Ring-opening Polymerization Of Rac-lactide Research Articles

An aniline-bridged bis(pyrazolyl)alkane ligand for dizinc-catalysed ring-opening polymerization.

We report an aniline ligand (1) with two bis(pyrazolyl)alkane arms, and its cationic, dizinc complexes. XRD, NMR, and modelling of the dizinc complexes resulted in an unprecedented, dynamic μ-anilide core. Compared with published μ-phenolate analogues, our μ-anilide complexes show higher activity and divergent counterion trends in ring-opening polymerization of rac-lactide.

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 L1-6H (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 ([L1-6H]:[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 (Pm = 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.

Bis(salicylamidine) Ligands (FAlen): A Variant of Salen with "à la Carte" Denticity.

Ethylene- and phenylene-bridged bis(salicylamidine) ligands have been readily prepared from ethylene or phenylenediamine and iminium chloride derivatives generated in situ from N,N-dialkylsalicylamides. The former, in its diprotonated form (FAlenH2), reacts with AlMe3 to afford a zwitterionic dimethyldiphenoxyaluminate complex with the FAlen ligand monoprotonated and in a bidentate κ2O,O' fashion. A phenylene-bridged proligand behaves differently, yielding a neutral methylaluminum complex bearing a κ3O,N,O'-coordinated FAlen ligand. From these complexes, methyl anion abstraction with B(C6F5)3 or a reaction with Schrock's alcohol leads to the corresponding aluminum cationic or alkoxy complexes in which a κ4O,N,N',O'-coordination mode of the FAlen ligand is observed. X-ray diffraction studies of the proligands and of the complexes show that the amidine functions feature a trans configuration when the N-amidine atom is not coordinated to the metal and conversely a cis configuration when it is. Density functional theory calculations show that trans-cis isomerization of the amidine functions occurs upon coordination with the metal ion with very low energy barriers. They also confirm the intuition that the denticity of the FAlen ligands in the complexes is directly related to the electron richness of the metal ion. At last, FAlen Al complexes are used as initiators for the controlled ring-opening polymerization of rac-lactide to afford poly(lactic acid) with slight isotactic bias.

Isoselective Ring-Opening Polymerization of rac-Lactide Catalyzed by Simple Potassium Amidate Complexes Containing Polycyclic Aryl Group

The isoselective ring-opening polymerization of rac-LA is a challenging goal. In this work, a series of potassium amidate complexes (K1–K10) were easily prepared and characterized using the 1H/13C NMR spectrum. The molecular structures of potassium complexes K2 and K10 were determined by X-ray diffraction, which showed that both were two-dimensional coordination polymers due to the adjacent π interactions of the aryl. In the presence of benzyl alcohol (BnOH), all of the potassium complexes exhibited a high catalytic activity toward the ring-opening polymerization of L-lactide and rac-LA, yielding linear polylactides capped with BnO or CH3O end groups. A significant solvent effect on the ROP of the L-LA was observed, with a superior efficiency in toluene than in THF and CH2Cl2. These complexes are iso-selective and act as active catalysts for the controlled ring-opening polymerization of rac-lactide, with a Pm from 0.54 to 0.76. This is a rare example of simple alkali metal complexes for the isoselective ROP of rac-lactide. The substituent greatly affected the monomer conversion and isoselectivities.

N-alkyl-β-ketoiminate zinc complexes: Synthesis, structure, and reactivity in ring-opening polymerization of lactide

A series of N-alkyl-β-ketoiminate monozinc complexes [{tBuCN(R)CHC(O)CtBu}2Zn] (R = Me, 2a; Et, 2b; Pr, 2c; iPr, 2d; Cy, 2e) were synthesized from the reaction of diethyl zinc with the corresponding β-ketimines. All these zinc complexes were characterized by NMR, elemental analysis and HRMS spectroscopy. The molecular structures of complexes 2b-2e were further confirmed by single-crystal X-ray diffraction. The structures exhibit that each zinc center is four-coordinate, and surrounded by two bidentate β-ketoiminate ligands, forming a distorted tetrahedral geometry. These homoleptic bis(β-ketoiminate) zinc complexes are active towards the ring-opening polymerization of rac-lactide in the absence of alcohol, yielding over 90 % conversions within 1 h at 70 °C in toluene with the ratio of monomer to catalyst being 100.

