Relation between Overall Rate of ATRP and Rates of Activation of Dormant Species

The rate of atom transfer radical polymerization (ATRP) depends on the rate constant of propagation (k(p)) and concentration of growing radicals. The latter is related to the ATRP equilibrium constant (K-ATRP) and concentrations of alkyl halides, activators, and deactivators. Activation of alkyl halides by Cu-I/L and deactivation of radicals by X-Cu-II/L are vital processes providing good control in ATRP. Rates of these reactions are typically identical throughout polymerization, since the ATRP equilibrium is maintained in essentially all ATRP systems. There are new ATRP processes carried out with ppm of Cu catalysts, such as activators regenerated by electron transfer (ARGET), initiators for continuous activator regeneration (ICAR), supplemental activators and reducing agents (SARA), and electrochemically or photochemically mediated ATRP (eATRP, photoATRP). In these processes, as in conventional radical polymerization (or in RAFT), concentration of radicals is established by balancing rates of radical generation (e.g., from thermal initiators, reduction rate or supplemental activation) and radical termination (i.e., reaching steady state). However, in these processes, the rate of activation of alkyl halides by Cu-I/L is still equal to the rate of deactivation of radicals by X-Cu-II/L. Can the rates of activation of alkyl halides (by Cu-I or by Cu-0) be directly related to the overall rate of ATRP? This report aims to clarify that rate of activation of alkyl halides by Cu species cannot be directly related to the overall rate of polymerization. There are many cases with the same rate of ATRP but dramatically different rates of activation and cases with similar activation rates but very different overall ATRP rates. Thus, both the analytical approach and PREDICI simulations clearly show that rates of normal ATRP with high catalyst loadings as well as rates of low ppm ATRP systems, such as ICAR ATRP and SARA ATRP, cannot be directly related with rates of activation of alkyl halides by Cu-I. In SARA ATRP, the activation of alkyl halides by Cu-I is always much faster than by Cu.