Search

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°). [ABSTRACT FROM AUTHOR]

In this study, guaiacolate derivatives‐containing tetranuclear sodium catalysts were synthesized, and their catalytic activity in l‐lactide (LA) polymerization was evaluated. Using LBF‐Na (sodium complex bearing 2,3‐dihydro‐2,2‐dimethyl‐7‐benzofuranolate) as a catalyst, the polymerization of LA was studied in various solvents. LBF‐Na polymerized LA in dichloromethane, tetrahydrofuran, and toluene, and the catalytic activity of LBF‐Na was similar in these solvents. Among the sodium complexes, LOH‐Na (sodium complex bearing guaiacolate) exhibited higher catalytic activity (5 min, conversion = 93%; MnGPC = 7200) than the other tetranuclear sodium complexes. Furthermore, LOH‐Na exhibited an "immortal" property after 20 equivalents of benzyl alcohol loading during LA polymerization and good controllability in polylactide production. Thus, guaiacolate ligands increase the catalytic activity of sodium catalysts in LA polymerization in both coordinating and halogen‐containing solvents and under the condition of extra alcohol loading. [ABSTRACT FROM AUTHOR]

A series of zinc complexes, [LXZnEt] (1–5) and [LXZn2(OAc)3] (6–9), associated with NNO‐tridentate Schiff base ligands (2‐(((2‐((cyclohexyl[methyl]amino)methyl)phenyl)imino)methyl)phenolate (CAP) derivatives), were synthesized, and their activity toward ring‐opening polymerization (ROP) of L‐lactide (LA) and the reaction of CO2 with cyclohexene oxide were also investigated. All of [LXZnEt] revealed excellent catalytic activity to ring‐opening polymerization (ROP) of LA in the presence of benzyl alcohol. Among them, [LHZnEt] (1) showed the highest activity with 82% conversation within 45 s. In contrast, [LXZn2(OAc)3] (6–9) were inactive in ROP of L‐lactide. In addition, all of these Zn complexes demonstrated moderate activity in the reaction of CO2 with cyclohexene oxide in the presence of Bu4NCl. [ABSTRACT FROM AUTHOR]

A series of bidentate ligands were examined to improve the catalytic activity of l-lactide ( LA) polymerization by using n-butyl lithium and BnOH. For LA polymerization, n-butyl lithium with tetraisopropyl methylenebis(phosphonate) ( L8 ) showed the greatest catalytic activity but with poor controllability of the polymer molecular weight. The polydispersity indices ( PDIs) could be improved without BnOH addition, but Mn GPC was much different from the Mn Cal . 1H NMR spectra confirmed that the cyclized PLA was obtained, thus implying that active chain-end mechanisms were operative in LA polymerization. [ABSTRACT FROM AUTHOR]

This study synthesized a series of titanium iminophenoxide complexes and investigated their suitability as catalysts for the ring-opening polymerization of L-lactide ( L-LA) and ε-caprolactone (CL). Complexes with bidentate ligands demonstrate higher catalytic activity than their tridentate counterparts since the third coordination atom needs to contend with L-LA and CL. Differences in the geometric framework of bidentate ligands also influence the catalytic activity. Type II ligands ( N, N-trans form of Ti complex) prevent the coordination of monomers to Ti thereby decreasing the initiation rate. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013 [ABSTRACT FROM AUTHOR]

This study investigated the synthesis of the biocatalyst, magnesium 2,6-di- tert-butyl-4-methylphenoxide (Mg(BHT)2) complex, and the ring-opening polymerization (ROP) of ε-caprolactone (CL). The complex demonstrates high catalytic activity and controllable of molecular weight for the ROP of CL in tetrahydrofuran at room temperature, even when polymerization was performed under air. Before this study, the polymerization of CL had never been performed using a magnesium catalyst under air at room temperature. Various forms of alcohols with different purposes were also used as initiators with Mg(BHT)2. The results show that the magnesium complex acts as a perfect catalyst because of its high catalytic activity and control ability without any cytotoxicity in the polymerization of CL, making it suitable for biomedical applications. In addition, nanoparticle formation, cytotoxicity, and phototoxicity of tri-2-hydroxyethyl ester [Ce6-(CH2CH2OPCL)3] were also studied in this article and Ce6-(CH2CH2OPCL)3 formed nanoparticle can act as a nanophotosensitizer for photodynamic therapy. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012 [ABSTRACT FROM AUTHOR]