Phthalocyanines with Non-Traditional Early Transition-Metals

The synthesis, spectroscopic and redox properties of new metallophthalocyanines (PcM) are active areas of research. The rings of PcM complexes can be successively reduced using chemical, electrochemical, or photochemical methods to give rise to species containing ring-reduced Pc(3-), Pc(4-) or Pc(5-) ligands ; in the other direction, both ring-oxidized Pc(-1) and Pc(0)-containing systems can be accessed. These species are usually generated and characterized in situ and have only recently begun to be isolated and structurally characterized. In particular, there are few examples of phthalocyanines with early-transition and f-block metals - despite the rich organometallic-type reactivity of these metals - and thus we targeted new PcM complexes in this underdeveloped area of the periodic table. Given their larger ionic size, the unusual structural feature of the metal centre protruding far out of the Pc cavity is observed. Hence, the exposure of the coordination sphere of these Lewis-acidic (often d0) metals makes these PcM complexes attractive catalysts; this cis-oriented axial ligation also drastically improves the solubility, despite the Pc-ring remaining unsubstituted.In this presentation the isolation and structural characterization of new PcM materials with M = scandium, zirconium, niobium and (time-permitting) molybdenum will be described, along with rare structurally characterized examples of their reduced Pc(4-) and Pc(3-)complexes. Their electronic structure and electrochemical behaviour will be discussed using a combination of spectroscopic and structural techniques. A series of moisture-sensitive, soluble PcZr(IV) and PcNb(V) alkoxides were prepared, characterized, and their catalytic activity towards the ring-opening polymerization of rac-lactide was studied and will be detailed. Reaction of PcScCl with LiO i Pr and NaO t Bu yielded two hydroxide complexes containing the PcScOH unit, obtained from the hydrolysis of the putative PcSc-alkoxide intermediate. The two structurally characterized systems have a tilted “butterfly” shape structure, reminiscent of bent metallocenes. The solubility of these early transition-metal based complexes present new opportunities for the advancement of this underdeveloped area of PcM chemistry. Time permitting, our related work on organometallic PcLnX complexes will also be described.ReferencesZhou, D.B. Leznoff, Chem. Commun., 2018, 54, 1829-1832. W. Zhou, J.R. Thompson, C.C. Leznoff, D.B. Leznoff, Chem. Eur. J., 2017, 23, 2323-2331; R. Platel, W. Zhou, T.T. Tasso, T. Furuyama, N. Kobayashi, D.B. Leznoff, Chem. Commun., 2015, 5986-89; D. McKearney, W. Zhou, V.E. Williams, D.B. Leznoff, Chem. Commun., 2019, 55, 6696-6699; Y. Ganga-Sah, E. Tajbakhsh, R.H. Platel, W. Zhou, D.B. Leznoff, J. Porph. Phthalocyanines, 2019, 23, 1592-1602; W. Zhou, D. McKearney, D.B. Leznoff, Chem. Eur. J., 2020, 26, 1027-31.

Effect of initiator on the catalytic performance of zinc(II) complexes supported by aminomethylquinoline and aminomethylpyridine derived ligands in stereoselective ring opening polymerization of rac-lactide

Zinc(II) complexes, namely [LnZnCl2] (Ln = LA–LD) supported with N,N'-bidentate aminomethylquinoline and aminomethylpyridine derived ligands, such as 2-(piperidin-1-ylmethyl)quinoline (LA), 4-(quinolin-2-ylmethyl)morpholine (LB), 2-(piperidin-1-ylmethyl)pyridine (LC), and 4-(pyridin-2-ylmethyl)morpholine (LD), were synthesized and structurally characterized. The structural data revealed that distorted tetrahedral geometries are adopted by Zn(II) metal ions by coordinating with corresponding ligands in a bidentate fashion. Catalytic studies conducted with these complexes in the presence of initiating reagents, i.e., LiMe, LiOCHMe2, and LiCl, showed that they can initiate ring-opening polymerization (ROP) of rac-lactide (rac-LA). All the tested initiators were active, with the in situ catalytic systems of [LnZnCl2] and [LiMe] displaying marked differences in the initiation, polymerization rate and control toward ROP of rac-LA compared to the in situ catalytic systems of [LnZnCl2] and [LiOiPr], giving an 87% conversion of rac-LA within 30 sec using [rac-LA]:[LBZnCl2]:[LiMe] = 100:1:2 at 0 ℃. Polylactides (PLAs) obtained were of low molecular weights with slightly higher polydispersity indices (PDI = 1.21–1.25) in all cases regardless of the initiating Zn(II) system and variations in ancillary ligands. Overall, the higher hetero-enriched PLAs were provided by the in situ catalytic systems of [LnZnCl2] and [LiMe] (the highest Pr up to 0.86 at 0 ℃) compared to that with the in situ system of [LnZnCl2] and [LiOiPr].

