Polymerization Of Rac-lactide Research Articles

Applications of low-valent compounds with heavy group-14 elements.

Over the last two decades, the low-valent compounds of group-14 elements have received significant attention in several fields of chemistry owing to their unique electronic properties. The low-valent group-14 species include tetrylenes, tetryliumylidene, tetrylones, dimetallenes and dimetallynes. These low-valent group-14 species have shown applications in various areas such as organic transformations (hydroboration, cyanosilylation, N-functionalisation of amines, and hydroamination), small molecule activation (e.g. P4, As4, CO2, CO, H2, alkene, and alkyne) and materials. This review presents an in-depth discussion on low-valent group-14 species-catalyzed reactions, including polymerization of rac-lactide, L-lactide, DL-lactide, and caprolactone, followed by their photophysical properties (phosphorescence and fluorescence), thin film deposition (atomic layer deposition and vapor phase deposition), and medicinal applications. This review concisely summarizes current developments of low-valent heavier group-14 compounds, covering synthetic methodologies, structural aspects, and their applications in various fields of chemistry. Finally, their opportunities and challenges are examined and emphasized.

Stereoselective ring-opening polymerization of rac-lactide catalyzed by the proton sponge bisphosphazene 1,8-bis(hexamethyltriaminophosphazenyl)naphthalene

Stereoselective ring-opening polymerization of rac-lactide was catalyzed by the proton sponge bisphosphazene HMPN, affording semicrystalline isotactic polylactide with Pm up to 0.93.

Zirconium complexes of an aminophenolate-etherphenolate ONO'O ligand: Synthesis, characterization and catalytic properties

A synthetic protocol to access new aminophenolate-etherphenolate ONO'O ligands is presented. Zirconium iso-propoxide and benzyl complexes of a prototypical tert-butyl substituted ONO'O ligand can be synthesized in high yields as single conformational isomers. Combined NMR spectroscopy and single-crystal X-ray diffraction studies on (ONO'O)Zr(OiPr)2 show that the ligand binds to the metal center in a fac-fac coordination mode, like the well-known bis(aminophenolate) ONNO analogue. The iso-propoxide complex (ONO'O)Zr(OiPr)2 proved to be a competent catalyst for L- and rac-lactide polymerization, exhibiting a productivity similar to that of the benchmark (ONNO)Zr(OiPr)2 in solution and in neat monomer. The benzyl complex (ONO'O)ZrBn2 is inactive in ethylene polymerization under the reaction conditions explored, unlike the benchmark (ONNO)ZrBn2. DFT studies suggest that a larger range of geometries is accessible for cationic active species with ONO'O ligands compared to ONNO or OO'O'O ligands.

Titanium and Vanadium Complexes of Tridentate Phenoxy-Imine and Phenoxy-Amine Ligands and Their Application in the Ring-Opening Polymerization of Cyclic Esters.

Phenoxy-imine and phenoxy-amine proligands, with the additional OH donor groups 2,4-tBu2-6-(2-CH2(OH)-C6H4N=CH)C6H3OH (L1H2), 6-(2-CH2(OH)-C6H4N=CH)C6H3OH (L2H2), and 2,4-tBu2-6-(2-CH2(OH)-C6H4NH-CH)C6H3OH (L3H2), were synthesized and their titanium (Ti-L1-Ti-L3) and vanadium (V-L1-V-L2) complexes were prepared in reactions with Ti(OiPr)4 and VO(OiPr)3, respectively. All new compounds were characterized with the use of FTIR, 1H, and 13C NMR spectroscopy; X-ray crystallography was also used to study proligands. All the complexes proved to be active catalysts in the ring-opening polymerization (ROP) of ε-caprolactone, rac-lactide, and L-lactide in the melt. The effects of the complex structure (transition metal type, presence of tBu substituents, and type of nitrogen donor group), as well as the polymerization time and temperature, on the monomer conversion and polymer properties were investigated in detail.

