In Situ Formation of Bis(phosphinimino)methanide Rare Earth Alkoxide Initiators for the Ring-Opening Polymerization of ε-Caprolactone

A series of pentamethylcyclopentadienyl bis(phosphinimino)methanide complexes of yttrium and the lanthanides, [{CH(PPh2NSiMe3)2}Ln(η5-C5Me5)Cl] (Ln = Y (1a), Sm (1b), Yb (1c)), were prepared by two different synthetic approaches. The compounds can be obtained either from [{CH(PPh2NSiMe3)2}LnCl2]2 (Ln = Y, Sm, Yb) and K(C5Me5) or in a one-pot reaction when K{CH(PPh2NSiMe3)2} is reacted with anhydrous rare earth trichlorides in the presence of K(C5Me5). When 1a−c were combined in situ with 1 equiv of 2-propanol, an active [Ln]-OiPr initiator was formed that enabled the pseudo-living ring-opening polymerization of ε-caprolactone to polymers with controlled molecular features (end groups, M̄ n) and very narrow molar mass distributions.