Change with temperature of the ESR spectra of methacrylic acid radicals

Abstract

In order to elucidate the structure of methacrylic acid radicals, the change with observation temperature of the ESR spectrum of free radicals trapped in solid methacrylic acid γ‐irradiated at −196°C was studied. Below −80°C, we found a 9‐line spectrum, which is similar to the ordinary 9‐line spectrum observed in irradiated poly(methacrylic acid) or poly(methyl methacrylate), but which differs in the stronger intensity of the so‐called 4‐line component. Our 9‐line spectrum changes reversibly into a 13‐line spectrum above −80°C. With broad‐line NMR measurements of methacrylic acid, it was found that there is such an unusual crystalline transition around −30°C that the line width is narrower in the lower‐temperature region (phase II) than that in the higher‐temperature region (phase I). The change of the ESR spectrum can be interpreted in terms of the exchange of the two β‐protons due to the hindered oscillation around the CαCβ bond of the single radical ···CβH2Cα(CH3)COOH if one assumes the gradual change of the hindering potential barrier caused by the crystalline transition and the lower barrier in phase II. The modified Bloch treatment gave the hindering potential barrier to be 7.2 kcal/mole in phase I and 1.5 kcal/mole in phase II. The difference between our 9‐line spectrum and the ordinary one with the very weak 4‐line component comes from the difference of the surrounding matrix.

Monomer- and polymer radicals of vinyl compounds: EPR and DFT studies of geometric and electronic structures in the adsorbed state

“…EPR spectra of this type of radicals observed in the solid phase with hyperfine splittings a CH3 = 22.2 G to the methyl protons and a β1 = 14.7 G, a β2 = 7.5 G of the methylene group [50] are very similar to that in Figure 4 (b).…”

“…A simulation taking account of the chemical exchange with the program ESREXN [52] is shown in Figure 5 (b). Rate constants of the same order were obtained from the 13 observed EPR spectra of methacrylic acid radicals in frozen solution [50]. Good agreement between experimental and simulated spectra has also been achieved by assuming a distribution of conformations of the methylene group [45].…”

“…The nine lines, designated as the 5 + 4 lines were assigned to the propagating radical -CH 2 Ċ(CH 3 )R. Details about the radical were obtained from the shape of the spectrum, especially the relative intensity of the lines which has been interpreted in terms of various factors such as that there is more than one conformation [49], there is a distribution of dihedral angles for a single conformation [45] or a site exchange motion around the -CH 2 -Ċ< bond [50].…”

Stable Free Radicals Produced in Acrylate and Methacrylate Free Radical Polymerization: Comparative EPR Studies of Structure and the Effects of Cross-Linking

“…One of the unresolved problems with the EPR spectra of acrylate and methacrylate polymers has been the absence or selective broadening of certain expected hyperfine lines. One hypothesis is a dynamic polymer motion that exchanges methylene proton positions, resulting in an alternating homogeneous line width [Sakai, Y.; Iwasaki, M. J. Polym. Sci.…”

Acrylic Polymer Radicals: Structural Characterization and Dynamics

“…The result of side-chain cleavage is a main-chain polymeric radical a and a smaller oxo-acyl radical b as a primary radical pair. Radical a has available to it a unimolecular decomposition pathway, namely, bscission, 22 to give a terminal alkene and the so-called propagating radical c. 23,24 This reaction is analogous to the reverse of the free radical polymerization reaction, that is, free radical addition to the alkene monomer. 25 Because of the rapid b-scission reaction, steady-state electron paramagnetic resonance (SSEPR) investigations carried out since 1951 on acrylate and methacrylate degradation were unable to confirm the chemistry shown in Scheme 14.1 as the first step.…”