Preparation and Characterization of Aluminum Alkoxides Coordinated on salen-Type Ligands: Highly Stereoselective Ring-Opening Polymerization of <i>rac</i>-Lactide
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A series of salen-type ligands (L1H2–L6H2) with sterically bulky cumyl groups have been synthesized. Reaction of these ligands with AlMe3 yields the mononuclear aluminum complexes [LAlMe] (1–3) or dinuclear species [L2Al2Me4] (4–6), respectively. Further reaction of [LAlMe] (1–3) with benzyl alcohol produces [LAl(OBn)] (1a–3a), respectively. Solid-state structural studies reveal that complexes 1a and 2a are mononuclear; however, complex 6 is a dinuclear species. Aluminum alkoxides 1a–3a are highly stereoselective in the ROP of rac-lactide, producing polylactide (PLA) with 94–97% enantiomeric selectivity (Pm) at high conversion. Their high enantioselectivity leads to PLA with high Tm (205 °C). The polymerization of l-lactide by these complexes also shows good living features with narrow PDI values (Mw/Mn = 1.06–1.25) signaling less or no transesterification, which can be further verified by MALDI-TOF mass spectrometric analysis.
