Improvement of catalytic activity of aluminum complexes for the ring-opening polymerization of ε-caprolactone: aluminum thioamidate and thioureidate systems

In this study, a series of Al complexes bearing amidates, thioamidates, ureidates, and thioureidates were synthesized and their catalytic activity for ε-caprolactone (CL) polymerization was evaluated. SPr-Al exhibited a higher catalytic activity than OPr-Al (3.2 times as high for CL polymerization; [CL] : [SPr-Al] : [BnOH] = 100 : 0.5 : 2; [SPr-Al] = 10 mM, conv. = 93% after 14 min at 25 °C), and USCl-Al exhibited a higher catalytic activity than UCl-Al (4.6 times as high for CL polymerization; [CL] : [USCl-Al] : [BnOH] = 100 : 0.5 : 2; [USCl-Al] = 10 mM, conv. = 90% after 15 min at 25 °C). Regardless of whether aluminum amidates or ureidates were present, thioligands improved the polymerization rate of aluminum catalysts. Density functional theory calculations revealed that the eight-membered ring [SPr-AlOMe2]2 decomposed into the four-membered ring SPr-AlOMe2. However, [OPr-AlOMe2]2 did not decompose because of its strong bridging Al-O bond. The overall activation energy required for CL polymerization was lower when using [SPr-AlOMe2]2 (18.1 kcal mol-1) as a catalyst than when using [OPr-AlOMe2]2 (23.9 kcal mol-1). This is because the TS2a transition state of SPr-AlOMe2 had a more open coordination geometry with a small N-Al-S angle (72.91°) than did TS3c of [OPr-AlOMe2]2, the crowded highest-energy transition state of [OPr-AlOMe2]2 with a larger N-Al-O angle (99.63°).

Controlled and effective ring-opening (co)polymerization of rac-lactide, ε-caprolactone and ε-decalactone by β-pyrimidyl enolate aluminum complexes

A series of β-pyrimidyl enolate aluminum complexes (1–6) were found to promote controlled and living ROP of rac-LA, ε-CL, and ε-DL. Six well-defined diblock copolymers and the perfect random copolymer poly(l-LA-r-CL) were successfully synthesized.

Aluminium complexes containing indolyl-phenolate ligands as catalysts for ring-opening polymerization of cyclic esters

A family of aluminium complexes supported by mono-anionic indolyl-phenolate ligands are described. Reactions of indolyl-phenolate based ligand precursors, IndHPh^ROH, with 1.0 or 0.5 equivalents of AlMe₂Cl in toluene afforded aluminium indolyl-phenolate complexes 1-4 and aluminium bis-indolyl-phenolate complexes 5-8 respectively. The molecular structure is reported for 5. Based on the NMR spectroscopic and X-ray crystallographic studies, a 1,3-hydrogen shift could happen from nitrogen to carbon on the five-membered ring of the indolyl group upon reacting with aluminium reagents. These novel aluminium complexes demonstrate catalytic activities toward the ring-opening polymerization of cyclic esters in the presence of alcohol.

Dinuclear Nickel Complexes Using Hexadentate Benzothiazole-Based Diamine-Bisphenolate Ligands: Highly Active Catalysts for Copolymerization of Carbon Dioxide with Epoxides

We reported for the first time the utilization of hexadentate benzothiazole-based diamine-bisphenolate ligands to synthesize structurally well-characterized dinickel dicarboxylate complexes and studied their catalysis for copolymerization of carbon dioxide with epoxides. Dinickel carboxylate complexes having a 1,3-diamine-bridged backbone were demonstrated to be high-performance catalysts for alternating copolymerization of CO₂ and cyclohexene oxide (CHO) with high product selectivity. Particularly, acetate-supported nickel complex 2 enabled us to promote such CO₂-copolymerization of this kind with a maximum turnover frequency of up to 2600 h^-1 and gave good molecular weight controllability under high-pressure conditions. It is worth noting that bimetallic Ni catalyst 2 was also capable of mediating the catalytic CO₂-polymerization of alicyclic epoxides at atmospheric pressure. Kinetic investigations of CO₂/CHO copolymerization by 2 allowed us to determine the rate equation of -d[CHO]/dt = k_p[2]¹[CHO]¹, and such catalysis exhibited a first-order dependence on both dinickel complex and CHO concentrations.

