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

The N‐heterocyclic carbene‐phosphinidene adducts (NHC)PH were reacted with AlMe(3) in toluene to afford the monoaluminum complexes [{(IDipp)PH}AlMe(3)] and [{(IMes)PH}AlMe(3)] (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 [{((Me)IMes)PH}(AlMe(3))(2)] was obtained for (Me)IMes=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 (P (m) up to 0.92, T (m) 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.