Determination of Polylactide Microstructure by Homonuclear Decoupled 1 H NMR Spectroscopy.

The ring-opening polymerization (ROP) of lactide (the dilactone of lactic acid) produces poly(lactide) commonly referred to as poly(lactic acid) (PLA). The monomer lactide, has two stereogenic centers and thus, three stereoisomers are possible, namely: D-(R,R), L-(S,S) and meso-lactide. The rac-lactide is an equimolar mixture of D- and L-enantiomers. Depending upon the relative configuration of the stereogenic centers in the polymeric chain, different tacticities (isotactic, syndiotactic, heterotactic and atactic) arise in the PLA chains. The study of the tacticity of a polymer is fundamental since it plays a crucial role in determining the physical properties of the polymer. NMR spectroscopy is a powerful analytical technique for the determination of the tacticity of PLA. This article describes in details the tacticity assignment for PLA derived from ROP of rac-lactide and meso-lactide, using homonuclear proton-decoupled 1 H NMR. The detailed tetrad level assignment pertinent to the methine hydrogen signal is the key for the determination of tacticity.

Polymetallic Group 4 Complexes: Catalysts for the Ring Opening Polymerisation of rac-Lactide

Five novel air- and moisture-stable polymetallic Ti and Zr amino acid-derived amine bis(phenolate) (ABP) complexes were synthesised and fully characterised, including X-ray crystallographic studies. The reaction of the ABP proligands with Ti or Zr alkoxides has resulted in the formation of polymetallic aggregates of different nuclearity. The steric bulk on the pendant arm of the ligand was found to play a critical role in establishing the nuclearity of the aggregated complex. Sterically, less-demanding groups, such as H or Me, facilitated the formation of tetrametallic Ti clusters, bridged by carboxylate groups, while increased steric bulk (tBu) led to the formation of binuclear μ-oxo-bridged species. The isolated complexes were employed as catalysts for the ring opening polymerisation (ROP) of rac-lactide. Overall, the Ti catalysts were all active with the smaller, bimetallic Ti aggregates exhibiting relatively faster rates. A monometallic, bis(ABP) Zr complex was found to exert remarkable ROP activity, albeit with limited control over the tacticity and molecular weight distribution of the polymer. A further oxo-bridged Zr cluster was shown to display a previously unprecedented trimetallic structure and achieved a moderate rate in the ROP of rac-lactide.

Towards a selective synthetic route for cobalt amino acid complexes and their application in ring opening polymerization of rac-lactide.

Catalysts based on cobalt amino acids and 2,2 bipyridine (bipy) present an attractive and cost-effective alternative as ring opening polymerization catalysts, yet this system remains underexplored despite the advantageous coordination properties of amino acids and bipy as ligands combined with the variety of accessible oxidation states and coordination geometries of cobalt. Here, metal complexes of type [Co(aa)2(bipy)] with amino acids (aa: glycine, leucine and threonine) as ligands are reported. The complexes were characterized spectroscopically (IR, UV-vis and 1H, 13C NMR for diamagnetic species), and by MS spectrometry and elemental analysis. The data reveal that the 2,2 bipyridine acts as a neutral bidentate donor coordinating to the metal ion through two nitrogen atoms and the amino acid acts as a bidentate ligand coordinating through the carboxylate and amino group forming a stable five membered ring and a pseudo-octahedral geometry around the Co center. The activity of the complexes for the ring opening polymerization (ROP) of rac-lactide is presented. The complexes are effective initiators for the ROP of rac-lactide (Kobs = 9.05 × 10−4 s−1) at 100 : 1 [rac-lactide] : [catalyst] 1 M overall concentration of lactide in toluene at 403 K.

Synthesis, structures, and catalytic efficiency in ring opening polymerization of rac-lactide with tridentate vs. bidentate cobalt(ii), zinc(ii), and cadmium(ii) complexes containing N-substituted N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)amine ligands.

