Yttrium Phosphasalen Initiators for rac-Lactide Polymerization: Excellent Rates and High Iso-Selectivities

Facile green ring-opening polymerization of l-lactide catalyzed by natural kaoline

Natural kaoline without any further modification was used as a novel catalyst for the facile green ring-opening polymerization of l-lactide.

Magnesium complexes containing biphenyl-based tridentate imino-phenolate ligands for ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate

A series of racemic 2-[(2'-(dimethylamino)biphenyl-2-ylimino)methyl]-4-R(2)-6-R(1)-phenols (L¹H-L⁴H) were reacted with {Mg[N(SiMe3)2]2}2 to provide four heteroleptic magnesium complexes (L¹⁻⁴)MgN(SiMe3)2·(THF)n (1, R(1) = (t)Bu, R(2) = Me, n = 1; 2, R(1) = R(2) = CMe2Ph, n = 0; 3, R(1) = CPh3, R(2) = (t)Bu, n = 1; 4, R(1) = Br, R(2) = (t)Bu, n = 0), which have been fully characterized. X-ray structural determination shows that complex 1 possesses a monomeric structure, but complex 4 is dimeric with C2-symmetry where the two metal centers are bridged by two phenolate oxygen atoms of the ligands. The coordination geometry around the magnesium center in these complexes can be best described as a distorted tetrahedral geometry. The heteroleptic complexes 1-4 efficiently initiate the ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate (α-MeTMC) and the polymerizations are better controlled in the presence of 2-propanol. In general, the introduction of a bulky ortho-substituent on the phenoxy unit results in increases of both the catalytic activity and the stereo- or regioselectivity of the corresponding magnesium complex. Microstructure analyses of the resulting PLAs revealed that P(r) values range from 0.46 to 0.81, depending on the catalyst and the polymerization conditions. For racemic α-MeTMC, detailed analyses using (1)H and (13)C NMR spectroscopy indicated the preferential ring-opening of α-MeTMC at the most hindered oxygen-acyl bond (X(reg) = 0.65-0.86).

Metal-size influence in iso-selective lactide polymerization

Iso-selective initiators for the ring-opening polymerization (ROP) of rac-lactide are rare outside of Group 13. We describe the first examples of highly iso-selective lutetium initiators. The phosphasalen lutetium ethoxide complex shows excellent iso-selectivity, with a P(i) value of 0.81-0.84 at 298 K, excellent rates, and high degrees of polymerization control. Conversely, the corresponding La derivative exhibits moderate heteroselectivity (P(s)=0.74, 298 K). Thus, the choice of metal center is shown to be crucial in determining the level and mode of stereocontrol. The relative order of rates for the series of complexes is inversely related to metallic covalent radius: that is, La>Y>Lu.

Zinc-Catalyzed Highly Isoselective Ring Opening Polymerization of rac-Lactide

A family of chiral zinc amido-oxazolinate complexes are shown to be highly active and isoselective initiators for the ring-opening polymerization (ROP) of rac-lactide, yielding isotactic stereoblock polylactides (PLA) with P_m up to 0.91. This represents the highest isoselectivity observed with zinc-based catalysts for ROP of rac-lactide.

Zirconium complexes of bipyrrolidine derived salan ligands for the isoselective polymerisation of rac-lactide

We report a series of Zr(iv) 2,2′-bipyrrolidine–salan derived complexes and their exploitation for the ROP of rac-lactide to afford highly isotactically enriched PLA.

Polymerization of nonfood biomass-derived monomers to sustainable polymers

The development of sustainable routes to fine chemicals, liquid fuels, and polymeric materials from natural resources has attracted significant attention from academia, industry, the general public, and governments owing to dwindling fossil resources, surging energy demand, global warming concerns, and other environmental problems. Cellulosic material, such as grasses, trees, corn stover, or wheat straw, is the most abundant nonfood renewable biomass resources on earth. Such annually renewable material can potentially meet our future needs with a low carbon footprint if it can be efficiently converted into fuels, value added chemicals, or polymeric materials. This chapter focuses on various renewable monomers derived directly from cellulose or cellulose platforms and corresponding sustainable polymers or copolymers produced therefrom. Recent advances related to the polymerization processes and the properties of novel biomass-derived polymers are also reviewed and discussed.

Chiral lanthanide complexes: coordination chemistry, spectroscopy, and catalysis

Luminescent lanthanide complexes bearing amido-bisoxazoline ligands are reported. They were studied using time-resolved luminescence spectroscopy, and were probed for their activity in hydroamination/cyclisation and ring-opening polymerisation catalysis.