A series of novel magnesium and zinc aminophenoxide complexes were successfully synthesized and one zinc complex was characterized by X-ray crystallography. They were also investigated as initiators for the ring opening polymerization of l-lactide. The complexes are effective in forming polylactides with good conversions. The nature and steric bulk of the ligands coordinated to the central metal ions enormously influenced the polymer properties. Among all the complexes, the zinc aminophenoxide complexes as initiators produced polymers with good molecular weight control and relatively narrow PDIs. [ABSTRACT FROM AUTHOR]

Abstract: A series of β-diketiminate zinc complexes has been synthesized and their reactivity for the ring-opening polymerization (ROP) of lactide has been studied. The reaction of β-diketimines (LH) with diethyl zinc forms the monomeric [LZnEt] complexes which further react with benzyl alcohol (BnOH) in toluene/hexane yielding dinuclear or trinuclear zinc complexes. These complexes have been characterized by single crystal X-ray diffraction, which showed the tri- and tetra-coordinated zinc complexes in which zinc atoms exhibit trigonal and tetrahedral geometry, respectively. All complexes have been tested as initiators for the ring-opening polymerization of lactide; they are all highly active. The rate of polymerization is heavily dependent on the N-aryl substituents with the order: alkyl group∼alkoxy>halide group>nitro group. The β-diketiminate zinc complexes allow controlled ring-opening polymerization as shown by the linear relationship between the percentage conversion and the number-average molecular weight. On the basis of literature reports, a mechanism for ROP of lactide has been proposed. [Copyright &y& Elsevier]

Abstract: A novel calcium complex, [(DAIP)2Ca]2 (where DAIP-H=2-[(2-dimethylamino-ethylimino)methyl]phenol), is prepared in a one flask reaction by condensation of Ca(OMe)2 with DAIP-H in toluene/THF. Experimental results show that in the presence of various alcohols, [(DAIP)2Ca]2 efficiently initiates the ring-opening polymerization of l-lactide in a controlled fashion, yielding polymers with expectative molecular weight and low polydispersity indexes. Furthermore, kinetic studies show a first-order dependency on both [LA] and [BnOH]. [Copyright &y& Elsevier]

• AMM associated Al complexes had higher activity than that of ketiminate ligands. • Steric hindrance groups in the ligands revealed the greater catalytic activity. • Electron-withdrawing groups in the ligands revealed the greater catalytic activity. • Ligands with chelating group reduced the reactivity of ring-opening polymerization. A series of Al complexes bearing diethyl 2-((arylamino)methylene)malonate and diethyl 2-((alkylamino)methylene)malonate ligands (AMM ligands) were synthesized and their application in the ring-opening polymerization of ε -caprolactone was studied. All the Al complexes bearing AMM ligands exhibited higher catalytic activity (approximately 2.8–16.5 fold) than those Al complexes bearing ketiminate ligands did, except OiPrAlMe 2 (2 fold lower catalytic activity). In addition, Al complexes with steric hindrance or electron-withdrawing groups in the ligands revealed the greater catalytic activity, and the chelating effect reduced the reactivity of ring-opening polymerization. [ABSTRACT FROM AUTHOR]

• Comparison study of CL polymerization by six-membered ring Al complexes. • Al complexes bearing aminophenolate ligands revealed the greatest catalytic activity. A series of 6-membered ring aluminum (Al) complexes bearing Schiff base, benzotriazole phenolate, aminophenolate, carboxamide phenolate, and carboxylate phenolate were synthesized, and their applications in ε-caprolactone (CL) polymerizations were studied. The decreasing catalytic rates for CL polymerization were ranked as follows: Al complexes bearing aminophenolate > benzotriazole phenolate > carboxylate phenolate > Schiff base > carboxamide phenolate. The Al complex bearing aminophenolate (LBu-prop) revealed an appreciably higher (10–100 fold) polymerization rate and a shorter induction period than the other 6-membered ring Al complexes in the CL polymerization. Further investigation of the steric effect in the phenolate ring of aminophenolate ligands demonstrated that Al complexes with the tert -butyl groups on 2,4 positions of the phenolate ring had the highest catalytic activity, followed by that of the cumyl groups, and it was also discovered that the hydrogen substituents were responsible for the largest decrease in the catalytic activity of Al complexes. The bulky substituents on the amino group increased the catalytic activity of associated Al complexes as well as the aminophenolate ligands with the third chelating atoms on the amino group. However, the aminophenolate ligands with the fourth chelating atoms on the amino group slightly decreased the catalytic activity. [ABSTRACT FROM AUTHOR]

