolymerization parameters, and hence a realistic kt,copo1,1 was successfully obtained. In the last simulation (simulation variant D), different chain length of the macroradicals were considered. The so-obtained kt,copo1,1 was equal to the diffusion limit which is considered to be unrealistic. Nonetheless, kt,copo1,1 was significantly higher than kt1,1 for the corresponding homopolymerizations in all simulation variants which agrees with results from previous works. This might be explained by a different chain flexibility of the copolymeric macroradicals compared to the homopolymeric case. Combing all the simulation results with the experimentally determined parameters, the whole kinetic picture of copolymerization system of MMA and styrene finally becomes clearer and more comprehensive. This combined method of SP–PLP–EPR and PREDICI® simulation opens up new perspectives for both experimental and theoretical approaches for the in-depth investigation into the kinetics of radical copolymerizations. | de |