Chirality-Induced Catalyst Aggregation: Insights into Catalyst Speciation and Activity Using Chiral Aluminum Catalysts in Cyclic Ester Ring-Opening Polymerization

A series
of chiral-at-metal aluminum complexes have been synthesized
using chiral catam ligands. Reactivity studies demonstrate the importance
of alkoxide bulkiness and complex chirality in inducing catalyst aggregation.
This has been exploited in cyclic ester ring-opening polymerization
(ROP). For ε-Cl ROP, enantiopure catalysts were found to outperform
a racemic mixture of catalysts, as the racemic mixture resulted in
lower activity heterodimeric catalytic species and reduced polymerization
rates. In the case of L-LA, one catalyst enantiomer was found to be
the most active of the series and outperformed both the other enantiomer
and the corresponding achiral catalyst. Very high activities were
observed and up to 9,200 equivalents of L-LA were polymerized in 4.5
min at 150 °C with TOF > 100,000 h–1 under
industrially relevant conditions. Analysis of the catalyst orders
for these reactions provided a meaningful catalyst speciation-activity
relationship enabling improved understanding of both catalyst speciation
and the activity of each catalytic species involved in cyclic ester
ROP using chiral aluminum catalysts.