A comparative time-resolved CW EPR and FT EPR investigation of the addition of 2-hydroxy-2-propyl radicals to acrylate and methacrylate monomers

Abstract

The addition of 2-hydroxy-2-propyl radicals to n-butyl acrylate and n-butyl methacrylate has been investigated by time-resolved continuous wave electron paramagnetic resonance (TR CW EPR) and time-resolved Fourier transform electron paramagnetic resonance (TR FT EPR). The 2-hydroxy-2-propyl radicals were generated by the photolysis of acetone in propan-2-ol (through hydrogen abstraction) and by the photolysis of a ketone 5 (through α-cleavage). The TR CW and TR FT spectra were experimentally equivalent for the addition of 2-hydroxy-2-propyl radicals to n-butyl acrylate to produce 3a. However, there are distinct differences between the TR CW and TR FT spectra for the addition of 2-hydroxy-2-propyl radicals to n-butyl methacrylate which produces the radical adduct 4a. In particular, a number of hyperfine lines clearly present in the TR CW spectra are much weaker in, or are absent from, the TR FT spectra. The differences in the TR CW and TR FT spectra are attributed to hindered rotation, which is important in the spectrum of the adduct of 2-hydroxy-2-propyl radicals to n-butyl methacrylate (4a), but not in the spectrum of the adduct of 2-hydroxy-2-propyl radicals to n-butyl acrylate (3a). The hindered rotation is shown to selectively shorten the spin–spin relaxation time, T₂, for certain hyperfine lines in the spectrum of 4a, resulting in broadening or disappearance of these lines and explaining the differences between the TR CW and TR FT spectra.