Controlled Oxidation of n-Eicosane and Low-Molecular-Weight Polyethylene.

Abstract

The aerobic oxidation of long-chain hydrocarbons provides a direct route to introduce oxygen-containing functionality into polyolefin backbones but is intrinsically coupled to C─C bond scission through radical autoxidation pathways. Here we examine the oxidation of n-eicosane (C₂₀) and low-molecular-weight polyethylene (PE4k) under O₂ in the presence of 2,2'-azobis(2-methylpropionitrile) (AIBN) or acetic acid (HOAc). AIBN and HOAc efficiently initiate autoxidation at 90-125 °C, producing ketone and alcohol products at moderate conversion. Increasing temperature and oxygen partial pressure enhance overall conversion to oxidized products but promote C─C bond cleavage and formation of lower-molecular-weight products consistent with a β-scission reaction pathway. Similar trends are observed for PE4k, where the additives promote CH₂ group oxidation but reduce molecular weight at high temperatures or with high oxygen pressure. Low levels of oxygen incorporation were achieved with low chain scission at reduced oxygen partial pressure. The AIBN and HOAc reactions proceed via different radical initiations but are proposed to converge to a common autoxidation radical chain propagation mechanism. Across both substrates, oxygen concentration and reaction temperature control the balance between functionalization and polymer backbone cleavage, defining an operational window for selective oxidative modification of polyolefins.