Highly Efficient Aluminum-Catalyzed Ring-Opening Polymerization of Cyclic Carbonates, Lactones, and Lactides, Including a Unique Crystallographic Snapshot of an Intermediate

The synthesis of a new well-defined aluminum-based catalyst (1) for the ring-opening polymerization of cyclic carbonates, lactones, and lactides is described. Compound 1 is supported by a cheap and readily available phenylene-diamine-based ligand set and features a nucleophilic dimethylamide ligand that is highly active toward polymerization chemistry. While trimethylene carbonate, ε-caprolactone, and δ-valerolactone can be polymerized in the absence of solvent and alcohol cocatalysts, polymers isolated from the solution-based polymerization of racemic lactide are indicative of living polymerization. Turnover frequencies of up to 36 900 h–1 demonstrate the high activity of Al toward these transformations and allow for the formation of polymers with molecular weights of up to 59 000 g/mol under mild conditions. The isolation and structural characterization of a polymerization intermediate (compound 2) revealed an extremely distorted coordination environment around the metal center, possibly explaining the high reactivity. Furthermore, hydrogen-bonding interactions between the coordinated Lewis base and the polymer end group provide the first structural insight into such polymerization reactions and may offer a mechanistic explanation for the formation of cyclic polymers.