Modeling the reversible addition-fragmentation chain transfer process in cumyl dithiobenzoate-mediated styrene homopolymerizations: Assessing rate coefficients for the addition-fragmentation equilibrium

A full kinetic scheme for the free-radical reversible addition-fragmentation chain transfer (RAFT) process is presented and implemented into the program package PREDICI®. With the cumyl dithiobenzoate-mediated bulk polymerization ofstyrene at 60°C as an example, the rate coefficients associated with the addition-fragmentation equilibrium are deduced by the careful modeling of the time-dependent evolution of experimental molecular weight distributions. The rate coefficient for the addition reaction of a free macroradical to a polymeric RAFT species (k_β) is approximately 5 · 10⁵ L mol^-1 s^-1, whereas the fragmentation rate coefficient of the formed macroradical RAFT species is close to 3 · 10^-2 s^-1. These values give an equilibrium constant of K = k_β/k_-β = 1.6 · 10⁷ L mol^-1. Conclusive evidence is given that the equilibrium lies well on the side of the macroradical RAFT species. The high value of k_β is comparable in size to the propagation rate coefficients reported for acrylates. The transfer rate coefficient to cumyl dithiobenzoate is close to 3.5 · 10⁵ L mol^-1 s^-1. A careful sensitivity analysis was performed, which indicated that the reported rate coefficients are accurate to a factor of 2.