Piano stool iron complexes with isocyanide ligands

Complexes [FeCpI(CO)(CNR)] [2a-f; R = 2,6-C6H3Me2 = Xyl, 4-C6H4OMe, CH2Ph, CH2P(O)(OEt)2, CH2SO2(4-C6H4Me), Me; Cp = η5-C5H5] and [FeCpI(CNR)2] [3a-f; R = Xyl, 4-C6H4OMe, CH2Ph, CH2P(O)(OEt)2, Me] were obtained in 58-73% yields from the thermal reactions of [FeCpI(CO)2] (1) in toluene with one/two equivalents of CNR, respectively, in the presence of [Fe2Cp2(CO)4] as catalyst. The 1:2 molar reaction of 1 with CNMe resulted in the prevalent formation of [FeCp(CO)(CNMe)2]I, 4f (86% yield). The reactions of 1 and 3a-c with an excess of CF3SO3CH3 afforded the triflate adducts [FeCp(OSO2CF3)L2] (L = CO, 1TF; Xyl, 3aTF; 4-C6H4OMe, 3bTF; CH2Ph, 3cTF), in 62-78% yields, via CH3I elimination. All products were characterized by elemental analysis, IR and multinuclear NMR spectroscopy, moreover the molecular structures of 2a, 2e and 3a were ascertained by single crystal X-ray diffraction. The potential of 1, 2a-f and 3a-f as catalysts in the bulk polymerization of rac-lactide was examined.

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.

Ring-Opening Polymerization of rac-Lactide Catalyzed by Octahedral Nickel Carboxylate Complexes

To date, nickel(II) complexes have not been practically investigated as catalysts in ring-opening polymerization (ROP) of lactide to produce biodegradable poly(lactic acid), which is in demand in biomedicine and industry. In this study, carboxylate complexes of nickel(II) containing various N-donor ligands with different nuclearity, metal core rigidity and nature of carboxylate ligands were synthesized and studied by infrared spectroscopy, X-ray diffraction, elemental and thermogravimetric analyses. The obtained complexes were examined in the ROP of the rac-lactide in bulk and in toluene solution with and without the addition of a benzyl alcohol initiator. In the series of complexes studied, the complex [Ni(DBED)2(O2CC(CH3)3)2]·(CH3)3CCO2H (DBED is N,N′-dibenzylethylenediamine) was a syndioselective catalyst and showed the highest catalytic ability in the polymerization without the addition of benzyl alcohol. For this complex, according to 1D DOSY 1H NMR spectroscopy and mass spectrometry with electrospray ionization, polymerization is initiated by a free secondary amine, DBED, leaving the metal’s inner coordination sphere. Based on the experimental data obtained, a comprehensive density functional theory (DFT) study of the ROP pathways including the initiation and first chain growth cycle steps with a detailed description of the intermediates and evaluation of the energy barriers of the steps was carried out. It was shown that one of the key roles in the reaction process is played by carboxylate ligands, which act as proton carriers from the initiator molecule and have a significant influence on the reactivity of the catalytic metal complexes.

Exploring ligand substituent effects on stereoselective polymerization of racemic lactide using aluminium salen-type complexes

A number of unreported aluminum complexes were utilized for the asymmetric kinetic resolution polymerization of rac-lactide.

Flexible Coordination of the Bis(amino-oxazoline) Ligand in Rare-Earth Metal Complexes: Synthesis, Structure, and Their Reactivity and Polymerization Performance.

Mononuclear rare-earth metal alkyl complexes supported by tetradentate dianionic bis(amino-oxazoline) ligands have been synthesized, and their reactivity toward small molecules and catalytic performance on ring-opening polymerization have been studied. Treatment of Ln(CH2SiMe3)3(THF)2 (Ln = Sc, Y; THF = tetrahydrofuran) with the bis(amino-oxazoline) proligand H2L afforded the corresponding rare-earth metal monoalkyl complexes L-Ln(CH2SiMe3)(THF)x (Ln = Sc, x = 0 (1); Ln = Y, x = 1 (2)). The isopropyl-substituted Sc alkyl complex L'-Sc(CH2SiMe3) (3) and the analogue Y silylamide complex L-Y[N(SiHMe2)2] (4) have been prepared by a similar method. Complexes 1 and 2 were stable in solution at room temperature but transformed gradually at elevated temperature to give a nucleophilic addition product for Sc (5) and an oxazoline ring-opened dimeric complex for Y (6). Reactions of 1 with elemental sulfur and selenium each led to insertion of one chalcogen into the Sc-C bond, and the corresponding six-coordinate mononuclear chalcogenolate complexes L-Sc(ECH2SiMe3)(THF) (E = S (7), Se (8)) were isolated. Treatment of 1 with an equimolar amount of aniline yielded the Sc anilide complex L-Sc(NHC6H5) (9), whereas the reaction of 1 with [NHEt3][BPh4] afforded the Sc ion-pair [L-Sc][BPh4] (10), which upon recrystallization led to formation of a THF-solvated product [L-Sc(THF)][BPh4] (11). Single-crystal X-ray diffraction analyses of complexes 1-3, 7-9, and 11 revealed the flexible coordination capability of the tetradentate bis(amino-oxazoline) ligand of upholding a mononuclear metal center via a torsion of the diaminobiphenyl axis. Complexes 1-4 were active catalysts for initiating the ring-opening polymerization of rac-lactide with good activity (TOF up to 3204 h-1) and heteroselectivity (Pr = 0.65-0.71). This study highlights the applicability of the well-defined tetradentate bis(amino-oxazoline) ligands for mononuclear rare-earth metal complexation and shed light on the new potential of rare-earth metal catalysts bearing this type of easily derivatizable polydentate ligand.