A series of titanium (Ti) complexes bearing ONO-tridentate Schiff bases were synthesized and their application to the ring-opening polymerization of L-lactide and ε-caprolactone was studied. Ti complexes bearing ONO-tridentate Schiff bases revealed a greater catalytic activity compared to other previously synthesized Ti complexes bearing multi-dentate phenolate ligands. We hypothesize that using ONO-tridentate Schiff bases as ligands reduces the occupied space and increases the monomer coordination around the Ti center, thereby increasing catalytic activity. The electronic effect of the phenolate group on the polymerization rate was also studied, showing that electron-donating substituents on the ligands could drastically increase polymerization activity. [ABSTRACT FROM AUTHOR]

A series of lithium complexes bearing NNO-tridentate Schiff base ligands were successfully synthesized, characterized by X-ray crystallography, and investigated as catalysts for the ring opening polymerization of l -lactide. The electronic effects and steric bulk of the ligands coordinated to Li atoms were determined to greatly influence its catalytic properties. Li complexes with electron-donating or bulky substituents of ligands both revealed higher catalytic activities than did the electron-withdrawing groups during polymerization. Among the Li complexes, [L Bu -Li] 2 with the two tert -butyl groups exhibited the highest catalytic activity and outstanding controllability. [ABSTRACT FROM AUTHOR]

• Ethyl salicylate-bearing Zn complexes exhibited a high polymerization rate during the polymerization of CL and LA. • The positive effect of substituents at the ortho position of phenolates was observed in CL polymerization. • Electronic donating substituents on phenolates enhanced the catalytic activity of Zn complexes in LA polymerization. • Bulky groups at the ortho position facilitated the detachment of the bridging alkoxide group to form the intermediate II-CL. • Ligands with electron-withdrawing groups may stabilize the intermediate trip. A series of zinc complexes bearing ethyl salicylates were synthesized, and their catalytic activity in the polymerization of ε -caprolactone (CL) and l -lactide (LA) was investigated. In CL polymerization, the catalytic activity of Zn complexes was promoted by ligands with the substituents at the ortho position of phenolates, such as the 2-methyl group of L6MeZnEt (40 min, 94% conversion at 25 °C, [CL] = 2.0 M) and the 2,4-di‑ tert ‑butyl groups of LBuZnEt (55 min, 93% conversion at 25 °C, [CL] = 2.0 M). In LA polymerization, the catalytic activity was promoted by ligands with electron-donating substituents, such as the 4-methyl group of L4MeZnEt (55 min, 90% conversion at 50 °C, [LA] = 0.5 M) and the 2,4-di‑ tert ‑butyl group of LBuZnEt (60 min, 90% conversion at 50 °C, [LA] = 0.5 M). Density functional theory calculations revealed that the polymerization requires the detachment of a bridging alkoxide group from one Zn center to form an intermediate II-CL ; bulky ligands may facilitate this unfavored dissociation process (from I-CL to II-CL). In LA polymerization, ligands with electron-withdrawing groups increase the Lewis acidity of Zn centers; the detachment of coordination atoms from the Zn atom is difficult; hence, the carbonyl group of ring-opening LA back binds to the Zn center to form a low-energy intermediate trip and then hinders another LA coordination. Ethyl salicylate-bearing Zn complexes exhibited a high polymerization rate during the polymerization of CL and LA. The positive effect of substituents at the ortho position of phenolates was observed in CL polymerization; by contrast, electronic donating substituents on phenolates enhanced the catalytic activity of Zn complexes in LA polymerization. DFT calculations revealed that bulky groups at the ortho position of phenolates facilitated the detachment of the bridging alkoxide group from the Zn center (I-CL) to form the intermediate II-CL in CL polymerization. However, the carbonyl group of ring-opening LA back binds to the Zn center to form the low-energy intermediate trip and then hindered another LA coordination. Ligands with electron-withdrawing groups may reduce the electron density on the Zn atom and increase the interaction between the Zn atom and the carbonyl group that stabilized the intermediate trip. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Multinuclear metal complexes improve the catalytic activity of catalysts for ROP of cyclic esters. • Multinuclear metal catalysts are cooperative processing of coordination-insertion reaction by metal centers. • Multinuclear metal catalysts improve the catalytic activity through the electronic (or inductive) effect. • An emphasis on comparing mononuclear and multinuclear metal catalysts was reported. Multinuclear metal complexes can effectively improve the catalytic activity of catalysts for ring-opening polymerization (ROP) of cyclic esters. If the catalytic metal centers are close enough to each other, they divide the labor of coordination-insertion reactions in polymerization processes. If the catalytic metal centers are too far apart, and the ligands connecting the catalytic metal centers can have resonance, their positive charge properties affect each other and thus affect the catalytic polymerization reaction in the same manner that electron-withdrawing groups do. This review focused on recent reports of multinuclear metal catalysts in ROP that exhibited higher catalytic activity than did mononuclear metal catalysts. The dilemma faced by the development of novel multinuclear metal complexes, and the strategies and the attention for designing catalysts are also provided. [ABSTRACT FROM AUTHOR]

