Metal-free Atom Transfer Radical Polymerization Research Articles

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

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.

The active role of excited states of phenothiazines in photoinduced metal free atom transfer radical polymerization: singlet or triplet excited states?

The active role of phenothiazine excited states in photoinduced metal-free atom transfer radical polymerization (ATRP) was investigated by using laser flash photolysis, fluorescence, phosphorescence and electron spin resonance spectroscopy.

Photoinduced Metal-Free Atom Transfer Radical Polymerization of Acrylonitrile.

Photoinduced metal-free atom transfer radical polymerization has been successfully extended to the synthesis of polyacrylonitrile (PAN) with predictable molecular weights and low dispersities. This was achieved using phenothiazine derivatives as photoredox catalysts, which activate dormant alkyl bromides to reversibly form propagating radicals. Both 1H NMR spectroscopy and chain-end extension polymerization show highly preserved Br chain-end functionality in the synthesized PAN.