Photoinitiated nitroxide-mediated radical polymerization

Diagram 3.2. General mechanism of nitroxide mediated radical polymerization 

Diagram 3.3. Photoinduced NMRP of vinyl monomers with an alkoxyamine possessing a chromophoric benzophenone molecule 

Yoshida et al. have reported the photopolymerization of MMA using azobis(4-methoxy-2,4-dimethylvaleronitrile) as an initiator with 4-methoxy-TEMPO (Me-TEMPO) as a mediator in the presence of a diaryliodonium salt namely, bis(alkylphenyl)iodonium hexafluorophosphate [YOS 12]. The diaryliodonium salt behaves as the photoacid generator and PMMA with a 

4 PDI value could be obtained with high conversion. It is understood that the mechanism of the reaction proceeds through an electron transfer between the excited diaryliodonium salt and the nitroxide during the propagation process [YOS 10a]. Similarly, the use of a triarylsulfonium salt, namely (4-tert-butylphenyl)-diphenylsulfonium triflate, as the photoacid generator in the same process for controlled photoinduced NMRP of MMA has also been reported [YOS 10b]. However, the mechanism still has unclear points as the function of the iodonium salt is unknown. 

---