Titanium Complexes of Salicylbenzoxazole and Salicylbenzothiazole Ligands for the Ring-Opening Polymerization of ε-Caprolactone and Substituted ε-Caprolactones and Their Copolymerizations

Two series of titanium complexes, including salicylbenzoxazole titanium complexes (1-4) and salicylbenzothiazole titanium complexes (5-8), were successfully synthesized and characterized by NMR spectroscopy, elemental analysis, and X-ray diffraction crystallography (for 2 and 5). The ¹H NMR spectra of complexes 7 and 8 reveal fluxional behavior in solution at room temperature, and the activation parameters were determined by lineshape analysis of variable-temperature (VT) NMR spectra in toluene-d₈: for 7, ΔH^⧧ = 73.0 ± 1.8 kJ mol^-1, ΔS^⧧ = 22.1 ± 5.5 J mol^-1 K^-1; for 8, ΔH^⧧ = 73.7 ± 1.2 kJ mol^-1, ΔS^⧧ = 20.3 ± 3.8 J mol^-1 K^-1. The positive values of ΔS^⧧ suggested that the isomerization occurred via a dissociative mechanism. All complexes were active initiators for the ring-opening polymerization of ε-caprolactone (ε-CL) and three substituted ε-CLs: γ-methyl-ε-caprolactone (γMeCL), γ-ethyl-ε-caprolactone (γEtCL), and γ-phenyl-ε-caprolactone (γPhCL). Of all complexes, complex 5 was found to be the most active initiator in this study. The copolymerizations between ε-CL and three substituted ε-CLs produced completely random copolymers. The polymerization was proposed to proceed via a dissociative coordination-insertion mechanism. The catalytic activity of the salicylbenzoxazole titanium complex was lower than that of its closely related salicylbenzothiazole titanium congener. Additionally, DFT calculations unveiled that the ligand decoordination step and the less steric congestion at the titanium center in the salicylbenzothiazole titanium complexes were the key factors in enhancing the catalytic rate.

Controlling polymer stereochemistry in ring-opening polymerization: a decade of advances shaping the future of biodegradable polyesters

This review highlights recent developments in the field of biodegradable polymeric materials intended to replace non-degradable conventional plastics, focusing on studies from the last ten years involving the stereoselective ring-opening polymerization of cyclic esters. This encompasses exciting advances in both catalyst design and monomer scope. Notably, the last decade has seen the emergence of metal-free stereocontrolled ROP for instance, as well as the synthesis and stereocontrolled polymerization of new types of chiral monomers. This study will emphasize recent stereoselective polymerization catalysts and chiral monomers and will focus on stereocontrol quantification, the mechanisms of stereocontrol and their differentiation if reported and studied for a specific catalyst system.

Titanium complexes of pyrrolylaldiminate ligands and their exploitation for the ring-opening polymerization of cyclic esters

A series of six-coordinate titanium complexes 1-6 supported by pyrrolylaldiminate ligands were prepared via the reaction of 2 equivalents of ligands and Ti(OⁱPr)₄ in toluene at 70 °C. The X-ray structure of 2 revealed that the two ligands were κ²-coordinated to the titanium center with the two pyrrole nitrogen atoms in trans positions and the two imine nitrogen atoms in cis positions. All complexes were active initiators for the ring-opening polymerization (ROP) of rac-lactide (rac-LA), ε-caprolactone (ε-CL), and three substituted ε-caprolactones (γ-methyl-ε-caprolactone (γMeCL), γ-ethyl-ε-caprolactone (γEtCL), and γ-phenyl-ε-caprolactone (γPhCL)). Polymerizations of all monomers were well controlled, affording predetermined molar masses and narrow dispersity values. Complex 5 exhibited the highest polymerization activities with rac-LA and ε-CL and its performance was comparable to other highly active six-coordinate titanium complexes reported thus far. Kinetic results revealed a first-order dependency on the monomer concentration, and the rate of polymerization was greatly influenced by the substituent on the imine nitrogen. End-group analysis of the isolated PLA and PCL suggested a coordination-insertion mechanism.

