Alkali and Alkaline-Earth-Metal-Based Catalytic Systems for Efficient Depolymerization of Polyesters and Polycarbonates

Abstract

The widespread use of plastic materials has led to severe contamination of ecosystems, posing significant risks to wildlife and human health. Therefore, the development of efficient, economical, and environmentally benign catalytic systems for the chemical recycling of plastic waste into value-added compounds is of great importance. We investigate the application of earth-abundant alkali and alkaline-earth metal complexes as catalysts for the depolymerization of ester- and carbonate-containing waste polymers. In particular, lithium bis(trimethylsilyl)amide [LiN(SiMe3)2]2·2THF, amidinate- and β-diketiminate (NacNac)-supported magnesium complexes, and Ca[N(SiMe3)2]2.2THF complexes were explored as efficient catalytic systems for the depolymerization of various plastic wastes such as PET, PBT, PCL, PLA, PES, PEA, PPC, and PCBPA. In the presence of pinacolborane (HBpin) as a hydride donor, these catalytic systems enable the conversion of biodegradable and non-biodegradable polyesters and polycarbonates into the corresponding borylated value-added alcohols under mild conditions without any pre-treatment, affording an excellent yield. Also, the lithium catalyst enabled gram-scale depolymerization of polyethylene terephthalate (PET) waste bottles to produce environmentally benign alcohols such as 1,4-benzene-dimethanol and ethylene glycol, via this strategy. The borylated 1,4-benzene-dimethanol intermediate obtained during PET depolymerization was characterized by single-crystal X-ray diffraction, NMR spectroscopy, and routine spectroscopic techniques. Furthermore, these catalytic systems demonstrated excellent efficiency in the depolymerization of mixed and contaminated plastic waste, producing selective and high yields of value-added products. Further, Mechanistic investigations using methyl benzoate as a model monomeric ester provided insight into the single-site catalytic pathway involved in the depolymerization process. The magnesium complexes efficiently facilitated the gram-scale depolymerization of polycaprolactone (PCL) and supported mechanistic insights through the hydroboration of monomeric esters, carbonates, and lactones, confirming the applicability of the catalytic system to polymeric materials. The Ca[N(SiMe3)2]2.2THF complexes were also employed to successfully depolymerize these polyesters and polycarbonates, providing high yields of the corresponding borylated alcohols. Stoichiometric experiments indicate that the active catalytic species [Ca]-H) in the reaction mixture is generated via the reaction of Ca[N(SiMe3)2]2.2THF and HBpin. Overall, this thesis highlights that Li-, Mg-, and Ca-based catalytic systems offer cost-effective, sustainable, and efficient approaches for the chemical recycling of plastic waste into valuable alcohol derivatives under mild conditions, with several applications not previously reported.