Stereocontrolled lactide polymerisation determined by the nuclearity of the yttrium initiator

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.

Cooperative Initiation in a Dinuclear Indium Complex for CO2 Epoxide Copolymerization

Dinuclear indium complexes have been synthesized and characterized. These include neutral and cationic indium complexes supported by a Schiff base ligand bearing a binaphthol linker. The new compounds were investigated for alternating copolymerization of CO₂ and cyclohexene oxide. In particular, the neutral indium chloride complex (±)-[(O_NapN_iN)InCl₂]₂ (4) showed high conversion of cyclohexene oxide and selectivity for poly(cyclohexene carbonate) formation without cocatalysts at 80 °C under various CO₂ pressures (2-30 bar). Importantly, the reactivity of the dinuclear indium chloride complex 4 is drastically different from that of the mononuclear indium chloride complex (±)-(NN_iO_tBu)InCl₂ (5), suggesting a cooperative initiation mechanism involving the two indium centers in 4.

Phosphasalalen Rare-Earth Complexes for the Polymerization of rac-Lactide and rac-β-Butyrolactone

A series of new phosphasalalen pro-ligands, analogues of salalen but with an iminophosphorane replacing the imine functionality, and their corresponding rare-earth alkoxide and siloxide complexes were synthesized. The multinuclear NMR spectra and X-ray diffraction analyses revealed that, for the tert-butoxide and ethoxide complexes, the resulting phosphasalalen rare-earth product was composed of a mononuclear alkoxide and a binuclear complex containing bridged alkoxo and hydroxo groups, while an analogous binuclear complex was isolated as the sole product for the siloxide complex. All the complexes could catalyze the heteroselective ring-opening polymerization (ROP) of rac-lactide (P_r up to 0.77) with high catalytic activities and a controlled polydispersity. Remarkably, the yttrium and lutetium phosphasalalen complexes could also efficiently catalyze the ROP of rac-β-butyrolactone to produce syndiotactic polymers (P_r up to 0.73) while their salalen analogues were inert, revealing the special effects of the iminophosphorane moiety. Detailed end-group analyses and kinetic investigations suggested that the alkoxo-hydroxo-bridged complexes maintained their binuclear structures in the polymerization.

The role of neutral donor ligands in the isoselective ring-opening polymerization of rac-β-butyrolactone

Isoenriched poly-3-hydroxybutyrate (P3HB) is a biodegradable material with properties similar to isotactic polypropylene, yet efficient routes to this material are lacking after 50+ years of extensive efforts in catalyst design. In this contribution, a novel lanthanum aminobisphenolate catalyst (1-La) can access isoenriched P3HB through the stereospecific ring-opening polymerization (ROP) of rac-β-butyrolactone (rac-BBL). Replacing the tethered donor group of a privileged supporting ligand with a non-coordinating benzyl substituent generates a catalyst whose reactivity and selectivity can be tuned with inexpensive achiral neutral donor ligands (e.g. phosphine oxides, OPR₃). The 1-La/OPR₃ (R = n-octyl, Ph) systems display high activity and are the most isoselective homogeneous catalysts for the ROP of rac-BBL to date (0 °C: P_m = 0.8, TOF ∼190 h⁻¹). Combined reactivity and spectroscopic studies provide insight into the active catalyst structure and ROP mechanism. Both 1-La(TPPO)2 and a structurally related catalyst with a tethered donor group (2-Y) operate under chain-end stereocontrol; however, 2-RE favors formation of P3HB with opposite tacticity (syndioenriched) and its ROP activity and selectivity are totally unaffected by added neutral donor ligands. Our studies uncover new roles for neutral donor ligands in stereospecific ROP, including suppression of chain-scission events, and point to new opportunities for catalyst design.

Simple achiral neutral donor ligands modify catalyst structure and function to enable access to isoenriched poly-3-hydroxybutyrate, a biodegradable material with properties similar to isotactic polypropylene.

Syntheses, structures and catalytic activity of tetranuclear Mg complexes in the ROP of cyclic esters under industrially relevant conditions

Tetranuclear magnesium imino(phenolate) complexes Mg₄(L^1–4)₄ are excellent catalysts for the ROP of bulk rac-lactide and ε-caprolactone under industrially relevant conditions.

Highly active Mg(ii) and Zn(ii) complexes for the ring opening polymerisation of lactide

Simple, highly active, Zn(ii) and Mg(ii) complexes for the industrial ring opening polymerisation (ROP) of lactide.

