Exceptionally high lactide polymerization activity of zirconium complexes with bridged diketiminate ligands

Abstract

A cyclohexanediyl-bridged, bis(N-xylyl) diketiminate ligand, (±)-C₆H₁₀(nacnac^XylH)₂, LH₂ (Xyl = 2,6-dimethylphenyl), was obtained from the reaction of [(2,6-dimethylphenyl)amino]-pent-3-en-2-one first with Meerwein's salt, then with (±)-cyclohexanediamine. The reaction of the ligand with Zr(NMe₂)₄ yielded LZr(NMe₂)₂. Protonation of the remaining diamide ligands with EtOH or [H₂NMe₂]Cl yielded LZr(OEt)₂ and LZrCl₂, respectively. The latter complex was also obtained by the reaction of LH₂ first with nBuLi and then with ZrCl₄(THF)₂. The dichloride complex yielded LZr(OEt)₂ and LZrMe₂ upon reaction with NaOEt or MeLi/AlMe₃, respectively. X-ray diffraction studies showed a trans-configuration of the ancillary ligands in LZrCl₂ and LZrMe₂, and a cis-configuration in LZr(NMe₂)₂ and LZr(OEt)₂. LZr(OEt)₂ was tested as a catalyst for the polymerization of rac-lactide. Kinetic investigations yielded a rate law first order in catalyst and monomer and a rate constant k = 14(1) L mol⁻¹ s⁻¹, the latter being orders of magnitude higher than typical activities for group 4 complexes in lactide polymerization. Analyses of the obtained polymer revealed an atactic polymer and broad polymer molecular weight distributions with sizeable fractions of cyclic oligomers. The influence of contaminants on the polymerization activity was examined: while lactic acid deactivates the catalyst, addition of up to 1 equiv. of water or para-toluenesulfonic acid revitalized catalysts not showing maximum activity.