Effects of initiator structure on activation rate constants in ATRP

Activation rate constants (kact) for a variety of initiators for Cu-mediated ATRP have been determined under the same conditions. The ratio of the activation rate constants for the studied alkyl (pseudo)halides exceeds 1 million times. The activation rate constants increase with initiator substitution (e.g., for primary, secondary, and tertiary α-bromoesters the ratios are ∼1:10:80), with the radical stabilizing α-substituent (e.g., alkyl bromides with −C(O)NEt2, −Ph, −C(O)OMe, and −CN groups the ratios are ∼1:4:8:600 but with both α-Ph and α-C(O)OEt ∼ 140 000), and with the leaving atom/group (e.g., for methyl 2-halopropionates:  chloro:bromo:iodo ∼1:20:35, but benzyl bromide is ∼10 000 more reactive than the corresponding isothiocyanate/thiocyanate).