Aqueous Depolymerization of Polyethylene at Ambient Temperature: <i>In Situ</i> Generation of Permanganate Using Ozone

The long sought-after goal of chemically recycling polyolefins at ambient temperature and pressure without the use of organic solvents is realized. Using ozone to regenerate a permanganate oxidant in situ in an aqueous environment, low-density polyethylene is converted to carboxylic acids and oligomeric wax. LDPE powder (with average particle sizes between 150 and 250 μm) serves as the substrate─reactions are conducted at atmospheric pressure and 30 °C. Water-soluble products are quantified using HPLC, with diacids having 4, 5, and 6 carbon units as the primary products. The remaining solid wax was analyzed for crystallinity by calorimetry (DSC), for acid number by titration, and for molecular functionality by ATR-IR and NMR spectroscopies. All three measured quantities increase with increasing reaction time. The acid number of the residual wax indicates a diacid carbon length of ∼30 (∼450 g/mol). Polymer oxidation occurs preferentially at side-chain branch points. Results suggest a two-reaction system in which branch-point tertiary carbons are selectively oxidized to yield linear carboxylic acids and ketones, followed by secondary depolymerization to yield water-soluble diacids. Experimental yields were 10% at 144 h.