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Spin Trapping of Radicals from the Reactions between Donor and Acceptor Olefins. Further Evidence for the Tetramethylene Diradical Intermediate as the Initiator of Spontaneous Copolymerization
journal contribution
posted on 2000-02-25, 00:00 authored by Lennart Eberson, Ola Persson, Henry K. Hall, Anne Buyle Padias, Peter J. SteelEPR spectroscopy and product isolation have been used to investigate the initiation of
spontaneous copolymerizations of donor and acceptor monomers. The results establish the presence of
tetramethylene diradicals as the only experimentally supported intermediates in the initiation for these
spontaneous copolymerizations. No evidence supporting a Mayo cycloaddition mechanism, electron
transfer, or charge-transfer complex initiation was found. The reaction of an olefin activated by three
electron-accepting groups, such as dimethyl cyanofumarate, methyl 3,3-dicyanoacrylate, or trimethyl
ethylenetricarboxylate, with a styrene having unsubstituted ortho positions was used in this study. In
the presence of the spin trap 2-methyl-2-nitrosopropane (t-BuNO), aminoxyls with characteristic EPR
spectra of high intensity were detected. Reaction of 4-methoxystyrene and dimethyl cyanofumarate in
the presence of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) led to a diamagnetic 1:1:1 adduct, 4-cyano-6-methoxy-3,4-dimethoxycarbonyl-1-piperidinyloxytetrahydronaphthalene, which was subjected to an
X-ray crystallographic study. Treatment of the TEMPO adduct with t-BuNO in chloroform gave a solution
containing TEMPO and the same aminoxyl as that formed from 4-methoxystyrene, dimethyl cyanofumarate, and t-BuNO. These aminoxyls were assigned the structure of spin adducts of tetrahydronaphthalen-1-yl radicals, formed by initial trapping of the diradical of the donor and acceptor olefin at the
more reactive radical center, followed by cyclization into the aromatic ring. This proposed mechanism
provides an alternative to the commonly accepted Mayo initiation mechanism.