New publication in ChemSusChem online

 

A Multitool for Circular Economy: Fast Ring‐Opening Polymerization and Chemical Recycling of (Bio)polyesters Using a Single Aliphatic Guanidine Carboxy Zinc Catalyst

Plastics are ubiquitous in our modern lives. Although these materials offer many benefits due to their tailored properties, the mass use of petroleum-based, non-degradable plastics in a linear economy has led to a massive and global pollution of the environment. Bioplastics are often cited as a solution to face this issue. These types of plastics can either be based on renewable raw materials, be biodegradable, or, like the polyester polylactide (PLA), have both properties. PLA is already present on today’s plastics market, mainly as packaging material, but also as disposable tableware or agricultural foils. The raw material for the production of PLA are starch-containing plants, e.g. corn. From these, lactic acid is obtained by fermentation, from which the monomer lactide is accessible. From this, polylactide is produced in a ring-opening polymerization. Due to the biodegradation of PLA to water and CO2, which is consumed by starch-containing plants through photosynthesis, a closed lifecycle is generated. However, the value of PLA is lost in the process. In addition, the industrial production of PLA is currently not yet fully sustainable. Here the toxic heavy metal catalyst stannous octanoate is used as standard for ring-opening polymerization. Accordingly, to exploit the full potential of PLA, new non-toxic, robust and at the same time highly active catalysts are needed. Furthermore, systems need to be implemented that enable efficient recycling of (bio-)plastics after their use, transforming the current linear economy into a circular one. Herein, waste is avoided and the plastics themselves serve as raw materials for new plastics and other products. Here, chemical recycling in particular shows great potential. In these processes, plastics are broken down into small molecules which can be reprocessed into plastics without any loss of quality or can also serve as the basis for new chemical products.

In this publication, we present a new N,O-hybrid guanidine zinc complex as a robust and highly active alternative to the toxic industrial catalyst stannous octanoate. The ligand used here has an aliphatic backbone, which significantly increases the catalytic activity of the catalyst, compared to analogous zinc catalysts with aromatic ligand backbones. Thus, the new catalyst achieves almost complete conversion after 70 seconds in lactide polymerization under industrially relevant conditions (monomer melt, 150 °C) at a [M]/[I] ratio of 1000:1. The catalyst retains this high activity even at lower catalyst loadings, which distinguishes it for industrial applications. Even in lactide polymerization at 100 °C in toluene, our new catalyst outperforms stannous octanoate by a factor of ten. The catalyst also excels in chemical recycling with its high activity and selectivity. Alcoholysis of polylactide to lactic acid esters was achieved, even at mild reaction conditions (60 °C in THF) and low catalyst loadings. Thus, in PLA methanolysis, complete polymer conversion was achieved after only 6 h with a methyl lactate yield of 88 %. The catalyst retains its high activity even in the presence of other polyesters (polycaprolactone or PET) and selectively catalyzes the degradation of PLA. By varying the reaction conditions (solvent-free, 150 °C to 180 °C), these polyesters could also be converted via alcoholysis. In the future, cascaded, selective recycling of plastics from mixtures can thus be implemented. Moreover, the catalyst retains its high activity even under these harsher reaction conditions and can itself be recycled up to four times. By combining sustainable polymerization catalysis and chemical recycling, our new catalyst thus becomes a multitool in the realization of a bio-based plastics circular economy. The front cover illustrates these material flows that, with our new catalyst regulating them in the form of a traffic circle and steering them along controlled paths.

The full article can be viewed on the publisher's website.