(633b) Oxidative Cleavage of Waste Polyolefins As a Route to Recycled-Carbon Polar Waxes

One of the strategies to reduce environmental issues caused by single-use plastics is chemical upcycling to value-added products. Conventional processes to produce polar waxes (used in coatings and emulsifiers) includes the oxidation of molten polyethylene (PE) under air/O₂ flow, radical grafting of maleic anhydride onto PE or polypropylene (PP) chains, and co-oligomerization of ethylene with a polar monomer [1]. These routes use fossil carbon-derived feedstocks, such as ethylene and propylene. In this presentation, I will describe how various PE and PP wastes can be converted directly to polar waxes via oxidative cleavage, using an alkylhydroperoxide as the oxidant. The hydroperoxide facilitates the formation of alkyl radicals by hydrogen atom abstraction from the polyolefin backbone, triggering C-C bond cleavage. GPC analysis confirms that M_n values of both LDPE and HDPE were decreased to ca. 1,000 g/mole (Ð ca. 2), which is typical for polar waxes used for coatings. For isotactic PP, M_n decreased to ca. 1,600 g/mol (Ð ca. 1.7). Similar materials are obtained for mixtures of PE and PP, as well as mixed post-consumer PP-PE waste (including dyes and labels), eliminating the need for mechanical separation. CHN analysis showed that oxygen contents of oxi-PE and oxi-PP are ca. 10 and 6 wt%, respectively. These values are 2-3 times larger than values for polar waxes produced by air oxidation of molten PE. According to IR, ¹H NMR, and 2D ¹H-¹³C HSQC/HMBC NMR, the functional groups in oxidized PE (oxi-PE) are predominantly internal ketones, while oxi-PP instead contains terminal methyl ketones, due to its different mechanism of chain cleavage. The mechanism for C-C bond cleavage and oxygen functionalization was further investigated using several model compounds (n-decane, 3-methylpentane, and squalane).

Reference

(1) Krendlinger, E., et al. "Waxes. Ullmann’s Encyclopedia of Industrial Chemistry." (2000).