Kinetik radikalischer Polymerisationen ionischer Monomere in wässriger Lösung: Spektroskopische Analyse und Modellierung

Abstract

This work targets the radical polymerization of ionic monomers in aqueous solution. Rate coefficients were determined by pulse-laser polymerization in conjunction with size-exclusion chromatography (PLP–SEC), with near-infrared detection after a single-pulse (SP–PLP–NIR) and with high time-resolved electron paramagnetic resonance spectroscopy (SP–PLP–EPR). The experiments were supported by modelling using the software package Predici®. The termination kinetic of the radical polymerization of the non-ionized monomer N-vinyl formamide in bulk and in aqueous solution between 40 and 70 °C from 500 to 2500 bar was determined by SP–PLP–NIR as a function of conversion. The impact of termination, transfer reactions and broadening on the formation of PLP-structures of arbitrary monomers during PLP–SEC experiments was investigated fundamentally by means of Predici® modelling. Conditions for the operative use of the PLP–SEC technique were identified, e.g. 19 to 92% of the formed radicals have to terminate in between two successive pulses to yield good and reliable PLP-structures. Due to the slow termination of ionic monomers (<19%) and potential intermolecular or intramolecular transfer reactions, the PLP–SEC technique can usually not be used for the identification of reliable propagation rate coefficients, kp. Thus, conversion-time traces of the radical polymerization of Trimethylaminoethylmethacrylate (TMAEMA) in D2O between 30 and 60 °C were fitted using a Predici® model to obtain kp. A model-free determination of the propagation and termination rate coefficients of TMAEMA under identical reaction conditions could be achieved by the combination of the coupled parameters <kt>/kp and kp/<kt>0.5 from SP–PLP–NIR and chemically initiated experiments, respectively. A Predici® model for the radical polymerization of Trimethylaminoethylacrylate (TMAEA) in aqueous solution between 20 and 84 °C was developed. The model emphasizes the significant importance of the mid-chain radical (MCR) kinetic for the radical polymerization of ionic acrylates. Predici® modelling of the radical polymerization of ionized methacrylic acid (MAA) in aqueous solution revealed, that kp values from literature PLP–SEC experiments are systematically overestimated by a factor of two. The impact of the counter cation concentration on kp of ionized MAA was investigated by fitting conversion-time traces of radical polymerizations of ionized MAA in D2O between 30 and 80 °C. The preexponential of kp increases towards higher counter cation concentrations, thus by addition of either monomer or salt. The termination kinetic of ionized MAA in aqueous solution between 20 and 80 °C from 1 to 500 bar was determined by SP–PLP–NIR as a function of conversion. NIR, SP–PLP–NIR, SP–PLP–EPR and 13C-NMR experiments revealed, that all rate coefficients of the radical polymerization of ionized acrylic acid (AA) in D2O are influenced by the counter cation concentration and increase upon addition of either monomer or salt. This can be explained by the formation of contact ion pairs. The involved rate coefficients lead to a very high MCR fraction of e.g. 98% at 50 °C. Nevertheless, the small amount of SPRs (2%) is responsible for the chain-growth during the polymerization (kps ≈ 103·kpt). Thus, minor changes of the SPR-MCR equilibrium cause strong effects on the polymerization rate. The kinetic coefficients presented in this study allow for a comprehensive simulation of conversion-time traces as well as molar mass distributions of the resulting products of the radical polymerization of ionized monomers in aqueous solution over a broad range of experimental conditions: Monomer concentration, degree of monomer conversion, type and concentration of the counter cation as well as temperature and pressure.