Guanidinate Zn(II) Complexes as Efficient Catalysts for Lactide Homo- and Copolymerization under Industrially Relevant Conditions

Poly(lactic acid) (PLA) is currently by far the most commercially relevant biodegradable polymer derived from bio-renewable sources. The industrial production of high molecular weight isotactic PLA is based on the ring-opening polymerization (ROP) of lactide (the dimeric lactone of lactic acid) promoted by Sn(II) 2-ethylhexanoate under solvent-free conditions at high temperatures (190–200 °C) in the presence of excess alcohol. Although a huge number of alternative catalysts have been reported in the literature, searching for better performance and lower toxicity, most studies were performed under conditions unsuitable for industrial processes, for example, at lower temperatures in solution. We report here the synthesis of two Zn(II) complexes supported by bulky monoanionic guanidinate ligands, which have been tested as catalysts for the ROP of l-lactide under both mild and industrially relevant conditions, that is, at 190 °C using technical grade l-lactide at a monomer/catalyst ratio up to 104 and excess alcohol. Interestingly, the Zn(II) catalysts also showed stability under the latter harsh conditions, showing activity comparable to that of the Sn catalyst and even lower monomer racemization. Moreover, they are able to copolymerize l-lactide with ε-caprolactone, affording either block or random copolymers depending on the reaction conditions.