Dithioester-mediated RAFT Polymerization: A Kinetic Study by Mathematical Modelling

Although reversible addition-fragmentation chain transfer (RAFT) polymerization has attracted great attention of many researchers over recent years, outstanding questions on the mechanism and kinetics of dithioester-mediated RAFT polymerization (especially dithiobenzoates) still have remained unsolved. In this work, based on experimental observations and exact theoretical predictions, the kinetic schemes of RAFT polymerization are extended to a wider range of reactions such as irreversible intermediate radical terminations and reversible transfer reactions. The reactions which have been labeled as kinetic schemes are theoretically the most probable existing reactions and are used for mathematical modelling. The detailed kinetic scheme is applied to three kinds of RAFT polymerization systems by utilizing the method of moments. Unknown kinetic rate constants are obtained by curve fitting of the modelling results and theoretical data, and applying the least square method; or estimation by considering the theoretical facts and experimental findings. The origin of the rate retardation and induction periods has been understood by studying the main and pre-equilibrium stages of dithiobenzoate-mediated RAFT homopolymerization. A copolymerization system in the presence of RAFT agent has also been examined to confirm the capability of introduced kinetic scheme in different monomer/RAFT agent systems. Although unknown parameters were obtained theoretically, their consistency with other researchers works shows the accuracy of the modelling procedure. The modelling results are in excellent agreement with experimental data which proves the validity and applicability of the detailed kinetic scheme. The results have shown that some reactions may not occur in many RAFT polymerization systems and can be eliminated and therefore more kinetic and mechanistic studies are required.