Kinetic EPR studies of the addition of carbohydrate-derived radicals to methacrylic acid

Abstract

EPR spectroscopy has been employed directly to monitor the initial stages in the free-radical copolymerisation of carbohydrates and methacrylic acid, initiated by a metal–peroxide couple. For the addition of a variety of oxygen-conjugated substrate-derived radicals (including those from myo-inositol, α- and β-D-glucose, D-fructose and sucrose) the rate constants are in the range (0.3–3.8)  10⁶ dm³ mol^–1 s^–1; in contrast ˙CH₂OH has a value of 6  10⁶ and α,α-dioxygen-substituted radicals have values of ca. 10⁷ dm³ mol^–1 s^–1.

The reaction is clearly assisted by the +M electronic effect of the α-oxygen and by bending at the radical centre (for α-dioxygen-substituted species). The reaction rate is, however, reduced by β-oxygen substituents, especially through a SOMO–σ* (β-C–O) interaction, giving a substantial stereoelectronic retardation for axial β-oxygen substituents and for acyclic β-OH groups where eclipsing geometry (with respect to the radical centre) is favoured.