Catalysts For Ring-opening Polymerization Research Articles

Boosting Highly Ordered Porosity in Lanthanum Metal-Organic Frameworks for Ring-Opening Polymerization of γ-Butyrolactone

<h2>Summary</h2> γ-butyrolactone (γ-BL), serving as the monomer of biocompatible and recyclable poly(γ-butyrolactone) plastic, faces a giant challenge in ring-opening polymerization (ROP) due to its low strain energy. The ROP of γ-BL is generally achieved under harsh conditions (e.g., high-pressure or gas-protected atmosphere) and is mostly based on homogeneous systems. Here, we synthesize a series of lanthanum metal-organic framework-based porous nanospheres and successfully apply them as heterogeneous catalysts in efficient ROP of γ-BL under mild conditions. Specifically, NNU-mIM-3 presents a yield of 24.6% with low polydispersity distribution centered at 3.6 kDa and the obtained PγBL can be completely and quantitatively recycled into monomers by simple heat treatment. Besides, the ROP catalysis systems have been intensively explored with various imine-based additives, proven to exhibit largely improved performance with PγBL yield of ~50% and Mw of ~8 kDa. This work paves a new way to explore novel heterogeneous catalysts in producing biocompatible and degradable plastics.

Synthesis and characterization of a series of phenyl piperazine based ligands

Piperazine based compounds are gaining more attention in today’s research as the piperazine nucleus is found in many biologically active compounds. Substitution in nitrogen atom of piperazine with a suitable fragment containing donor atoms, make it unique for versatile binding possibilities with metal ion. Piperazine derived ligands and their metal complexes have shown applications in different fields like antimicrobial, antioxidant, antihistaminic, anticancer, DNA binding and protein binding, catalyst in ring opening polymerization (ROP), etc. Metal-organic framework derived from piperazine based ligands has also been reported in the literature. This paper presents the synthesis, and characterization of a series of piperazine based ligands. The asymmetrical ligands have been synthesized by cyclization of bis-chloroethyl amine with suitable amine. Some of the representative metal complexes are also synthesized and characterized.

Bimetallic Aluminum Complexes Bearing Binaphthyl-Based Iminophenolate Ligands as Catalysts for the Synthesis of Polyesters

Two dinuclear aluminum complexes bearing dinaphthalene bridging Schiff bases have been synthesized and investigated as catalysts in ring-opening polymerization (ROP) of cyclic esters such as rac-lactide (rac-LA) and ε-caprolactone (ε-CL) and in the copolymerization (ROCOP) of phthalic anhydride (PA) with cyclohexene oxide (CHO) and limonene oxide (LO). The polymerizations of cyclic esters were living, producing polymers with narrow molar mass distributions. Kinetic studies showed that the polymerizations were first order with respect to the monomers. Cooperative effects between two metal centers, located in proximal positions, are invoked to rationalize the high activities toward both monomers, although the rigid backbone of the complexes enhances the reactivity of less encumbered caprolactone in comparison to lactide. Good activities were achieved also in the copolymerization of phthalic anhydride with cyclohexene epoxide and with the bioderived limonene oxide.

Controlled and highly effective ring‐opening polymerization of α‐chloro‐ε‐caprolactone using Zn‐ and Al‐based catalysts

