Hydroxyl-radical-induced reactions of poly(vinyl methyl ether): a pulse radiolysis, EPR and product study in deoxygenated and oxygenated aqueous solutions

Abstract

Hydroxyl radicals were generated radiolytically in N₂O-saturated aqueous solutions in the presence of poly(vinyl methyl ether) (PVME, 6 × 10⁴ Da, 10⁻³–10⁻² mol dm⁻³ in monomer units). As measured by pulse radiolysis, they react (k = 2.2 × 10⁸ dm³ mol⁻¹ s⁻¹) with PVME by giving rise to mainly α-alkoxyalkyl radicals (≈72%) that reduce (k ≈ 2 × 10⁹ dm³ mol⁻¹ s⁻¹) Fe(CN)₆³⁻, IrCl₆²⁻ or tetranitromethane. Based on the formaldehyde yield in the presence of the latter two oxidants (≈40% of ˙OH), it is concluded that OH radicals undergo H-abstraction at ROCH₂–H, R₃C–H and R₂HC–H with probabilities of ≈40, ≈32 and ≈28%, respectively. The momentary rate constant of the decay of the PVME radicals depends on the number of radicals per polymer chain and drops as they decay. The yield of intermolecular crosslinks, as measured by an increase in the molecular weight, strongly increases with decreasing dose rate, and it is concluded that the majority of crosslinks occur intramolecularly, even at the lowest dose rate used [0.0015 Gy s⁻¹, G(intermolecular crosslinks) = 0.62 × 10⁻⁷ mol J⁻¹]. In the presence of dioxygen, the primary PVME radicals are converted into their corresponding peroxyl radicals. They undergo efficient autoxidation via intramolecular H-abstraction [e.g.G(dioxygen uptake) ≈ 110 × 10⁻⁷ mol J⁻¹ at 0.0015 Gy s⁻¹]. Most of the hydroperoxides are unstable, i.e. the high dioxygen-uptake yield is not matched by the organic hydroperoxide yield [G(organic hydroperoxide) ≈ 34 × 10⁻⁷ mol J⁻¹ at 0.0015 Gy s⁻¹]. As a consequence of the instability of some of the organic hydroperoxides, chain scission also strongly depends on dose rate [e.g.G(chain breaks) ≈ 15 × 10⁻⁷ mol J⁻¹ at 0.0015 Gy s⁻¹], and prompt chain breakage due to processes occurring in the bimolecular decay of the peroxyl radicals is minor compared to the former pathway.