A series of Co(ii), Zn(ii), and Cd(ii) complexes supported by 1-(3,5-dimethyl-1H-pyrazol-1-yl)-N-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-N-(furan-2-ylmethyl)methanamine (LA) and N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-4-isopropylaniline (LB) were synthesized. The direct chelation of CoCl2·6H2O, ZnCl2, and CdBr2·4H2O by the ligands produced [LnMX2] (Ln = LA or LB; M = Zn or Co, with X = Cl; M = Cd, with X = Br) complexes in high yields. Structural studies revealed that [LBCoCl2] and [LBZnCl2] adopted distorted tetrahedral geometries, as LB coordinated the metal centers in a bidentate fashion, while LA coordinated the metal centers in a tridentate fashion through the nitrogen atoms of the pyrazole and amine moieties, so that [LACoCl2] and [LAZnCl2] exhibited trigonal bipyramidal geometries and [LACdBr2] a square pyramidal geometry. [LBCdBr2] has two Cd-containing structures per unit cell, whereby one Cd center adopted a distorted tetrahedral geometry and the other exhibited square bipyramidal geometry. The in situ-generated alkyl derivatives of the synthesized complexes were assessed in the ring-opening polymerization of rac-lactide. Heterotactic polylactides (PLAs) were furnished with all complexes. The [LBZnCl2]/MeLi system produced PLA with a superior heterotactic bias (Pr up to 0.94) at −25 °C. PLAs with wide-ranging polydispersity indices (1.16–2.23) and low molecular weights were produced in all cases, irrespective of the specific M(II) center and ancillary ligand utilized.

Chiral 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)-Catalyzed Stereoselective Ring-Opening Polymerization of rac-Lactide: High Reactivity for Isotactic Enriched Polylactides (PLAs).

Chiral 4,8-diphenyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (DiPh-TBD) was synthesized and applied to a ring-opening polymerization of rac-lactide (rac-LA). The chiral DiPh-TBD promoted the synthesis of isotactic enriched polylactides (PLAs) with controlled molecular weight and narrow molecular weight distributions under mild, metal-free conditions. When the [rac-LA]/[Cat.] ratio was 100/1, full monomer conversion was achieved within only 1 min and a moderate probability of 0.67 meso dyads (Pm) was obtained at room temperature. A chain-end control mechanism (CEC) was found to be responsible for the isoselectivity based on the homodecoupled 1H NMR spectrum, the chiral HPLC measurement, and kinetic studies.

Copper(II) complexes containing N′-aromatic group substituted N,N′,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)amines: Synthesis, structures, polymerization of methyl methacrylate and ring opening polymerization of rac-lactide

A series of Cu(II) complexes, [LnCuCl2] (Ln = LA–LF), supported by N′-aromatic-group-substituted N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methylamine ligands have been synthesized. Variations of substituents at the ortho position of the aniline moiety influenced the solid-state structures of these complexes. The X-ray structures of [LnCuCl2] (Ln = LA–LC and LF) revealed that ligands are coordinated in a N,N,N-tridentate fashion to Cu(II) center and adopted a distorted square-pyramidal geometry, while [LDCuCl2] with N,N-bidentate coordination adopts distorted tetrahedral geometry. These complexes were capable of polymerizing methyl methacrylate (MMA) in the presence of modified methylaluminoxane (MMAO), with [LFCuCl2] displaying the highest activity (2.81 × 104 g PMMA (mol Cu)−1h−1). Regardless of ligand architecture, syndio-enriched PMMAs have been furnished with a slightly broader polydispersity index (PDI). Additionally, the in situ generated dimethyl derivatives, polymerized rac-LA and furnished PLA with mediocre heterotacticities at room temperature. Importantly, the catalytic efficiencies of the Cu(II) complexes studied have been found to be influenced by the steric and electronic properties of ligand.

