Combining alkali metals and zinc to harness heterometallic cooperativity in cyclic ester ring-opening polymerisation

Heterometallic cooperativity is an emerging strategy to elevate polymerisation catalyst performance. Here, we report the first heterotrimetallic Na/Zn-2 and K/Zn-2 complexes supported by a ProPhenol ligand, which deliver "best of both" in cyclic ester ring-opening polymerisation, combining the outstanding activity (Na/K) and good control (Zn-2) of homometallic analogues. Detailed NMR studies and density-functional theory calculations suggest that the Na/Zn-2 and K/Zn-2 complexes retain their heterometallic structures in the solution-state. To the best of our knowledge, the K/Zn-2 analogue is the most active heterometallic catalyst reported for rac-lactide polymerisation (k(obs) = 1.7 x 10(-2) s(-1)), giving activities five times faster than the Na/Zn-2 complex. These versatile catalysts also display outstanding performance in epsilon-caprolatone and delta-valerolactone ring-opening polymerisation. These studies provide underpinning methodologies for future heterometallic polymerisation catalyst design, both in cyclic ester polymerisation and other ring-opening (co)polymerisation reactions.