Mechanistic Investigations of the Stereoselective Rare Earth Metal-Mediated Ring-Opening Polymerization of β-Butyrolactone

Aluminium complexes of phenoxy-azo ligands in the catalysis of rac-lactide polymerisation

Fourteen new phenoxy-azo aluminium complexes comprising two series, namely, dimethyl{phenoxy-azo}aluminium complexes 1a-7a and monomethyl{phenoxy-azo}aluminium complexes 1b-7b, were successfully synthesised and characterised. The molecular structure of complex 4a, determined using X-ray diffraction analysis, displayed a distorted tetrahedral geometry. The 1H NMR spectrum of complex 5b revealed fluxional behaviour caused by isomeric transformation that occurs in the solution at room temperature. The activation parameters determined by lineshape analysis of variable-temperature 1H NMR spectra in toluene-d8 are as follows: ΔH‡ = 70.05 ± 1.19 kJ mol-1, ΔS‡ = 21.78 ± 3.58 J mol-1 K-1 and ΔG‡ (298 K) = 63.56 ± 0.11 kJ mol-1. All aluminium complexes are active initiators for the ring-opening polymerisation of rac-lactide, and the polymerisations proceeded in a controlled manner and were living. In comparison, the catalytic activity of the dimethyl{phenoxy-azo}aluminium complexes was insignificantly different from that of the corresponding monomethyl{phenoxy-azo}aluminium complexes. The steric factor of the ortho-phenoxy substituent was observed to exert a decelerating effect on the catalytic rate. Kinetic investigations revealed first-order dependency on both monomer and initiator concentrations. Comparative catalytic investigations conducted on phenoxy-azo aluminium and phenoxy-imine aluminium complexes revealed that the former complexes exhibited lower catalytic activity.

Merging σ-Bond Metathesis with Polymerization Catalysis: Insights into Rare-Earth Metal Complexes, End-Group Functionalization, and Application Prospects

Polymers with well-defined structures, synthesized through metal-catalyzed processes, and having end groups exhibiting different polarity and reactivity than the backbone, are gaining considerable attention in both scientific and industrial communities. These polymers show potential applications as fundamental building blocks and additives in the creation of innovative functional materials. Investigations are directed toward identifying the most optimal and uncomplicated synthetic approach by employing a combination of living coordination polymerization mediated by rare-earth metal complexes and C-H bond activation reaction by σ-bond metathesis. This combination directly yields catalysts with diverse functional groups from a single precursor, enabling the production of terminal-functionalized polymers without the need for sequential reactions, such as termination reactions. The utilization of this innovative methodology allows for precise control over end-group functionalities, providing a versatile approach to tailor the properties and applications of the resulting polymers. This perspective discusses the principles, challenges, and potential advancements associated with this synthetic strategy, highlighting its significance in advancing the interface of metalorganic chemistry, polymer chemistry, and materials science.

Highly Isoselective Ring-Opening Polymerization of rac-β-Butyrolactone: Access to Synthetic Poly(3-hydroxybutyrate) with Polyolefin-like Material Properties

We report the highly isoselective ring-opening polymerization (ROP) of racemic β-butyrolactone (β-BL) using in situ-generated catalysts based on Y[N(SiHMe₂)₂]₃(THF)₂ and salan-type pro-ligands. The catalyst system produces isotactic poly(3-hydroxybutyrate) (PHB) with record productivity (TOF up to 32 000 h^-1) and the highest isoselectivity (P_m up to 0.89) in ROP of β-BL achieved to date. In contrast to bacterial PHB, the chemically synthesized PHB has beneficial material properties, such as increased melt processing window attributed to a lowered melting temperature (T_m ≈ 140 °C) and drastically reduced brittleness. The produced PHB showed an elongation at break of 392%, thus demonstrating promising polyolefin-like thermomechanical material properties.

Synthesis of N-Methyl-o-phenylenediamine-Bridged Bis(phenolato) Lanthanide Alkoxides and Their Catalytic Performance for the (Co)Polymerization of rac-Butyrolactone and l-Lactide

A series of lanthanide alkoxo complexes supported by ONNO salalen ligands were synthesized and characterized. A one-pot reaction of LH₂ (L = (2-O-C₆H₂-^tBu₂-3,5)CH═N-C₆H₄-N(CH₃)CH₂(2-O-C₆H₂-^tBu₂-3,5)) with LnCp₃(THF) in a 1:1 molar ratio followed by the addition of 1 equiv of ROH (R = Bn, ⁱPr, and CF₃CH₂), afforded the dimeric lanthanide alkoxo complexes [LLn(μ-OCH₂Ph)]₂ [Ln = Lu (1), Yb (2), Sm (3), Nd (4)], [L²Yb(μ-OⁱPr)]₂ (5), and [L²Yb(μ-OCH₂CF₃)]₂ (6) in good isolated yields. All these lanthanide complexes were characterized by elemental analysis and FT-IR spectroscopy. In addition, complex 1 has been characterized by NMR spectroscopy. Single-crystal X-ray diffraction analysis of complexes 1, 2, 5, and 6 showed that these lanthanide alkoxo complexes are dimeric in the solid state. Complexes 1-6 showed good activity toward the homopolymerization of rac-butyrolactone (rac-BBL) to give atactic PHB, and ionic radii of central metals have profound influence on the polymerization. The polymerization behavior of l-lactide (l-LA) initiated by complex 2 was also explored. The kinetic study revealed that the polymerizations of rac-BBL and l-LA initiated by salalen lanthanide akoxide are first order for both the monomer and the initiator concentrations. Furthermore, it was found that complexes 1 and 2 showed good activity in the copolymerization of l-LA and rac-BBL, affording gradient copolymers.

