Use of Titanium Complexes Bearing Diphenolate or Calix[n]arene Ligands in α-Olefin Polymerization and the ROP of Cyclic Esters

Crafting sustainable solutions: architecting biodegradable copolymers through controlled ring-opening copolymerization

Polylactic acid (PLA) is a biodegradable and biocompatible polymer with versatile applications in packaging and medicine. It is derived from lactic acid and thus represents an eco-friendly option sourced from renewable raw materials. Despite its advantages, PLA exhibits few drawbacks, such as brittleness and relatively high melting and glass transition temperatures. However, these limitations can be addressed through copolymerization with other monomers like ε-caprolactone (ε-CL), resulting in a composite material with improved physical properties. This paper comprehensively reviews achievements in PLA-PCL copolymerization using organometallic catalysts, discussing scientific findings and various copolymer architectures obtained, including random or block configurations. It also demonstrates various sustainable catalysts for achieving the required microstructure under mild reaction conditions without the aid of any external initiator.

Reaction of Ph2C(X)(CO2H) (X = OH, NH2) with [VO(OR)3] (R = Et, nPr): structure, magnetic susceptibility and ROP capability

Reaction of [VO(OR)3] (R = Et, nPr) with 2,2'-diphenylglycine afforded the alkoxide-bridged dimers {[VO(OR)(μ-OR)][Ph2C(NH2)(CO2)]}2, whereas use of benzilic acid, in the presence of alkali metals, afforded 16-membered metallocycles {V8(O)4M(OR)8[Ph2C(OH)(CO2)]12} (M = <1 Na, K). For the ring systems, magnetic susceptibility data is consistent with mixed-valence vanadium with an average oxidation state of 3.5. The dimer and ring systems are capable of the ring opening polymerisation (ROP) of ε-caprolactone under N2, air, or as melts affording mostly low to medium molecular weight cyclic and linear products.

Improvement of catalytic activity of aluminum complexes for the ring-opening polymerization of ε-caprolactone: aluminum thioamidate and thioureidate systems

In this study, a series of Al complexes bearing amidates, thioamidates, ureidates, and thioureidates were synthesized and their catalytic activity for ε-caprolactone (CL) polymerization was evaluated. SPr-Al exhibited a higher catalytic activity than OPr-Al (3.2 times as high for CL polymerization; [CL] : [SPr-Al] : [BnOH] = 100 : 0.5 : 2; [SPr-Al] = 10 mM, conv. = 93% after 14 min at 25 °C), and USCl-Al exhibited a higher catalytic activity than UCl-Al (4.6 times as high for CL polymerization; [CL] : [USCl-Al] : [BnOH] = 100 : 0.5 : 2; [USCl-Al] = 10 mM, conv. = 90% after 15 min at 25 °C). Regardless of whether aluminum amidates or ureidates were present, thioligands improved the polymerization rate of aluminum catalysts. Density functional theory calculations revealed that the eight-membered ring [SPr-AlOMe2]2 decomposed into the four-membered ring SPr-AlOMe2. However, [OPr-AlOMe2]2 did not decompose because of its strong bridging Al-O bond. The overall activation energy required for CL polymerization was lower when using [SPr-AlOMe2]2 (18.1 kcal mol-1) as a catalyst than when using [OPr-AlOMe2]2 (23.9 kcal mol-1). This is because the TS2a transition state of SPr-AlOMe2 had a more open coordination geometry with a small N-Al-S angle (72.91°) than did TS3c of [OPr-AlOMe2]2, the crowded highest-energy transition state of [OPr-AlOMe2]2 with a larger N-Al-O angle (99.63°).

Ring Opening Polymerization of Lactides and Lactones by Multimetallic Titanium Complexes Derived from the Acids Ph2C(X)CO2H (X = OH, NH2)

The reactions of the titanium alkoxide [Ti(OR)4] (R = Me, nPr, iPr, tBu) with the acids 2,2’-Ph2C(X)(CO2H), where X = OH and NH2, i.e., benzilic acid (2,2’-diphenylglycolic acid, L1H2), and 2,2’-diphenylglycine (L2H3), have been investigated. The variation of the reaction stoichiometry allows for the isolation of mono-, bi-, tri or tetra-metallic products, the structures of which have been determined by X-ray crystallography. The ability of the resulting complexes to act as catalysts for the ring opening polymerization (ROP) of ε-caprolactone (ε-CL) and r-lactide (r-LA) has been investigated. In the case of ε-CL, all catalysts except that derived from [Ti(OnPr)4] and L2H3, i.e., 7, exhibited an induction period of between 60 and 285 min, with 7 exhibiting the best performance (>99% conversion within 6 min). The PCL products are moderate- to high-molecular weight polymers. For r-LA, systems 1, 3, 4 and 7 afforded conversions of ca. 90% or more, with 4 exhibiting the fastest kinetics. The molecular weights for the PLA are somewhat higher than those of the PCL, with both cyclic and linear PLA products (end groups of OR/OH) identified. Comparative studies versus the [Ti(OR)4] starting materials were conducted, and although high conversions were achieved, the control was poor.

