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

Metal Complexes in the Synthesis of Biodegradable Polymers: Achievements and Prospects

This review describes recent advances in the synthesis of homopolymers of lactide and related cyclic esters via ring-opening polymerization (ROP) in the presence of metal complexes based on group 1, 2, 4, 12, 13 and 14 metals. Particular attention is paid to the influence of the initiator structure on the properties of the obtaining homo- and copolymers. Also, a separate chapter is devoted to the study of metal complexes in the synthesis of copolymers of lactide and lactones. This review highlights the efforts made over the last ten years or so, and shows how main-group metals have received increasing attention in the field of the polymerization of lactide and related cyclic esters.

Isoselective Polymerization of rac-Lactide by Magnesium Initiators Bearing Achiral Di(2-pyridyl)methyl Substituted Aminophenolate Ligands

Reactions of achiral di(2-pyridyl)methyl substituted aminophenols L^1-6H (2-{N-R³-N-[di(2-pyridyl)methyl]aminomethyl}-4-R¹-6-R²-C₆H₂OH: R¹ = R² = ^tBu, R³ = ⁿBu (L¹H), R³ = ⁿhexyl (L²H), R³ = cyclohexyl (L³H); R¹ = R² = cumyl, R³ = ⁿBu (L⁴H), R³ = ⁿhexyl (L⁵H), R³ = cyclohexyl (L⁶H)) with {Mg[N(SiMe₃)₂]₂}₂ ([L^1-6H]:[Mg] = 1:1) afforded a series of magnesium silylamido complexes 1-6. In the solid state, the magnesium center of 3, 4, and 6 is penta-coordinated by the tetradentate aminophenloate ligand and one silylamido ligand to form a seriously distorted square-pyramidal geometry as confirmed by X-ray crystallography diffraction analysis. VT ¹H NMR and ROESY experiments further indicate that these magnesium complexes are also five-coordinated in solutions where the coordination of either of the two pyridyl pendants to the magnesium center is maintained. Complexes 1-6 are highly active toward the ring-opening polymerization of rac-lactide (rac-LA) at r.t. both in toluene and in tetrahydrofuran, capable of polymerizing 500 equiv of monomer to high conversions just within minutes. Among them, complex 3 exhibited the highest iso-stereoselectivity, affording moderately isotactic polylactide in toluene (P_m = 0.75). It is found that the isoselectivities and activities of these magnesium complexes toward the polymerization of rac-LA are closely associated with the substituents at the ortho-position of the phenoxide unit and on the skeleton nitrogen atom of the ligand. On the basis of NMR spectroscopic studies, the formation of isotactic PLAs with dominant stereoblock sequences was witnessed by using these magnesium complexes as initiators, and the inequivalent coordination of two pyridyl pendant arms in these magnesium complexes might be the source of exerting isoselective control.

A Single Catalyst for Promoting Reverse Processes: Synthesis and Chemical Degradation of Polylactide

A simple zinc catalyst showing high activity for both the synthesis of polylactide, a biodegradable polymer produced from renewable feedstock, and its degradation was described. In the ring-opening polymerization of lactides, the zinc catalyst showed one of the highest activities reported in the literature for reactions carried out in solution at room temperature. This excellent performance was preserved even when the process was performed under industrial conditions: at high temperature, in the absence of solvent, and by using a low catalyst loading with unpurified monomers. The same complex revealed high efficiency also in depolymerization of polylactide by alcoholysis, a process that occurred efficiently at room temperature and in the absence of solvent, conditions that reduce costs and guarantee low environmental impact.

Recent progress in the application of group 1, 2 & 13 metal complexes as catalysts for the ring opening polymerization of cyclic esters

This review focuses on recent developments concerned with the use of well-defined main group complexes as (pre-)catalysts for the ROP of cyclic esters to give aliphatic polyesters; factors influencing catalytic activity, selectivity and polymer properties are all discussed.

New homoleptic bis(pyrrolylpyridiylimino) Mg(ii) and Zn(ii) complexes as catalysts for the ring opening polymerization of cyclic esters via an "activated monomer" mechanism

The reaction of MgBu₂ and ZnEt₂ or Zn{N[Si(CH₃)₃]₂}₂ with a tridentate monoanionic pyrrolylpyridiylimino [N^-,N,N] proligand gave homoleptic species, as exclusive products, in high yields. The complexes were characterized in solution by 1D and 2D NMR analysis and by single crystal X-ray crystallography. The new homoleptic complexes were tested as initiators in the polymerization of ε-caprolactone and lactide in the presence of an exogenous alcohol. For both complexes, the polymerizations proceed via an "activated monomer" mechanism that, in the case of the magnesium complex, was correlated with the coordinative flexibility of the ligands, resulting in extremely high productivities under mild conditions.