Light Induced Polymer and Polymerization Reactions. 14. Radical Photopolymerization by Potassium Trisoxalato Cobaltate Arylonium Salts Systems.
The photopolymerization of acrylamide (AA) by trisoxalato cobaltate ([Co(ox)3]3⊖) is inefficient because the produced nucleophilic ·CO2⊖‐radicals are oxidized or they terminate with polymer radicals. If onium compounds are added to this system the initiation efficiency is increased as follows : p‐CH3OC6H4N2⊕>Ph2I⊕>PhCOCH2S(CH3)2⊕>Ph3S⊕. By means of quantum yield measurements a mechanism was proposed including the electron transfer from ·CO2⊖ to onium compounds. The produced radicals also initiate the polymerization of AA. The exponent m of the polymerization rate (vp ≈︁ [AA]m) is decreased from 1.4 to 1.1 in the presence of Ph2I⊕, deciding the decrease of termination reaction between ·CO2⊖ and polymer radicals. In comparison with K3[Fx(ox)3] the corresponding cobalt complex possesses higher polymerization quantum yields for AA.
2,4,5‐Triphenyloxazole – New Preparative Pathway and Properties in Photoinduced Electron Transfer Reactions
The reaction between desylchloride and sodium azide in boiling methanole leads to 2,4,5‐triphenyl oxazole (TPO) c in good yields. This new synthetic course can be used for the synthesis of other triaryl substituted oxazoles too. The shortlived excited singlet state of TPO acts as electron donor (towards several types of onium compounds) and as electron acceptor (towards triethanole amine), and is quenched very efficiently.
The photo‐induced electron transfer reaction with onium salts results in the homolytic decomposition of the neutral radicals of these compounds with moderate quantum yields. All systems investigated can be used as photo‐initiators for the free radical polymerization of methyl methacrylate.
3-R-Alkoxycarbonyl-5-formyl-2,3-dihydro-6H-1,3-oxazines IIa-IIc (R = methyl, ethyl, isopropyl) as primary photoproducts formed by a rearrangement of 3-R-alkoxycarbonyl-3a,4,6,6a-tetrahydrofuro[3,4-d]isoxazoles Ia-Ic undergo cyclization to bicyclic lactones IIIa-IIIc due to further irradiation. The photochemical transformation of compounds II into III constitutes a new kind of photo-induced rearrangement. The quantum yields are very little dependent on the presence of oxygen and polarity of the solvent. The proposed concerted mechanism of the photorearrangement is discussed.
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