Catalysts For Ring-opening Polymerization Research Articles

Developments in the use of rare earth metal complexes as efficient catalysts for ring-opening polymerization of cyclic esters used in biomedical applications

AbstractBiodegradable polymers represent a class of particularly useful materials for many biomedical and pharmaceutical applications. Among these types of polyesters, poly(ε-caprolactone) and polylactides are considered very promising for controlled drug delivery devices. These polymers are mainly produced by ring-opening polymerization of their respective cyclic esters, since this method allows a strict control of the molecular parameters (molecular weight and distribution) of the obtained polymers. The most widely used catalysts for ring-opening polymerization of cyclic esters are tin- and aluminium-based organometalliccomplexes; however since the contamination of the aliphatic polyesters by potentially toxic metallic residues is particularly of concern for biomedical applications, the possibility of replacing organometallic initiators by novel less toxic or more efficient organometallic complexes has been intensively studied. Thus, in the recent years, the use of highly reactive rare earth initiators/catalysts leading to lower polymer contamination has been developed. The use of rare earth complexes is considered a valuable strategy to decrease the polyester contamination by metallic residues and represents an attractive alternative to traditional organometallic complexes.

Triclocarban: Commercial Antibacterial and Highly Effective H-Bond Donating Catalyst for Ring-Opening Polymerization.

The antibacterial compound, triclocarban (TCC), is shown to be a highly effective H-bond donating catalyst for ring-opening polymerization (ROP) when applied with an H-bond accepting base cocatalyst. These ROPs exhibit the characteristics of "living" polymerizations. TCC is shown to possess the high activity characteristic of urea (vs thiourea) H-bond donors. The urea class of H-bond donors is shown to remain highly active in H-bonding solvents, a trait that is not displayed by the corresponding thiourea H-bond donors. Two H-bond donating ureas that are electronically similar to TCC are evaluated for their efficacy in ROP, and a mechanism of action is proposed. This "off-the-shelf" H-bond donor is among the most active and most controlled organocatalysts for the ROP of lactones.

Synthesis, characterisation, and X-ray structures of Zn(II) complexes containing bis-camphoryldiimine ligands: Application to polymerisation of rac-lactide

Novel Zn(II) complexes, (N1E,N2E)-N1,N2-bis(1,7,7-trimethylbicyclo [2.2.1]heptan-2-ylidene)butane-1,2-diamine (BCE) ZnCl2 and (N1E,N3E)-2,2-dimethyl-N1,N3-bis(1,7,7-trimethylbicyclo [2.2.1]heptan-2-ylidene)pentane-1,3-diamine (BCP) ZnCl2, were synthesised. X-ray diffraction analysis revealed that the geometry around the Zn(II) centre is distorted tetrahedral. The alkoxy derivatives, generated in situ, were highly efficient catalysts for ring-opening polymerisation (ROP) of rac-lactide (rac-LA), yielding polylactide (PLA) with good number-average molecular weights and narrow polydispersity indices. (BCP)Zn(OCHMe2)2 exhibited activity for stereoselective ROP of rac-LA, and complete conversion occurred within 8min to attain heterotactic PLA (Pr=0.78).

Lactide as the Playmaker of the ROP Game: Theoretical and Experimental Investigation of Ring-Opening Polymerization of Lactide Initiated by Aminonaphtholate Zinc Complexes

A family of homo- and heteroleptic zinc complexes bearing aminonaphtholate ligands was synthesized and fully characterized. Using NMR spectroscopy and DFT calculation, bis-alkoxy-bridged complexes [LZn(μ-OR)]2 were confirmed to have dimeric structures in solution, analogous to those obtained via X-ray crystallography. Surprisingly, a detailed experimental and theoretical study of the catalytic activity of [LZn(μ-OR)]2 in the ring-opening polymerization (ROP) of lactides showed that although well-defined alkoxy dimers possess a single-site structural motif, the most active initiator is obtained during in situ alcoholysis of the alkylzinc precursor. These results indicate that rational ancillary and alkoxy ligand design that takes into account its mutual interaction on monomer coordination may be key to the synthesis of new high-performance ROP catalysts.

