Stereoselective polymerization of rac -lactide catalyzed by zwitterionic calcium complexes

A reaction of heteroscorpionated ligands HL1 and HL2 (L1 = (3,5-Me2Pz)2CHP(Ph)2NPh, L2 = (3,5-Me2Pz)2CHP(Ph)2NPh(2-OMe), Pz = pyrazole) with homoleptic calcium amide Ca[N(SiMe3)2]2(THF)2, respectively, afforded novel zwitterionic calcium complexes L1CaN(SiMe3)2(THF) (1) and L2CaN(SiMe3)2 (2) through protolysis. 1 and 2 were characterized by NMR spectroscopy. The chemical shifts of the protons of pyrazole and phosphino phenyl within 1 are similar to those in complex 2, while the chemical shift of phosphine in complex 2 (28.66 ppm) is a little higher than that in complex 1 (11.43 ppm). The structures of complexes 1 and 2 were determined by X-ray diffraction analysis. Both 1 and 2 adopt a distorted trigonal bipyramidal geometry. L1 and L2 chelate to the calcium ion through a κ3 and κ4 fashion, respectively. 1 and 2 were assayed towards the ROP of rac-lactide. Both of them catalyzed the polymerization in a controlled manner. With the increase of the feeding ratio of the monomer to catalyst, the molecular weights increased linearly. At 25 °C, these two calcium complexes catalyzed the ROP of rac-lactide to afford atactic polylactides. Surprisingly, at −75 °C, complex 1 gave a heterotactic sequence enriched polylactide (Pr = 0.84); in contrast, complex 2 produced an isotactic sequence enriched polymer. The reason behind was elucidated.