Are Well Performing Catalysts for the Ring Opening Polymerization of l‑Lactide under Mild Laboratory Conditions Suitable for the Industrial Process? The Case of New Highly Active Zn(II) Catalysts

Poly­(lactic acid)
(PLA) is one of the best candidates as a sustainable
plastic material for a circular economy, being biodegradable, bio-based,
recyclable, and displaying good thermal and mechanical properties.
The industrial production of PLA is mainly based on the ring opening
polymerization (ROP) of l-lactide (l-LA) promoted
by tin­(II) 2-ethylhexanoate [Sn­(Oct)2] in a continuous
solvent-free process operating at temperatures between 180 and 200
°C, above the melting point of the resulting isotactic polymer.
Despite the huge efforts in the research of alternative catalysts
based on less toxic metals, resulting in a plethora of highly active
catalysts under laboratory mild conditions, very few candidates can
compete with Sn­(Oct)2 under industrially relevant conditions.
We report a family of new Zn­(II) complexes, bearing variously substituted
monoanionic [N,O–] (imidazole­[1,5-a]­pyrid-3-yl)­phenolate ligands, as catalysts for the ROP of l-LA under both mild (20 °C, solvent) and industrially relevant
(190 °C, in the melt, technical grade unpurified monomer, very
low catalyst loading) conditions. Interestingly, the best performing
catalyst under mild conditions is the worst performing under harsh
conditions, and, on the contrary, the less active catalysts under
mild conditions compete well with Sn­(Oct)2 under industrially
relevant conditions. Kinetic and DFT mechanistic investigations shed
light on the non-trivial role of the 2-pyridine substituent in the
catalytic performances at different temperatures. Preliminary depolymerization
tests on commercial PLLA samples suggested that the new catalysts
can also be a suitable candidate for the chemical recycling of PLA
under mild conditions.