A Comparison of Gallium and Indium Alkoxide Complexes as Catalysts for Ring-Opening Polymerization of Lactide.

The impact of the metal size and Lewis acidity on the polymerization activity of group 13 metal complexes was studied, and it was shown that, within the same ligand family, indium complexes are far more reactive and selective than their gallium analogues. To this end, gallium and aluminum complexes supported by a tridentate diaminophenolate ligand, as well as gallium complexes supported by N,N'-ethylenebis(salicylimine)(salen) ligands, were synthesized and compared to their indium analogues. Using the tridentate ligand set, it was possible to isolate the gallium chloride complexes 3 and (±)-4 and the aluminum analogues 5 and (±)-6. The alkoxygallium complex (±)-2, supported by a salen ligand, was also prepared and characterized and, along with the three-component system GaCl3/BnOH/NEt3, was tested for the ring-opening polymerization of lactide and ε-caprolactone. The polymerization rates and selectivities of both systems were significantly lower than those for the indium analogues. The reaction of (±)-2 with 1 equiv of lactide forms the first insertion product, which is stable in solution and can be characterized at room temperature. In order to understand the differences of the reactivity within the group 13 metal complexes, a Lewis acidity study using triethylphosphine oxide (the Gutmann-Beckett method) was undertaken for a series of aluminum, gallium, and indium halide complexes; this study shows that indium halide complexes are less Lewis acidic than their aluminum and gallium analogues. Density functional theory calculations show that the Mulliken charges for the indium complexes are higher than those for the gallium analogues. These data suggest that the impact of ligands on the reactivity is more significant than that of the metal Lewis acidity.

Multimetallic Lithium Complexes Derived from the Acids Ph2C(X)CO2H (X=OH, NH2): Synthesis, Structure and Ring Opening Polymerization of Lactides and Lactones

AbstractReaction of LiOR (R=t‐Bu, Ph) with the acids 2,2/‐Ph2C(X)(CO2H), X=OH (benzH), NH2 (dpgH) was investigated. For benzH, one equivalent LiOt‐Bu in THF afforded [Li(benz)]2⋅2THF (1⋅2THF), which adopts a 1D chain structure. If acetonitrile is used (mild conditions), another polymorph of 1 is isolated; LiOPh also led to 1. Robust work‐up afforded [Li7(benz)7(MeCN)] 2MeCN THF (2⋅2MeCN⋅THF). Use of LiOt‐Bu (2 equivalents) led to {Li8(Ot‐Bu)2[(benz)](OCPh2CO2CPh2CO2t‐Bu)2(THF)4} (3), the core of which comprises two open cubes linked by benz ligands. For dpgH, two equivalents of LiOt‐Bu in THF afforded [Li6(Ot‐Bu)2(dpg)2(THF)2] (4), which contains an Li2O2 6‐step ladder. Similar reaction of LiOPh afforded [Li8(PhO)4(dpg)4(MeCN)4] (5). Complexes 1–5 were screened for their potential as catalysts for ring opening polymerization (ROP) of ϵ‐caprolactone (ϵ‐CL), rac‐lactide (rac‐LA) and δ‐valerolactone (δ‐VL). For ROP of ϵ‐CL, conversions > 70 % were achievable at 110 °C with good control. For rac‐LA and δ‐VL, temperatures of at least 110 °C over 12 h were necessary for activity (conversions > 60 %). Systems employing 2 were inactive.

Zirconium and hafnium complexes bearing pyrrolidine derived salalen-type {ONNO} ligands and their application for ring-opening polymerization of lactides.