Ring-opening polymerization of rac-lactide, copolymerization of rac-lactide and ε-caprolactone by zinc complexes bearing pyridyl-based tridentate amino-phenolate ligands

A series of zinc complexes supported by tridentate aminophenolate ligands bearing a substituted pyridyl ring, LZn[N(SiMe3)2] (L = {2-[(6-R3-pyrid-2-yl)CH2N(R4)CH2]-4-R2-6-R1-C6H2O}, 1: R1 = Trityl, R2 = Me, R3 = Me, R4 = Bn; 2: R1 = Trityl, R2 = Me, R3 = Me, R4 = nHex; 3: R1 = Trityl, R2 = Me, R3 = Me, R4 = Cy; 4: R1 = Trityl, R2 = Me, R3 = Ph, R4 = Cy; 5: R1 = Trityl, R2 = Me, R3 = tBu, R4 = Cy; 6: R1 = R2 = tBu, R3 = Ph, R4 = Cy), were synthesized via the reactions of Zn[N(SiMe3)2]2 and the corresponding aminophenols. The X-ray diffraction analysis of typical complex 2 confirmed its monomeric nature, where the metal centre is tetra-coordinated by three donors of the aminophenolate ligand and one silylamido group in a distorted tetrahedral geometry. All these zinc complexes were moderately to highly active initiators toward the ring-opening polymerization (ROP) of rac-lactide (rac-LA). The substituents on the ortho-positions of the phenolate and pyridyl rings as well as on the skeleton nitrogen atom of the ligand show significant influences on the catalytic activities and the stereoselectivities of the corresponding complexes toward the polymerization of rac-LA, leading to the production of atactic polylactides (PLAs) via complex 5 (Pm = 0.53) and moderate isotatic bias PLAs via complexes 1–4, 6 (Pm = 0.63–0.66). In addition, complexes 2 and 4 could efficiently initiate the copolymerization of rac-LA and ε-CL at 70 °C to afford random copolymers with high molecular weights and moderate polydispersities.

Lanthanide bis(borohydride) complexes coordinated by tetradentate phenoxide ligand: Synthesis, structure, and catalytic activity in ring-opening polymerization of rac-lactide and ε-caprolactone

New lanthanide bis(borohydride) complexes coordinated by monoanionic tetradentate phenoxide ligand [(C5H4NCH2)2NCH2(3,5-tBu2C6H2O)]Ln(BH4)2 [Ln = Y (1), Nd (2), Sm (3)] were synthesized by the reactions of Ln(BH4)3(THF)3 with an equimolar amount of the lithium phenoxide [(C5H4NCH2)2NCH2(3,5-tBu2C6H2O)Li] in THF at 60 °C. The complexes were isolated after recrystallization from toluene in 75, 64, and 69% yields, respectively. X-ray analysis revealed that in 1 the phenoxide ligand is tetradentate, while two borohydride groups are coordinated to the Y3+ ion in a κ3-fashion. Compounds 1–3 initiate ring-opening polymerization (ROP) of ɛ-caprolactone (ɛ-CL) and racemic lactide (rac-LA) in toluene at 25 °C. The obtained polylactides are characterized by atactic structures Pr = 0.54–0.58, and rather narrow polydispersities (Mw/Mn = 1.1–1.5). For ɛ-caprolactone polymerization, the resulting polymers have moderate polydispersities (Mw/Mn = 1.4–2.4) and molecular weights (6.2–54.1·103).

Tetranuclear complexes of group 12 and 13 supported on a polynucleating ligand and activity studies in the ROP of rac-lactide

A polynucleating ligand (L-H4) containing four N,O bidentate sites was cleanly metallated by four equivalents of AlMe3, GaMe3 and ZnEt2 resulting in tetranuclear complexes L(MMe2)4 (M = Al (1), Ga (2)) and L(ZnEt(CH3CN)0.5)4 (3). The Al complex shows the best catalytic activity for the ring-opening polymerization (ROP) of rac-lactide (rac-LA) in the present family of tetranuclear compounds. Polymerization studies indicate the lack of cooperativity between metal centers in the ROP of rac-LA, despite having a ligand platform which binds to four metal centers.

Binuclear Schiff-base zinc(II) complexes: Synthesis, crystal structures and reactivity toward ring opening polymerization of rac-lactide

