Gradient Isotactic Multiblock Polylactides from Aluminum Complexes of Chiral Salalen Ligands

Recent advances in enantioselective ring-opening polymerization and copolymerization

Precisely controlling macromolecular stereochemistry and sequences is a powerful strategy for manipulating polymer properties. Controlled synthetic routes to prepare degradable polyester, polycarbonate, and polyether are of recent interest due to the need for sustainable materials as alternatives to petrochemical-based polyolefins. Enantioselective ring-opening polymerization and ring-opening copolymerization of racemic monomers offer access to stereoregular polymers, specifically enantiopure polymers that form stereocomplexes with improved physicochemical and mechanical properties. Here, we highlight the state-of-the-art of this polymerization chemistry that can produce microstructure-defined polymers. In particular, the structures and performances of various homogeneous enantioselective catalysts are presented. Trends and future challenges of such chemistry are discussed.

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 L^1-6H (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 ([L^1-6H]:[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 (P_m = 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.

Bayesian-optimization-assisted discovery of stereoselective aluminum complexes for ring-opening polymerization of racemic lactide

Stereoselective ring-opening polymerization catalysts are used to produce degradable stereoregular poly(lactic acids) with thermal and mechanical properties that are superior to those of atactic polymers. However, the process of discovering highly stereoselective catalysts is still largely empirical. We aim to develop an integrated computational and experimental framework for efficient, predictive catalyst selection and optimization. As a proof of principle, we have developed a Bayesian optimization workflow on a subset of literature results for stereoselective lactide ring-opening polymerization, and using the algorithm, we identify multiple new Al complexes that catalyze either isoselective or heteroselective polymerization. In addition, feature attribution analysis uncovers mechanistically meaningful ligand descriptors, such as percent buried volume (%V_bur) and the highest occupied molecular orbital energy (E_HOMO), that can access quantitative and predictive models for catalyst development.

Salen-like Chromium and Aluminum Complexes as Catalysts in the Copolymerization of Epoxides with Cyclic Anhydrides for the Synthesis of Polyesters

Chromium and aluminum complexes bearing salalen ligands were explored as catalysts for the ring-opening copolymerization (ROCOP) of succinic (SA), maleic (MA), and phthalic (PA) anhydrides with several epoxides: cyclohexene oxide (CHO), propylene oxide (PO), and limonene oxide (LO). Their behavior was compared with that of traditional salen chromium complexes. A completely alternating enchainment of monomers to provide pure polyesters was achieved with all the catalysts when used in combination with 4-(dimethylamino)pyridine (DMAP) as the cocatalyst. Poly(propylene maleate-block-polyglycolide), a diblock polyester with a precise composition, was obtained by switch catalysis, in which the same catalyst was able to combine the ROCOP of propylene oxide and maleic anhydride with the ring-opening polymerization (ROP) of glycolide (GA) through a one-pot procedure, starting from an initial mixture of the three different monomers.

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.

Stereogradient Poly(Lactic Acid) from meso-Lactide/L-Lactide Mixtures

The production of L-lactide from L-lactic acid involves a substantial formation of meso-lactide as an impurity, and, upon polymerization with the industrial catalyst tin octanoate, results in poly(L-lactic acid) of reduced crystallinity due to stereoerrors randomly distributed along the polymer chains. We describe a new approach wherein, instead of avoiding stereoerrors by removing the meso-lactide prior to polymerization, the stereoerrors in the polymer are tolerated, by crowding them in a stereogradient copolymer. A zirconium complex of an amine tris(phenolate) ligand is found to exhibit very high syndioselectivity in the ring opening polymerization catalysis of meso-lactide at room temperature, and gives rise to stereogradient copolymers in the polymerization of mixtures of meso-lactide/L-lactide in the melt at 180 °C. Relative to the stereo-random copolymers obtained with tin octanoate, the stereogradient copolymers exhibit enhanced crystallinities manifested in lower solubilities and higher melting temperatures and enthalpies.