The reactions of pyrrolidine derived salalen-type {ONNO} ligands (S)-L1-3-H2 with 1 equiv. M(OiPr)4(HOiPr) (M = Zr or Hf) in diethyl ether yielded complexes [L1-3M(OiPr)2] (L = L1, M = Zr (1); L = L2, M = Zr (2), Hf (3); L = L3, M = Zr (4), Hf (5)). All of these complexes were well characterized by NMR spectroscopy, elemental analyses and single-crystal X-ray analysis in the case of complexes 1 and 3-5. X-ray structural determination revealed that these complexes were analogous mononuclear species and had a similar structure in which the metal centers were six-coordinated to two oxygen atoms and two nitrogen atoms of one ligand and two oxygen atoms of two isopropoxy groups. All of these complexes efficiently initialized the ring-opening polymerization of lactides to afford polymers with controlled molecular weight and narrow polydispersity. Furthermore, the ring-opening polymerization of rac-lactide catalyzed by complexes 1-5 afforded isotactic-enriched polymers in solution (Pm = 0.74-0.80) and under melt conditions (Pm = 0.63-0.72).

The role of ligand redox non-innocence in ring-opening polymerization reactions catalysed by bis(imino)pyridine iron alkoxide complexes.

The reactivity of iron-based ring opening polymerization catalysts is compared when the catalyst is in three different oxidation states. Formally iron(i) monoalkoxide complexes 3a (p-methoxyphenoxide) and 3b (neopentoxide) supported by bis(imino)pyridine ligands were synthesized and investigated as catalysts for the ring opening polymerization and copolymerization of various monomers. For most monomers, 3a and 3b were superior catalysts compared to analogous, formally iron(ii) and iron(iii) complexes (1a/1b and 2a/2b, respectively) for the ring opening polymerization of various cyclic ester and cyclic carbonate monomers. Experimental and computational investigation into the electronic structures of 3a and 3b revealed that they are most accurately described as containing a high spin iron(ii) center that is antiferromagnetically coupled to a singly reduced bis(imino)pyridine ligand. This electronic structure leads to increased electron density near the metal center without modulating the apparent metal oxidation state, which results in superior catalytic performance for the more highly reduced 3a and 3b compared to the increasingly more oxidized complexes (i.e.1a/1b and 2a/2b, respectively) in ring opening polymerization reactions. These findings have significant ramifications for the emerging field of redox-switchable polymerization catalysis.

Titanium alkoxides immobilized on magnetic mesoporous silica nanoparticles and their characterization by solid state voltammetry techniques: Application in ring opening polymerization

In this paper, we present the synthesis of mesoporous silica coated Fe3O4 core–shell nanoparticles or magnetic mesoporous silica nanoparticles (MMSNs) via a surfactant templated sol–gel method using magnetite nanoparticles (MNPs) stabilized with tartrate as nucleation seeds. The synthetic parameters have been carefully controlled in order to scale up the synthetic procedure to obtain more than 1 g of stable, high quality core–shell and biocompatible MMSNs per synthesis. MMSNs have been used as support to prepare heterogeneous titanium catalysts for ring opening polymerization (ROP) of ε-caprolactone. The prepared materials have been fully characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF) N2 adsorption-desorption, transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TG-DTG), UV–vis and DRUV-vis techniques. In addition, extensive solid state electrochemical studies have been carried out in aqueous and organic medium demonstrating the coating of MNPs with a layer of mesoporous silica and the immobilization of titanium as electroactive species.

Cadmium acetate as a ring opening polymerization catalyst for the polymerization of rac-lactide, ε-caprolactone and as a precatalyst for the polymerization of ethylene

In the present study, we have discussed the bulk ring opening polymerization (ROP) of rac-lactide (rac-LA) and e-caprolactone (e-CL) using Cd(OAc)2. Cd(OAc)2 appeared to be a good catalyst for the polymerization of rac-LA and e-CL yielding high molecular weight (Mn) polymers with narrow molecular weight distributions (MWDs). The catalytic activity of the system can be increased markedly upon using catalytic amount of BnOH as external alcoholic initiator. There is a first order dependence of the rate constant with respect to monomer concentrations as understood from the kinetic studies. The rate was found to be faster in the presence of BnOH. The polymerization process was controlled. The polymerization proceeded via the coordination-insertion mechanism without BnOH as well as activated monomer mechanism in the presence of BnOH. In the absence of BnOH, the acetyl group initiated the polymerization as understood from the 1H NMR and MALDI-TOF analysis. The benzyloxy group initiated the polymerization in the presence of BnOH. Moderate activity towards the polymerization of ethylene was observed using MAO as alkyl aluminum activator. The polymerization parameters towards the polymerization of ethylene were widely investigated.

