Implications of monomer and initiator structure on the dissociative electron‐transfer step of SET‐LRP

The heterolytic dissociation process associated with the activation of Single Electron‐Transfer Living Radical Polymerization is examined through the use of energy profile modeling. Monomer and initiator structure is correlated with the approximate activation barriers, energies of electrostatic ion‐radical pair formation, and stability of ion‐radical pair generated from the counteranion halide leaving group and the radical atom with partial positive charge density induced by its electron‐withdrawing substituent. Energy profiles permit access not just to one, but to all local minima, in the dissociation pathway and the identification of a global minimum. The location and energy of this global minimum allows for the placement of various initiators and dormant propagating macroradicals on the spectrum between stepwise and concerted dissociative electron‐transfer. The barrier for the activation step for alkyl‐halides derived from acrylates, vinyl halides, and styrenes, as well as from initiators bearing electron‐withdrawing groups is decreased in comparison to relatively more electron‐rich alkyl halides. This rate enhancement is explained through the sticky dissociative model wherein electron‐transfer is accelerated by the formation of strong ion‐radical pairs between radicals with partial positive charge density and their counteranion leaving group. Greater electron‐withdrawing capacity of the alkyl halide substituent increases the stability of the ion‐radical pair, reduces its equilibrium bond length, and accelerates electron‐transfer.