Radical Polymerization Kinetics in Systems with Transfer Reactions Studied by Pulsed-Laser-Polymerization and Online EPR-Detection

Abstract

The SP-PLP-EPR technique has been developed for very detailed kinetic investigations into radical polymerization. The method which combines single-pulse-pulsed-laser-polymerization (SP-PLP) with electron paramagnetic resonance spectroscopy (EPR) allows for direct online monitoring of radical concentration with a time-resolution of ìs after applying an intense laser SP. Polymerizations have been carried out at ambient pressure within a broad temperature range in bulk and in solutions, including water and ionic liquids as solvents. SP-PLP-EPR provides access to the highly complex diffusion-controlled termination kinetics of radicals as a function of chain-length during polymerization. A unique feature of the method is that different types of radicals may be followed independently. Kinetic fitting of measured concentration vs. time traces via PREDICI® simulation yields reliable rate coefficients. Monitoring of midchain radicals formed via backbiting of chain-end radicals in acrylic acid and acrylate polymerizations allows for studying intramolecular transfer and individual termination steps of both species. Tracing of propagating and of intermediate radicals during reversible addition fragmentation transfer (RAFT) polymerization provides access to RAFT addition, fragmentation and to termination reactions. By directly measuring the lifetime of the intermediate radical during dithiobenzoate-mediated acrylate polymerization, a reliable fragementation rate coefficient has been determined which value excludes the so-called slow-fragementation from explaining rate retardation. SP-PLP-EPR in the presence of copper(II)-complexes is suitable for measuring deactivation rate coefficients during atom transfer radical polymerization (ATRP).A physically based description of chain-length-dependent termination has been developed via SP-PLP-EPR experiments on methacrylate and acrylate monomers. The termination rate coefficient of macroradicals of identical degree of polymerization is adequately represented via a composite model. The decay of termination rate coefficient with increasing chain length is described by power-laws with higher and lower (negative) exponent values for chain-lengths below and above a crossover chain-length of about 50 monomer units. Termination in both regimes proceeds against the friction of monomer and solvent. Hence the absolute value and the temperature dependence of the termination reaction are determined by the fluidity of the monomer-solvent mixtures. Rate coefficients for transfer of chain-end radicals to form midchain radicals (backbiting) and for monomer addition to midchain radicals have been measured for aqueous-phase polymerization of non-ionized and fully ionized acrylic acid. These data are important for modeling of industrially relevant polymerization processes.