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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
Author(s) -
Smith Peter,
Donovan William H.,
Mader Charles E.,
Dominguez Luis M.,
Koscielniak Walter T.
Publication year - 1995
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1260330512
Subject(s) - chemistry , radical , electron paramagnetic resonance , acrylonitrile , monomer , methacrylonitrile , methylene , methyl acrylate , medicinal chemistry , photochemistry , polymer chemistry , copolymer , nuclear magnetic resonance , organic chemistry , polymer , physics
Using rapid‐mixing, continuous‐flow TiCl 3 ‐based techniques at 25°C monomeric addition radicals of general formula · CX(Y)CH 2 C * HR 1 R 2 , where R 1 and R 2 are different groups, neither being H, so that C * is a chiral center, were characterized by electron paramagnetic resonance. These species were generated by adding · CH(CH 3 )OH, CH(CH 3 )OCH 2 CH 3 ,.CH(CH 3 )OC(O)H,.CH(CH 3 )OC(O)CH 3 and radicals in turn to each of the monomers, of general structure CX(Y)CH 2 , 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 1,1‐difluoroethylene did not yield detectable levels of the expected addition radicals. In every addition radical observed, the splittings of the β‐CH 2 protons were found to be unequal and the size of this inequivalence varied qualitatively with the bulkiness of X, Y, R 1 and R 2 . Similar measurements on monomeric addition radicals of general formula.CX(Y)CH 2 Z, where Z is achiral, generated in turn from the same monomers by adding.OH,.NH 2 and, showed no evidence for the β‐CH 2 proton splittings 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 1,1′‐disubstituted ethylenes.