AbstractsChemistry

Polyesters derived from the ring-opening polymerization (ROP) of lactides formed from inexpensive renewable resources constitute one important class of biodegradable and biocompatible polymers. Herein, the preparation and characterization of a series of closely related magnesium and zinc compounds that are active for ROP of lactides are reported using three ß-diiminate ligands L¹, L² and L³ where L¹ = CH(CMeNC₆H₃-2,6-ⁱPr₂)₂, L² = CH(CMeNC₆H₄-2-^tBu)₂ and L³ = CH(CMeNC₆H₄-2-OMe)₂. For the L¹ ligand, L¹Mg(NⁱPr₂)•(THF), L¹Zn(NⁱPr₂), L¹Mg(O^tBu)•(THF), L¹Zn(O^tBu) and L¹Zn(OSiPh₃)•(THF) have been synthesized. All compounds initiate and sustain ROP of lactides. For a related series L¹MX(THF)_n, where n = 0 or 1, the reactivity follows the order M = Mg > Zn and X = O^tBu > NⁱPr₂ > N(SiMe₃)₂ > OSiPh₃. Heterotactic polylactide (PLA) is produced from ROP of rac-lactide by the zinc catalysts in CH₂Cl₂. The magnesium catalysts produce atactic and heterotactic PLA in CH₂Cl₂ and THF, respectively. The resting states for Zn and Mg are proposed to be L¹Zn(¿²-OCHMeC(O)OP) and L¹Mg(µ-OP)₂MgL¹. For the L² ligand, L²MgNⁱPr₂•(THF) and L²ZnNⁱPr₂ are prepared. In solution, L²ZnNⁱPr₂ exists as a mixture of syn- and anti-rotamers while L²MgNⁱPr₂•(THF) exists only as a syn-rotamer. In the zinc compound, the syn-conformer is shown to be more reactive than the anti–conformer and is responsible for the polymerization. Heterotactic PLA is obtained in CH₂Cl₂ for the zinc compound and in THF for the magnesium compound. For the L³ ligand, L³MgN(SiMe₃)₂, [L³MgO^tBu]₂, L³ZnN(SiMe₃)₂, L³ZnOⁱPr and L³CaN(SiMe₃)₂•(THF) are prepared. The OMe groups of ligand L³ show little affinity toward zinc and in the ROP of rac-lactide give PLA with only moderately enhanced heterotactic tetrads. The OMe groups are shown to bind to Mg reversibly and display significant enhancement of heterotactic PLA. Herein, the first single-site calcium complexes of L¹ and tris(pyrazolyl/indazolyl)borate ligands are reported and shown to initiate and sustain ROP of lactide: L¹CaNR¹₂•(THF), Tp^R2CaNR¹₂•(THF)_n,…