[Display omitted] In this study, a series of mononuclear complexes and dinuclear aluminum complexes bridged by 1,3- and 1,4-substituted phenyl ligands were synthesized, and their activities for the ring-opening polymerization (ROP) of ε-caprolactone (CL) were investigated. All the dinuclear aluminum complexes exhibited higher catalytic activity than did the corresponding mononuclear Al complexes (2–6-fold for the k obs values), and most of the dinuclear Al complexes bearing 1,3-substituted phenyl ligands exhibited higher activity than did the corresponding dinuclear Al complexes bearing 1,4-substituted phenyl ligands (1.5–3-fold for the k obs values). The only exceptions were the Al complexes bearing benzothiazole ligands, for which the dinuclear Al complex with 1,4-bis(benzothiazol-2-yl)benzene exhibited higher catalytic activity than did the corresponding 1,3- bis(benzothiazol-2-yl)benzene complex. This higher activity of the dinuclear Al complexes was attributed to one of the Al atoms in the dinuclear Al system being regarded as an electron-withdrawing group. This group influenced the other Al atom through associated aromatic ligands to enhance the catalytic activity for the ROP of CL. [ABSTRACT FROM AUTHOR]

This study synthesized and examined a series of titanium aminophenoxide complexes as catalysts for the ring-opening polymerization of l -lactide and ɛ-caprolactone. These Ti complexes are more active for LA than for CL. The interaction between coordinating atoms of the ligands and the central metal ions has a considerable influence on the resulting catalyses. The complex with thiophenyl groups demonstrated the highest catalytic activity, due to the lability of thiophenyl groups. Rapid changes between association and dissociation can be used to tune the electronic density of Ti in order to avoid contending with the coordination of l -lactide and ɛ-caprolactone to increase activity. In addition, kinetic results indicate a first-order dependency on [ L OMe Ti(O i Pr) 2 ] and a first-order dependency on [CL] and [LA] respectively. [ABSTRACT FROM AUTHOR]

A series of titanium (Ti) compounds bearing 2,6-bis(o -hydroxyalkyl)pyridine ligands were synthesized and investigated as catalysts for the ring-opening polymerization of ε-caprolactone (CL) and L -lactide (LA). The Ti complexes with electron-withdrawing groups in pyridine rings or steric bulky groups in hydroxymethyl groups reduced the catalytic activity in CL and LA polymerizations, and Me L Ph -TiOPr 2 exhibited the highest catalytic activity ([CL] = 0.5 M, [CL]:[Cat] = 100:8, 60 °C, 7.5 h, conversion = 93%; [LA] = 0.5 M, [LA]:[Cat] = 100:8, 60 °C, 3.5 h, conversion = 98%). The density functional theory calculation results revealed that CL interacted with the ligand through weak hydrogen bonds, and then approached the Ti center. After being attacked by isopropyl oxide, the carbonyl group of CL bonded to the Ti center. After the rearrangement and ring opening of CL ring, CL was converted to poly-ε-caprolactone. Image 1 • Ti compounds bearing 2,6-bis(o -hydroxyalkyl)pyridine ligands were synthesized. • LA and CL polymerizations were investigated. • Me L Ph -TiOPr 2 exhibited the highest catalytic activity. [ABSTRACT FROM AUTHOR]