A tetracoordinated phosphasalen nickel(III) complex

The oxidation of a Ni(II) complex bearing a tetradentate phosphasalen ligand, which differs from salen by the presence of an iminophosphorane (PN) in place of an imine unit, was easily achieved by addition of a silver salt. The site of this oxidation was investigated with a combination of techniques (NMR, EPR, UV/Vis spectroscopy, X-ray diffraction, magnetic measurements) as well as DFT calculations. All data are in agreement with a high-valent Ni(III) center concentrating the spin density. This markedly differs from precedents in the salen series for which oxidation on the metal was only observed at low temperature or in the presence of additional ligands or anions. Therefore, thanks to the good electron-donating properties of the phosphasalen ligand, [Ni(Psalen)](+) represents a rare example of a tetracoordinated high-valent nickel complex in presence of a phenoxide ligand.

Heteroleptic tin(II) initiators for the ring-opening (co)polymerization of lactide and trimethylene carbonate: mechanistic insights from experiments and computations

The tin(II) complexes {LO(x)}Sn(X) ({LO(x)}(-) =aminophenolate ancillary) containing amido (1-4), chloro (5), or lactyl (6) coligands (X) promote the ring-opening polymerization (ROP) of cyclic esters. Complex 6, which models the first insertion of L-lactide, initiates the living ROP of L-LA on its own, but the amido derivatives 1-4 require the addition of alcohol to do so. Upon addition of one to ten equivalents of iPrOH, precatalysts 1-4 promote the ROP of trimethylene carbonate (TMC); yet, hardly any activity is observed if tert-butyl (R)-lactate is used instead of iPrOH. Strong inhibition of the reactivity of TMC is also detected for the simultaneous copolymerization of L-LA and TMC, or for the block copolymerization of TMC after that of L-LA. Experimental and computational data for the {LO(x)}Sn(OR)complexes (OR=lactyl or lactidyl) replicating the active species during the tin(II)-mediated ROP of L-LA demonstrate that the formation of a five-membered chelate is largely favored over that of an eight-membered one, and that it constitutes the resting state of the catalyst during this (co)polymerization. Comprehensive DFT calculations show that, out of the four possible monomer insertion sequences during simultaneous copolymerization of L-LA and TMC: 1) TMC then TMC, 2) TMC then L-LA, 3) L-LA then L-LA, and 4) L-LA then TMC, the first three are possible. By contrast, insertion of L-LA followed by that of TMC (i.e., insertion sequence 4) is endothermic by +1.1 kcal mol(-1), which compares unfavorably with consecutive insertions of two L-LA units (i.e., insertion sequence 3) (-10.2 kcal mol(-1)). The copolymerization of L-LA and TMC thus proceeds under thermodynamic control.

Synthesis and characterization of bimetallic lanthanide-alkali metal complexes stabilized by aminophenoxy ligands and their catalytic activity for the polymerization of 2,2-dimethyltrimethylene carbonate

Electronic properties of the aminophenolate groups have obvious effect on the synthesis of aminophenolate lanthanide-lithium complexes. Amine elimination reactions of Ln[N(SiMe3)2]3(μ-Cl)Li(THF)3 with lithium aminophenolates [ArNHCH2(3,5-(t)Bu2C6H2-2-O)Li(THF)]2 (Ar = p-ClC6H4, [ONH](Cl-p); p-BrC6H4, [ONH](Br-p)) in tetrahydrofuran (THF) in a 1 : 2 molar ratio gave the bimetallic lanthanide-lithium amido complexes [NO](Cl-p)2Ln[N(SiMe3)2][Li(THF)]2 (Ln = Y (1), Yb (2)), and [NO](Br-p)2Ln[N(SiMe3)2][Li(THF)]2 (Ln = Y (3), Yb (4)). When the Ar groups are p-MeOC6H4, ([ONH](MeO-p)) and o-MeOC6H4 ([ONH](MeO-o)), similar reactions generated the homoleptic lanthanide-lithium complexes [NO](MeO-p)3Ln[Li(THF)]3 (Ln = Y (5), Yb (6)) and [NO](MeO-o)2Ln[Li(THF)] (Ln = Y (7), Yb (8)) in high isolated yields, respectively. Whereas the bimetallic lanthanide-lithium amido complexes [NO](Cl-o)2Ln[N(SiMe3)2][Li(THF)]2 (Ln = Y (9), Yb (10)) can be obtained in good yields, when the Ar group is o-ClC6H4 ([ONH](Cl-o)). All of these complexes were well characterized. X-ray structure determination revealed that these complexes have solvated monomeric structures. In complexes 1-4, 9, and 10, the lanthanide atom is five-coordinated by two oxygen atoms and two nitrogen atoms from two aminophenoxy ligands and one nitrogen atom from N(SiMe3)2 group to form a distorted trigonal bipyramidal geometry, whereas in complexes 5-8, the central lanthanide atom is six-coordinated by oxygen atoms, and nitrogen atoms from the aminophenoxy ligands to form a distorted octahedron. It was found that complexes 1-10 are highly efficient initiators for the ring-opening polymerization of 2,2-dimethyltrimethylene carbonate (DTC), affording the polymers with high molecular weights, and the homoleptic heterobimetallic lanthanide complexes showed apparently high activity.