New publication in ChemPlusChem online

06/04/2022
 

Guanidine carboxy zinc complexes for the chemical recycling of renewable polyesters

Plastics are ubiquitous in our lives. However, facing the increasing environmental pollution caused by petroleum-based, non-degradable plastics is one of the greatest global challenges of the near future. Bioplastics, such as the bio-based and fully biodegradable polyester polylactide (PLA), can make a decisive contribution to solving this problem. Unfortunately, in the current linear plastics economy, PLA can be a danger to the environment, since the toxic catalyst stannous octanoate, currently used for industrial production, is released from the polymer, when degraded and can accumulate in nature. Therefore, extensive research has been made by the Herres-Pawlis group to develop non-toxic, robust and highly active catalysts for the ring-opening polymerization of lactide to produce PLA. Various iron and zinc guanidine catalysts have already been developed that are significantly more active than the toxic tin catalyst using industrially preferred melt conditions. In addition to the development of non-toxic catalysts for production, efficient recycling of PLA after its use in desirable, to exploit its full potential in a circular plastics economy. Due to the reactive ester bond in the polymer chain, PLA is particularly well suited for chemical recycling methods, such as alcoholysis.

In the current work, we show that three robust guanidine-carboxy-zinc complexes, previously developed in our group and suitable for lactide polymerization, are also catalytically active in the alcoholysis of PLA. Herein, all tested catalysts transform PLA to methyl lactate in a transesterification reaction using mild reaction conditions (60 °C in THF). Methyl lactate can be cyclized to the monomer lactide to recycle PLA, used as a solvent or as a starting material for the synthesis of new compounds. Electron density donating substituents on the ligand increased the catalytic activity, whereas electron density withdrawing substituents decrease the catalytic activity. The most active catalyst [ZnCl2(TMG5NMe2asme)] has additionally been tested towards applicability in industry. Both the scalability of the reaction and the recycling of the catalyst itself are fulfilled. Furthermore, the conversion of other alcohols, the selective recycling of PLA from plastic mixtures and blends, and the recycling of the bioplastic polycaprolactone can be realised using the catalyst. Using solvent free conditions at 150 °C, accelerated alcoholysis of PLA, reaching full conversion after only 1 h. which further demonstrates the industrial relevance of the catalyst. Therefore, guanidine carboxy Zn catalysts can be a vital tool in the implementation of a sustainable circular bioplastics economy.

The article Guanidine Carboxy Zinc Complexes for the Chemical Recycling of Renewable Polyestersis available on the webpage of the publisher.