NSSN-Type Group 4 Metal Complexes in the Ring-Opening Polymerization of l-Lactide

A new class of zirconium and hafnium complexes coordinated by linear dianonic tetradentate NSSN ligands is reported. The ligands feature two amide functions coupled with two thioether groups linked by a central flexible ethane bridge and two lateral rigid phenylene bridges and differ for the substituents on the aniline nitrogen atoms, i.e., isopropyl, cyclohexyl, or mesityl substituents: NSSN-iPr, NSSN-Cy, or NSSN-Mes. They were prepared by reacting 2-aminothiophenol with dibromoethane to afford the NSSN ligands without substituents on the aniline nitrogen atoms, which were subsequently alkylated through a reductive amination of acetone or cyclohexanone or palladium-catalyzed cross-coupling reaction with mesityl bromide. The corresponding zirconium and hafnium complexes 1-5 were obtained through a transamination reaction between the neutral ligands and Zr(NMe₂)₄ or Hf(NMe₂)₄ [(NSSN-iPr)Zr(NMe₂)₂ (1), (NSSN-Cy)Zr(NMe₂)₂ (2), (NSSN-Mes)Zr(NMe₂)₂ (3), (NSSN-iPr)Hf(NMe₂)₂ (4), and (NSSN-Cy)Hf(NMe₂)₂ (5)]. They were characterized in solution by NMR spectroscopy and in solid state by X-ray diffraction analysis (except for 3). All complexes present an octahedral coordination geometry with a fac-fac ligand wrapping and a cis relationship between the other two monodentate ligands. The catalytic performances of 1-5 in the ring-opening polymerization of cyclic esters were investigated. Complex 1 was the most active: its polymerization activity was superior to those generally displayed by zirconium complexes featuring OSSO ligands and compared well with those of the most active group 4 complexes operating in a toluene solution.

Dinuclear aluminum complexes bearing methylene-bridged phenoxy-imine ligands and their application in the ring-opening polymerization of rac-lactide

The first report on the use of dinuclear aluminum complexes supported by methylene-bridged phenoxy-imine ligands for the ring-opening polymerization of rac-lactide.

Coordination Ring-Opening Polymerization of Cyclic Esters: A Critical Overview of DFT Modeling and Visualization of the Reaction Mechanisms

Ring-opening polymerization (ROP) of cyclic esters (lactones, lactides, cyclic carbonates and phosphates) is an effective tool to synthesize biocompatible and biodegradable polymers. Metal complexes effectively catalyze ROP, a remarkable diversity of the ROP mechanisms prompted the use of density functional theory (DFT) methods for simulation and visualization of the ROP pathways. Optimization of the molecular structures of the key reaction intermediates and transition states has allowed to explain the values of catalytic activities and stereocontrol events. DFT computation data sets might be viewed as a sound basis for the design of novel ROP catalysts and cyclic substrates, for the creation of new types of homo- and copolymers with promising properties. In this review, we summarized the results of DFT modeling of coordination ROP of cyclic esters. The importance to understand the difference between initiation and propagation stages, to consider the possibility of polymer-catalyst coordination, to figure out the key transition states, and other aspects of DFT simulation and visualization of ROP have been also discussed in our review.