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.

Zinc Complexes of Sequential Tetradentate Monoanionic Ligands in the Isoselective Polymerization of rac-Lactide.

Zinc complexes of {ONNN}-type sequential tetradentate monoanionic ligands reacted with diethylzinc to give the mononuclear ethylzinc complexes. The benzyloxy complexes were formed readily and were found to be highly active as well as living/immortal catalysts for ring-opening polymerization of rac-lactic acid with a clear isospecific inclination. Chiral gas chromatography analysis revealed a mild preference for a given lactide enantiomer by the chiral catalysts.

Ring-opening polymerization of ε-caprolactone catalysed by (pyridyl)benzoazole Zn(II) and Cu(II) complexes

This paper describes the synthesis of (pyridyl)benzoazole Zn(II) and Cu(II) complexes and their applications as catalysts in ring-opening polymerization (ROP) of e-caprolactone (e-CL). Reactions of 2-(3-pyridyl)-1H-benzimidazole (L1), 2-(2-pyridyl)-1H-benzothiazole (L2) and 2-(2-pyridyl)-1H-benzimidazole (L3) with Zn(II) and Cu(II) acetates produced the corresponding complexes; [Zn2(L1)2(OAc)4)] (1), [Cu2(L1)2(OAc)4] (2), [Zn(L2)(OAc)2)] (3), [Zn(L3)(OAc)2)] (4) and [Cu(L3), (OAc)2)] (5). Molecular structures of complexes 2 and 5a revealed that while L1 adopts a monodentate binding mode, through the pyridyl nitrogen atom, L3 exhibits a bidentate coordination mode. All the complexes formed active catalysts in the ROP of e-CL to afford moderate molecular weight polymers. The kinetics of the ROP reactions of e-CL were pseudo-first-order with respect to monomer and catalysts.

On thermal behaviour of DMC catalysts for ring opening polymerization of epoxides

The thermoanalytical studies of two types of the double metal cyanide catalysts were carried out.The identity of the catalysts was characterized by an elemental analysis and FTIR. The thermoanalytical examinations performed with the use of TG-DSC, EGA-FTIR, EGA-MS shed new light on views of the bonding of the ligands in the DMC. The chemical justification of the processes responsible for the TG/DSC steps was proposed. Several organic ligand complexing states were found in both types of catalysts. The ligand-DMC bondings were decomposing stepwise in relatively low temperature ranges, which could suggest their negligible role at catalytic reaction being performed usually at higher temperatures.The stoichiometric quantitative correlation between the values of TG steps responsible for the ligands evolution, and the concentration of total carbon, nitrogen and cobalt was found.The relationship was used as a base for the quantitative method of characterization of the DMC catalysts.

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.

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

The development of well-defined homogeneous catalysts for the ring-opening polymerization (ROP) of cyclic esters has made enormous progress over the past decade. This perspective focuses on some recent advances in the field of discrete metal complexes modified by various aryloxide or arylamido ligands bearing the nitrogen-containing heterocycle moiety, and their catalytic applications in ROP of lactones. It mainly highlights aryloxide/arylamido ligands that are directly installed by the N-heterocyclic group. The complex structure-ROP performance relationships and the observed trends with respect to their catalytic efficiency affected by ligand modifications are also discussed.

Dual Catalysis for Selective Ring-Opening Polymerization of Lactones: Evolution toward Simplicity.