A series of aluminum complexes bearing heterocyclic amine ligands were synthesized, and their application in ε -caprolactone (CL) and L -Lactide (LA) polymerization and copolymerization were studied. The CL polymerization results revealed that ligands containing the higher electronegativity atom increased the polymerization rate of CL polymerization. Among these Al complexes, NON-AlMe 3 bearing benzofurazan ligand was the best catalyst in CL polymerization ([ NON-AlMe 3 ] = 10 mM, conversion = 94% after 4 min at room temperature). In LA polymerization, the catalytic trend of Al complexes was random, and NCN-AlMe 3 bearing 1-benzylbenzimidazole ligand was the best catalyst ([ NCN-AlMe 3 ] = 10 mM, conversion = 90% after 45 min at 80 °C). The selectivity of the production of a PLA- gradual -PCL copolymer could be controlled by different Al complexes. Our study revealed that the four-coordinated trimethyl Al complexes bearing heterocyclic amine ligands were easily synthesized and constitute an ideal catalyst design in CL polymerization. Image 1 • NON-AlMe 3 was the best catalyst in CL polymerization. • NCN-AlMe 3 was the best catalyst in LA polymerization. • PLA- gradual -PCL copolymer could be controlled by different Al complexes. [ABSTRACT FROM AUTHOR]

• 5-membered ring Mg complexes had higher activity in lactide polymerization. • Mg complexes with thiophene groups revealed the highest catalytic activity. • BnOH approaches LA through the hydrogen bond with phenolate, and then initiates LA. A series of magnesium complexes bearing 2-(arylideneamino)phenolate (forming 5-membered ring Mg complexes) and 2-((arylimino)methyl)phenolate (forming 6-membered ring Mg complexes) ligands were synthesized, and their application in the ring-opening polymerization of L -lactide (LA) was investigated. The Mg complexes with 5-membered rings exhibit a higher polymerization rate and controllability for polylactide production than those with 6-membered rings do (one-fold to three-fold for LA polymerization). The density function theory calculation results revealed that benzyl alcohol approaches LA through the hydrogen bond with phenolate oxygen, and then initiates LA, with the proton transferring to phenolate oxygen to form the LA-OBn adduct. The LA-OBn adduct was activated using the proton present on phenolate oxygen to open the ring of the LA-OBn adduct. Moreover, the LA polymerization using L5ThioMg as a catalyst needs to overcome a 17.7 kcal/mol activation free energy that is lower than that of L6ThioMg (21.0 kcal/mol) [ABSTRACT FROM AUTHOR]

A series of pyridine and polypyridine-zinc complexes were synthesized, and their application in ε-caprolactone and l -lactide polymerization was studied. In ε-caprolactone polymerization, zinc complexes bearing 1,10-phenanthroline derivatives exhibited a higher polymerization rate than did diethyl zinc (2–10 times). In l -lactide polymerization, zinc complexes bearing 1,10-phenanthroline derivatives exhibited a considerably higher polymerization rate than did diethyl zinc (2–220 times). According to density functional theory results, the ortho -hydrogens of polypyridine ligand can co-activate the carbonyl group of monomers with zinc metal center to increase the catalytic activity of zinc catalysts. Image 1 • The polypyridne-zinc complexes were synthesized. • 1,10-Phenanthroline derivatives enhanced the catalytic activity of Zn complexes. • Hydrogens of polypyridine co-activate the carbonyl group of monomers. [ABSTRACT FROM AUTHOR]

The ring-opening polymerizations of ε-caprolactone (CL) using 5- and 6-membered ring Zn complexes bearing Schiff base as catalysts were comparatively studied. The 5-membered ring Zn complexes exhibited a considerably higher polymerization rate than the 6-membered ring Zn complexes (2–12 fold for CL polymerization) did. X-ray data revealed that 5-membered ring L 5-Ph 2 Zn creates larger empty space around the Zn center than 6-membered ring L 6-Bn 2 Zn. This allows higher interaction between the initiator and Zn complex and easy CL coordination. Image 1 • The 5-membered ring Zn complexes exhibited a considerably higher polymerization rate. • X5-membered ring L5-Ph 2 Zn creates larger empty space around the Zn center than 6-membered ring L6-Bn 2 Zn [ABSTRACT FROM AUTHOR]

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 -O i Pr) 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. Image 1 • N -Heterocyclic plumbylene is an efficient catalyst in ring-opening polymerization of l -lactide. • 97% conversion of L- LA was accomplished in 10 min under a catalyst loading of merely 0.3 mol%. • N -Heterocyclic plumbylene reveals outstanding controllability in l -lactide polymerization. [ABSTRACT FROM AUTHOR]