Reaching High Stereoselectivity and Activity in Organocatalyzed Ring-Opening Polymerization of Racemic Lactide by the Combined Use of a Chiral (Thio)Urea and a N-Heterocyclic Carbene

Stereochemical control during polymerization is a key strategy of polymer chemistry to achieve semicrystalline engineered plastics. The stereoselective ring-opening polymerization (ROP) of racemic lactide (rac-LA), which can lead to highly isotactic polylactide (PLA), is one of the emblematic examples in this area. Surprisingly, stereoselective ROP of rac-LA employing chiral organocatalysts has been under-leveraged. Here we show that a commercially available chiral thiourea (TU1), or its urea homologue (U1), can be used in conjunction with an appropriately selected N-heterocyclic carbene (NHC) to trigger the stereoselective ROP of rac-LA at room temperature in toluene. Both a high organic catalysis activity (>90% monomer conversion in 5-9 h) and a high stereoselectivity (probability of formation of meso dyads, P_m, in the range 0.82-0.93) can be achieved by thus pairing a NHC and a chiral amino(thio)urea. The less sterically hindered and the more basic NHC, that is, a NHC bearing tert-butyl substituents (NHC_tBu), provides the highest stereoselectivity when employed in conjunction with the chiral TU1 or U1. This asymmetric organic catalysis strategy, as applied here in polymerization chemistry, further expands the field of possibilities to achieve bioplastics with adapted thermomechanical 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.

Gradient isoselective ring-opening polymerization of racemic cyclic diolide driven by chiral phosphoric acid catalysis

Commercially available binaphthol-derived chiral phosphoric acid organocatalysts were demonstrated to enable chemo- and stereoselective ROP of rac-cyclic diolide, yielding gradient isotactic multiblock poly(3-hydroxybutyrate) (P3HB).

Simple Zn(ii) complexes for the production and degradation of polyesters

Nine new complexes based on thioether appended iminophenolate (ONS) ligands have been prepared and fully characterized in solution by NMR spectroscopy. Solid-state structures were also obtained for seven complexes. In solution, all complexes were monomeric. The complexes were highly active for the polymerization of purified rac-lactide ([M] : [Zn] : [BnOH] = 10 000 : 1 : 30 at 180 °C) reaching TOF values up to 250 000 h⁻¹. The kinetics of the polymerization have been probed by in situ Raman spectroscopy. The rate of reaction was dramatically reduced using technical grade rac-lactide with increased initiator loading. To move towards a circular economy, it is vital that catalysts are developed to facilitate chemical recycling of commodity and emerging polymeric materials. In this vein, the complexes have been assessed for their ability to break down poly(lactic acid) and poly(ethylene terephthalate). The results from both the polymerization and degradation reactions are discussed in terms of ligand functionality.

Nine new complexes based on thioether appended iminophenolate (ONS) ligands have been prepared and fully characterized in solution by NMR spectroscopy.

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.

Active in Sleep: Iron Guanidine Catalyst Performs ROP on Dormant Side of ATRP

Copolymers are the answer to property limitations of homopolymers. In order to use the full variety of monomers available, catalysts active in more than one polymerization mechanism are currently investigated. Iron guanidine catalysts have shown to be extraordinarily active in ROP of lactide and herein prove their versatility by also promoting ATRP of styrene. The presented iron complex is the first polymerizing lactide and styrene simultaneously to a defined block copolymer in a convenient one-pot synthesis. Both mechanisms work hand in hand with ROP using the dominantly present FeII species on the dormant side of the ATRP equilibrium. This orthogonal copolymerization by a benign iron catalyst opens up new pathways to biocompatible polymerization procedures broadening the scope of ATRP applications.

Breaking the symmetry: C1-salans with (N-H) backbones

We disclose a synthetic route that providess an unprecedented library of C₁ salan ligands endowed with (N-H) backbones, previously limited to N-methylated backbones. Efforts to identify a generic complexation protocol to yield the corresponding Cu(II)-salan complexes demonstrate the scope and limitations of this approach.

