Catalytic Oxidative Deformylation of Polyethylene Glycols with the Participation of Molecular Oxygen

Abstract

The kinetics of oxidation of diethylene glycol, triethylene glycol, and polyethylene glycols (PEGs) with molecular weights ranging from 400 to 2000 in the presence of Cu(II) ions and bases was studied. It was found that ethylene glycols can be oxidized by molecular oxygen in anhydrous media in a temperature range of 30–60°C at anomalouosly high rates which are higher than the rates of chain-radical PEG autoxidation by several orders of magnitude. Only terminal hydroxyl groups were subjected to oxidation. The reaction occurs with the cleavage of a C–C bond and results in the formation of formic acid and a PEG with the number of –(CH2CH2O)– groups lower than that in the parent compound by unity. The rate and selectivity of PEG oxidation were found to strongly depend on the molecular weight of the polymer; from diethylene glycol to PEG 2000, the specific rate of oxidation increased by a factor of 60 in terms of terminal hydroxyl groups. An oxidation mechanism was suggested, which involves the formation of ternary complexes [Cu2+···A–···O2], which undergo further degradation by a many-electron concerted mechanism to form formic acid and, probably, an unstable hemiacetal {RO–CH2OH}. The rapid oxidative degradation of the latter leads to the formation of PEG with a lower molecular weight.