Beyond stereoselectivity, switchable catalysis: some of the last frontier challenges in ring-opening polymerization of cyclic esters

Investigation of redox switchable titanium and zirconium catalysts for the ring opening polymerization of cyclic esters and epoxides

The synthesis and characterization of (thiolfan*)Zr(O^tBu)₂ (thiolfan* = 1,1′-di(2,4-di-tert-butyl-6-thiophenoxide)ferrocene) is reported, as well as its activity toward the ring-opening polymerizations of l-lactide and ε-caprolactone.

Investigation of the dinuclear effect of aluminum complexes in the ring-opening polymerization of ε-caprolactone

Al complexes bearing hydrazine-bridging Schiff base ligands showed the best catalytic activity, approximately 3- to 11-fold higher than that of dinuclear Al complexes bearing Salen ligands and mononuclear Al complexes bearing Schiff base ligands.

Aluminium complexes containing salicylbenzothiazole ligands and their application in the ring-opening polymerisation of rac-lactide and ε-caprolactone

This paper is the first report on the use of aluminium salicylbenzothiazole complexes for the ROP of rac-LA and ε-CL.

Mono- and binuclear chiral N,N,O-scorpionate zinc alkyls as efficient initiators for the ROP of rac-lactide

Activity enhancement in the ROP ofrac-LA through the cooperative effect of two remote zinc metals in N,N,O-scorpionate binuclear trisalkyls.

A redox-switchable ring-closing metathesis catalyst

A ring-closing metathesis catalyst was arrested upon reduction of a redox-active ligand; subsequent oxidation restored catalytic activity.

Palladium complexes of ferrocene-based phosphine ligands as redox-switchable catalysts in Buchwald–Hartwig cross-coupling reactions

Redox-switchable catalysis: Palladium(ii) complexes of two differently substituted [1]phosphaferrocenophanes FcPR (R = Mes, biaryl) and of diphenylferrocenyl phosphine Ph₂PFc were applied in redox-switchable Buchwald–Hartwig cross-coupling reactions.

Preparation of zirconium and hafnium complexes containing chiral N atoms from asymmetric tertiary amine ligands, and their catalytic properties for polymerization of rac-lactide

Preparation of Zr and Hf complexes containing chiral N atoms, and their use in polymerization of rac-lactide.

Continuous flow ring-opening polymerizations

This minireview briefly highlights recent important progress, challenge and the future outlook for continuous flow ring-opening polymerizations.

Impact of aryloxy initiators on the living and immortal polymerization of lactide

This report describes two different methodologies for the synthesis of aryl end-functionalized poly(lactide)s (PLAs) catalyzed by indium complexes.

The Role of Nitrogen Donors in Zinc Catalysts for Lactide Ring-Opening Polymerization

The electronic effects of nitrogen donors in zinc catalysts for ring-opening polymerization of cyclic esters were investigated. Alkyl and benzyloxy zinc complexes supported by tridentate diamino- and aminoimino phenolate ligands were synthesized, and their solid-state and solution structures characterized. The solution-state structures showed that the alkyl complexes are mononuclear, while the alkoxy complexes are dimeric with the ligands coordinated with different denticities depending on the nature of the ligand donors. The catalytic activities of these compounds toward the ring-opening polymerization of racemic lactide were studied and showed that catalysts with secondary and imine nitrogen donors are more active than analogues with tertiary amines.

Bis- and Tris-Urea H-Bond Donors for Ring-Opening Polymerization: Unprecedented Activity and Control from an Organocatalyst

A new class of H-bond donating ureas was developed for the ring-opening polymerization (ROP) of lactone monomers, and they exhibit dramatic rate acceleration versus previous H-bond mediated polymerization catalysts. The most active of these new catalysts, a tris-urea H-bond donor, is among the most active organocatalysts known for ROP, yet it retains the high selectivity of H-bond mediated organocatalysts. The urea cocatalyst, along with an H-bond accepting base, exhibits the characteristics of a "living" ROP, is highly active, in one case, accelerating a reaction from days to minutes, and remains active at low catalyst loadings. The rate acceleration exhibited by this H-bond donor occurs for all base cocatalysts examined. A mechanism of action is proposed, and the new catalysts are shown to accelerate small molecule transesterifications versus currently known monothiourea catalysts. It is no longer necessary to choose between a highly active or highly selective organocatalyst for ROP.

