Mechanism of Photoinduced Metal-Free Atom Transfer Radical Polymerization: Experimental and Computational Studies

TLDR

The authors investigated photoinduced metal‑free ATRP of methyl methacrylate using phenothiazine derivatives and related compounds as photoredox catalysts. They examined the redox properties and radical‑cation stability of the catalysts by cyclic voltammetry, measured lifetimes and activity of photoexcited catalysts with laser flash photolysis, and used DFT calculations to elucidate the structures and energy profiles of deactivation pathways. The study shows that activation of metal‑free ATRP proceeds via a dissociative electron‑transfer mechanism with the excited catalyst, while deactivation is favored by an associative electron‑transfer pathway, and that not all phenothiazine derivatives efficiently participate in deactivation, insights that guide the design of improved photoredox catalysts.

Abstract

Photoinduced metal-free atom transfer radical polymerization (ATRP) of methyl methacrylate was investigated using several phenothiazine derivatives and other related compounds as photoredox catalysts. The experiments show that all selected catalysts can be involved in the activation step, but not all of them participated efficiently in the deactivation step. The redox properties and the stability of radical cations derived from the catalysts were evaluated by cyclic voltammetry. Laser flash photolysis (LFP) was used to determine the lifetime and activity of photoexcited catalysts. Kinetic analysis of the activation reaction according to dissociative electron-transfer (DET) theory suggests that the activation occurs only with an excited state of catalyst. Density functional theory (DFT) calculations revealed the structures and stabilities of the radical cation intermediates as well as the reaction energy profiles of deactivation pathways with different photoredox catalysts. Both experiments and calculations suggest that the activation process undergoes a DET mechanism, while an associative electron transfer involving a termolecular encounter (the exact reverse of DET pathway) is favored in the deactivation process. This detailed study provides a deeper understanding of the chemical processes of metal-free ATRP that can aid the design of better catalytic systems. Additionally, this work elucidates several important common pathways involved in synthetically useful organic reactions catalyzed by photoredox catalysts.