Alkali and Alkaline Earth Metal Complexes as Versatile Catalysts for Ring-Opening Polymerization of Cyclic Esters

Biodegradable polyesters such as poly(ϵ-caprolactone) (PCL) and poly(lactic acid) (PLA) have been considered for use in several areas, such as drug delivery devices, sutures, tissue engineering, and GBR membranes, due to its bio-renewability, biodegradability, and biocompatibility. Several synthetic techniques for the preparation of polyesters have been reported in the literature, amongst which the ring-opening polymerization (ROP) of cyclic esters is the most efficient. A convenient approach to access iso-selective PLAs is polymerization of racemic lactide (rac-LA), which shows excellent stereoregularity without the need for costly chiral auxiliaries or ligands. In this personal account, we review a series of methods that have been practiced to the synthesis of biodegradable polyesters from various cyclic monomers using alkali and alkaline earth metal complexes as efficient catalysts.

Isoselective Polymerization of rac-Lactide by Aluminum Complexes of N-Heterocyclic Carbene-Phosphinidene Adducts

The N-heterocyclic carbene-phosphinidene adducts (NHC)PH were reacted with AlMe3 in toluene to afford the monoaluminum complexes [{(IDipp)PH}AlMe3 ] and [{(IMes)PH}AlMe3 ] (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). In contrast, the dialuminum complex [{(Me IMes)PH}(AlMe3 )2 ] was obtained for Me IMes=1,3-bis(2,4,6-trimethylphenyl)-4,5-dimethylimidazolin-2-ylidene. These complexes served as initiators for the efficient ring-opening polymerization of rac-lactide in toluene at 60 °C. High degrees of isoselectivity were found for the poly(rac-lactide) obtained in the presence of the monoaluminum complexes (Pm up to 0.92, Tm up to 191 °C), whereas almost atactic polymers were produced by the dialuminum complex. Detailed mechanistic studies reveal that the polymerization proceeds via a coordination-insertion mechanism with the carbene-phosphinidene ligands acting as stereodirecting groups.

Phosphasalalen Rare-Earth Complexes for the Polymerization of rac-Lactide and rac-β-Butyrolactone

A series of new phosphasalalen pro-ligands, analogues of salalen but with an iminophosphorane replacing the imine functionality, and their corresponding rare-earth alkoxide and siloxide complexes were synthesized. The multinuclear NMR spectra and X-ray diffraction analyses revealed that, for the tert-butoxide and ethoxide complexes, the resulting phosphasalalen rare-earth product was composed of a mononuclear alkoxide and a binuclear complex containing bridged alkoxo and hydroxo groups, while an analogous binuclear complex was isolated as the sole product for the siloxide complex. All the complexes could catalyze the heteroselective ring-opening polymerization (ROP) of rac-lactide (P_r up to 0.77) with high catalytic activities and a controlled polydispersity. Remarkably, the yttrium and lutetium phosphasalalen complexes could also efficiently catalyze the ROP of rac-β-butyrolactone to produce syndiotactic polymers (P_r up to 0.73) while their salalen analogues were inert, revealing the special effects of the iminophosphorane moiety. Detailed end-group analyses and kinetic investigations suggested that the alkoxo-hydroxo-bridged complexes maintained their binuclear structures in the polymerization.

Make or break: Mg(ii)- and Zn(ii)-catalen complexes for PLA production and recycling of commodity polyesters

A series of Mg(ii) and Zn(ii) catalen complexes have been prepared for PLA formation and recycling.

Stereogradient polycaprolactones formed by asymmetric kinetic resolution polymerization of 6-methyl-ε-caprolactone

Chiral phosphoric acid catalyzed asymmetric kinetic resolution polymerizations of 6-methyl-ε-caprolactone were developed to synthesize stereogradient polycaprolactones.

Stereoselective polymerization of rac-lactide catalyzed by zwitterionic calcium complexes

Two zwitterionic calcium complexes L¹CaN(SiMe₃)₂(THF) (1) and L²CaN(SiMe₃)₂ (2) via protolysis reaction were synthesized. At −75 °C, 1 gave a heterotactic sequence enriched polylactide, whilst 2 produced an isotactic sequence enriched polymer.

Chiral salenCo(iii) complexes with bulky substituents as catalysts for stereoselective alternating copolymerization of racemic propylene oxide with carbon dioxide and succinic anhydride

Stereoregular poly(propylene carbonate)s and poly(propylene succinate-block-carbonate)s were synthesized with new chiral salenCo(iii) catalysts carrying bulky substituents.

