Ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate catalyzed by magnesium and zinc complexes derived from binaphthyl-based iminophenolate ligands

Abstract

A series of racemic 2-[(2′-(dimethylamino)binaphthyl-2-ylimino)methyl]-4-R2-6-R(1)-phenols (L1H–L6H) were reacted with 1 equiv. of Mg[N(SiMe3)2]2 or Zn[N(SiMe3)2]2 to provide three heteroleptic magnesium silylamido complexes [(L4–(6))MgN(SiMe3)2] (4a: R(1) = R(2) = cumyl; 5a: R(1) = CPh3, R(2) = Me; 6a: R(1) = CPh3, R(2) = (t)Bu) and one heteroleptic zinc silylamido complex [(L(4))ZnN(SiMe3)2] (4b: R(1) = R(2) = cumyl). The molecular structures of complexes 4a, 4b and a homolepic zinc complex (L(5))2Zn (5b′) were further confirmed by X-ray diffraction studies. In the solid state, the heteroleptic magnesium complex 4a has a four-coordinated metal core chelated by three donor atoms of the ligand and one bis(trimethylsilyl)amido group; however, the heteroleptic zinc complex 4b has a three-coordinated metal core chelated by two donor atoms of the ligand and one bis(trimethylsilyl)amido group, where the NMe2 group of the iminophenolate ligand is not coordinated to the zinc center. Magnesium complexes 4a–6a efficiently initiated the ring-opening polymerization (ROP) of rac-lactide in the presence/absence of 2-propanol, affording atactic polylactides in toluene and heterotactic bias polylactides in THF. Zinc complex 4b exhibited the highest stereoselectivity for the ROP of rac-lactide, affording substantially heterotactic polylactides in both solvents (Pr = 0.80–0.84). The kinetic studies of the polymerization revealed that the rate of polymerization is first-order in both monomer and catalyst concentrations. All these complexes were also very active in the polymerization of racemic α-methyltrimethylene carbonate (α-MeTMC). Detailed analyses using 1H and 13C NMR spectroscopy indicated the preferential ring-opening of α-MeTMC at the most hindered oxygen–acyl bond and zinc complex 4b gave the highest regioselectivity (Xreg = 0.98).

Magnesium and Zinc Complexes Supported by N,N,O Tridentate Ligands: Synthesis and Catalysis in the Ring‐Opening Polymerization of rac‐Lactide and α‐Methyltrimethylene Carbonate

“…Therefore, it is conceivable that these magnesium and zinc complexes have the same configuration in solution as in the solid state (see below). This is in contrast to our previously reported binaphthyl‐based iminophenolate magnesium complexes, for which dissociation of the amino group was witnessed in solution, and implies the formation of a less flexible complex geometry in this work.…”

“…As shown in Figure , complex 5a has a monomeric structure in the solid state in which the magnesium center is fourfold coordinated by three heteroatom donors of the tridentate ligand and one bis(trimethylsilyl)amido group in a distorted tetrahedral geometry. In contrast to the iminophenolate analogues, the R a configuration of the binaphthyl moiety in the ligand leads exclusively to the R configuration of the magnesium center, and the S a configuration to the S configuration. The Mg1–N2 [2.202(2) Å] and Mg1–N3 [2.034(2) Å] bond lengths in complex 5a are comparable to those in the iminophenolate magnesium analogue bearing the same substituents on the phenoxide ring [ 4a in ref ,.…”

“…Previously, we synthesized a series of magnesium and zinc silylamido complexes supported by racemic binaphthyl‐based iminophenolate ligands, which showed moderate heteroselectivities toward the polymerization of rac ‐LA ( P r = 0.72–0.84) and moderate to high regioselectivities toward the polymerization of racemic α‐MeTMC ( X reg = 0.78–0.98) . The NMe 2 group on the binaphthyl ring of these complexes is dissociated from the metal center in solution, and this is considered to weaken the chiral induction effect of the binaphthyl moiety during the polymerization.…”

Cubane‐Type Polynuclear Zinc Complexes Containing Tridentate Schiff Base Ligands: Synthesis, Characterization, and Ring‐Opening Polymerization Studies of rac‐Lactide and ε‐Caprolactone

“…ROP processes offer excel control in terms of molecular weight ( M n ), polydispersity (MWDs) and microstructure . Several complexes have been successfully employed in the ROP of lactide, including lithium, sodium, potassium, magnesium, calcium, strontium, aluminum, tin, zinc, some transition metals, and rare‐earth metals . Since complete elimination of catalyst rests from the polymer is virtually impossible, it is highly preferable to use non‐ or low toxic metals (for instance for biomedical applications).…”

Non-symmetric β-Diketiminate Magnesium Complexes as Initiators for Ring-Opening Polymerization/Copolymerization of Lactide, ε-Caprolactone

“…镁金属中心与配体骨架中 [49,50,56] , 说明在固体 Figure 2 The molecular structure of complex 1 [68] . [57,58,68,78] . 鉴于该系列镁硅氨基络合物对质子源非常敏感, 没 有进一步研究异丙醇存在下的 ε-己内酯聚合反应.…”