Photo-Induced Micellization of Block Copolymers

We found novel photo-induced micellizations through photolysis, photoelectron transfer, and photo-Claisen rearrangement. The photolysis-induced micellization was attained using poly(4- tert-butoxystyrene)- block-polystyrene diblock copolymer (PBSt- b-PSt). BSt- b-PSt showed no self-assembly in dichloromethane and existed as isolated copolymers. Dynamic light scattering demonstrated that the copolymer produced spherical micelles in this solvent due to irradiation with a high-pressure mercury lamp in the presence of photo-acid generators, such as bis(alkylphenyl)iodonium hexafluorophosphate, diphenyliodonium hexafluorophosphate, and triphenylsulfonium triflate. The 1H NMR analysis confirmed that PBSt- b-PSt was converted into poly(4-vinylphenol)- block-PSt by the irradiation, resulting in self-assembly into micelles. The irradiation in the presence of the photo-acid generator also induced the micellization of poly(4-pyridinemethoxymethylstyrene)- block-polystyrene diblock copolymer (PPySt- b-PSt). Micellization occurred by electron transfer from the pyridine to the photo-acid generator in their excited states and provided monodispersed spherical micelles with cores of PPySt blocks. Further, the photo-Claisen rearrangement caused the micellization of poly(4-allyloxystyrene)- block-polystyrene diblock copolymer (PASt- b-PSt). Micellization was promoted in cyclohexane at room temperature without a catalyst. During micellization, the elimination of the allyl groups competitively occurred along with the photorearrangement of the 4-allyloxystyrene units into the 3-allyl-4-hydroxystyrene units.