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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]

In this study, a series of sodium complexes with amidate ligands, including sulfonamide, acylamide, and aryl carbamate, were synthesized. These sodium complexes were used as catalysts to study the L-lactide (LA) polymerization in high polar solvent (dimethyl sulfoxide: tetrahydrofuran = 1:4). Among these sodium complexes, CTolOMe-Na (5) exhibited the highest catalytic activity (10 s, conversion > 99% with the ratio [LA]:[CTolOMe-Na (5)]:[BnOH] = 800:1:120, [CTolOMe-Na (5)] = 1.25 mM at 0 °C) and controllability (Đ = 1.10). To prove that CTolOMe-Na (5) can polymerize LA with special initiators in polar solvent, 1,4-benzenedimethanol and pentaerythritol were used as initiators in LA polymerization. The polymerization results revealed that end-chain-modified poly(lactides) with a precise molar mass (MnGPC: 3000–8800) and narrow dispersity (1.04–1.28) can be synthesized. [ABSTRACT FROM AUTHOR]

A series of ligands were tested to meliorate the catalytic activity of ε-caprolactone (CL) and l-lactide (LA) polymerization by Sn(Oct) and BnOH. There are seven ligands (methyl 2-(dibenzylamino)acetate (MDBAA), di(1 H-pyrazol-1-yl)methane, benzanide, 8-aminoquinoline, 2-bromo-1-phenylethanone, 2,6-di- tert-butyl-4-methylphenol, and dipyridine) were found to be useful in increasing the polymerization rate for CL polymerization. For LA polymerization, there are only two ligands (8-aminoquinoline and p-thiocresol) effectively raised the polymerization rate. Moreover, the kinetic study reveals a first-order dependency on [CL] and second-order dependency on [LA], indicating different polymerization mechanisms in Sn(Oct) catalytic system. The mechanistic study also explains that MDBAA could potentially modify the framework of the catalytic intermediate and produce the hydrogen, thereby increasing the rate of CL polymerization. Graphical Abstract: [Figure not available: see fulltext.] [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 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]

[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]

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]

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]