Time-Resolved EPR Studies of Main Chain Radicals from Acrylic Polymers. Structural Characterization at High Temperatures

Abstract

Main chain radicals from several acrylic polymers are characterized in liquid solution at high temperatures (∼100 °C) using time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The radicals are produced by laser flash photolysis (248 nm) and subsequent loss of the side chain ester functionality by Norrish I α-cleavage. At these temperatures fast-motion spectra with conformationally averaged hyperfine interactions are observed. The spectra are strongly spin polarized from the triplet mechanism (TM) of chemically induced dynamic electron spin polarization (CIDEP). Computer simulation of the TREPR spectra leads to unambiguous structural characterization of both the main chain radical and the side chain oxo-acyl radical. Hyperfine coupling constants, g-factors, and line widths are reported and discussed for radicals produced from five different acrylic polymers. Variation of the side chain on the polymer backbone leads to significant changes in the observed hyperfine coupling constants. For the main chain radical from poly(fluorooctyl methacrylate) (PFOMA), fast motion of the polymer chain is not accessible at these temperatures. The side chain oxo-acyl radical from PFOMA exhibits long-range 19F hyperfine interactions. Additional TREPR experiments on small molecule model compounds and gel permeation chromotography results of the photolyzed polymers support the conclusion that the primary photodegradation mechanism proposed in this paper is general for acrylic polymers.