Affordable and Clean Energy (SDG 7), Responsible Consumption and Production (SDG 12) N-heterocyclic Phosphines as Pre-catalysts for the Selective Degradation of Poly(lactic acid) Laura English, 1,2 Matthew Jones, 1,2 David Liptrot 1,2 1 Department of Chemistry, University of Bath, Bath, BA2 7AY, UK. Email: lee33@bath.ac.uk 2 Centre for Sustainable and Circular Technologies, Bath, BA2 7AY, UK. N-heterocyclic Phosphines as Pre-catalysts for the Selective Degradation of Poly(lactic acid) Laura English* 1,2 Matthew Jones, 1,2 David Liptrot 1,2 1 Department of Chemistry, University of Bath, Bath, UK 2 Centre for Sustainable and Circular Technologies, Bath, UK. In recent years an influx of research has been conducted on the production of polymers derived from renewable resources and their breakdown into re-useable resources or value-added chemicals. 1 One such polymer, poly (lactic acid) (PLA), can be produced from the ring-opening polymerization of lactide which is derived from lactic acid, a renewable product of the fermentation of starch rich feedstocks such as corn. While PLA is mechanically recyclable, and under certain conditions biodegradable, 2 recently research has focused on the catalytic degradation of the polymer to form alkyl lactates via alcoholysis. 3 Some of the compounds reported to act as catalysts for the production of PLA, have also been reported to be active for its alcoholysis, and even the alcoholysis of other waste plastics such as poly(ethylene terephthalate) (PET) and poly(caprolactone) (PCL). 4 In recent years an influx of research has been conducted on the production of polymers derived from renewable resources and their breakdown into re-useable resources or value- added chemicals.[1] One such polymer, poly(lactic acid) (PLA), can be produced from the ring-opening polymerization of lactide which is derived from lactic acid, a renewable product of the fermentation of starch rich feedstocks such as corn. While PLA is mechanically recyclable, and under certain conditions biodegradable,[2] recently research has focused on the catalytic degradation of the polymer to form alkyl lactates via alcoholysis.[3] Some of the compounds reported to act as catalysts for the production of PLA, have also been reported to be active for its alcoholysis, and even the alcoholysis of other waste plastics such as poly(ethylene terephthalate) (PET) and poly(caprolactone) (PCL).[4]
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Figure 1: Kinetic profile of N-heterocyclic phosphine catalysed degradation reaction. In this work, an air-stable N-heterocyclic phosphine was shown to selectively degrade PLA in the presence of an alcohol while remaining unreactive towards PET and PCL, with no competing ring-opening polymerization of lactide.[5] Kinetic analysis of the reaction provided insight into the mechanism of the reaction, comparable to that reported in the literature and the efficacy of the reaction was tested in multiple solvents. The identity of the active catalyst species was then investigated and shown to be the enediamine ligand produced by the reaction of the pre-catalyst with alcohols. The kinetics of the reaction were then probed using an authentic sample of the active catalyst and the reaction was shown to proceed much faster. Isolation of the active catalyst is not trivial, however, and the compound is highly air-sensitive. Thus the reaction benefits from the employment of the air-stable N-heterocyclic phosphine pre-catalyst which is, by comparison, much easier to synthesise and provides the same selectivity as the active species. References 1. Zhu, J. B.; Chen, E. Y. X. Angew. Chem. Int. Ed. 2018, 57, 12558-12562. 2. Auras, R.; Harte, B.; Selke, S. Macromol. Biosci. 2004, 4, 835-864. 3. a) McKeown, P.; Roman-Ramirez, L. A.; Bates, S.; Wood, J.; Jones, M. D. ChemSusChem, 2019, 12, 5233-5238; b) Payne, J.; McKeown, P.; Jones, M. D. Polym. Degrad. Stab., 2019, 165, 170-18 4. a) McKeown, P.; Kamran, M.; Davidson, M. G.; Jones, M. D; Roman-Romirez, L. A.; Wood, J. Green Chem., 2020, 22, 3721- 3726; b) Payne, J.; McKeown, P.; Mahon, M. F.; Emanuelsson, E. A. C.; Jones, M. D., Polym. Chem., 2020, 11, 2381–2389. 5. English, L. E.; Jones, M. D.; Liptrot, D. J. ChemCatChem, 2022, 14, e202101904. Figure 1: Kinetic profile of N-heterocyclic phosphine catalysed degradation reaction. In this work, an air-stable N-heterocyclic phosphine was shown to selectively degrade PLA in the presence of an alcohol while remaining unreactive towards PET and PCL, with no competing ring-opening polymerization of lactide.[5] Kinetic analysis of the reaction provided insight into the mechanism of the reaction, comparable to that reported in the literature and the efficacy of the reaction was tested in multiple solvents. The identity of the active catalyst species was then investigated and shown to be the enediamine ligand produced by the reaction of the pre-catalyst with alcohols. The kinetics of the reaction were then probed using an authentic sample of the active catalyst and the reaction was shown to proceed much faster. Isolation of the active catalyst is not trivial, however, and the compound is highly air- sensitive. Thus the reaction benefits from the employment of the air-stable N-heterocyclic phosphine pre-catalyst which is, by comparison, much easier to synthesise and provides the same selectivity as the active species. ______________ References: [1] Zhu, J. B.; Chen, E. Y. X. Angew. Chem. Int. Ed. 2018, 57, 12558-12562. [2] Auras, R.; Harte, B.; Selke, S. Macromol. Biosci. 2004, 4, 835-864. [3] a) McKeown, P.; Roman-Ramirez, L. A.; Bates, S.; Wood, J.; Jones, M. D. ChemSusChem, 2019, 12, 5233-5238; b) Payne, J.; McKeown, P.; Jones, M. D. Polym. Degrad. Stab., 2019, 165, 170-18 [4] a) McKeown, P.; Kamran, M.; Davidson, M. G.; Jones, M. D; Roman-Romirez, L. A.; Wood, J. Green Chem., 2020, 22, 3721-3726; b) Payne, J.; McKeown, P.; Mahon, M. F.; Emanuelsson, E. A. C.; Jones, M. D., Polym. Chem., 2020, 11, 2381–2389. [5] English, L. E.; Jones, M. D.; Liptrot, D. J. ChemCatChem, 2022, 14, e202101904.
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