Depolymerisation of poly(lactide) under continuous flow conditions

Poly(L-lactic acid) (PLLA) is recognized as a commercially successful bio-based plastic, with end-of-life materials typically subjected to industrial composting. As an alternative aimed at enabling a circular economy, chemical recycling to monomer (CRM) has been investigated, whereby direct depolymerisation to L-lactide can be achieved. However, CRM of PLLA has been hindered by thermal decomposition and side reactions, attributed to the high ceiling temperature (T_c) of PLLA in bulk (>600 °C), which has prevented large-scale implementation. As a result, catalytic strategies have been developed that operate under vacuum or in high dilution using high boiling point solvents.

In the present study, a commercially available Sn(II) catalyst was employed with low boiling point solvents across a range of temperatures and concentrations to facilitate CRM of PLLA in a continuous flow process. Among the solvents tested, tetrahydrofuran (THF) was found to yield the best results, with up to 92% conversion of lactide and 92–97% selectivity for L-lactide formation at temperatures between 150–170 °C. Furthermore, inline monitoring of monomer and polymer concentrations in the flow system was performed, allowing the depolymerisation rate coefficient (k_depo) to be determined. The activation energy associated with k_depo was calculated to be 129.4 kJ mol⁻¹.

• Ellis, S
• Buchard, A
• Junkers, T

• Polymer Reaction Design Group, School of Chemistry, Monash University 17 Rainforest Walk Clayton VIC 3800 Australia

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