Stereoselective polymerization of rac-lactide catalyzed by zwitterionic calcium complexes

Abstract

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 (P_r = 0.84); in contrast, complex 2 produced an isotactic sequence enriched polymer. The reason behind was elucidated.