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.

Phenoxy-Amidine Ligands: Toward Lactic Acid-Tolerant Catalysts for Lactide Ring-Opening Polymerization

The replacement of the imine functionality in the ubiquitous phenoxy-imine (FI) ligands by a more robust and donor N,N,N′-trisubstituted amidine function was examined and gave rise to the synthesis of five new phenoxy-amidine (FA) ligands (L1–L5). The solid-state structure of four proligands has been determined by X-ray diffraction analysis and showed that the amidine moiety is in a trans-configuration. The reaction of the phenol-amidine proligands with AlMe3 afforded mononuclear (L1–L5)AlMe2 (1a–5a). A similar alkane elimination route was used from ZnEt2 and led to dinuclear [(L1–L5)ZnEt]2 complexes (1b–5b) or to homoleptic (L2/L4)2Zn complexes (2b′, 4b′) depending on the metal/ligand ratio used. The structure of these complexes has been determined by NMR spectroscopy (1H, 13C, HMBC, HSQC, DOSY, and NOESY experiments) and X-ray diffraction study for seven of them. The crystal structure of the Al complexes showed FA ligands coordinated in a chelate fashion via the O atom of the aryloxy group and the imino-N atom, indicating that the amidine function has undergone trans–cis isomerization upon coordination. A similar chelating coordination mode was observed for the FA ligands with Zn metal ions. These complexes were used as initiators for the ring-opening polymerization of rac-lactide. FA–Zn complexes gave the best performance, affording polylactic acid with a narrow molecular weight distribution and heterotactic bias (Pr up to 0.75). Remarkably, some of these complexes were able to tolerate the presence of a large amount of lactic acid to the point of using it as a co-initiator during the polymerization reaction.

Mono(imidazolin-2-iminato) Hafnium Complexes: Synthesis and Application in the Ring-Opening Polymerization of ε-Caprolactone and rac-Lactide

Mono-substituted imidazolinX-2-iminato hafnium(IV) complexes (X = iPr, tBu, Mesityl, Dipp) were synthesized and fully characterized, including solid-state X-ray diffraction analysis. When the X group is small (iPr), a dimeric structure is obtained. In all the monomeric complexes, the Hf-N bond can be regarded as a double bond with similar electronic properties. The main difference among the monomeric complexes is the cone angle of the ligand, which induces varying steric hindrances around the metal center. When the monomeric complex of mono(bis(diisopropylphenyl)imidazolin-2-iminato) hafnium tribenzyl was reacted with three equivalents (equiv) of iPrOH, the benzyl groups were easily replaced, forming the corresponding tri-isopropoxide complex. However, when BnOH was used, dimeric complexes were obtained. When five equivalents of the corresponding alcohols (BnOH, iPrOH) were reacted with the monomeric complex, different dimeric complexes were obtained. Regardless of the high oxophilicity of the hafnium complexes, all complexes were active catalysts for the ring-opening polymerization (ROP) of ε-caprolactone. Dimeric complexes 5 and 6 were found to be the most active catalysts, enabling polymerization to occur in a living, immortal fashion, as well as the copolymerization of ε-caprolactone with rac-lactide, producing block copolymer PCL-b-LAC. The introduction of imidazolin-2-iminato ligands enables the tailoring of the oxophilicity of the complexes, allowing them to be active in catalytic processes with oxygen-containing substrates.

Isoselective mechanism for asymmetric kinetic resolution polymerization of rac-lactide catalyzed by chiral tridentate bis(oxazolinylphenyl)amido ligand supported zinc complexes

Chiral tridentate bis(oxazolinylphenyl)amido ligand supported zinc complexes were synthesized and utilized for the asymmetric kinetic resolution polymerization of rac-lactide. These polymerizations catalyzed by chiral catalysts were performed efficiently, resulting in isotactic polylactide with stereogradient microstructure and narrow dispersity. The asymmetric kinetic resolution polymerization and block polymerization experiments indicated that the combination of enantiomorphic site control and chain-end control should be responsible for the stereocontrol mechanism during the polymerization process.

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.

Investigating the Ring-Opening Polymerization Activity of Niobium and Tantalum Ethoxides Supported by Phenoxyimine Ligands.