AbstractThe present study details the highly effective and controlled ring‐opening polymerization (ROP) of α‐chloro‐ε‐caprolactone (1, αClεCL), a cyclic ester that has been little explored thus far in ROP catalysis, using Zn‐ and Al‐based catalysts [Zn(C6F5)2(toluene)] (4), [N,N′‐bis(3,5‐di‐tert‐butylsalicylidene)1,3‐diaminopropanato]aluminium(III)benzyloxide (5) and [N,N′‐bis(3,5‐di‐tert‐butylsalicylidene)1,3‐diamino‐2,2′‐dimethylpropanato]aluminium(III)benzyloxide] (6). Chain‐length‐controlled PαClεCL material is produced under solution ROP conditions, as deduced from GPC, NMR, MALDI‐TOF, and kinetic data. In contrast, the ROP of 1 is ill‐defined under bulk ROP conditions due to partial thermal degradation of the polymer chain (presumably through C–Cl cleavage), reflecting the limited stability of PαClεCL. The Al Catalysts 5 and 6 are highly active ROP catalysts of αClεCL at room temperature (TOF up to 2,400 hr−1) to afford well‐defined P(αClεCL). In the case of Catalyst 6, carrying out the ROP of αClεCL under immortal conditions (with BnOH as chain transfer agent) is clearly beneficial to ROP activity and control, with no apparent side‐reaction of chloro‐functionalized PCL chains as the ROP proceeds. The controlled character of these ROPs was further exploited for the production of chain‐length‐controlled PLLA‐b‐PαClεCL diblocks through sequential ROP of l‐lactide and αClεCL, affording copolymers with improved thermal and biodegradable properties.

Recent Studies on Single Site Metal Alkoxide Complexes as Catalysts for Ring Opening Polymerization of Cyclic Compounds

The selection of the catalyst is central to being able to control features and to better understand microstructures of polymers. New single-site metal alkoxide catalysts allow chemists to prepare regio/stereo regular polymers or copolymers that meet increasingly demanding performance requirements. These catalysts produce high molecular weights polymers with narrow polydispersity indexes or living properties and essentially regular polymers in structures. This review includes the synthesis, activity, and mechanistic aspects of especially single-site metal alkoxide catalysts with examples from my previous studies and recently published similar articles, beginning with an extensive survey on the aluminum, titanium, zirconium, and tin catalysts used to make polyethers, polyesters, and their derivatives. This review also compares the effects of ligands, substituents on ligands, and central metal atoms on ROP reactions. This review will provide the basis for the researchers who seek the new synthesis and application of catalysts in the future.

Phosphorus pentoxide as a cost-effective, metal-free catalyst for ring opening polymerization of ε-caprolactone.

The ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) using phosphorus pentoxide (P2O5) as a metal-free catalyst and isopropanol (iPrOH) as initiator resulted in the preparation of poly(ε-caprolactone) with narrow weight distribution. NMR spectroscopy analyses of the prepared PCL indicated the presence of the initiator residue at the end of the polymer chain, implying the occurrence of the ε-CL-catalysis ROP through a monomer activation mechanism. Kinetic experiments confirmed the controlled/living nature of ε-CL ring-opening catalyzed by phosphorus pentoxide. The commercial availability of phosphorus pentoxide and its easy-handling provide additional opportunities for polymer synthesis and nanocomposite manufacturing.

Cationic indium catalysts for ring opening polymerization: tuning reactivity with hemilabile ligands.

This is a comprehensive study of the effects of rationally designed hemilabile ligands on the stability, reactivity, and change in catalytic behavior of indium complexes. We report cationic alkyl indium complexes supported by a family of hemi-salen type ligands bearing hemilabile thiophenyl (2a), furfuryl (2b) and pyridyl (2c) pendant donor arms. Shelf-life and stability of these complexes followed the trend 2a < 2b < 2c, showing direct correlation to the affinity of the pendant donor group to the indium center. Reactivity towards polymerization of epichlorohydrin and cyclohexene oxide followed the trend 2a > 2b > 2c with control of polymerization following an inverse relationship to reactivity. Surprisingly, 2c polymerized racemic lactide without an external initiator, likely through an alkyl-initiated coordination-insertion mechanism.

Stable abnormal N-heterocyclic carbenes and their applications.

