Lactide polymerization catalyzed by Mg and Zn diketiminate complexes with flexible ligand frameworks
Diketimine ligands bearing N-benzyl, N-9-anthrylmethyl and N-mesitylmethyl substituents (nacnacBnH, nacnacAnH, and nacnacMesH) 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 ClnacnacBnH and ClnacnacAnH. The ligands were reacted with Zn(TMSA)2 (TMSA = N(SiMe3)2) to yield nacnacAnZn(TMSA) and ClnacnacBnZn(TMSA). Protonation with isopropanol afforded nacnacAnZnOiPr and ClnacnacBnZnOiPr. Reaction of the diketimines with Mg(TMSA)2 afforded nacnacAnMg(TMSA), nacnacMesMg(TMSA), ClnacnacBnMg(TMSA) and ClnacnacAnMg(TMSA). Subsequent protonation with tert-butanol produced nacnacMesMgOtBu and ClnacnacBnMgOtBu, 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 nacnacAnMg(TMSA)/BnOH and ClnacnacAnMg(TMSA)/BnOH were moderately to highly active in rac-lactide polymerization (90% conversion in 30 s to 3 h). nacnacAnZnOiPr produced highly heterotactic polymers (Pr = 0.90), ClnacnacBnMgOtBu/BnOH produced slightly isotactic polymers at −30 °C (Pr = 0.43), and all other catalysts produced atactic polymers with a slight heterotactic bias.