Molecular Structure of Poly(vinyl alcohol)-Derived Carbon-Centered Radicals Studied by Rapid-Flow and Spin-Trapping ESR Measurements: A Short-Lived Intermediate Radical in the Initial Stage of the Graft Polymerization Reaction

Continuous rapid-flow electron spin resonance (ESR) was used to detect poly(vinyl alcohol) (PVA)-derived carbon-entered radicals, in order to characterize the short-lived intermediate radicals in the initial stage of the graft polymerization of PVA. ESR spectra were recorded by mixing PVA, hydrogen peroxide, and Ti-2(SO4)(3) at pH 2.0. The observed spectrum indicated three paramagnetic species: a minor species A, and two major species B-1, and B-2. Based on the observed proton hyperfine coupling constant (hfcc) values, species A was assigned to the carbon-centered radical of PVA produced by hydrogen atom abstraction from the methine carbon bearing a hydroxyl group. Species B-1 and B-2 were ascribed to two structural isomers of the PVA-derived, carbonyl-conjugated radical produced by acid-catalyzed radical reaction at the vicinal diol moiety (i.e., head-to-head structure) of PVA. Spintrapping ESR measurements were performed at 80 degrees C using a water-soluble spin-trapping reagent 3,5-dibromo-4-nitroso-benzenesulfonate sodium salt (DBNBS). Based on the hfcc values observed for the spin-adducts, species B-1 and B-2 were confirmed to be the major PVA main-chain-derived radicals produced under the graft reaction conditions. These findings demonstrated that the PVA-derived, carbonyl-conjugated radicals are one of the intermediate radicals in the initial stage of the graft polymerization reaction under acidic conditions.