Metal–Ligand Catalyzed Upgrading of Waste Polyethylene for Industrial Applications

The accumulation of polyethylene (PE) waste presents significant environmental and economic challenges, particularly in developing regions where plastic valorisation infrastructure remains limited. In this work, waste polyethylene was upgraded through coordination-catalyzed oxidative functionalization using earth-abundant Schiff base metal complexes of iron, cobalt, manganese, and copper with salen and salophen ligands. The process enables selective incorporation of oxygen-containing functional groups while largely preserving polymer molecular integrity, offering a material-oriented alternative to fuel-focused plastic recycling. Fourier transform infrared spectroscopy confirmed the formation of carbonyl and hydroxyl functionalities, with the carbonyl index (CI) increasing from 0.02 ± 0.01 for untreated polyethylene to 0.48 ± 0.04 and 0.42 ± 0.03 for Fe(salen)Cl and Co(salen) catalysts, respectively, under identical conditions. Salophen-based complexes consistently exhibited slightly higher oxidation efficiencies than their salen analogues. Gel permeation chromatography revealed controlled molecular weight reduction, with number-average molecular weight (Mₙ) decreasing from 62.4 × 10³ g•mol⁻¹ (untreated PE) to 56.8 × 10³ and 54.9 × 10³ g•mol⁻¹ for Fe- and Co-based systems, while dispersity remained within polymer-grade ranges. Differential scanning calorimetry and thermogravimetric analysis showed only minor changes in melting temperature and thermal stability. Surface-sensitive X-ray photoelectron spectroscopy confirmed oxidation localized primarily at the polymer surface, while atomic absorption spectroscopy indicated residual metal contents below 10 ppm. Catalyst reusability studies demonstrated sustained activity over multiple cycles. Overall, this coordination-catalyzed strategy provides a scalable and industrially relevant pathway for upgrading polyethylene waste into value-added functional polymers, with strong potential for integration into emerging circular polymer economies in Nigeria and other African regions.