Low-Symmetry β-Diketimine Aryloxide Rare-Earth Complexes: Flexible, Reactive, and Selective

We report the synthesis, characterization, and reactivity of a new low-symmetry β-diketimine featuring a pendant amino(methyl)phenol donor and its corresponding heteroleptic rare-earth (RE) complexes. This includes the first structurally characterized examples of alcoholysis and insertion from an isolated RE^III amide in a β-diketimine framework. The flexible methylene linkage leads to RE^III complexes with tunable dynamic solution behavior that defines their stoichiometric and catalytic reactivity. The addition of a strong neutral donor ligand, tricyclohexylphosphine oxide, suppresses a prevalent catalyst degradation pathway (base-promoted elimination) and dramatically enhances the catalyst performance in the stereospecific ring-opening polymerization of rac-β-butyrolactone. Our results further demonstrate the importance of ligand reorganization in the stoichiometric and catalytic activity of RE^III ions.

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.

Biodegradable Polyhydroxyalkanoates by Stereoselective Copolymerization of Racemic Diolides: Stereocontrol and Polyolefin-Like Properties

Bacterial polyhydroxyalkanoates (PHAs) are a unique class of biodegradable polymers because of their biodegradability in ambient environments and structural diversity enabled by side-chain groups. However, the biosynthesis of PHAs is slow and expensive, limiting their broader applications as commodity plastics. To overcome such limitation, the catalyzed chemical synthesis of bacterial PHAs has been developed, using the metal-catalyzed stereoselective ring-opening (co)polymerization of racemic cyclic diolides (rac-8DL^R , R=alkyl group). In this combined experimental and computational study, polymerization kinetics, stereocontrol, copolymerization characteristics, and the properties of the resulting PHAs have been examined. Most notably, stereoselective copolymerizations of rac-8DL^Me with rac-8DL^R (R=Et, Bu) have yielded high-molecular-weight, crystalline isotactic PHA copolymers that are hard, ductile, and tough plastics, and exhibit polyolefin-like thermal and mechanical properties.

(Co)polymerization of (−)-menthide and β-butyrolactone with yttrium-bis(phenolates): tuning material properties of sustainable polyesters

Sustainable thermoplastic elastomers derived from block copolymers of syndiotactic poly(3-hydroxybutyrate) and poly((−)-menthide) were synthesized via yttrium-mediated ring-opening polymerization.

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.

Yttrium-Mediated Ring-Opening Copolymerization of Oppositely Configurated 4-Alkoxymethylene-β-Propiolactones: Effective Access to Highly Alternated Isotactic Functional PHAs

The ring-opening copolymerization (ROCOP) of functional 4-alkoxymethylene-β-propiolactones (BPL^OR s) by yttrium-bisphenolate complexes was investigated. The ROCOP of equimolar mixtures of BPL^OR s of opposite configurations, namely (R)-BPL^OR1 /(S)-BPL^OR2 [R¹ , R² =OMe, OAllyl, OCH₂ Ph (=OBn), OSiMe₂ tBu (=OTBDMS)], by the syndioselective Y{ONOO^cum }/iPrOH catalyst/initiator system affords P(HB^OR1 -alt-HB^OR2 ) copolymers with high alternation degrees (altern.=89-94 %), as determined by comprehensive kinetic, ¹³ C{¹ H} NMR spectroscopy, MALDI-ToF MS and ESI MS/MS fragmentation studies. The ROCOP of the (R)-BPL^OMe /(S)-BPL^OTBDMS combination, featuring a large difference in the substituents' bulkiness, constitutes the only observed exception to this trend (altern.=64 %). On the other hand, the isoselectivity of the Y{ONNO^Cl }/iPrOH catalyst/initiator system has been exploited to generate, in a one-pot/one-step procedure, original mixtures of isotactic poly(hydroxyalkanoate)s (PHAs). This system efficiently transforms equimolar mixtures of (R)-BPL^OAll /(S)-BPL^OMe into a 1:1 mixture of the corresponding isotactic iso-(R)-PHB^OAll and iso-(S)-PHB^OMe homopolymers; almost no copolymerization defects are observed. This new approach has been extended successfully to the ROCOP of equimolar mixtures of racemic monomers, rac-BPL^OAll /rac-BPL^OMe .

