Abstract
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.
Highlights
Stimuli-responsive polymers have attracted considerable attention because these polymers have many industrial applications, such as sensors [1,2,3], drug carriers [4,5,6], artificial muscles [7,8], optical data storage [9], and electric devices with molecular switches [10,11]
It has been reported that chemical reactions inducing self-assembly of the polymers; the oxidation [27], reduction [28,29,30], and disproportionation [31,32] of stable nitroxyl radicals supported on the side chains of a block copolymer, promote the micellization of the copolymer to provide micelles that function as oxidants and reductants
The photolysis-induced micellization was determined for a PBSt-b-PSt diblock copolymer (Scheme 1)
Summary
Stimuli-responsive polymers have attracted considerable attention because these polymers have many industrial applications, such as sensors [1,2,3], drug carriers [4,5,6], artificial muscles [7,8], optical data storage [9], and electric devices with molecular switches [10,11]. The structure change by the irreversible photoreaction has been investigated on the photolysis of diazosulfonates [42,43,44], 1-iminopyridinium ylides [45], [4(4'-alkoxybenzoyl)phenylmethyl]phosphonic acids [46], and didecyl-2-methoxy-5-nitrophenyl phosphate [47]. The former three kinds of surfactants lose their surface-active ability by photolysis, resulting in destruction of the micelles and vesicles. This review describes the novel micellizations promoting the irreversible reactions of these photoreactive groups attached to the block copolymers
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