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 (nacnac(Bn)H, nacnac(An)H, and nacnac(Mes)H) 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(Bn)H and Clnacnac(An)H. The ligands were reacted with Zn(TMSA)2 (TMSA = N(SiMe3)2) to yield nacnac(An)Zn(TMSA) and Clnacnac(Bn)Zn(TMSA). Protonation with isopropanol afforded nacnac(An)ZnOiPr and Clnacnac(Bn)ZnOiPr. Reaction of the diketimines with Mg(TMSA)2 afforded nacnac(An)Mg(TMSA), nacnac(Mes)Mg(TMSA), Clnacnac(Bn)Mg(TMSA) and Clnacnac(An)Mg(TMSA). Subsequent protonation with tert-butanol produced nacnac(Mes)MgOtBu and Clnacnac(Bn)MgOtBu, 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(An)Mg(TMSA)/BnOH and Clnacnac(An)Mg(TMSA)/BnOH were moderately to highly active in rac-lactide polymerization (90% conversion in 30 s to 3 h). nacnac(An)ZnOiPr produced highly heterotactic polymers (P(r) = 0.90), Clnacnac(Bn)MgOtBu/BnOH produced slightly isotactic polymers at -30 °C (P(r) = 0.43), and all other catalysts produced atactic polymers with a slight heterotactic bias.

Square-planar Cu(II) diketiminate complexes in lactide polymerization.

Cu(OiPr)2 was reacted with several ß-diketimine ligands, nacnac(R)H. Sterically undemanding ligands with N-benzyl substituents afforded the dimeric heteroleptic complexes [nacnac(Bn)Cu(µ-OiPr)]2 and [3-Cl-nacnac(Bn)Cu(µ-OiPr)]2 (Bn = benzyl). With sterically more demanding amines, dimerization was not possible, and the putative nacnacCuOiPr intermediate underwent ligand exchange to the homoleptic bisdiketiminate complexes Cu(nacnac(ipp))2 and Cu(nacnac(Naph))2 (ipp = 2-isopropylphenyl, Naph = 1-napthyl). Homoleptic complexes were also prepared with N-benzyl ligands to yield Cu(nacnac(Bn))2 and Cu(3-succinimido-nacnac(Bn))2. All complexes were characterized by single-crystal X-ray diffraction. Even bulkier ligands with N-anthrylmethyl, N-mesitylmethyl, or N-methylbenzyl substituents failed to react with Cu(OiPr)2. In the case of nacnac(dipp)CuOiPr, putative nacnac(dipp)CuOiPr decomposed by ß-hydride elimination. Heteroleptic complexes [nacnac(Bn)Cu(µ-OiPr)]2 and [3-Cl-nacnac(Bn)Cu(µ-OiPr)]2 are very highly active rac-lactide polymerization catalysts, with complete monomer conversion at ambient temperature in solution in 0.5-5 min. In the presence of free alcohol, the homoleptic complexes seem to be in equilibrium with small amounts of the respective heteroleptic complex, which are sufficient to complete polymerization in less than 60 min at room temperature. All catalysts show high control of the polymerization with polydispersities of 1.1 and below. The obtained polymers were essentially atactic, with a slight heterotactic bias at ambient temperature and at -17 °C.

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.

Nacnac(Bn)CuOiPr: a strained geometry resulting in very high lactide polymerization activity.

N,N'-Dibenzyl diketiminate copper isopropanolate, (nacnac(Bn)CuOiPr)(2), polymerizes rac- and S,S-lactide in the presence or absence of isopropanol as a chain-transfer reagent with very high activity (k(2) = 32 M(-1) s(-1)), narrow polydispersities and without evidence of side reactions such as transesterification, epimerization or catalyst decomposition.

Nacnac(Bn)MgOtBu: a diketiminate-based catalyst for the polymerisation of rac-lactide with slight isotactic preference.

The reaction of N,N'-dibenzyl-2-amino-4-imino-pent-2-ene, nacnac(Bn)H, with 1 or 2 equiv. of MgnBu2 afforded the homoleptic complex nacnac(Bn)2Mg. The reaction of nacnac(Bn)H with Mg(N(SiMe3)2)2 yielded nacnac(Bn)MgN(SiMe3)2, which reacted with tert-butanol to form nacnac(Bn)MgOtBu. The latter complex crystallizes as an alkoxide bridge dimer and is active in the ring-opening polymerisation of rac-lactide. Polymerisations at room temperature afforded atactic polylactide, while polymerisations at -17 and -26 °C afforded polylactide with a small isotactic bias (P(m) = 0.52, and 0.55, respectively).