Time-Resolved EPR Studies of Main Chain Radicals from Acrylic Polymers. Dynamic Effects Due to Conformational Motion

Abstract

The temperature dependencies of the time-resolved electron paramagnetic resonance (TREPR) spectra of five main chain acrylic free radicals are presented and discussed in terms of conformational dynamics. The radicals are produced in liquid solution at temperatures ranging from 25 °C to over 100 °C by direct excitation (248 nm) of the ester group in the polymers leading to Norrish I α-cleavage of the side chain ester moiety. Restricted rotational motion near the radical center leads to modulation of the β-hyperfine coupling constants, which manifests itself in some of the spectra as an alternating line width effect near room temperature. A standard hyperfine modulation model (two-site exchange) is proposed and has been added to the simulation routine. The model works especially well for two of the polymers (poly(ethyl acrylate) and poly(methyl d3-methacrylate), for which activation barriers for rotation are extracted from Arrhenius plots. The model is somewhat successful for poly(ethyl cyanoacrylate) but fails for poly(ethyl methacrylate) and poly(methyl methacrylate). This failure is discussed in terms of radical structure and dynamics and the possibility that jumping between more than two low-energy conformational sites is involved. Disruption of the symmetry of the hyperfine couplings at intermediate temperatures supports this explanation.