Synthesis and characterization of dinuclear rare-earth complexes supported by amine-bridged bis(phenolate) ligands and their catalytic activity for the ring-opening polymerization of l-lactide

Reactions of amine-bridged bis(phenolate) protio-ligands N,N-bis(3,5-di-tert-butyl-2-hydroxybenzyl)aminoacetic acid (L(1)-H3) and N,N-bis[3,5-bis(α,α'-dimethylbenzyl)-2-hydroxybenzyl]aminoacetic acid (L(2)-H3), with 1 equiv. M[N(SiMe3)2]3 (M = La, Nd, Sm, Gd, Y) in THF at room temperature yielded the neutral rare-earth complexes [M2(L)2(THF)4] (L = L(1), M = La (), Nd (), Sm (), Gd (), Y (); L = L(2), M = La (), Nd (), Sm (), Gd (), Y ()). All of these complexes were characterized by single-crystal X-ray diffraction, elemental analysis and in the case of yttrium and lanthanum complexes, (1)H NMR spectroscopy. The molecular structure of revealed dinuclear species in which the eight-coordinate lanthanum centers were bonded to two oxygen atoms of two THF molecules, to three oxygen atoms and one nitrogen atom of one L(1) ligand, and two oxygen atoms of the carboxyl group of another. Complexes were also dinuclear species containing seven-coordinate metal centers similar to , albeit with bonding to one rather than two carboxyl group oxygens of another ligand. Further treatment of with excess benzyl alcohol provided dinuclear complex [La2(L(1))2(BnOH)6] (), in which each lanthanum ion is eight-coordinate, bonded to three oxygen atoms and one nitrogen atom of one ligand, three oxygen atoms of three BnOH molecules, as well as one oxygen atom of bridging carboxyl group of the other ligand. In the presence of BnOH, complexes efficiently catalyzed the ring-opening polymerization of l-lactide in a controlled manner and gave polymers with relatively narrow molecular weight distributions. The kinetic and mechanistic studies associated with the ROP of l-lactide using /BnOH initiating system have been performed.

Dinucleating Ligand Platforms Supporting Indium and Zinc Catalysts for Cyclic Ester Polymerization

The synthesis of the first alkoxide-bridged indium complex supported by a chiral dinucleating ligand platform (1), along with its zinc analogue (2), is reported. Both complexes are synthesized in a one-pot reaction starting from a chiral dinucleating bis(diamino)phenolate ligand platform, sodium ethoxide, and respective metal salts. The dinucleating indium analogue (7) based on an achiral ligand backbone is also reported. Indium complexes bearing either the chiral or achiral ligand catalyze the ring-opening polymerization of racemic lactide (rac-LA) to afford highly heterotactic poly(lactic acid) (PLA; Pr > 0.85). The indium complex bearing an achiral ligand affords essentially atactic PLA from meso-LA. The role of the dinucleating ligand structure in catalyst synthesis and polymerization activity is discussed.

Diastereoselective synthesis of chiral aminophenolate magnesium complexes and their application in the stereoselective polymerization of rac-lactide and rac-β-butyrolactone

A series of magnesium silylamido complexes supported by chiral aminophenolate ligands were synthesized, and their catalytic behaviors toward the ring-opening polymerization of rac-lactide (LA) and rac-β-butyrolactone (BBL) were explored.

Controlling the stereoselectivity of rac-LA polymerization by chiral recognition induced the formation of homochiral dimeric metal alkoxides

Chiral recognition of monomeric Me₂MOR units resulting in the formation of homochiral dimeric species [Me₂M(μ-OR)]₂(M = Ga, In), leads to heteroselective catalysts for the ring opening polymerization ofrac-lactide (rac-LA).

Synthesis and characterization of rare-earth metal complexes supported by a new pentadentate Schiff base and their application in heteroselective polymerization of rac-lactide

Both rare-earth metal salen aryloxides and amides can efficiently initiate the ring-opening polymerization of rac-lactide to give heterotactic-rich polylactides.

Comparing a series of 8-quinolinolato complexes of aluminium, titanium and zinc as initiators for the ring-opening polymerization of rac-lactide

The preparation, characterization and properties of a series of 8-hydroxy quinolinolate complexes of Ti(iv), Al(iii) and Zn(ii) are presented. The complexes are active initiators for rac-lactide polymerizations: their performances are compared.

