ESR study of irradiated acrylic acid and reaction of the produced radical

A single crystal of acrylic acid CH2CHCOOH was irradiated at −196°C. and its ESR spectrum was measured at that temperature with various crystal orientations with respect to the external magnetic field. At −196°C. the spectrum appears as a double quartet, the doublet showing anisotropy. By analyzing the spectrum, the hyperfine coupling tensor was derived. The result showed that the radical was CH3ĊHCOOH produced by hydrogen addition to the double bond. Referring to the molecular orientation of acrylic acid in the crystal, it was found that the radical was oriented in the crystalline lattice in the same way as the undamaged molecule. When the temperature was raised from −196°C. the double quartet spectrum began to decay at −100°C., and there remained a triplet spectrum showing no anisotropy. The triplet may be attributed to a propagating type radical In order to compare the ESR result with polymerization process, monomer preirradiated at −196°C. was postpolymerized at various elevated temperatures. It was found that although the radical decayed at −78°C. no discernible polymer was obtained at that temperature. These results suggest that, although the radical can move and be reactive at −78°C. which is far below the melting point, it does not seem sufficiently mobile to permit growth of the propagating chain to any observable extent.