A series of four- and five-coordinate aluminum complexes bearing pyrrolide-imine ligands have been synthesized and characterized for the ring-opening polymerization of rac-lactide. Reactions of a 1:1 molar ratio of pyrrolide-imine ligands, (ArN=CH)C4H3NH where Ar = C6H5 (HL1 ); 4-F-C6H4 (HL2 ); 4-Me-C6H4 (HL3 );  4-MeO-C6H4 (HL4 ); 2-Me-C6H4 (HL5 ); 2-tert-Bu-C6H4 (HL6 ), and trimethylaluminum were conducted in toluene at room temperature to generate the four-coordinate aluminum complexes, [(ArN=CH)C4H3N]AlMe2 where Ar = C6H5 (1a ); 4-F-C6H4 (2a ); 4-Me-C6H4 (3a );  4-MeO-C6H4 (4a ); 2-Me-C6H4 (5a ); 2-tert-Bu-C6H4 (6a ). The five-coordinate aluminum complexes, [(ArN=CH)C4H3N]2AlMe  where Ar = C6H5 (1b ); 4-F-C6H4 (2b ); 4-Me-C6H4 (3b ); 4-MeO-C6H4 (4b ); 2-Me-C6H4 (5b ); 2-tert-Bu-C6H4 (6b ), were prepared by the reaction between their corresponding ligands (HL1 -HL6 ) and trimethylaluminum in a 2:1 molar ratio in toluene at 110 �C. In the presence of an equimolar concentration of benzyl alcohol, these complexes were found to be efficient single-site initiators for the ring-opening polymerization of rac-lactide at 70 �C, providing polymers with plausibly controlled molecular weight and narrow polydispersity. The polymerization rates initiated by the five-coordinate aluminum complexes were slightly faster than those by the four-coordinate aluminum counterparts. According to the density functional theory (DFT) calculation study, the rate of polymerization could be related to the Al-CH3 bond stiffness of initiators. In addition, the polymerization rate was strongly affected by the aromatic substituent (Ar) in the imino group. The order of decreasing rate was 4-F-C6H4 > 4-MeO-C6H4 > C6H5 > 4-Me-C6H4 > 2-Me-C6H4 > 2-tert-Bu-C6H4. Complexes 6a  and 6b  gave heterotactic bias PLAs while the others produced atactic polymers.

Discrete four- and five-coordinate aluminum complexes bearing pyrrolide-imine ligands for ring-opening polymerization of <i>rac</i>-lactide

243rd National Meeting (2012)

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Discrete four- and five-coordinate aluminum complexes bearing pyrrolide-imine ligands for ring-opening polymerization of rac-lactide