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.

Chemoselective Polymerizations from Mixtures of Epoxide, Lactone, Anhydride, and Carbon Dioxide

Controlling polymer composition starting from mixtures of monomers is an important, but rarely achieved, target. Here a single switchable catalyst for both ring-opening polymerization (ROP) of lactones and ring-opening copolymerization (ROCOP) of epoxides, anhydrides, and CO2 is investigated, using both experimental and theoretical methods. Different combinations of four model monomers-ε-caprolactone, cyclohexene oxide, phthalic anhydride, and carbon dioxide-are investigated using a single dizinc catalyst. The catalyst switches between the distinct polymerization cycles and shows high monomer selectivity, resulting in block sequence control and predictable compositions (esters and carbonates) in the polymer chain. The understanding gained of the orthogonal reactivity of monomers, specifically controlled by the nature of the metal-chain end group, opens the way to engineer polymer block sequences.

Block Copolymerization of Lactide and an Epoxide Facilitated by a Redox Switchable Iron-Based Catalyst

A cationic iron(III) complex was active for the polymerization of various epoxides, whereas the analogous neutral iron(II) complex was inactive. Cyclohexene oxide polymerization could be "switched off" upon in situ reduction of the iron(III) catalyst and "switched on" upon in situ oxidation, which is orthogonal to what was observed previously for lactide polymerization. Conducting copolymerization reactions in the presence of both monomers resulted in block copolymers whose identity can be controlled by the oxidation state of the catalyst: selective lactide polymerization was observed in the iron(II) oxidation state and selective epoxide polymerization was observed in the iron(III) oxidation state. Evidence for the formation of block copolymers was obtained from solubility differences, GPC, and DOSY-NMR studies.

Aluminum alkyl complexes: synthesis, structure, and application in ROP of cyclic esters

Aluminum alkyl complexes have very useful applications as catalysts or reagents in small molecule transformations and as cocatalysts in olefin polymerization. This short review focuses on some recent developments in the design, synthesis and structure of aluminum(iii) alkyl complexes supported by various ligands bearing nitrogen, oxygen, sulfur or phosphorus atoms, and their catalytic applications in the ring-opening polymerization (ROP) of cyclic esters. The coordination chemistry of the Al metal centre and the catalytic activity changes of the complexes caused by ligand modifications are also discussed.

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.

Poly(trimethylene carbonate)/Poly(malic acid) Amphiphilic Diblock Copolymers as Biocompatible Nanoparticles

Amphiphilic polycarbonate-poly(hydroxyalkanoate) diblock copolymers, namely, poly(trimethylene carbonate) (PTMC)-b-poly(β-malic acid) (PMLA), are reported for the first time. The synthetic strategy relies on commercially available catalysts and initiator. The controlled ring-opening polymerization (ROP) of trimethylene carbonate (TMC) catalyzed by the organic guanidine base 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), associated with iPrOH as an initiator, provided iPrO-PTMC-OH, which served as a macroinitiator in the controlled ROP of benzyl β-malolactonate (MLABe) catalyzed by the neodymium triflate salt (Nd(OTf)3). The resulting hydrophobic iPrO-PTMC-b-PMLABe-OH copolymers were then hydrogenolyzed into the parent iPrO-PTMC-b-PMLA-OH copolymers. A range of well-defined copolymers, featuring different sizes of segments (Mn,NMR up to 9300 g mol(-1) ; ÐM =1.28-1.40), were thus isolated in gram quantities, as evidenced by NMR spectroscopy, size exclusion chromatography, thermogravimetric analysis, differential scanning calorimetry, and contact angle analyses. Subsequently, PTMC-b-PMLA copolymers with different hydrophilic weight fractions (11-75 %) self-assembled in phosphate-buffered saline upon nanoprecipitation into well-defined nano-objects with Dh =61-176 nm, a polydispersity index <0.25, and a negative surface charge, as characterized by dynamic light scattering and zeta-potential analyses. In addition, these nanoparticles demonstrated no significant effect on cell viability at low concentrations, and a very low cytotoxicity at high concentrations only for PTMC-b-PMLA copolymers exhibiting hydrophilic fractions over 47 %, thus illustrating the potential of these copolymers as promising nanoparticles.

