A Theoretical Study of Radical Formation Mechanisms in PET-RAFT Polymerization Using Porphyrins as Photoredox Catalysts

Mechanisms of photoinduced electron/energy transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerizations using zinc tetraphenylporphyrin (ZnTPP) or tetraphenylporphyrin (TPP) as photoredox catalysts (PCs) were studied using density functional theory calculations. To explain the selectivity of ZnTPP for trithiocarbonate compounds, the radical generation mechanisms of two chain transfer agents (CTAs), a trithiocarbonate (BTPA) and a dithiobenzoate (CPADB), were compared. The results suggest that the reaction mechanism (i.e., electron or energy transfer) depends on both the PC and CTA. For the most efficient combination, ZnTPP and BTPA, the reaction proceeds via an electron transfer mechanism. In contrast, TPP reacts with CPADB via an energy transfer mechanism. Furthermore, the formation of a stable complex between intermediates is identified for the reaction of ZnTPP and BTPA. These findings reveal the detailed mechanism and will offer insight into improving the yield and selectivity of PET-RAFT polymerization using porphyrins as PCs.