Salen-type aluminum and zinc complexes as two-faced Janus compounds: contribution to molecular sensing and polymerization catalysis

The aim of the present review is to highlight the most recent achievements in different fields of application of salen-based zinc and aluminum complexes. More specifically this article focuses on the use of aluminum and zinc salen-type complexes as optical probes for biologically relevant molecules, as catalysts for the ring opening polymerization (ROP) of cyclic esters and co-polymerization of epoxides and anhydrides (ROCOP) and in the chemical fixation of carbon dioxide (CO2). The intention is to provide an overview of the most recent results from our group within the framework of the state-of-art-results in the literature.

Highly Isoselective Ring-Opening Polymerization of rac-Lactide Using Chiral Binuclear Aluminum Catalyst

The microstructure can significantly affect the physical and mechanical properties of polymers. Five chiral binuclear aluminum methyl complexes (rac1 to rac5) are synthesized by using chiral binaphthalene diamine ligand and characterized by both ¹ H and ¹³ C NMR spectroscopy. The crystal structures of rac3 and rac5 are also identified by single crystal X-ray diffraction. In the presence of ⁱ PrOH initiator, these aluminum complexes catalyze the ring-opening polymerization (ROP) of rac-lactide (rac-LA) in a controlled manner and produce polymers with high to excellent isoselectivity (P_m up to 0.93). Even with a catalyst loading as low as 0.1%, polylactide with a P_m of 0.83 can still be obtained. Kinetic studies reveal the first-order dependence on monomer concentration whereas kinetic resolution polymerization provides the evidence to support that these binuclear aluminum catalysts catalyzed ROP of rac-LA adopts enantiomorphic site control mechanism. Furthermore, this strategy can also be applied to the ROP of both ε-caprolactone and δ-valeroactone.

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.

Active Ga-catalysts for the ring opening homo- and copolymerization of cyclic esters, and copolymerization of epoxide and anhydrides

A series of gallium complexes L12Ga4Me8 (1), L22Ga4Me8 (2), and L32Ga4Me8 (3) was synthesized by reaction of GaMe3 with Schiff base ligands L1-3H2 (L1H2 = 2,4-di-tert-butyl-6-{[(3-hydroxypropyl)imino]methyl}phenol; L2H2 = 2,4-dichloro-6-{[(3-hydroxypropyl)imino]methyl}phenol; L3H2 = 4-tert-butyl-2-{[(3-hydroxypropyl)imino]methyl}phenol) and characterized by 1H, 13C NMR, IR spectroscopy, elemental analysis and single crystal X-ray analysis (1, 2), proving their tetranuclear structure in the solid state. Complexes 1-3 showed good catalytic activity in the ring opening homopolymerization (ROP) and ring opening copolymerization (ROcoP) of lactide (LA) and ε-caprolactone (ε-CL) in the presence of benzyl alcohol (BnOH) in toluene at 100 °C, yielding polymers with the expected average molecular weights (Mn) and narrow molecular weight distributions (MWD), as well as a high isoselectivity for the ROP of rac-lactide (rac-LA), yielding isotactic-enriched PLAs with Pm values up to 0.78. Kinetic studies with complex 1 proved the first order dependence on monomer concentration, while mechanistic studies confirmed the coordination insertion mechanistic (CIM) pathway. Sequential addition of monomers gave well defined diblock copolymers of PCL-b-PLLA and PLLA-b-PCL, proving the living character of the polymerization reactions. The catalysts also showed perfect selectivity for the copolymerization of cyclohexene oxide (CHO) with both succinic anhydride (SA) and maleic anhydride (MA) in the presence of BnOH and produced >99% alternating block copolymers.

