An ESR approach to the estimation of the rate constants of the addition and fragmentation processes involved in the RAFT polymerization of styrene

The reversible-addition-fragmentation chain transfer (RAFT) controlled radical polymerization of such vinylic monomers as styrene (=ethenylbenzene) has gained increasing popularity in current years. While there is a general agreement on the mechanism of RAFT polymerization, there is an ongoing debate about the values of the rate constants of its key steps, i.e., the addition of the propagating radicals to the mediator and the fragmentation of the resulting spin adducts. By carrying out an ESR spectroscopic investigation of the AIBN-initiated polymerization of styrene (AIBN = 2,2′-azobis[2-methyl- propanenitrile]), mediated by benzyl (diethoxyphosphoryl)dithioformate (5) as RAFT agent, we were able to detect and characterize four different radical species involved in the process. By reproducing their concentration-time profiles through a kinetic model, the addition and fragmentation rate constants at 90° of the propagating radicals to and from the mediator were estimated to be ca. 10⁷ M^-1 s^-1 and ca. 10³ s^-1, respectively. The validity of the kinetic model was supported by hybrid meta DFT calculations with the BB1K functional that predicted addition- and fragmentation-rate-constant values in good agreement with those estimated from the ESR experiments.