Efficient synthesis of polylactide and copolymers under industrial conditions by multinuclear β-ketoimide zinc complexes

Abstract

Polylactic acid (PLA), a representative degradable aliphatic polyester, has the advantages of biodegradability, biocompatibility, and good thermal and mechanical properties. The industrial production of high-molecular-weight PLA is achieved via the ring-opening polymerization (ROP) of L-lactide (L-LA) catalyzed by Sn(II)-2-ethyl-hexanoate (Sn(Oct)₂) under melt and bulk conditions. Although huge efforts have been made to develop organometallics with low toxicity, and many catalysts that are highly active under mild laboratory conditions have been found, very few candidates can compete with Sn(Oct)₂ under industrially relevant conditions. Here, we report novel multinuclear β-ketoiminate zinc complexes as efficient catalysts for L-LA polymerization under industrially relevant conditions, with a turnover frequency as high as 5880 h⁻¹. The catalysts exhibited good stability and activity and compete well with Sn(Oct)₂ under industrial conditions, affording colorless PLLA with high crystallinity. Preliminary copolymerization experiments suggested that the zinc catalyst can also catalyze the random copolymerization of L-LA with ε-caprolactone and dioxanone under melt and bulk conditions.