Although N-heterocyclic carbenes (NHCs) have been known as ligands for organometallic complexes since the 1960s, these carbenes did not attract considerable attention until Arduengo et al. reported the isolation of a metal-free imidazol-2-ylidene in 1991. In 2001 Crabtree et al. reported a few complexes featuring an NHC isomer, namely an imidazol-5-ylidene, also termed abnormal NHC (aNHCs). In 2009, it was shown that providing to protect the C-2 position of an imidazolium salt, the deprotonation occurred at the C-5 position, affording imidazol-5-ylidenes that could be isolated. Over the last ten years, stable aNHCs have been used for designing a range of catalysts employing Pd(ii), Cu(i), Ni(ii), Fe(0), Zn(ii), Ag(i), and Au(i/iii) metal based precursors. These catalysts were utilized for different organic transformations such as the Suzuki-Miyaura cross-coupling reaction, C-H bond activation, dehydrogenative coupling, Huisgen 1,3-dipolar cycloaddition (click reaction), hydroheteroarylation, hydrosilylation reaction and migratory insertion of carbenes. Main-group metal complexes were also synthesized, including K(i), Al(iii), Zn(ii), Sn(ii), Ge(ii), and Si(ii/iv). Among them, K(i), Al(iii), and Zn(ii) complexes were used for the polymerization of caprolactone and rac-lactide at room temperature. In addition, based on the superior nucleophilicity of aNHCs, relative to that of their nNHCs isomers, they were used for small molecules activation, such as carbon dioxide (CO2), nitrous oxide (N2O), tetrahydrofuran (THF), tetrahydrothiophene and 9-borabicyclo[3.3.1]nonane (9BBN). aNHCs have also been shown to be efficient metal-free catalysts for ring opening polymerization of different cyclic esters at room temperature; they are among the most active metal-free catalysts for ε-caprolactone polymerization. Recently, aNHCs successfully accomplished the metal-free catalytic formylation of amides using CO2 and the catalytic reduction of carbon dioxide, including atmospheric CO2, into methanol, under ambient conditions. Although other transition metal complexes featuring aNHCs as ligand have been prepared and used in catalysis, this review article summarize the results obtained with the isolated aNHCs.

An unprecedented cobalt(ii)-containing Wells–Dawson-type tungstovanadate-based metal–organic framework as an efficient catalyst for ring-opening polymerization of ε-caprolactone

An unprecedented organic–inorganic hybrid material based on Wells–Dawson-type tungstovanadate building blocks and cobalt(ii)–organic framework with a bis(triazole) ligand was prepared and employed to catalyze solvent-free ROP of caprolactone.

Organocatalytic ring-opening polymerization of disulfide functional macrocyclic carbonates: An alternative strategy to enzymatic catalysis

Ring-opening polymerization (ROP) of macrocyclic carbonates (≥12 ring) is still challenging due to the associated low ring-strain. Although organometallic and enzyme based catalysts have been studied, disadvantages such as organometallic residue and enzymic nature limit their applications. After screening commercially available organocatalysts, we report the use of organocatalysts for ROP of disulfide-containing macrocyclic carbonates (MSS, 16 ring). It was found that organocatalysts (TBD) presented high active and living ROP of MSS, as evidenced by kinetic studies, yielding main chain disulfide-containing polycarbonates with tailor-made structures and predictable molecular weights with low molecular weight distribution. Copolymerizations with trimethylene carbonate (TMC) generated random copolymers with controlled components, regulating the density of disulfide functional groups. By comparing with the behaviors of enzyme catalysis in kinetic studies and (co-)polymerization, it is observed that organic catalyzed ROP showed more efficient (~ ten times faster), milder condition and more controlled behaviors than enzyme catalyzed ROP (N-435). Therefore, we believe this organic catalyzed strategy will provide an alternative to the current enzymatic and organometallic catalyst for ROP of macrocyclic carbonates.

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.

Chromium Amino-bis(phenolate) Complexes as Catalysts for Ring-Opening Polymerization of Cyclohexene Oxide

The chromium(III) amino-bis(phenolate) “ate” complex, {Na[CrCl2L1]}, 1, where L1 = 6,6′-((1,4-diazepane-1,4-diyl)bis(methylene))bis(2,4-di-tert-butylphenolato) and its derivatives 2–4 (where the ph...