Rare-earth/zinc heterometallic complexes containing both alkoxy-amino-bis(phenolato) and chiral salen ligands: synthesis and catalytic application for copolymerization of CO2 with cyclohexene oxide

Ten rare-earth/zinc heterometallic complexes were synthesized and their catalytic application for copolymerization of CO₂ with cyclohexene oxide was explored.

Steric vs. electronic stereocontrol in syndio- or iso-selective ROP of functional chiral β-lactones mediated by achiral yttrium-bisphenolate complexes

Electronic vs. steric effects of ligand ortho-substituents in (bis)phenolate yttrium-based complexes for preparation of a large scope of stereoregular poly(3-hydroxyalkanoate)s.

Highly Syndiotactic or Isotactic Polyhydroxyalkanoates by Ligand-Controlled Yttrium-Catalyzed Stereoselective Ring-Opening Polymerization of Functional Racemic β-Lactones

Reported herein is the first stereoselective controlled ROP of a specific family of racemic functional β-lactones, namely 4-alkoxymethylene-β-propiolactones (BPL^OR s). This process is catalyzed by an yttrium complex stabilized by a nonchiral tetradentate amino alkoxy bisphenolate ligand {ONOO^R'2 }^2- , which features both a good activity and a high degree of control over the molar masses of the resulting functional poly(3-hydroxyalkanoate)s. A simple modification of the R' substituents in ortho and para position on the ligand platform allows for a complete reversal from virtually pure syndioselectivity (P_s up to 0.91 with R'=cumyl) to very high isoselectivity (P_i up to 0.93 with R'=Cl), as supported by DFT insights. This is the first example of a highly isoselective ROP of a racemic chiral β-lactone.

Effects of nucleophilic ligands on the chain initiation efficiency of polar monomer polymerizations catalyzed by 2-methoxyethylaminobis(phenolate)yttrium complexes: a DFT study

DFT calculations suggest that less steric hindrance and a good orbital match induced by nucleophilic ligands are beneficial to the chain initiation of ROP and GTP, respectively.

Highly Stereocontrolled Ring-Opening Polymerization of Racemic Alkyl β-Malolactonates Mediated by Yttrium [Amino-alkoxy-bis(phenolate)] Complexes

Yttrium [amino-alkoxy-bis(phenolate)]amido complexes have been used for the ring-opening polymerization (ROP) of racemic alkyl β-malolactonates (4-alkoxycarbonyl-2-oxetanones, rac-MLA(R) s) bearing an allyl (All), benzyl (Bz) or methyl (Me) lateral ester function. The nature of the ortho-substituent on the phenolate rings in the metal ancillary dictated the stereocontrol of the ROP, and consequently the syndiotactic enrichment of the resulting polyesters. ROP promoted by catalysts with halogen (Cl, Br)-disubstituted ligands allowed the first reported synthesis of highly syndiotactic PMLA(R) s (Pr ≥ 0.95); conversely, catalysts bearing bulky alkyl and aryl ortho-substituted ligands proved largely ineffective. All polymers have been characterized by (1) H and (13) C{(1) H} NMR spectroscopy, MALDI-ToF mass spectrometry and DSC analyses. Statistical and thermal analyses enabled the rationalization of the chain-end control mechanism. Whereas the stereocontrol of the polymerization obeyed a Markov first-order (Mk1) model for the ROP of rac-MLA(Bz) and rac-MLA(All) , the ROP of rac-MLA(Me) led to a chain end-control of Markov second-order type (Mk2). DFT computations suggest that the high stereocontrol ability featured by catalysts bearing Cl- and Br-substituted ligands does not likely originate from halogen bonding between the halogen substituent and the growing polyester chain.

Mechanistic Studies of ε-Caprolactone Polymerization by (salen)AlOR Complexes and a Predictive Model for Cyclic Ester Polymerizations

Aluminum alkoxide complexes (2) of salen ligands with a three-carbon linker and para substituents having variable electron-withdrawing capabilities (X = NO₂, Br, OMe) were prepared, and the kinetics of their ring-opening polymerization (ROP) of ε-caprolactone (CL) were investigated as a function of temperature, with the aim of drawing comparisons to similar systems with two-carbon linkers investigated previously (1). While 1 and 2 exhibit saturation kinetics and similar dependences of their ROP rates on substituents X (invariant K_eq, similar Hammett ρ = +1.4(1) and 1.2(1) for k₂, respectively), ROP by 2 was significantly faster than for 1. Theoretical calculations confirm that, while the reactant structures differ, the transition state geometries are quite similar, and by analyzing the energetics of the involved distortions accompanying the structural changes, a significant contribution to the basis for the rate differences was identified. Using this knowledge, a simplified computational method for evaluating ligand structural influences on cyclic ester ROP rates is proposed that may have utility for future catalyst design.

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.