Single-site calcium initiators for the controlled ring-opening polymerization of lactides and lactones
Single-site calcium initiators containing chelating tmhd (H-tmhd = 2,2,6,6-tetramethylheptane-3,5-dione) ligands [(THF)Ca(tmhd)](2)[mu-N(SiMe3)(2)](mu-tmhd) (2) and [(THF)Ca(tmhd)](2)[mu-OCH(Me)Ph](mu-tmhd) (3) have been synthesized and applied for the ring-opening polymerization of L-lactide and epsilon-caprolactone. Both 2 and 3 were highly reactive and promoted a fast polymerization of L-lactide and epsilon-caprolactone to high monomer conversions under mild conditions (TEF as a solvent, room temperature). More importantly, results showed that the ring-opening polymerizations of lactides and lactones initiated by either 3 or 2 in the presence of equivalent 2-propanol are living, to provide polymers and block copolymers of controlled molecular weights and tailored end-groups. The polymerizations were first-order in monomer up to high conversions, in which the in situ initiating system 2/2-propanol revealed no induction period and much faster polymerization kinetics as compared to 3.