In-depth analysis of radical polymerization kinetics and mechanism by highly time-resolved electron paramagnetic resonance spectroscopy in conjunction with pulsed-laser initiation

• Applicant: Professor Dr. Michael Buback
• Subject Area: Polymer Materials ; Technical Chemistry
• Term: from 2016 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 314868524

Detailed knowledge about radical polymerization kinetics on the basis of accurate rate coefficients for the individual steps of propagation, termination and transfer is of enormous relevance for the fundamental understanding of these elementary reactions and for the simulation of technical polymerization processes. Within our group, the SP-PLP-EPR method has been developed, which connects pulsed laser polymerization (PLP) induced by a single pulse (SP) with electron paramagnetic resonance (EPR) spectroscopy. Radical concentrations after laser pulsing are directly monitored with a time resolution of microseconds to milliseconds. The method provides perfect access to the chain-length-dependence of radical termination, as has been demonstrated for methyl methacrylate, styrene and vinyl acetate. As the concentration of more than one type of radicals may be monitored, the SP-PLP-EPR technique also allows for studies into the termination and transfer kinetics of acrylate polymerization and of acrylamide polymerization in aqueous solution. With these monomers, secondary chain-end radicals, at typical polymerization conditions, may undergo internal transfer by a 1,5-hydrogen shift reaction (backbiting) thus producing midchain radicals with very different kinetic properties. Analysis of several radical species also allows for detailed kinetic and mechanistic analyses of reversible deactivation polymerizations such as RAFT and ATRP.The favourable signature of the SP-PLP-EPR method is due to the almost instantaneous production of an intense burst of small radicals in conjunction with the highly sensitive EPR detection of the time evolution of radicals originating from the primary photoinitiator-derived species. The method is applicable within the entire field of radical polymerization, i.e., to a wide range of monomers and solvent environments.The submitted project proposal focuses on SP-PLP-EPR studies into the following topics:1. Measurement of termination rate coefficients for bulk radical polymerizations up to high degrees of monomer conversion2. Deducing termination, transfer and propagation rate coefficients for radical polymerizations of fully and partially ionized monomers from a single SP-PLP-EPR experiment 3. Design of double-pulse PLP-EPR experiments for testing cross-termination models of radicals differing in chain length 4. Measurement of the backbiting rate of acrylates as a function of alkyl ester sizeThe project aims at demonstrating the enormous and unrivalled potential of the SP-PLP-EPR technique toward the detailed and accurate measurement of relevant rate coefficients for the entire field of radical polymerization.

• DFG Programme: Research Grants