ESR single‐crystal study of radical and radical‐pair formation in some γ‐irradiated vinyl monomers

Single crystals of methyl methacrylate (MMA), methyl acrylate (MA), and acrolein (A) have been prepared by a low‐temperature technique. After irradiation with γ‐rays at 77°K the paramagnetic species were identified by ESR spectroscopy. MMA gave a seven‐line single spectrum from radicals formed by hydrogen addition. The hyperfine coupling constants are slightly anisotropic with a mean value of 22 G. Radical pairs were observed as ΔMs = 1 and ΔMs = 2 transitions; the hyperfine coupling was 11 G. From the strongly anisotropic dipolar interaction, upper limits for the distances between the pair components were calculated to be 5.45 Å and 6.3 Å. MA gave a five‐line main spectrum with the same hyperfine coupling values and two radical pairs, one with a distance 5.9 Å between the components. In a there was also a strongly anisotropic interaction. The hyperfine coupling of the ΔMs = 2 transition was 9.8 G. The number of radical pairs compared to the total number of radicals increases only slightly with the radiation dose. This makes it likely that pair formation occurs in the spurs and blobs formed by the γ‐radiation. At an increased temperature the radical pairs disappeared; the spectrum of MMA changed to that characteristic of propagating polymer radicals.