Synthesis of Ultrahigh Molecular Weight PLAs Using a Phenoxy-Imine Al(III) Complex

l- and d-lactide polymerization kinetics using phenoxy-imine ligands of the type Me2Al[O-2-tert-Bu-6-(C6F5N=CH)C6H3] in the presence of n-butanol and benzyl alcohol by ring-opening polymerization into polylactide are investigated. Effects of initiator concentration, catalyst concentration, polymerization temperature, and time on the molecular weight of poly-l-lactide are also investigated. Purification and drying of l-lactide are found to significantly influence the polymerization kinetics and the final molecular weight achieved. Ultrahigh molecular weight poly(l-lactic acid) PLLA (M w = 1.4 × 106 g/mol with Đ = 1.8) and ultrahigh molecular weight poly(d-lactic acid) PDLA (M w = 1.3 × 106 g/mol with Đ = 2.0) are obtained when polymerization is performed with a molar ratio of monomer to catalyst (LA/Al) of 8000 for 72 h at 120 °C in the presence of benzyl alcohol with conversions of 96 and 91%, respectively. We report for the first time the synthesis of ultrahigh molecular weight poly-l- and d-lactide using the Me2Al[O-2-tert-Bu-6-(C6F5N=CH)C6H3] catalyst. The identified catalyst is found to be suitable for the synthesis of a broad range of molecular weights.

Mechanistic Insight into the Stereoselective Cationic Polymerization of Vinyl Ethers

The control of the tacticity of synthetic polymers enables the realization of emergent physical properties from readily available starting materials. While stereodefined polymers derived from nonpolar vinyl monomers can be efficiently prepared using early transition metal catalysts, general methods for the stereoselective polymerization of polar vinyl monomers remain underdeveloped. We recently demonstrated asymmetric ion pairing catalysis as an effective approach to achieve stereoselective cationic polymerization of vinyl ethers. Herein, we provide a deeper understanding of stereoselective ion-pairing polymerization through comprehensive experimental and computational studies. These findings demonstrate the importance of ligand deceleration effects for the identification of reaction conditions that enhance stereoselectivity, which was supported by computational studies that identified the solution-state catalyst structure. An evaluation of monomer substrates with systematic variations in steric parameters and functional group identities established key structure-reactivity relationships for stereoselective homo- and copolymerization. Expansion of the monomer scope to include enantioenriched vinyl ethers enabled the preparation of an isotactic poly(vinyl ether) with the highest stereoselectivity (95.1% ± 0.1 meso diads) reported to date, which occurred when monomer and catalyst stereochemistry were fully matched under a triple diastereocontrol model. The more complete understanding of stereoselective cationic polymerization reported herein offers a foundation for the design of improved catalytic systems and for the translation of isotactic poly(vinyl ether)s to applied areas.

Synthesis of Biodegradable Polymers: A Review on the Use of Schiff-Base Metal Complexes as Catalysts for the Ring Opening Polymerization (ROP) of Cyclic Esters

This review describes the recent advances (from 2008 onwards) in the use of Schiff-base metal complexes as catalysts for the ring opening polymerization (ROP) of cyclic esters. The synthesis and structure of the metal complexes, as well as all aspects concerning the polymerization process and the characteristics of the polymers formed, will be discussed.

Alkaline Earth Metal-Mediated Highly Iso-selective Ring-Opening Polymerization of rac-Lactide

Alkaline earth (Ae) metal complexes of the aminophosphine borane ligand are highly active and iso-selective catalysts for the ring-opening polymerization (ROP) of rac-lactide (LA). The polymerization reactions are well controlled and produce polylactides with molecular weights that are precise and narrowly distributed. Kinetic studies reveal that the ROP of rac-LA catalyzed by all Ae metal complexes had a first-order dependency on LA concentration as well as catalyst concentration. A plausible reaction mechanism for Ae metal complex-mediated ROP of rac-LA is discussed, based on controlled kinetic experiments and molecular chain mobility.

Aluminium complexes of salanol ligands: coordination chemistry and stereoselective lactide polymerization

Aluminium complexes of the new salanol ligands give rise to all PLA tacticities by different stereocontrol mechanisms.

Syntheses, structures and catalytic activity of tetranuclear Mg complexes in the ROP of cyclic esters under industrially relevant conditions

Tetranuclear magnesium imino(phenolate) complexes Mg₄(L^1–4)₄ are excellent catalysts for the ROP of bulk rac-lactide and ε-caprolactone under industrially relevant conditions.

Indium phosphasalen catalysts showing high isoselectivity and activity in racemic lactide and lactone ring opening polymerizations

Study of a series of phosphasalen indium alkoxide complexes reveals that the substitution pattern at the phosphorus atoms can deliver outstanding isoselectivity with high rates.