Synthesis of a Zirconium Complex of an N,O-type p-tert-Butylcalix[4]arene and Its Application in Some Multicomponent Reactions

The synthesis and characterization of a new octahedral Zr(IV) complex of oxygen-depleted N,O-type calixarene ligand comprising two distal-functionalized pyrazole rings have been reported. The cone shape and structure of the prepared complex were confirmed univocally by single-crystal X-ray diffraction and NMR studies. The Zr metal lies at 2.091 Å from the plane of the calixarene ring. This complex has been utilized as an efficient catalyst for the synthesis of Biginelli adducts, bis(indolyl)methanes, and coumarins. This complex (Cl₂Zr-calixarene) showed superior activity for these multicomponent reactions in comparison to the corresponding Ti(IV) and Zn(II) analogues. Ferrocene-appended bis(indolyl)methane, prepared using this catalyst, was also evaluated for its anticancer activity against the A-172 cell line.

Trinuclear zinc calix[4]arenes: synthesis, structure, and ring opening polymerization studies

The trinuclear zinc calix[4]arene complexes [Zn₃(O₂CCH₃)₂(L(O)₂(OMe)₂)₂·xMeCN (x = 7.5, 1; x = 6, 1'), [Zn₃(O₂CCH₃)₂(L(O)₂(OnPr)₂)₂·5MeCN (2·5MeCN), [Zn₃(OEt)₂(L(O)₂(OMe)₂)₂]·4MeCN (3·4MeCN), [Zn₃(OEt)₂(L(Opentyl)₂)₂]·4.5MeCN (4·4.5MeCN) and [Zn₃(OH)₂(L(O)₂(On-pentyl)₂]·8MeCN (5·8MeCN) have been isolated from reaction of [(ZnEt)₂(L(O)₂(OR)₂)₂] (L(OH)₂(OR)₂ = 1,3-dialkoxy-4-tert-butylcalix[4]arene; R = methyl, n-propyl or pentyl) and the reagents acetic acid, ethanol, and presumed adventitious water, respectively. Attempts to make 5via a controlled hydrolysis led only to the isolation of polymorphs of (L(OH)₂(Opentyl)₂·MeCN. Reaction of [Zn(C₆F₅)₂] with L(OH)₂(Opentyl)₂, in the presence of K₂CO₃, led to the isolation of the complex [Zn₆(L(On-pentyl))₂(OH)₃(C₆F₅)₃(NCMe)₃]·3MeCN (6·3MeCN). The molecular structures of 1-6 reveal they all contain a near linear (163 to 179°) Zn₃ motif. In 1-5, a central tetrahedral Zn centre is flanked by trigonal bipyramidal Zn centres, whilst in 6, for the linear Zn₃ unit, a central distorted octahedral zinc centre is flanked by trigonal planar and a tetrahedral zinc centres. Screening for the ring opening polymerization (ROP) of ε-caprolactone at 90 °C revealed that they are active with moderate to good conversion affording low to medium molecular weight products with at least two series of ions. For comparative studies, the trinuclear aminebis(phenolate) complex [Zn₃(Oi-Pr)₂L^/] (L^/ = n-propylamine-N,N-bis(2-methylene-4,6-di-tert-butylphenolate) I was prepared. Kinetics revealed the rate order I > 4 > 6 ≈ 2 ≈ 1 > 3.

An ultrastable Ti-based metallocalixarene nanocage cluster with photocatalytic amine oxidation activity

Polyhedral metallocalixarene nanocage clusters based on pure Ti(IV) ions are to our knowledge unknown hitherto. Herein we report the first Ti(IV)-based metallocalixarene nanocage cluster by assembling a [Ti₁₃O₁₄] cage with six t-butylcalix[4]arene molecules. Notably, the cluster exhibits extraordinary stability in high-concentration acid/alkali solutions and can act as a stable photocatalyst to catalyze the oxidation of ammonia to imines.

