EPR study of β‐methylene proton magnetic inequivalence in monomeric addition radicals derived from mono‐ and 1,1′‐disubstituted ethylenes with chiral and non‐chiral substituents

Abstract

Using rapid-mixing, continuous-flow TiCI,-based techniques at 25 "C monomeric addition radicals of general formula 'CX(Y)CH,C*HRIRZ, where R' and RZ are different groups, neither being H, 90 that C* is a chiral center, were characterized by electron paramagnetic resonance. These species were generated by adding 'CH(CH,)OH, 'CH(CH,)OCH,CH, , 'CH(CH,)OC(O)H, 'CH(CH,)OC(O)CH, and 'kHCHzOCHzCHzO radicals in turn to each of the monomers, of general structure CX(Y)=CH, , acrylonitrile, acrylic acid, tert-butyl acrylate, butyl acrylate, methacrylonitrile, methacrylic acid, itaconic acid and but-3-ene-1,2,3-tricarboxylic acid. Similar reactions of carbon-centered radicals with vinyl fluoride and 1,l-difluoroethylene did not yield detectable levels of the expected addition radicals. In every addition radical observed, the splittings of the B-CH, protoy were found to be unequal and the size of this inequivalence varied qualitatively with the bulkiness of X, Y, R' and RZ. Similar measurements on monomeric addition radicals of general formula 'CX(Y)CH,Z, where Z is achiral, generated in turn from the same monomers by adding 'OH, 'NH, and 'CHOCH,OCH,d, showed no evidence for the fl-CH, proton splitting to be unequal. These results demonstrate the importance of an awareness of chirality effects in interpreting EPR spectra of monomeric and polymeric addition radicals of mono-and 1,l'-disubstituted etbylenes.