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

Hydroxyl radicals were generated radiolytically in N2O-saturated aqueous so lutions in the presence of poly(vinyl methyl ether) (PVME, 6 x 10(4) Da, 10 (-3)-10(-2) mol dm(-3) in monomer units). As measured by pulse radiolysis, they react (k = 2.2 x 10(8) dm(3) mol(-1) s(-1)) with PVME by giving rise t o mainly alpha-alkoxyalkyl radicals (similar to 72%) that reduce (k approxi mate to 2 x 10(9) dm(3) mol(-1) s(-1)) Fe(CN)(6)(3-), IrCl62- or tetranitro methane. Based on the formaldehyde yield in the presence of the latter two oxidants (similar to 40% of (OH)-O-.), it is concluded that OH radicals und ergo H-abstraction at ROCH2-H, R3C-H and R2HC-H with probabilities of appro ximate to 40, similar to 32 and similar to 28%, respectively. The momentary rate constant of the decay of the PVME radicals depends on the number of r adicals per polymer chain and drops as they decay. The yield of intermolecu lar crosslinks, as measured by an increase in the molecular weight, strongl y increases with decreasing dose rate, and it is concluded that the majorit y of crosslinks occur intramolecularly, even at the lowest dose rate used [ 0.0015 Gy s(-1), G(intermolecular crosslinks) = 0.62 x 10(-7) mol J(-1)]. I n the presence of dioxygen, the primary PVME radicals are converted into th eir corresponding peroxyl radicals. They undergo efficient autoxidation via intramolecular H-abstraction [e.g. G(dioxygen uptake) approximate to 110 x 10(-7) mol J(-1) at 0.0015 Gy s(-1)]. Most of the hydroperoxides are unsta ble, i.e. the high dioxygen-uptake yield is not matched by the organic hydr operoxide yield [G(organic hydroperoxide) approximate to 34 x 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) approximate to 15 x 10(-7) mol J(-1) at 0.0015 Gy s(- 1)], and prompt chain breakage due to processes occurring in the bimolecula r decay of the peroxyl radicals is minor compared to the former pathway.