Titanium, zirconium and hafnium complexes bearing amino-benzotriazole phenolate ligands as efficient catalysts for ring-opening polymerization of lactides

Group 4 catalysts incorporated in 2-(2H-benzotriazol-2-yl)-4-methyl-6-(morpholinomethyl)phenolate ligand (C1MOBTP) were synthesized and structurally characterized. The reaction of Ti(OiPr)4 or M(OiPr)4(iPrOH) (M = Zr, Hf) with C1MOBTP-H (2.0 M equiv.) in toluene produced the hexa-coordinated bis-adduct metal complex [(C1MOBTP)2M(OiPr)2] (M = Ti for 1, Zr for 2 and Hf for 3). Experimental results indicate that hafnium complex 3 displayed the higher catalytic activity than those of di-C1MOBTP-featuring complexes 1 and 2 for the controlled L-LA polymerization. Hafnium alkoxide 3 catalyzes the ring-opening polymerization effectively not only in a “controlled” fashion but also an “immortal” manner, giving the corresponding polymers with the expected molecular weights and narrow molecular weight distributions.

Benzannulated N-heterocyclic plumbylene: An efficient catalyst in ring opening polymerization of l-lactide

Main-group metal-based catalysts have attracted considerable attentions due to their complementary reactivities to transition metals. In searching for the catalytic application of heavy divalent Group 14 derivatives, we came to discover that the benzannulated N-heterocyclic plumbylene (1) could serve as an effective catalyst for ring-opening polymerization of l-lactide (L-LA) at room temperature. In the presence of benzyl alcohol (BnOH), 97% conversion of L-LA was accomplished in 10 min under a catalyst loading of merely 0.3 mol% ([LA]:[1]:[BnOH] = 400:1.2:1.2, [1] = 1.2 mM). Compared to the polymeric plumbylene ([Pb(μ-OiPr)2]∞), the excellent catalytic activity of 1 at ambient temperature may be ascribed to the steric and electronic stabilizations of the Pb2+ center by the organic backbone. Thus, thermal activation is no longer required to expose the Lewis acidic metal center for polymerization.

Aluminium complexes as catalysts for ring-opening polymerization of ε-caprolactone bearing Schiff base ligands derived from 4,4′-Methylenebis (2,6-diisopropylaniline)

The mono and binuclear compounds, [4,4’ methylene bis (2,6-diisopropylaniline) (3,5-di-tert-butylsalicylaldehyde) AlEt2] (1), (2,6-diisopropylaniline) bis- (3,5-tBu2C6H2-O-2) AlEt2], obtained by reacting between triethyl-aluminium and Schiff base derived from reaction of 4,4’ methylene bis (2,6-diisopropylaniline with one and two equivalent of 3,5-Di-tert-butylsalicylaldehyde to form L1H and L2H2 respectively. The reaction of L1H with triethyl aluminium lead to form monometallic complex, while L2H reacted with two equivalent of triethyl aluminium formed bimetallic complex (2). Both ligands and catalysts have been prepared and characterized by IR, 1HNMR and CHN analysis. The compounds 1 and 2 are investigated as catalysts for the polymerization of ε-caprolactone in the presence of benzyl alcohol. Both complexes showed moderate activity as pre-catalysts for ε-caprolactone polymerisation.

Organotin-bridged ionic liquid as a solvent-free, leaching-resistive catalyst for ring opening polymerization of ε-caprolactone

An easy synthesis provides a monoalkyltin trichloride grafted onto an ionic liquid. The catalyst paves the way to nontoxic biologically relevant materials.

