Ligand-free scandium alkyl and alkoxide complexes for immortal ring-opening polymerization of lactide

Ligand-free Ln (CH2SiMe3)(3) (THF)(2) (1: Ln = Sc; 2: Ln = Lu; 3: Ln = Er; 4: Ln = Y) and La(CH2C6H4-o-NMe2)(3) (5) are employed as the initiators for ring-opening polymerization (ROP) of rac-lactide (rac-LA). Complex 4 solely is active but uncontrolled, while complex 1 is inert. In presence of excess benzhydrol, complexes 1-5 can promote the ROP of LA smoothly, in which the molecular weight distribution values of the resultant polymer are related to the radii of the rare-earth metals. Complex 1 displays high activity for the controlled ROP of LA, in which the benzhydrol act a dual role as chain-transfer agent and activator. The polymerizations proceed smoothly and efficiently in a wide range of benzhydrol feeding (3-150 equiv). The molecular weights of the obtained PLAs can be precisely controlled by the monomer loading with respect to that of benzhydrol, while the molecular weight distribution (MWD) maintains a very narrow level (PDI = 1.05-1.18). Every scandium centre by means of chain-transfer reaction with benzhydrol to generate the active species Sc(OCHPh2)(3)center dot THF (6), grows apparently up to 150 PLA polymer chains, showing a typical feature of an immortal polymerization. (C) 2018 Published by Elsevier B.V.