Dinuclear magnesium, zinc and aluminum complexes supported by bis(iminopyrrolide) ligands: synthesis, structures, and catalysis toward the ring-opening polymerization of ε-caprolactone and rac-lactide

Synthesis and characterization of novel dinuclear magnesium, zinc and aluminum complexes supported by bis(iminopyrrolide) ligands and their catalysis toward the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) and rac-lactide (rac-LA) were carried out. The ligand precursors [(5-Bu(t)-2-C4H2NH)CH=N(CH2)2]2NH (H2L(Bu), 1) and [(2-C4H3NH)CH=N(CH2)2]2NH (H2L(H), 2) were prepared by condensation of diethylene triamine with 5-tert-butyl-1H-pyrrole-2-carbaldehyde and 1H-pyrrole-2-carbaldehyde, respectively. Treatment of 1 with 2 equiv. of Bu(n)MgOBn in toluene at 70 °C produced dinuclear magnesium complex [(μ-OBn)2Mg2L(Bu)] (3). Similar treatment of 1 with EtZnOBn generated [(μ-OBn)2Zn2L(Bu)] (4). Reaction of 2 with 2 equiv. of Me2AlOBn in toluene at 70 °C afforded aluminum complex [Me2Al(μ-OBn)2AlL(H)] (5). Complexes 3-5 were characterized by (1)H and (13)C NMR spectroscopy, elemental analyses, and single crystal X-ray diffraction techniques. Each of complexes 3-5 is active for the ROP of ε-CL and rac-LA. Kinetic studies of the polymerization reactions were performed.

Ring opening polymerization of rac-lactide by group 4 tetracarbamato complexes: activation, propagation and role of the metal

A series of group 4 metal tetracarbamates M(O(2)CNR(2))(4) (M = Ti, R = Et, 1a; M = Zr, R = Et, 1b; (i)Pr, 1c; M = Hf, R = Et, 1d; R = (i)Pr, 1e) were studied as catalytic precursors in the solution polymerization of rac-lactide. The titanium complex but not the zirconium and hafnium ones increase the activity by addition of (i)PrOH. The structure of the carbamato ligand markedly influences the molar mass of polymer; the complexes with isopropyl carbamato ligands produced PLA with molar masses up to 94,000 g mol(-1). The main mechanistic aspects of the initial stages of the polymerization reactions were outlined by spectroscopic and computational analyses. In the case of zirconium and hafnium complexes, an interaction between a carbamato ligand and the CH unit of one lactide molecule is established at room temperature. This interaction is followed by the high temperature proton transfer from the lactide to the carbamato O-atom, affording a catalytic active alkoxy complex with release of CO(2) and NHR(2). The polymerization mediated by Ti(O(2)CNEt(2))(4) involves the release of a radical fragment [O(2)CNEt(2)]˙, with consequent generation of a Ti(III) center. The propagating chain is an alcoholate ligand coordinated to a Ti(IV) centre and containing a radical mainly localized at the tail of the chain (DFT, EPR).

Mechanism of living lactide polymerization by dinuclear indium catalysts and its impact on isoselectivity

A family of racemic and enantiopure indium complexes 1-11 bearing bulky chiral diaminoaryloxy ligands, H(NNO(R)), were synthesized and fully characterized. Investigation of both the mono- and the bis-alkoxy-bridged complexes [(NNO(R))InX](2)[μ-Y][μ-OEt] (5, R = (t)Bu, X = Y = Cl; 8, R = Me, X = I, Y = OEt) by variable temperature, 2D NOESY, and PGSE NMR spectroscopy confirmed dinuclear structures in solution analogous to those obtained by single-crystal X-ray crystallography. The dinuclear complexes in the family were highly active catalysts for the ring-opening polymerization (ROP) of lactide (LA) to form poly(lactic acid) (PLA) at room temperature. In particular, complex 5 showed living polymerization behavior over a large molecular weight range. A detailed investigation of catalyst stereoselectivity showed that, although (R,R/R,R)-5 is highly selective for l-LA, only atactic PLA is obtained in the polymerization of racemic LA. No such selectivity was observed for complex 8. Importantly, the selectivities obtained for the ROP of racemic LA with (R,R/R,R)-5 and (R,R/R,R)-8 are different and, along with kinetics investigations, suggest a dinuclear propagating species for these complexes.