EPR 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.