Depolymerisation of poly(lactide) under continuous flow conditions

Area of Science:

• Polymer Chemistry
• Sustainable Materials
• Chemical Engineering

Background:

• Poly(l-lactic acid) (PLLA) is a widely used bio-based plastic with end-of-life options including industrial composting.
• Chemical recycling to monomer (CRM) offers a circular economy approach by depolymerizing PLLA back to its l-lactide monomer.
• Traditional CRM methods face challenges due to PLLA's high ceiling temperature and susceptibility to thermal decomposition and side reactions, often requiring vacuum or high dilution conditions.

Purpose of the Study:

• To investigate the chemical recycling of PLLA to l-lactide using a commercially available tin(II) catalyst.
• To explore the efficacy of CRM in a continuous flow process utilizing low boiling point solvents.
• To optimize reaction conditions including temperature and catalyst concentration for efficient PLLA depolymerization.

Main Methods:

• A continuous flow process was employed for the chemical recycling of PLLA.
• A commercially available tin(II) catalyst was utilized in conjunction with low boiling point solvents.
• Tetrahydrofuran (THF) was identified as the optimal solvent, and reaction parameters such as temperature and concentration were systematically varied.

Main Results:

• Up to 92% conversion of PLLA to lactide was achieved using THF as the solvent at temperatures between 150-170 °C.
• High selectivity for l-lactide formation, ranging from 92% to 97%, was observed under optimized conditions.
• Inline monitoring in the flow process allowed for the determination of the depolymerization rate coefficient (k_depo) and its activation energy (129.4 kJ mol⁻¹).

Conclusions:

• The developed continuous flow process using a Sn(II) catalyst and THF is effective for the chemical recycling of PLLA.
• This catalytic approach provides a scalable and efficient method for PLLA depolymerization, yielding high-purity l-lactide.
• The findings contribute to the development of sustainable practices and a circular economy for PLLA-based materials.