Critical Assessment of RAFT Equilibrium Constants: Theory Meets Experiment

Equilibrium constants, K-eq, for the reversible addition-fragmentation chain transfer (RAFT) polymerization of butyl acrylate mediated by trithiocarbonate and dithiobenzoate RAFT agents have been estimated by quantum chemical calculations, namely by a combination of density functional structure optimizations, corrections for solvent effects plus the double harmonic approximation as well as high-level ab-initio local correlation methods. Individual contributions to K-eq are analyzed. The results are compared to experimental K-eq measured by microsecond time-resolved electron paramagnetic resonance (EPR) spectroscopy. Dithiobenzoate (DTB) RAFT agents are of particular interest, as earlier quantum chemical calculations resulted in K-eq values differing by several orders of magnitude from the numbers deduced via EPR measurements. This mismatch between theory and experiment is overcome by new quantum chemical estimates. The major factors behind the improved agreement are the application of dispersion-corrected density functional theory (DFT) functionals for the equilibrium structures and full-system coupled cluster calculations. The so-obtained ab initio K-eq data provide clear evidence for rate retardation with DTB-mediated acrylate polymerizations being due to cross-termination rather than to slow fragmentation of the RAFT intermediate radical.