Sustainable approaches for the synthesis and the chemical degradation of polyesters

Abstract

Petrochemical-based plastics have a significant impact on modern society, as underlined by their high production and countless applications. Indeed, the production of plastics has increased over the past 50 years, reaching 390.7 million tonnes in 2021.1 Although these materials have been designed to last over time, every year around 40% of the world's plastic production is used in the short-term packaging field and about 50 million tons of plastic waste are retained in landfills or spread into the environment.2 Severe consequences due to the accumulation of plastic waste have encouraged current scientific research to develop new strategies to address these problems. Some examples are the chemical recycling of traditional plastics, which would reduce accumulation and help reuse the raw material,3

but also the introduction of new bio-based polymers, for which an effective waste management strategy is defined beforehand. Both of these approaches are consistent with a circular economy vision, in which the material retains its value even after its use.4 In this context, polylactide (PLA) is considered the most promising material because it combines good mechanical properties, biodegradability and biocompatibility. Although these characteristics design it as a green material, there are some weaknesses in terms of sustainability regarding the production and management of waste. The catalyst used industrially to produce PLA is tin(II) bis(2-ethylhexanoate) [Sn(Oct)2], which is classified as a toxic compound, and the waste management strategy of PLA is still aligned with an economic model linear. A promising end-of-life path for PLA is its chemical recycling through alcoholysis in which the product, alkyl lactate, can be converted into lactide, thus realizing a circular economy process, or used as a green solvent in industries.5

The aim of this PhD thesis was the development of new catalytic systems able of synthesizing PLA using non- toxic and robust metals enough to withstand even industrially relevant reaction conditions. Specifically, new

heteroleptic and homoleptic complexes of zinc and magnesium supported by pyridyl-phenoxy-imine ligands were synthesized. The family of Zn heteroleptic complexes showed the highest activities and a good control of the polymerization process, ranking among the most efficient catalysts reported in the literature and comparable to the industrial tin catalyst. Recent literature studies have shown that bimetallic complexes can show a peculiar catalytic behavior compared to monometallic analogues, thanks to cooperation effects that can be established between the two

metal centers. The introduction of a binaphthol backbone between the two pyridyl-phenoxy-imine moieties allowed to obtain hexacoordinate ligands, which promoted the synthesis of bimetallic complexes. These catalytic systems, in addition to being highly active in the lactide polymerization, showed a good control of the polymerization obtaining linear polymers with expected end groups. When the reaction is carried out at high temperature, cyclic PLA is selectively obtained. The same complexes were also used in the degradation reaction of PLA, by alcoholysis. The Zn heteroleptic complexes showed the highest activities and from mechanistic studies two different pathways were highlighted according to the degradation conditions. In THF solution, the degradation occurred through a two-step process in which the random scission of the polymer chains leads to the formation of oligomers, which progressively convert into alkyl lactate. Under solvent-free conditions, the degradation occurred through a progressive erosion of the chain-end with the direct formation of alkyl lactate. Subsequently simple and commercially available compounds, such as amides of various non-toxic metals, have been explored in the degradation reactions of PLA and polyethylene terephthalate (PET), one of the most commercially widespread polymers. This work has allowed us to highlight the potential of different metals in these still underdeveloped reactions. For example, Zr amide proved to be the best catalyst in PET glycolysis reaching activities comparable to the best reported in the literature for metal catalysts (78% BHET yield within one hour at 180°C). Kol’s research group of Tel Aviv University, where I carried out a visiting research period, had synthesized a new hexacoordinate complex of Zr supported by aminotris(phenolate) ligand, which showed the highest activity ever recorded for metal complexes in lactide polymerization under industrial conditions.6 In this context, my work has focused on the synthesis of new Zr complexes with different substituents in the phenoxy moieties and the study of their behavior in the stereoselective polymerization of PLA. [edited by Author]