Azo-methoxy-calix[4]arene complexes with metal cations for chemical sensor applications: Characterization, QTAIM analyses and dispersion-corrected DFT- computations

In this work, the structures, quantum chemical descriptors, morphologic characterization of the azo-methoxy-calix[4]arene were investigated. The analyses and interpretation of the theoretical and the experimental IR spectroscopy results for the corresponding compounds was performed. The complexation of the azo-methoxy-calix[4]arene with Zn²⁺,Hg²⁺ , Cu²⁺ , Co²⁺, Ni²⁺ , Pb²⁺ and Cd²⁺metal cations has been calculated by the dispersion corrected density functional theory (DFT-D3). The values of the interaction energies show that the specific molecule is more selective to the Cu²⁺ cation. The study of the reactivity parameters confirms that the azo-methoxy-calix[4]arene molecule is more reactive and sensitive to the Cu²⁺ cation than that Co²⁺ and Cd²⁺. In addition, the investigation of the electrophilic and nucleophilic sites has been studied by the molecular electrostatic potential (MEP) analysis. The Hirshfeld surface (HS) analysis of the azo-methoxy-calix[4]arene-Cu²⁺ interaction have been used to understand the Cu⋯hydrogen-bond donors formed between the cation and the specific compound. The Quantum Theory of Atoms in Molecules (QTAIM) via Non covalent Interaction (NCI) analysis was carried out to demonstrate the nature, the type and the strength of the interaction formed between the Cu²⁺ cation and the two symmetrical ligands and the cavity. Finally, the chemical sensor properties based on the Si/SiO₂/Si₃N₄/Azo-methoxy-calix[4]arene for detection of Cu²⁺ cation were studied. Sensing performances are determined with a linear range from 10^-5.2 to 10^-2.2 M. The Si/SiO₂/Si₃N₄/azo-methoxy-calix[4]arene structure is a promoter to have a good performance sensor.

Copolymerization of L-Lactide and ε-Caprolactone promoted by zinc complexes with phosphorus based ligands

The zinc complexes 1 and 2 bearing chelating phosphorous based pincer ligands were found to catalyze the ring-opening copolymerization of L-lactide and ε-caprolactone in the melt at 110 °C to obtain a series of random copolymers in which the monomer distributions were coherent with the monomer ratio in the feed. All the obtained copolymers showed high molecular masses with monomodal and moderately narrow distributions. The thermal properties of the achieved copolymers (T_g, glass transition temperature, and T_m, melting temperature) were strongly dependent on their composition. A linear dependence of T_g with molar percentage of lactide in the copolymers was observed over a temperature range from -59 °C (pure polycaprolactone) to 55 °C (pure polylactide).

Lithium calix[4]arenes: structural studies and use in the ring opening polymerization of cyclic esters

We have structurally characterized a number of lithiated calix[4]arenes, where the bridge in the calix[4]arene is thia (–S–, L^SH₄), sulfinyl (–SO–, L^SOH₄), sulfonyl (–SO₂–, L^SO2H₄), dimethyleneoxa (–CH₂OCH₂–, L^COCH₄) or methylene (–CH₂–, LH₄). In the case of L^4SH₄, interaction with LiOtBu led to the isolation of the complex [Li₈(L^4S)₂(THF)₄]·5THF (1·5THF), whilst similar interaction of L^4SOH₄ led to the isolation of [Li₆(L^4SOH)₂(THF)₂]·5(THF) (2·5THF). Interestingly, the mixed sulfinyl/sulfonyl complexes [Li₈(calix[4]arene(SO)(SO₂)(SO_1.68)₂)₂(THF)₆]·8(THF) (3·8THF) and [Li₅Na(L^SO/3SO2H)₂(THF)₅]·7.5(THF) (4·7.5(THF) have also been characterized. Interaction of LiOtBu with L^SO2H₄ and L^COCH₄ afforded [Li₅L^4SO2(OH)(THF)₄]·2THF (5·2THF) and [Li₆(L^COC)₂(HOtBu)₂]·0.78THF·1.22hexane (6·0.78THF·1.22hexane), respectively. In the case of LH₄, reaction with LiOtBu in THF afforded a monoclinic polymorph [LH₂Li₂(thf)(OH₂)₂]·3THF (7·3THF) of a known triclinic form of the complex, whilst reaction of the de-butylated analogue of LH₄, namely de-BuLH₄, afforded a polymeric chain structure {[Li₅(de-BuL)(OH)(NCMe)₃]·2MeCN}_n (8·2MeCN). For comparative catalytic studies, the complex [Li₆(L^Pr)₂(H₂O)₂]·hexane (9 hexane), where L^Pr2H₂ = 1,3-di-n-propyloxycalix[4]areneH₂, was also prepared. The molecular crystal structures of 1–9 are reported, and their ability to act as catalysts for the ring opening (co-)/polymerization (ROP) of the cyclic esters ε-caprolactone, δ-valerolactone, and rac-lactide has been investigated. In most of the cases, complex 6 outperformed the other systems, allowing for higher conversions and/or greated polymer M_n.

