Developing new catalysts for racemic lactide (rac-LA) ring opening polymerization (ROP) is important to understand the key features controlling the production of the bio-degradable polymer and to improve its physical and chemical properties. Our focus has been to investigate zinc catalysts and recently, we reported dizinc bis(ethyl) macrocycle pre-catalysts which showed good rates in rac-LA ROP. One limitation was that an induction period was observed due to the slow reaction of the Zn-Et with alcohol sources to generate the active alkoxide initiator. Here, new di-zinc bis(amido) pre-catalysts are synthesized, and are shown to rapidly form alkoxide complexes upon addition of alcohol. The structure-activity relationships of two di-Zn bis(amido) macrocycles, differing in ring sizes, and their corresponding alkoxides are explored. The activity of the series is remarkably high, being more than an order of magnitude higher than the other known zinc catalyst under comparable conditions. The catalysts are highly tolerant and maintain high activity even at very low catalyst loading (≥ 0.002 mol %). The catalysts show high levels of control affording aliphatic polyesters with controlled molecular weights and narrow dispersities. Catalyst comparisons reveal that the structure of the complex exerts a strong effect on the rate of the polymerization. There is a positive effect of two metal centres being in close proximity; the dinuclear macrocyclic catalysts are significantly better than their mononuclear acyclic ligand analogues.

Dizinc lactide polymerization catalysts: Hyperactivity by control of ligand conformation and metallic cooperativity

253rd National Meeting (2017)

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