PHOTO-INDUCED RADICAL COPOLYMERIZATIONS OF ELECTRON-RICH OLEFINS WITH ELECTRON-POOR OLEFINS.
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PublisherThe University of Arizona.
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AbstractThis study is a systematic investigation of the parameters and conditions necessary for photo-induced radical copolymerizations of donor olefins with acceptor olefins in the absence of an initiator. Very few cases have been previously reported and no mechanistic details of the initiation have been proposed in the literature. Our results show that the photoinitiation depends on the relative donor and acceptor strengths of the monomers, as well as the solvent. The highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor must be at the appropriate energy levels in order to produce a radical initiating species upon photoexcitation of the electron donor-acceptor (EDA) complex. If the donor-acceptor interaction is too weak, no copolymerization occurs. The excited complex (contact ion pair) presumably decays back to the ground state faster than producing an initiating species. If the donor-acceptor interaction is too strong, the excited complex dissociates to the free ions which could initiate ionic homopolymerization rather than radical copolymerization. The solvent may also determine the course of the reaction. In two cases, copolymerizations, which could be photo-induced in 1,2-dichloroethane, could not be photo-induced in acetonitrile. Dissociation of the excited complex (contact ion pair) is favored in polar solvents, such as acetonitrile, which are able to stabilize the ion radicals. This initiation method produces high molecular weight copolymers that may be cast into transparent films.