A VISIBLE-LIGHT INITIATING SYSTEM FOR FREE-RADICAL PROMOTED CATIONIC POLYMERIZATION

A visible light initiator system for the photoinduced cationic polymerization of epoxides is reported. The developed system consists of a xanthene dye, an aromatic amine, and a diaryliodonium salt. We describe it as a free radical promoted cationic chain polymerization in that radicals generated from the visible light absorber start a chain process which, after oxidation of the formed radicals, produces a cationic chain initiating carbocation. The system can efficiently photoinitiate the polymerization of certain cyclic ethers at any wavelength of the visible spectrum depending on the dye's absorption spectrum and its oxidation/reduction potentials. Epoxides do not polymerize in the absence of one of the three components or in the absence of light. Aromatic amines of low basicity containing alpha-hydrogens are the preferred coinitiators for the polymerization of cyclohexene oxide. Studies on photochemical reactions between each two of the three components demonstrate that both oxidative bleaching of the dye by the diaryliodonium salt and reductive bleaching by the aromatic amine contribute to alpha-amino radical formation. The radical formed can be easily oxidized into an alpha-aminocarbocation by an onium salt of proper potential. Characterization of the polymer formed from cyclohexene oxide indicates that all the xanthene dye and appreciable amounts of the aromatic amine are incorporated in the polymer. It is believed that the alpha-aminocarbocation initiates the ring opening cationic polymerization. Dye molecules are incorporated into the polymer in chain terminating processes.