Upon crystallization from DMF/diethyl ether or CH2Cl2/pyridine of tetracoordinated Zn(II) complexes containing either 4-fluorophenyl or 4-anisyl substituted O,N,O-tridentate Schiff base ligands and pyridine as auxiliary ligand, three new doubly phenoxide-bridged dimeric Zn(II) complexes were isolated and their catalytic properties for the ring opening polymerization (ROP) of rac-lactide were explored. The single-crystal XRD analysis allowed to corroborate the molecular structures of the binuclear complexes, showing that the pentacoordinated Zn(II) metal ion adopts a slightly distorted square pyramidal geometry. The basal plane Is formed by the O,N,O-donor set of the tridentate Schiff base ligand and a phenoxo bridging oxygen, while a pyridine or a DMF molecule is located at the apex of the pyramid. The binuclear complexes were employed as catalysts in the ROP of rac-lactide for polylactide (PLA) synthesis. Polymerization conditions such as reaction time, catalyst concentration and use of benzyl alcohol as cocatalyst were studied, as well as their influence on the conversion rate, average molecular weight and polydispersity of the obtained PLA. The catalyst substituted with fluorine was more reactive, but with a lower control on the molecular weight compared to the methoxy-substituted catalyst. On the other hand, the presence of pyridine as an auxiliary ligand generates an improvement in the polydispersity of PLA.

Aluminum complexes with Schiff base bridged bis(indolyl) ligands: synthesis, structure, and catalytic activity for polymerization of rac-lactide.

A series of Schiff base bridged bis(indolyl) ligands were developed for aluminum chemistry. The reactions of AlEt3 or AlMe3 with the Schiff base bridged bis(indolyl) proligands R1(-N[double bond, length as m-dash]CHC8H5NH)2 (R1 = -CH2CH2- (H2L1); -CH2CH2CH2- (H2L2); -CH2CMe2CH2- (H2L3); rac-Cy (H2L4); and R,R-Cy (H2L5)) were studied leading to the synthesis of a series of aluminum alkyl complexes L1AlEt (1)-L5AlEt (5) and L3AlMe (3b) in good yields, while the reaction of H2L3 with Al(OiPr)3 gave the aluminum alkoxide complex L3AlOiPr (3a). These aluminum complexes were characterized by spectroscopic methods and elemental analyses. The solid state structures of the aluminum complexes 1-5 and 3a were confirmed by the X-ray diffraction study. X-ray analyses revealed that the aluminum centre in these complexes is five-coordinated. The coordination geometry is between square pyramidal and trigonal bipyramidal. In the presence of 1 equiv. of isopropanol, the aluminum alkyl complexes exhibited notable activity towards the ring-opening polymerization of rac-lactide at 70 °C in toluene, with good control over molecular weights and dispersities. The substituents and the length of the bridging part between the two Schiff base nitrogen atoms have an influence on either the tacticity of isolated polymers or the rate of polymerization. The kinetics of complex L3AlOiPr (3a) in C6D6 was also investigated, and the experimental results revealed that the rate of polymerization was first-order with respect to rac-lactide.

Facile Preparation of Stereoblock PLA From Ring-Opening Polymerization of rac-Lactide by a Synergetic Binary Catalytic System Containing Ureas and Alkoxides.

Ring-opening polymerization (ROP) of cyclic esters/lactones by efficient catalysts is a powerful method for synthesis of biodegradable and biocompatible polyesters with well-defined structures. To develop catalytic systems that are fast, selective and controlled is a persistent effort of chemists. In this contribution, we report a binary urea/alkoxide catalytic system that could catalyze ROP of rac-LA in a fast (over 90% conversion within 1–2 min), stereoselective (Pi up to 0.93) and controlled manner, indicated by narrow MW distributions, linear relationship between the monomer conversions and Mns, end-group fidelity, and chain extension experiments. Remarkably, the catalytic system described here is simple, easily prepared, and structurally tunable and thus has versatile catalytic performances.

Aggregated initiators: defining their role in the ROP of rac-lactide.

Reported examples of aggregated initiators for the ring-opening polymerisation (ROP) of lactide often lack detailed investigations as to the nature of the active species, making it difficult to reconcile ligand design with performance. Here, we offer additional stability to the polynuclear titanium complexes, TiL(OiPr) (L = 9-14), through a bridging carboxylate anchored to the supporting amine bis(phenolate) ligands. An in-depth study of solution-state behaviour determined the process of assembly was driven by interactions between the carboxylate and a vacant site on a neighbouring titanium centre. Furthermore, we establish that mononuclear units form dynamic mixtures of polynuclear aggregates, with a clear relationship between nuclearity of the aggregates and the steric bulk on the ligand. Smaller aggregates displayed increased activity towards the ROP of rac-lactide. Furthermore, addition of a chiral centre, on the ligand framework, was investigated as a route to influence the selectivity of the polymerisation via easily-accessible initiators.