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

A cyclohexanediyl-bridged, bis(N-xylyl) diketiminate ligand, (+/-)-C6H10(nacnac(Xyl)H)(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.