Novel Li-calix[n]arene complexes (n = 3, 4) having (–S–), (–SO–), (–SO₂–), (–CH₂OCH₂–) or (–CH₂–) bridges have been synthesized and fully characterized. Their catalytic activity in the ring opening polymerization of lactones has been studied.

Use of pyrazoles as ligands greatly enhances the catalytic activity of titanium iso-propoxide for the ring-opening polymerization of l-lactide: a cooperation effect

Using TiOⁱPr₄ with a pyrazole ligand for one-pot LA polymerization improved catalytic activity compared with using TiOⁱPr₄ only. At 60 °C, TiOⁱPr₄ with ^furPz exhibited a higher catalytic activity (approximately 3-fold) than TiOⁱPr₄. At room temperature, TiOⁱPr₄ with ^BuPz exhibited a higher catalytic activity (approximately 17-fold) than TiOⁱPr₄. High molecular mass PLA (M _nGPC = 51 100, and Đ = 1.10) could be produced by using TiOⁱPr₄ with ^furPz in melt polymerization ([TiOⁱPr₄] : [^furPz] = 1000 : 1 : 1 at 100 °C, 240 min). The crystal structure of ^MePz₂Ti₂OⁱPr₇ revealed the cooperative activation between two Ti atoms during LA polymerization.

INSIGHTS into the structures adopted by titanocalix[6 and 8]arenes and their use in the ring opening polymerization of cyclic esters

Interaction of p-tert-butylcalix[6]areneH6, L1H6, with [TiCl4] afforded the complex [Ti2Cl3(MeCN)2(OH2)(L1H)][Ti2Cl3(MeCN)3(L1H)]·4.5MeCN (1·4.5MeCN), in which two pseudo-octahedral titanium centres are bound to one calix[6]arene. A similar reaction but employing THF resulted in the THF ring-opened product [Ti4Cl2(μ3-O)2(NCMe)2(L)2(O(CH2)4Cl)2]·4MeCN (2·4MeCN), where LH4 = p-tert-butylcalix[4]areneH4. Interaction of L1H6 with [TiF4] (3 equiv.) led, after work-up, to the complex [(TiF)2(μ-F)L1H]2·6.5MeCN (3·6.5MeCN). Treatment of p-tert-butylcalix[8]areneH8, L2H8, with [TiCl4] led to the isolation of the complex [(TiCl)2(TiClNCMe)2(μ3-O)2(L2)]·1.5MeCN (4·1.5MeCN). From a similar reaction, a co-crystallized complex [Ti4O2Cl4(MeCN)2(L2)][Ti3Cl6(MeCN)5(OH2)(L2H2)]·H2O·11MeCN (5·H2O 11MeCN) was isolated. Extension of the L2H8 chemistry to [TiBr4] afforded, depending on the stoichiometry, the complexes [(TiBr)2(TiBrNCMe)2(μ3-O)2(L2)]·6MeCN (6·6MeCN) or [[Ti(NCMe)2Br]2[Ti(O)Br2(NCMe)](L2)]·7.5MeCN (7·7.5MeCN), whilst use of [TiF4] afforded complexes containing Ca2+ and Na+, thought to originate from drying agents, namely [Ti8CaF20(OH2)Na2(MeCN)4(L2)2]·14MeCN (8·14MeCN), [Na(MeCN)2][Ti8CaF20NaO16(L2)2]·7MeCN (9·7MeCN) or [Na]6[Ti8F20Na(MeCN)2(L2)][Ti8F20Na(MeCN)0.5(L2)]·15.5(C2H3N) (10·15.5MeCN). In the case of [TiI4], the ladder [(TiI)2(TiINCMe)2(μ3-O)2(L2)]·7.25CH2Cl2 (11·7.25CH2Cl2) was isolated. These complexes have been screened for their potential to act as catalysts in the ring opening polymerization (ROP) of ε-caprolactone (ε-CL), δ-valerolactone (δ-VL) and rac-lactide (r-LA), both in air and N2. For ε-CL and δ-VL, moderate activity at 130 °C over 24 h was observed for 1, 9 and 11; for r-LA, only 1 exhibited reasonable activity. In the case of the co-polymerization of ε-CL with δ-VL, the complexes 1 and 11 afforded reasonable conversions and low molecular weight polymers, whilst 4, 6, and 9 were less effective. None of the complexes proved to be active in the co-polymerization of ε-CL and r-LA under the conditions employed herein.

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.