Single-Site Calcium Initiators for the Controlled Ring-Opening Polymerization of Lactides and Lactones

Summary Single-site calcium initiators containing chelating tmhd(H-tmhd = 2,2,6,6-tetramethylheptane-33-dione) ligands[(THF)Ca(tmhd)_]2[-N(SiMe₃)₂](-tmhd)(2) and[(THF)Ca(tmhd)]₂[-OCH(Me)Ph](-tmhd)(3) have been synthesized andapplied for the ring-opening polymerization of L-lactide and-caprolactone. Both 2 and3 were highly reactive andpromoted a fast polymerization of L-lactide and -caprolactoneto high monomer conversions under mild conditions (THF as asolvent, room temperature). More importantly, results showedthat the ring-opening polymerizations of lactides and lactonesinitiated by either 3 or2 in the presence of equivalent2-propanol are living, to provide polymers and block copolymersof controlled molecular weights and tailored end-groups. Thepolymerizations were first-order in monomer up to highconversions, in which the insitu initiating system 2/2-propanol revealed no induction periodand much faster polymerization kinetics as compared to3.