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

EPR spectroscopy and product isolation have been used to investigate the in itiation of spontaneous copolymerizations of donor and acceptor monomers. T he results establish the presence of tetramethylene diradicals as the only experimentally supported intermediates in the initiation for these spontane ous copolymerizations. No evidence supporting a Mayo cycloaddition mechanis m, 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 ethylenet ricarboxylate, 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 pr esence 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-piperidinyloxytetr ahydronaphthalene which was subjected to an X-ray crystallographic study. T reatment of the TEMPO adduct with t-BuNO in chloroform gave a solution cont aining TEMPO and the same aminoxyl as that formed from 4-methoxystyrene, di methyl cyanofumarate, and t-BuNO. These aminoxyls were assigned the structu re of spin adducts of tetrahydronaphthalen-1-yl radicals, formed by initial trapping of the diradical of the donor and acceptor olefin at the more rea ctive radical center, followed by cyclization into the aromatic ring. This proposed mechanism provides an alternative to the commonly accepted Mayo in itiation mechanism.