Much work has been directed to the design of complex single-site catalysts for ring-opening polymerization (ROP) to enhance both activity and selectivity. More simply, however, cooperative effects between Lewis acids and organocatalytic nucleophiles/Lewis bases provide a powerful alternative. In this study we demonstrate that the combination of N-heterocyclic carbenes, 1,8-diazabicycloundec-7-ene (DBU) and 4-dimethylaminopyridine (DMAP) with simple Lewis acids enables the ROP of the macrolactone pentadecalactone in a rapid and efficient manner. Remarkably, regardless of the nature of the nucleophile, the order of activity was observed to be MgX2 ≫ YCl3 ≫ AlCl3 and MgI2 > MgBr2 > MgCl2 in every case. The minimal influence of the organobase on polymerization activity allows for the use of simple and inexpensive precursors. Furthermore, extension of the study to other cyclic (di)ester monomers reveals the choice of Lewis acid to lead to monomer selective ROP activity and hence control over copolymer composition by choice of Lewis acid. This approach could lead to the realization of complex polymer structures with tunable physical properties from simple catalyst combinations.

Magnesium Pyrazolyl-Indolyl Complexes as Catalysts for Ring-Opening Polymerization of L-Lactide

A series indole-based ligand precursors, PzRIndH (R = H, PzHIndH; R = Me, PzMeIndH;R = t-Bu, PztBuIndH; and R = Ph, PzPhIndH), have been synthesized via copper-catalyzed N-arylation (for PzHIndH) or the Bartoli indole synthesis (for PzMeIndH, PztBuIndH and PzPhIndH) reactions with moderate to high yield. Reactions of these ligand precursors with 0.7 equivalentof MgnBu2 in THF (for 1) or hexane (for 2–4) afforded the bis-indolyl magnesium complexes 1–4,respectively. All the ligand precursors and related magnesium complexes have been characterizedby NMR spectroscopy and elemental analyses. The molecular structure is reported for compound 1.These novel magnesium complexes demonstrate efficient catalytic activities for the ring-opening polymerization of L-lactide in the presence of alcohol.

Rare-Earth Complexes Supported by Tripodal Tetradentate Bis(phenolate) Ligands: A Privileged Class of Catalysts for Ring-Opening Polymerization of Cyclic Esters

Tripodal dianionic diamino- or amino-alkoxy-bis(phenolate) ligands {ONXOR1,R2} (X = NR2, OR) constitute a privileged class of ligands which has found much interest in combination with group 3 and group 4 metals for generating highly efficient polymerization catalysts. This account article describes the structural variety of and synthetic routes toward trivalent rare-earth complexes incorporating such {ONXOR1,R2} ancillaries along with reactive amido, alkyl, alkoxide, amidinate, guanidinate, halide, and borohydride groups. The chemistry of related divalent rare-earth complexes is also included. This class of Ln{ONXOR1,R2}(R) complexes features outstanding performance in the ring-opening polymerization (ROP) of cyclic esters. Examples of their high reactivity, which has allowed enlarging the scope of ROP reactions to monomers that are difficult to ring-open, and of their ability to control reactions are provided. A particular emphasis is given on their propensity to fine-tune the stereoselectivity of ROP re...

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

Metal-based catalysts and initiators have played a pivotal role in the ring-opening polymerization (ROP) of cyclic esters, thanks to their high activity and remarkable ability to control precisely the architectures of the resulting polyesters in terms of molar mass, dispersity, microstructure, or tacticity. Today, after two decades of extensive research, the field is slowly reaching maturity. However, several challenges remain, while original concepts have emerged around new types or new applications of catalysis. This Review is not intended to comprehensively cover all of these aspects. Rather, it provides a personal overview of the very recent progress achieved in some selected, important aspects of ROP catalysis--stereocontrol and switchable catalysis. Hence, the first part addresses the development of new metal-based catalysts for the isoselective ROP of racemic lactide towards stereoblock copolymers, and the use of syndioselective ROP metal catalysts to control the monomer sequence in copolymers. A second part covers the development of ROP catalysts--primarily metal-based catalysts, but also organocatalysts--that can be externally regulated by the use of chemical or photo stimuli to switch them between two states with different catalytic abilities. Current challenges and opportunities are highlighted.