Redox-triggered crosslinking of a degradable polymer

A unique redox-triggered crosslinking reaction is disclosed that capitalizes on the orthogonal reactivity of an iron-based catalyst for the ring opening polymerization of cyclic diesters and epoxides.

Traceless switch organocatalysis enables multiblock ring-opening copolymerizations of lactones, carbonates, and lactides: by a one plus one approach in one pot

A switch of organocatalysis from cationic to base/conjugate-acid bifunctional mechanisms made ring-opening copolymerizations of lactones/carbonates to lactides success.

New perspectives in organolanthanide chemistry from redox to bond metathesis: insights from theory

A fifteen year contribution of computational studies carried out in close synergy with experiments is summarized.

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.

Reversible modulation of the activity of thiourea catalysts with anions: a simple approach to switchable asymmetric catalysis

A simple and straightforward approach to switchable asymmetric catalysis is presented, based on the interactions of thiourea catalysts with anions.

Controlling the stereoselectivity of rac-LA polymerization by chiral recognition induced the formation of homochiral dimeric metal alkoxides

Chiral recognition of monomeric Me₂MOR units resulting in the formation of homochiral dimeric species [Me₂M(μ-OR)]₂(M = Ga, In), leads to heteroselective catalysts for the ring opening polymerization ofrac-lactide (rac-LA).

The ring-opening polymerization of ε-caprolactone and l-lactide using aluminum complexes bearing benzothiazole ligands as catalysts

A series of aluminum complexes bearing benzothiazole ligands was synthesized and the ring-opening polymerization of ε-caprolactone (CL) and l-lactide (LA) using these aluminum complexes as catalysts was studied.

Photoswitchable ring-opening polymerization of lactide catalyzed by azobenzene-based thiourea

An azobenzene-based thiourea compound 1 as a catalyst was successfully used in the ring-opening polymerization of rac-lactide. The reactivity of a catalytic polymerization system using photoresponsive azobenzene-based thiourea/PMDETA as a catalyst could be switched between slow and fast states by alternating exposure to UV and ambient light.

Aluminum complexes containing salicylbenzoxazole ligands and their application in the ring-opening polymerization of rac-lactide and ε-caprolactone

Two series of aluminum complexes supported by salicylbenzoxazole ligands were successfully synthesized and applied as active initiators for the ring-opening polymerization of rac-LA and ε-CL.

Metal complexes containing nitrogen-heterocycle based aryloxide or arylamido derivatives as discrete catalysts for ring-opening polymerization of cyclic esters

This perspective highlights some recent advances in the field of discrete N-heterocyclic aryloxide/arylamido metal complexes for ROP of lactones.

Macrocyclic Dizinc(II) Alkyl and Alkoxide Complexes: Reversible CO2 Uptake and Polymerization Catalysis Testing

The synthesis of three new dizinc(II) complexes bearing a macrocyclic [2 + 2] Schiff base ligand is reported. The bis(anilido)tetraimine macrocycle reacts with diethylzinc to form a bis(ethyl)dizinc(II) complex, [L(Et)Zn2Et2] (1). The reaction of complex 1 with isopropyl alcohol is reported, forming a bis(isopropyl alkoxide)dizinc complex, [L(Et)Zn2((i)PrO)2] (2). Furthermore, complex 1, with 2 equiv of alcohol, is applied as an initiator for racemic lactide ring-opening polymerization. It shows moderately high activity, resulting in a pseudo-first-order rate coefficient of 9.8 × 10(-3) min(-1), with [LA] = 1 M and [initiator] = 5 mM at 25 °C and in a tetrahydrofuran solvent. Polymerization occurs with good control, as evidenced by the linear fit to a plot of molecular weight versus conversion, the narrow dispersities, and the limited transesterification. The same initiating system is inactive for the ring-opening copolymerization of carbon dioxide (CO2) and cyclohexene oxide at 80 °C and 1 bar of CO2 pressure. However, stoichiometric reactions between complex 2 and CO2, at 1 bar pressure, result in the reversible formation of new dizinc carbonate species, [L(Et)Zn2((i)PrO)((i)PrOCO2)] (3a) and [L(Et)Zn2((i)PrOCO2)2] (3b), and the reaction was studied using density functional theory calculations. All of the new complexes, 1-3b, are fully characterized, including NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction.