| 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 β-diiminate ligands L1, L2 and L3 where L 1 = CH(CMeNC6H3-2,6-iPr 2)2, L2 = CH(CMeNC6H4-2- tBu)2 and L3 = CH(CMeNC6H 4-2-OMe)2. For the L1 ligand, L1Mg(N iPr2)·(THF), L1Zn(NiPr 2), L1Mg(OtBu)·(THF), L 1Zn(OtBu) 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 = OtBu > NiPr2 > N(SiMe3)2 > OSiPh 3. 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 CH2Cl 2 and THF, respectively. The resting states for Zn and Mg are proposed to be L1Zn(η2-OCHMeC(O)OP) and L 1Mg(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 L2MgN iPr2·(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. (Abstract shortened by UMI.)... |
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