Isoselective Polymerization of rac-Lactide by Aluminum Complexes of N-Heterocyclic Carbene-Phosphinidene Adducts

Abstract

The N-heterocyclic carbene-phosphinidene adducts (NHC)PH were reacted with AlMe3 in toluene to afford the monoaluminum complexes [{(IDipp)PH}AlMe3] and [{(IMes)PH}AlMe3] (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). In contrast, the dialuminum complex [{(MeIMes)PH}(AlMe3)2] was obtained for MeIMes=1,3-bis(2,4,6-trimethylphenyl)-4,5-dimethylimidazolin-2-ylidene. These complexes served as initiators for the efficient ring-opening polymerization of rac-lactide in toluene at 60 °C. High degrees of isoselectivity were found for the poly(rac-lactide) obtained in the presence of the monoaluminum complexes (Pm up to 0.92, Tm up to 191 °C), whereas almost atactic polymers were produced by the dialuminum complex. Detailed mechanistic studies reveal that the polymerization proceeds via a coordination-insertion mechanism with the carbene-phosphinidene ligands acting as stereodirecting groups.