Mechanistic Insight into the Ring-Opening Polymerization of ε-Caprolactone and L-Lactide Using Ketiminate-Ligated Aluminum Catalysts

The reactivity and the reaction conditions of the ring-opening polymerization of ε-caprolactone (ε-CL) and L-lactide (LA) initiated by aluminum ketiminate complexes have been shown differently. Herein, we account for the observation by studying the mechanisms on the basis of density functional theory (DFT) calculations. The calculations show that the ring-opening polymerization of ε-CL and LA are rate-determined by the benzoxide insertion and the C–O bond cleavage step, respectively. Theoretical computations suggest that the reaction temperature of L–LA polymerization should be higher than that of ε-CL one, in agreement with the experimental data. To provide a reasonable interpretation of the experimental results and to give an insight into the catalyst design, the influence of the electronic, steric, and thermal effects on the polymerization behaviors will be also discussed in this study.

Four- and five-coordinate aluminum complexes supported by N,O-bidentate β-pyrazylenolate ligands: synthesis, structure and application in ROP of ε-caprolactone and lactide

Two series of alkyl aluminum complexes LAlMe2 and L2AlMe supported by N,O-bidentate β-pyrazylenolate ligands have been synthesized and applied to the ROP of ε-CL and rac-LA.

Organo-Aluminum and Zinc Acetamidinates: Preparation, Coordination Ability, and Ring-Opening Polymerization Processes of Cyclic Esters

The reaction of the highly sterically demanding NNN'-heteroscorpionate protioligands pbp^tamd-H, tbp^tamd-H, and phbp^tamd-H (a) and the low sterically hindered analogs pbpamd-H, tbpamd-H, and phbpamd-H (b), with 1 equiv of AlR₃ (R = Me, Et) proceed in high yields to give two families of complexes: the mononuclear dialkyl aluminum bidentate-acetamidinates [AlR₂(κ²- N' N')] (κ²- N' N' = pbp^tamd, R = Me 1, Et 2; tbp^tamd, R = Me 3, Et 4; phbp^tamd, R = Me 5, Et 6) and the monodentate-acetamidinates [AlR₂(κ²- NN')] (κ²- NN' = tbpamd, R = Me 7; phbpamd, R = Me 8, Et 9). In complexes 7-9, the presence of two possible CH-NH tautomers as low extended π-N-C-N'(sp²)-Al and high extended π-HN-C₂-N'(sp²)-Al complexes, respectively, could be identified. Moreover, the reaction of aluminum dimethyls 7 and 8 with ZnMe₂ afforded the isolation of the more stable scorpionate zinc monoalkyls [Zn(Me)(κ³- NNN')] ( NNN' = tbpamd 10 and phbpamd 11), through a very unusual ligand exchange process, involving a zinc-to-aluminum transmetalation of an alkyl group. The X-ray crystal structures of 1, 3, 7, and 8, as well as that of 11, confirmed unambiguously the different κ²-arrangements proposed for bi- or monodentate acetamidinate dialkyls 1-6 and 7-9, respectively, the presence of NH tautomer in 7 and 8, and a κ³- NNN' coordination in monoalkyl 11. Density functional theory calculations were used to explore the three different favored κ²-arrangements found in acetamidinate aluminum dialkyls 1-9, the relative stability of both CH-NH tautomers, and the ligand transfer reaction leading to the formation of κ³- NNN' zinc monoalkyls 10 and 11. Interestingly, dialkyls 1, 5, 7, and 8 can act as highly efficient single-component living initiators for the ring-opening polymerization of ε-caprolactone and rac-lactide in mild conditions after hours. These initiators efficiently mediated the immortal polymerization in the presence of excess of benzyl alcohol (up to 20 equiv), as evidenced by the narrow dispersity values and the good agreement between the experimental M_n values and monomer/benzyl alcohol ratios. In addition, the most sterically hindered initiator, 5, exhibits enhanced levels of heteroselectivity on the produced PLAs, reaching P_s values up to 0.70.