Iminophosphorane neodymium(III) complexes as efficient initiators for lactide polymerization

Bis(iminophosphoranyl)methanide ({CH(PPh₂NR)}^-, R = iPr or Ph) neodymium(III) complexes were prepared from NdI₃(THF) _3.5. The steric bulk of the ligand controlled the stoichiometry of the resulting complexes. Thus, three new complexes, bearing one or two ancillary ligands, were prepared and characterized using various spectroscopic techniques and by single-crystal X-ray diffraction. Reaction of the heteroleptic neodymium iodide complexes with amido groups yielded viable initiators for the ring-opening polymerization of lactide. The polymerizations were conducted using either the heteroleptic neodymium amido complexes or the in situ generated alkoxide complexes. Using such conditions, the neodymium complexes showed very fast and well-controlled polymerizations, with complete conversion being obtained in only a few minutes and yielding polylactide with controlled molecular weight and narrow polydispersity index. An initiating system comprising a rare neodymium-alkyl-carbene complex [Nd{C(PPh₂NiPr) ₂}{CH(PPh₂NiPr)₂}] and externally added iPrOH was also an unexpected catalyst for the ROP of lactide.