Thermal degradation and crystallization kinetics studies on synthesized calcium mercaptosuccinate end-capped poly(ε-caprolactone) nanocomposite

The calcium mercaptosuccinate (Ca-MS) functionalized poly(e-caprolactone) (PCL) was successfully synthesized by ring-opening polymerization (ROP) method at 160 °C for 2 h under nitrogen atmosphere with mild stirring in the presence of stannous octoate (SO) as ROP catalyst. The synthesized Ca-MS functionalized PCL was characterized by FTIR spectroscopy, NMR spectroscopy, atomic force microscopy, HRTEM and polarized optical microscopy (POM) like analytical techniques. The microstructure of Ca-MS nucleated PCL crystals was analysed by POM. The thermal properties of Ca-MS functionalized PCL were examined using differential scanning calorimetry and thermogravimetric analysis techniques. The non-isothermal crystallization and degradation kinetic studies were carried out to assess the crystallinity as well as the energy of activation (Ea) of Ca-MS functionalized PCL. The FTIR spectrum showed a peak at 1720 cm−1 corresponding to the carbonyl stretching of PCL. The 1H-NMR spectrum showed an alkoxy proton signal at 4.1 ppm. The non-isothermal crystallization kinetic study showed the 3D crystal growth with the Ea value of 142.4 kJ/mol.

HNTs/GO composite as efficient catalyst for ring-opening polymerization of ε-Caprolactone

ABSTRACTPolycaprolactone (PCL) has been widely applied based on the biocompatibility and biodegradability. It is important to investigate the safe and efficient catalyst system for ring-opening polymerization of monomer ε-caprolactone (ε-CL). In our work, Halloysite/Graphene Oxide (HNTs/GO) composite is successfully prepared by electrostatic self-assembly and used as an effective catalyst for the ring-opening polymerization of ε-CL. The structures, morphologies and thermal properties of the products are subsequently characterized. The results show that the synthesized PCL are semi-crystalline and observed to be lamellar, which depend on the template effects of HNTs/GO composite. The HNTs/GO composite serves as an effective catalyst and a reinforcing filler simultaneously to improve the thermal stability of the prepared PCL.

High Efficiency Organic Lewis Pair Catalyst for Ring-Opening Polymerization of Epoxides with Chemoselectivity

Ring-opening polymerization (ROP) is a versatile approach to well-defined polymers. Achieving both high efficiency and precise control remains a major challenge. We herein report room-temperature living ROP of epoxides catalyzed by combined organobase and triethylborane (Et3B) with water or alcohols as initiators. Extremely high catalytic efficiency (turnover frequency up to 6000 h–1) is exhibited despite the relatively mild Lewis basicity and acidity, and the catalyst loading can be minimized to 44 ppm. Poly(ethylene oxide) products are noncytotoxic toward a variety of cell lines even without purification. Poly(propylene oxide) with molar mass as high as 210 kg mol–1 and low dispersity (ĐM < 1.1) can be achieved owing to the absence of detrimental transfer reactions. The chemoselectivity allows the catalyst to be readily applied to living ROP of glycidyl ethers, tailored synthesis of diverse functional/block/nonlinear (co)polyether structures, and one-pot one-catalyst synthesis of polyether-based polyure...

Guanidinium as bifunctional organocatalyst for ring-opening polymerizations

Typical organocatalyst promoted ring-opening polymerization (ROP) by activation of monomer or polymer chain end via the acidic or basic mechanism. Dual activating of both the monomer and the propagating polymer chain end suggested an ideal strategy of widely tuneable catalysis of high efficiency. Stoichiometric Brønsted acid–base adduct was rarely employed in ROPs since general Brønsted acid and Brønsted base quench each other. We proposed that an acid–base adduct may work bifunctionally in catalysis. The cationic part of the acid–base adduct as H-bond donor activated the monomer, and the counter anion part as H-bond acceptor activated initiator/chain end. A series of guanidiniums, derived from commercially available Brønsted acids and guanidine, were employed as catalysts in ROPs of carbonate, lactide, and lactone. Guanidinium triazabicyclodecene hydrochloride exhibited outstanding catalytic performances in the respect of the rate and controlled in the ROPs of l-lactide, trimethylene carbonate, and δ-valerolactone with nearly full conversions, predicted molecular weights, as well as narrow dispersity of Đ = 1.07. These experimental results indicated that guanidinium is an efficient organocatalyst in controlled/living ring-opening polymerization.