Plastics are an indispensable material with numerous benefits and advantages compared to traditional materials, such as glass and paper. However, their widespread use has caused significant environmental pollution and most plastics are currently nonrenewable. Biobased polymers represent an important step for tackling these issues, however, the end-of-life disposal of such materials needs to be critically considered to allow for a transition to a circular economy for plastics. Poly(lactic acid) (PLA) is an important example of a biobased polymer, which is also biodegradable. However, industrial composting of PLA affords water and carbon dioxide only and in the natural environment, PLA has a slow biodegradation rate. Therefore, recycling processes are important for PLA, particularly chemical recycling, which affords monomers and useful platform chemicals, maintaining the usefulness and value of the material. This review covers the different methods of PLA chemical recycling, highlighting recent trends and advances in the area.

Poly(lactic acid) (PLA) was degraded to methyl lactate (Me-La) by an imino monophenolate Zn(1)2 catalyst in the presence of THF, as the solvent, and methanol, as the protic source. As well as solution based polymerization and degradation, catalyst stability was also assessed and discussed. The chemical degradation of four different commercial samples of PLA, varying in molecular weight, was studied. The effect of PLA concentration (0.05 to 0.2 g mL-1), reaction temperature (40 to 130 °C), and catalyst concentration (4 to 16 wt%) on conversion, yield and selectivity were studied and statistically analyzed. Mass transfer limitations were examined by utilizing two different PLA particle sizes and altering the stirring speed. Results revealed that the main variables affecting PLA degradation by Zn(1)2 are temperature and catalyst concentration. It was possible to observe Me-La formation even at 40 °C although the reaction times were significantly longer compared with the highest temperatures. Conversions of 100%, as determined by 1H NMR spectroscopy and GPC, were possible in short times (<15 min) depending on the reaction temperature and catalyst concentration. A reaction mechanism for the production of Me-La from PLA degradation, which considers oligomers as an intermediate product, is presented and values of the kinetic constants provided from this modelling. The activation energy for the initial PLA degradation step was found to be 42 – 66 kJ mol-1, generally decreasing with catalyst loading. This work presents a discrete metal complex catalyst that can be used for the chemical recycling of PLA to Me-La.

The use of a simple, cheap and effective organocatalyst, has been exploited for the transesterification/degradation of commercial polymers.

Herein we report the synthesis and characterisation of a series of Zr(IV) 2,2′-bipyrrolidine–salan derived complexes and their exploitation for the ring opening polymerisation of rac-lactide to afford highly isotactically enriched polymers.

In this paper we have prepared a series of Ti(IV), Hf(IV) and Al(III) complexes based on bipyrrolidine salan pro-ligands. The Hf(IV) complexes have all been characterised in the solid-state, the chiral ligands coordinate to Hf(IV) in an α-cis manner whereas the meso ligand coordinates in a β-cis geometry. The Hf(IV) complexes are all active for the ROP of rac-lactide in the melt, with the fluxional meso complex affording a strong isotactic bias P_m = 0.84. As expected Hf(3)(OⁱPr)₂ polymerised L-LA faster than rac-LA (k_app = 5.9 × 10⁻³ min⁻¹vs. 3.8 × 10⁻³ min⁻¹). For Ti(IV) complexes atactic PLA was formed. The salan pro-ligands have also been complexed to Al(III), and the novel Al–Me and Al-OⁱPr complexes were characterised in the solid and solution state. Al(1)(OⁱPr) was fluxional on the NMR timescale, whereas Al(3)(OⁱPr) was locked in solution with no exchange. Interestingly, the Al(III) complexes of 3H₂ produce PLA with a very strong heterotactic bias P_r upto 0.87, whereas atactic PLA is produced with 1H₂. For Al(3)(OⁱPr) a linear relationship is observed with M_n and conversion. Experiments with the addition of an equivalent of rac-LA to the selective initiators have also been performed and are discussed.