A variety of metal catalysts from around the periodic table have been studied for the ring-opening polymerization (ROP) of cyclic esters. Within this field, group V catalysts have been rarely explored. To better understand the effect the choice of metal and ligand has on ROP activity, a series of 10 niobium and tantalum alkoxide catalysts, supported by a range of phenoxyimine ligands, were synthesized. The electronics and steric bulk of the ligands were varied on the phenoxy group (tBu, Cl, and OMe) and the imine group (Ph; 2,6-diMePh; 2,6-diiPrPh; and 2,4,6-tritBuPh) to probe their effect on the catalyst structure and activity. Catalysts were characterized with 1D, 2D, and variable-temperature NMR techniques to determine their structure in solution. Single crystal X-ray diffraction studies were conducted to establish their solid-state structure. The 10 catalysts are pseudo-octahedral, and each shows ligand coordination through phenoxy-oxygen and imine-nitrogen (O,N). In the case of the o-vanillin ligand set, however, evidence was found for O,O-coordination of the ligand when the steric encumbrance of the imine-nitrogen was increased. Each catalyst was active for the ring-opening polymerization of both rac-lactide (LA) and ε-caprolactone (CL) in the absence of solvent at 140 °C. In the case of CL, the catalysts supported by chloro-containing ligands showed the most polymerization control based on final polymer molecular weight and dispersity. Ligand trends were less clear for the polymerization of LA, though in all cases the catalysts were more controlled than the parent homoleptic alkoxide [M(OEt)5; M = Nb or Ta]. The most promising catalyst in the family was tested for copolymerization activity of LA and CL in one pot. Copolymerization of the two monomers was successful and yielded random poly(caprolactone-co-lactide).

Template Synthesis of NPN' Pincer-type Ligands at Titanium Using an Ambiphilic Phosphide Scaffold.

Ti-imido complex [TiCl(NtBu)(BIPP)] [1; BIPP = bis(iminophosphoranyl)phosphide ligand] reacts with terminal alkynes R-C≡CH (R = phenyl, isopropenyl, cyclopropyl, and 2-pyridyl) via P-P bond cleavage of the BIPP ligand. The resulting complexes [TiCl(NPN')(NPhPPh2)] (2a-d) contain a pincer-type NPN' phosphide ligand that incorporates the terminal alkyne and the imido ligand from complex 1. Complexes 2a-d feature two chiral centers (Ti and P) with interdependent absolute configurations; thus, they are formed stereoselectively. Complex 2a (R = phenyl) undergoes chloride abstraction with [Et3SiHSiEt3][B(C6F5)4], yielding [Ti(NPN')(NPhPPh2)][B(C6F5)4] (3). Complex 3 is a moderately active and stereoselective initiator for the ring-opening polymerization of rac-lactide. Complex 3 activates the C═O bond of 4-iodobenzaldehyde to give complex 4 as a single diastereomer despite the presence of three chiral centers. Complex 3 undergoes transmetallation with SbCl3, yielding [Sb(NPN')][B(C6F5)4] (5) and [TiCl3(NPhPPh2)] (6) selectively. The bonding situation in 3 and 5 was analyzed using Bader's atoms in molecules and the electron localization function, showing that the nitrogen atoms of the NPN' ligand are electronically similar, and that the metal-phosphide interaction is more polar in the case of titanium.

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].

Synthesis and characterisation of an enantiomerically pure scandium pentadienyl complex and its application in the polymerisation of rac-lactide

The alkyl-functionalised scandium complex [(pdl*SiMe2NtBu)Sc(thf)(CH2SiMe3)] (2) was synthesised in enantiomerically pure form and characterised by NMR spectroscopy and X-ray diffraction analysis. Complex 2 features a chiral constrained geometry ligand derived from the natural compound (1R)-(-)-myrtenal, in which the pentadienyl (pdl*) fragment coordinates in η3:η2-allyl-en fashion to the scandium atom. Compound 2 catalyses the polymerisation of rac-lactide at 30 °C and 50 °C yielding amorphous poly(lactide) with slightly heterotactic enchainment (Pm = 0.36 and 0.37). In agreement with the data obtained from GPC and DSC measurements, a chain-end control mechanism is proposed with fast chain propagation relative to the initiation, which leads to broad molecular weight distributions (Đ ≈ 1.80) and higher than expected molecular weights. Furthermore, chain transfer processes are observed, but only small amounts of transesterification and racemisation occur. Kinetic studies reveal a second-order dependence in rac-lactide (monomer) concentration and a first-order dependence in the concentration of catalyst 2.