N-Heterocyclic Phosphines as Precatalysts for the Highly Selective Degradation of Poly(lactic acid)

An N-heterocyclic phosphine (NHP) has been investigated as a catalyst for transesterification of a range of commercial polymer samples. NHP catalysed degradation of poly(lactic acid) with methanol provides access to methyl lactate (MeLA) in high yields over the course of days. In situ NMR spectroscopy and kinetic analysis has provided quantitative assessment of the catalyst, and a solvent screen is reported. Surprisingly attempts to depolymerise polycaprolactone (PCL) and polyethylene terephthalate (PET) were unsuccessful, and reactions mixed PLA/PCL and PLA/PET provided completely specific degradation of the PLA alone. NMR analysis of the catalyst provides insight into its solution speciation, indicating that the NHP does not remain intact under the transesterification conditions. The enediamine, tBuN(H)CH=CHN(H)tBu, was shown to be formed via methanolysis of the NHP and proved to be a selective catalyst for PLA degradation. Assessment of the depolymerisation activity of by-products of the NHP methanolysis suggest this is the active catalyst.