Reaction of the 2-hydroxy-2-propyl radical with acrylate type molecules in aqueous solution: Radical addition or electron transfer

The reaction of 2-hydroxy-2-propyl (IP) radical with 12 acrylate type monomers in aqueous solutions was investigated by means of pulse radiolysis: the molecular structure effects and the mechanisms were evaluated. Radicals were generated either in the reaction of hydrated electron with acetone and subsequent protonation or in the H abstraction reaction by OH radicals from i-propanol. IP radical reacts with acrylamides and acrylic acid esters in radical addition reaction of the type: (CH3)2(OH)C · + CH2 =CH-C(O)R → (CH3)2(OH)C-CH2-C ·H-C(O)R with rate coefficients between 3.2 x 107 and 1.9 x 108 mol-1 dm3 s-1. Rate coefficients of the reaction with maleic acid (neutral), dimethyl- and diethyl maleate are between 1.3 x 108 and 4.7 x 108 mol-1 dm3 s-1. The values for dimethyl- and diethyl fumarate are ca. one order of magnitude higher. However with maleates and fumarates there is also a few percent contribution from the electron transfer reaction of the type (CH3)2(OH)C · + ROOC-CH=CH-COOR + H2O → ROOC-CH=CH-C ·O-OR + (CH3)2CO + H3O+. In neutral solutions and at pH ∼3 and ∼9, the same rate coefficients were measured. However, in dialkyl maleate and -fumarate solutions between pH 9.5 and 10.5, a base catalyzed decomposition of the IP radical adduct was observed forming electron adduct: ROOC-C((CH3)2(OH)C)H-C ·H-COOR + OH-→ ROOC-CH=CH-C ·O-OR + (CH3)2CO + H2O. Molecular structure effects were evaluated using log k-σ p and log k-LUMO plots