(327h) Predictive Model for Catalyst Effect of Photo-Induced and Copper-Catalyzed Atom Transfer Radical Polymerization (ATRP) Reaction

Authors: 
Fang, C., University of Pittsburgh
Atom transfer radical polymerization (ATRP) is among the most powerful and robust controlled radical polymerization techniques that facilitates macromolecular engineering by synthesis of polymers with precise molecular weights, low dispersities, and well-controlled architectures. Good control over polymer structures via ATRP is largely attributed to the fast activation/deactivation equilibrium that can be governed by transition metal catalysts (particularly with copper catalysts) or photoredox catalysts. However, the detailed mechanism of ATRP has not been studied thoroughly at the atomic level, which hinders the rational design of better ATRP catalysts. In this study, we employ density functional theory (DFT) and Marcus Theory calculations to investigate all possible reaction pathways for photoinduced ATRP and Copper-catalyzed ATRP (Cu-ATRP). In photoinduced ATRP, we discovered that the reaction occurs via an out-sphere electron transfer (OSET) process where the excited photoredox catalyst will be oxidized to a radical cation and the alkyl halide substrate will be reduced to radical anion that rapidly dissociate to radical for polymerization. On the other hand, inner-sphere electron transfer (ISET) pathway is most favored in Cu-ATRP in which the halogen atom in substrate transfers to the catalyst via a concerted transition state. Notably, the ISET halogen transfer transition states are first time investigated by computation in this study. For both cases, calculations provided good agreement with experimental activation rate constants. In addition, we investigated the origin of reactivity differences for a representative set of catalysts for both ATRP system by studying their electronic and steric factors. We found that for photoinduced ATRP, the E(HOMO) of catalyst can be a good predictor for the reactivity, while in Cu-ATRP, the catalyst E(HOMO), flexibility, and percentage of buried volume contribute to the reactivity deviation.