posted on 2016-01-28, 00:00authored byXiangcheng Pan, Cheng Fang, Marco Fantin, Nikhil Malhotra, Woong Young So, Linda A. Peteanu, Abdirisak A. Isse, Armando Gennaro, Peng Liu, Krzysztof Matyjaszewski
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.