Lactide polymerization catalyzed by Mg and Zn diketiminate complexes with flexible ligand frameworks

Abstract

Diketimine ligands bearing N-benzyl, N-9-anthrylmethyl and N-mesitylmethyl substituents (nacnac^BnH, nacnac^AnH, and nacnac^MesH) were prepared from condensation of the amine with either acetyl acetone or its ethylene glycol monoketal. Chlorination with N-chlorosuccinimide in the 3-position yielded Clnacnac^BnH and Clnacnac^AnH. The ligands were reacted with Zn(TMSA)₂ (TMSA = N(SiMe₃)₂) to yield nacnac^AnZn(TMSA) and Clnacnac^BnZn(TMSA). Protonation with isopropanol afforded nacnac^AnZnOiPr and Clnacnac^BnZnOiPr. Reaction of the diketimines with Mg(TMSA)₂ afforded nacnac^AnMg(TMSA), nacnac^MesMg(TMSA), Clnacnac^BnMg(TMSA) and Clnacnac^AnMg(TMSA). Subsequent protonation with tert-butanol produced nacnac^MesMgOtBu and Clnacnac^BnMgOtBu, but only decomposition was observed with N-anthrylmethyl substituents. Most complexes were characterized by X-ray diffraction studies. TMSA complexes were monomeric and alkoxide complexes dimeric in the solid state. All alkoxide complexes, as well as nacnac^AnMg(TMSA)/BnOH and Clnacnac^AnMg(TMSA)/BnOH were moderately to highly active in rac-lactide polymerization (90% conversion in 30 s to 3 h). nacnac^AnZnOiPr produced highly heterotactic polymers (P_r = 0.90), Clnacnac^BnMgOtBu/BnOH produced slightly isotactic polymers at −30 °C (P_r = 0.43), and all other catalysts produced atactic polymers with a slight heterotactic bias.