"Like Recycles Like": Selective Ring-Closing Depolymerization of Poly(L-Lactic Acid) to L-Lactide.

Area of Science:

• Polymer Chemistry
• Sustainable Materials
• Chemical Engineering

Background:

• Chemical recycling of poly(L-lactic acid) (PLA) to L-lactide is challenging due to low selectivity and side reactions like epimerization.
• The high ceiling temperature (Tc) of L-lactide necessitates harsh conditions or multi-step processes for effective PLA recycling.

Purpose of the Study:

• To overcome limitations in PLA chemical recycling by decreasing the ceiling temperature of L-lactide.
• To develop a more efficient and selective method for converting high-molecular-weight PLA back to its cyclic monomer.

Main Methods:

• Investigated the effect of solvent interactions on the monomer-polymer equilibrium to lower L-lactide's ceiling temperature.
• Analyzed the relationship between solvent Hildebrand solubility parameter and observed ceiling temperature, identifying a "like recycles like" correlation.
• Utilized strong monomer-solubilizing solvents like dimethyl formamide and gamma-valerolactone to facilitate recycling.

Main Results:

• Achieved chemical recycling of high-molecular-weight poly(L-lactic acid) directly to L-lactide within 1-4 hours at 140°C.
• Demonstrated high conversion rates (>95%) and excellent selectivity (98-99%) for L-lactide.
• Successfully isolated and repolymerized the recycled L-lactide with precise control over molecular weight and dispersity.

Conclusions:

• Solvent selection is crucial for reducing the ceiling temperature and enabling efficient, selective PLA chemical recycling.
• This method provides a viable pathway for closing the polymer loop, promoting sustainable plastic management.
• The ability to repolymerize recycled L-lactide with high fidelity supports circular economy principles for polylactic acid.