To Be or Not To Be… Cooperative? Trimetallic Catalysts for the Ring‐Opening Polymerisation of Cyclic Esters

Multimetallic catalysts have emerged as a promising route to enhance catalyst performance in cyclic ester ring-opening polymerisation (ROP), as these complexes can outperform the monometallic analogues in terms of reactivity and/or polymerisation control. Such enhancements are often attributed to “multimetallic cooperativity”, yet the origins of this cooperativity often remain unclear. Here, we report the synthesis and characterisation of two trimetallic Al-salen complexes (L1(AlMe)3 and L1(AlEt)3), containing three Al-salen subunits in close proximity, and their monometallic analogues (L2(AlMe) and L2(AlEt)). These complexes have been applied as catalysts for cyclic ester ROP under a range of conditions, to probe their potential for cooperative behaviour. Trimetallic L1(AlMe)3 and L1(AlEt)3 both display high activities in rac-lactide ROP, giving kobs values of up to 11 x 10-3 min-1. Yet careful benchmarking against the monometallic complexes reveals that whether these systems display multimetallic cooperativity or not depends upon the reaction conditions. Detailed kinetic and spectroscopic studies into the origins of cooperativity revealed that the neighbouring metals play a key role in the initiation step, rather than propagation. Overall, these studies highlight the importance of understanding the reaction conditions in order to accurately define whether a catalyst displays multimetallic cooperativity.