Radical Cations of Phenoxazine and Dihydrophenazine Photoredox Catalysts and Their Role as Deactivators in Organocatalyzed Atom Transfer Radical Polymerization

Radical cations of photoredox catalysts
used in
organocatalyzed atom transfer radical polymerization (O-ATRP) have
been synthesized and investigated to gain insight into deactivation
in O-ATRP. The stability and reactivity of these compounds were studied
in two solvents, N,N-dimethylacetamide
and ethyl acetate, to identify possible side reactions in O-ATRP and
to investigate the ability of these radical cations to deactivate
alkyl radicals. A number of other factors that could influence deactivation
in O-ATRP were also probed, such as ion pairing with the radical cations,
radical cation oxidation potential, and halide oxidation potential.
Ultimately, these studies enabled radical cations to be employed as
reagents during O-ATRP to demonstrate improvements in polymerization
control with increasing radical cation concentrations. In the polymerization
of acrylates, this approach enabled superior molecular weight control,
a decrease in polymer dispersity from 1.90 to 1.44, and an increase
in initiator efficiency from 78 to 102%. This work highlights the
importance of understanding the mechanism and side reactions of O-ATRP,
as well as the importance of catalyst radical cations for successful
O-ATRP.